moldovite - a review by milan_trnka & stanislac_ houzar

7/30/2019 Moldovite - a review by Milan_Trnka & Stanislac_ Houzar

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A number of general surveys about moldavites were pub-lished in Czech. They include books by Rost Vltavny a tek-tity [Moldavites and tektites], 1972, by Trnka and HouzarMoravsk vltavny [Moravian moldavites], 1991, and byBouka Tajemn vltavny [Mysterious moldavites], 1992.Bouka also published two books in English:Moldavites (to-gether with Konta, 1986), which describes mostly South Bo-

hemian moldavites, and Moldavites the Czech Tektites,1994, unfortunately a very poorly accessible book. Lange(1995) wrote a very detailed study about Lusatian mol-davites (in German). There is also a number of papers aboutmoldavite occurrences, geology of moldavite-bearing sedi-ments, properties and origin of moldavites etc., published inCzech, only with English or German summaries (e.g. Pro-ceedings on Moldavites Conferences IIVIII).

An overview of substrewn fields of moldavites

The deposits of moldavites are known from a few dis-crete regions (Fig. 1). The present boundaries and the sizeof substrewn fields are determined by the geological devel-opment of the whole territory and do not correspond to thedistribution of moldavites at the time of their fall. In view ofintensive denudation of a predominant part of the Bohemi-an Massif and adjacent regions, the present occurrences ofmoldavites are only relicts of the initial strewn field.

The nature of the original surface on which moldavitesfell and where they were buried by sediments has neverbeen identified reliably. Immediately after the moldavitesfell on the earths surface, they were transported to sec-ondary deposits. The oldest moldavite-bearing sedimentsare colluvio-fluvial clays and sands, which are designatedas sediments of the strewn field. In these sediments, mol-davites were transported over very short distances.Streams were of decisive importance for the transport of

moldavites to faraway places. A majority of moldavitesconnected with fluvial sediments is supposed to have beentransported over a distance of about one to ten kilometres.However, with even longer transport of moldavites, thechances of their preservation substantially decrease.

Individual areas with moldavite occurrences partly dif-fer in their geological evolution. Therefore, also the char-acter and age of moldavite-bearing sediments can bedifferent. Fig. 2 shows a comparison of stratigraphic posi-tions of the main types of moldavite-bearing sedimentsfrom all substrewn fields. Moldavites from separate areasalso differ in their properties (Table 1).

The richest accumulations of moldavites are usuallyfound in deposits in strewn-field sediments. With a longertransport the concentration of moldavites strongly decreas-es. Sporadic finds of moldavites from places outside themain occurrences, e.g. Jindichv Hradec, Chrany andStar in esk stedoho, Jevinves (Bouka et al. 1999),Praha-blice (ebera 1972) and Skryje near Beroun inBohemia, Zntnek and Moravsk Brnice in Moravia, can

Milan Trnka Stanislav Houzar

284

Fig.3 Fig.5

N

E

49

50

51

161412

52

100 km0

N

Praha

Berlin

Wien

Ries

Cheb area

Lusatian area

Austrian area

Moravian area

South Bohemian area

Fig. 1. A map of the moldavite strewn field in central Europe (with indi-cated positions of Figs 3 and 5).

Properties Souhern Bohemia Radomilice Cheb Western Lusatian area, Horn area,(except Radomilice area) area area Moravia Germany Austria

Predominant colour bottle green pale and bottle bottle green olive green and olive green and bottle green(80%) green (90%) (80%) brown (89%) bottle green (71%)

Number of found pieces 10 000,000 50,000 1,200 20,000 300 20Maximum weight (g) 122 172 36 258 74 104Muong Nong type found not found not found not found not found not foundSphericity lower higher lower higher higher not determinedSiO2 wt% 78.6 82.6 78.7 79.3 79.3 79.7Al2O3 wt% 10.1 8.2 10.1 11.0 10.5 9.8FeO wt% 1.62 1.18 1.62 2.26 1.84 1.54CaO + MgO wt% 5.31 4.20 5.10 3.03 3.75 4.13HCa / Mg types found not found not found not found not found not found18O 11.29 11.42 not determined 11.13 not determined not determinedHomogeneity lower higher lower higher higher not determinedLechatelierite abundance higher lower higher lower medium not determinedBubble abundance higher lower higher lower lower not determined

Crystalline inclusions rare rare not found not found not found not foundStrewn field sediments found not found not found found not found found (?)

Table 1. A comparison of some typical features of moldavites from individual substrewn fields.


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represent pieces transported over a long distance or verysmall relicts of other, earlier substrewn fields. On the otherhand, it is also possible that these data are not reliable.

Even the prehistoric man contributed, to a small extent,to the present-day distribution of moldavites. The oldestarchaeological finds of moldavites are dated back to thePalaeolithic. Worked pieces of moldavites, associated withPaleolithic prehistoric remains, were found at the Gude-nushhle (Mousterien) and Willendorf (Aurignacien,Gravettien) in the Waldviertel in Austria (Koeberl et al.1988). But worked moldavites are characteristic especial-ly of Neolithic people in western Moravia. A polished

disc-shaped moldavite was found in a small pot togetherwith a sculpture of a bulls head in Skpina fortifications(erven and Frhlich 1990, Vok 1999).

Southern Bohemia

The largest area of moldavite occurrences is the SouthBohemian substrewn field. It covers the area of both Ter-tiary basins (esk Budjovice and Tebo Basin) and ofthe adjacent crystalline complex, to the west and the southof the basins. The deposits cover a total area of more than2000 km2. The area is defined by the connecting lines ofthe towns of Psek, Vesel nad Luic, esk Velenice,esk Krumlov and Prachatice. The rough overview of the

main moldavite deposits in Southern Bohemia is indicatedin Fig. 3.

The character of moldavites is not the same in thewhole area. Bouka (1997) has set aside the Radomilicearea as a discrete part in the NW of the Budjovice Basin.Somewhat unusual are also the moldavites found nearHorusice, which are, moreover, spatially separated fromother localities of the South Bohemian substrewn field.Certain differences have also been found in the moldavitesfrom the southern part of the Tebo Basin. In spite of this,it is more logical to understand the whole South Bohemi-an area as a one substrewn field and to explain minor or

major differences among the moldavites by the variabilityof the source material or by the variability of the condi-tions of their origin.

The South Bohemian moldavite-bearing sediments be-long to several stratigraphic levels. The oldest ones, usual-ly classified to the Middle Miocene, are marked as theVrbe beds and Koroseky gravels and sands. These twogroups of sediments of different facies lie close to, andpass into, each other. They were formed in different depo-sitional environments in the neighbourhood of the shoresof receding lakes (ebera 1967, 1977).

The Vrbe beds are deposited on crystalline rocks oron the Middle Miocene sediments of the Mydlovary For-mation. Most of the sediments are colluvio-fluvial sandy

Moldavites: a review

285

Era

Lower

Lower

Upper

16.5

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fie

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ithre

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ites

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lifluc

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Gravel

san

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lsan

dso

fRauno

Forma

tion(L

ower

Lusa

tia

)

OlderS

en

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berg

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ls(loc.

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lla,

Brauna

)

Bau

tze

nElbegrave

ls(loc.W

iesa

)

Glacila

cus

trinnese

dimen

ts(loc.

Saa

lhausen

)

Imfritz-

Ra

dessen

Forma

tion

Age (Ma) WesternMoravia Cheb area(Western Bohemia)

Lusatian area(Germany)

Tertiary

Southern Bohemia

Pliocene

Miocene

Middle

Upper

Pannonian 8.8

Pontian 5.6

Horn area

(Austria)

Quaternary

Holocene

Pleistocene

Sarma

tian

Ba

den

ian

Period

Dacian 3.7

Romanian 1.8

0.01

Moldavite fall (different determination from 14.4 to 15.1 Ma)

Fig. 2. Stratigraphic scale with a comparison of geological positions of moldavite-bearing sediments from individul substrewn fields.


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clays and/or clayey sands with an admixture of angularpsephitic material. They fill stream depressions, ravines orform dejection cones. The thickness of these sediments is

usually several metres. The total length of transport of themoldavites from the place of their fall is estimated at about1 km at the most. Therefore, the Vrbe beds are markedas strewn-field sediments.

Koroseky gravels and sands represent fluvial, fluviola-custrine to deltaic facies of the Vrbe beds, in which mol-davites underwent a longer transport, usually of a fewkilometres. The thickness of the sediments reaches about25 m. The main component of these gravels and sands arequartz and feldspars. The usual size of pebbles is 12 cm,up to 10 cm at the most.

Later, some of the Middle Miocene sediments were de-structed and the moldavites were redeposited into younger

sediments. Therefore, we can find moldavites also in thePliocene to Recent fluvial sediments and in the Pleis-tocene solifluction soils (Bouka 1972).

The character of moldavites is particularly connectedwith their geological history, in the same way as in othersubstrewn fields. The moldavites from the Vrbe bedsare angular, with deep, well-preserved sculpture and lowaverage weight. In the Koroseky gravels and sands, themoldavites were partially rounded and the number ofpieces with flat pit sculpture increased. Moldavites,which underwent a longer transport in the Pliocene toPleistocene ages obtained pebble-like shapes and their

surface was abraded or only slightly corroded (finely en-larged fissures).

Southern part of the eskBudjovice Basin and itssurroundings

This is the area with the rich-est moldavite occurrences,where more than 99% of all

moldavites were found. This isthe reason why the majority ofinformation about the physical,chemical and structural proper-ties is based on the study of mol-davites from this area.

The most important depositsare concentrated in the NW-SEstrip along the western marginof the esk Budjovice Basin.This strip stretches from Neto-lice to Besednice and is a few

kilometres wide and more than35 km long. Most occurrencesare connected with the Vrbebeds and the Koroseky gravelsand sands.

Relicts of the moldavite-bearing Vrbe beds can befound in whole length of thestrip, mainly (from NW to SE)at Brusn, Doln Chrany,

Jankov, Kvtkovice, Hab, Lip, Slave (near eskBudjovice), Vrbe (the locality of Nov Hospoda),Bukovec, Krasejovka, Besednice, Slave (near Trhov

Sviny) and Zlu. Moldavites in these sediments are de-posited very irregularly. Extremely rich accumulationswere found at Jankov, Vrbe and Slave (near TrhovSviny). It is estimated that approximately more than threetonnes of moldavites were dug out at the last mentionedlocality.

More significant occurrences of the Koroseky gravelsand sands with moldavites are known from the localities ofitn, Tebanice, Lhenice, Koroseky, Vrbe, Zhorice,Holkov, Loenice, Chlum nad Mal (Fig. 4), Nesm andDobrkovsk Lhotka. In younger fluvial sediments the mol-davites are less frequent. These sediments were found to

the east of other localities towards to basin centre, e.g. thePliocene occurrences near Kamenn jezd or Quaternarysandy gravels at Ronov near esk Budjovice. The up-per parts of older moldavite-bearing sediments were rede-posited by solifluction in the Pleistocene age.

Radomilice area

Moldavites with specific characteristics were found ata few localities in the northern part of the eskBudjovice Basin (localities of Radomilice, Strp, Zblat,Zblatko, Dubenec, Bez and others). These moldavitesare relatively rich in SiO2 (the average content is 83%),

part of them are typically light green coloured, they have arelatively the low content of lechatelierite and bubbles, a


286

TEBO

PSEK

ESKESKKRRUMLOMLOV

ESKBUDJOVICE

VESELNAD LUNIC

BLANICE

VLVL

VLTAVA

STRSTR

OPNICE

OPNICE

OTAVA

NERKA

MA

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Jakule

Besednice

Lhenice

Jehndno

Radomilice

JankovVrbe

Horusice

BorovanyBor

Protivn

Smrmrkovice

StrpZblat

Maletice Tn nadn nadVltaltavouu

Tebanice

Koroseky

OpaliceLoeniceoenice

Kamennamennjezd

Vrto

OleniceleniceTebe

Kramoln

TrhovSviny

Holkov

Chrany

Slave

Hrdloezy

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TeboBasinNetolice

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LUNICE

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jezd

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Chrany

Slave

Hrdloezy

PRACHATICE

Budjo

viceBasin

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Netolice

N

0 10 km

LUNICE

Fig. 3. Sketch map of moldavite localities in southern Bohemia (modified according to Bouka 1994).


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high sphericity and a high aver-age weight (Bouka 1997). Themajority of South Bohemianmoldavites with the weight over100 g come exactly from theselocalities. Therefore, this areaused to be delimited as a dis-crete substrewn field.

Oval moldavites with abrad-ed surface are connected withthe Quaternary alluvial-fangravels and sands (Mindel-Riss,Schovnek et al. 1981). As indi-cated by sporadic finds of well-preserved moldavites at manylocalities in this area, we mayspeculate about the existence ofrelicts of the Koroseky gravelsand sands in their footwall. This

would be in agreement with the description of the finds ofmoldavites in a section in rusty brown gravels nearRadomilice (Woldich 1888). Deeply sculptured and well-preserved moldavites even prevail at some localities.

To the north of the Radomilice area, very poor occur-rences of moldavites are scattered near Protivn, Maletice,Tn nad Vltavou, Psek, Jehndno, Podolsko, erven andothers. Absolute lack of data on these moldavites does notpermit any considerations about their similarity with otheroccurrences.

Tebo Basin

Most localities in the Tebo Basin are situated in itssouthwestern part. Several thousands of moldavites are es-timated to come from this area. At the localities of Bor andJakule, the moldavites are connected with coarse-grainedclayey gravelly sands rich in quartz and feldspars, whichprobably represent Middle Miocene sediments (Korosekygravels and sands). Clayey sands usually form their foot-wall. The character of the moldavites is in agreement withthe character of the sediments. The moldavites are angulartiny fragments, usually with the weight under 3 grams.The primary shapes are usually drops. The most corrodedare green-brown to brown moldavites but bottle green or

olive-green moldavites generally prevail.Moldavites from Borovany occur at two stratigraphi-

cally different levels. The older gravels and sands lie in thehangingwall of the Mydlovary Formation and are desig-nated as the Domann Formation probably an equivalentto the Vrbe beds (Vrna et al. 1983). The younger levelis found inside the Ledenice Formation of Pliocene ageand is formed by coarse-grained rusty-brown sands.

Gravels with subangular quartz pebbles are younger,Pliocene to Pleistocene in age. Slightly water-abraded mol-davites with higher weight (often exceeding 20 g) werefound near Hrdloezy. Individual, longer-transported mol-davites come from the Holocene sediments of the LuniceRiver, e.g. near Majdalena (Vamberov and evk 1990).

Anomalous moldavite concentration (more than 1,000pieces) was found in gravel and sand deposits at the Horu-sice locality near Vesel nad Lunic, at the northeasternmargin of the Tebo Basin. The moldavites come from abasal gravel bed 1 m in thickness. Subangular pebbles ofvaricoloured quartz, less frequently of gneiss, granite andamphibolite, are the main constituents of the gravels. Themoldavites are light green, bottle green, olive green tobrown green and their weight is usually 520 g. The mol-davite sculpture is deep or slightly worn. The chemical da-ta and physical properties of the moldavites prove theprimary high variability of the local part of the substrewn

field. These moldavites are not a mixture of pieces trans-ported from various areas, as it was claimed by Boukaand evk (1990). Some other localities with rare mol-davites are known from the gravels of the Lunice River,from the surrounding of Vesel nad Lunic (ebera 1977).

Cheb Basin (western Bohemia)

The first moldavites from the Cheb Basin came fromthe gravelly shore of the Jesenice reservoir (Okrouhl nearCheb Bouka et al. 1995). But the main present-day de-posit is the gravel pit of the TEKAZ Cheb company, notfar from Denice, where the moldavite-bearing sediments

have been well exposed. Some sporadic pieces were foundalso at other localities in the near surroundings.

In the bottom of the sandpit, sediments of the MiddleMiocene Cypris Formation are recovered. Moldavite-bear-ing sediments are Pliocene in age and belong to theVildtejn Formation. These are fluvio-lacustrine, poorlysorted gravels and sands, rich in quartz and often contain-ing clasts of kaolinized feldspars, granite, gneiss, quartziteand phyllite. The presence of alternating beds of coarseand fine material, lenticular beds and the frequent interfin-gering and cross bedding indicate a rapid sedimentation(Bouka et al. 1995).

The gravels also include pebbles of andalusite andother heavy minerals such as ilmenite, tourmaline, diop-


287

1 m01 2 3 4 5 6 7 8

-3 m

-2 m

-1 m

Fig. 4. A section of moldavite-bearing sediments of the Koroseky type in the sand-pit near Chlum nadMal, southern Bohemia (according to ebera in Vrna et al. 1984).1 topsoil, 2 slightly humic grey gravelly sand, 3 rusty brown gravelly sand, 4 rusty brown limonite-rich gravelly sand, 5 greyish white gravelly sand, 6 rusty yellow gravelly sand with limonite aggre-gates, 7 greyish white fine-grained sand, 8 position of moldavites


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side, garnet, zircon and anatase. In the gravel pit, mol-davites were found in a bed at the depth of 1722 metres(445450 m above sea level). The bed is overlain withblack Mn-bearing sandy gravels (ada et al. 1998).

It can be assumed that the Cheb moldavites form asmall substrewn field or that streams brought them downfrom the area SE of the Cheb Basin. The extent of the sub-strewn field of the Cheb moldavites can be determined bythe observation of the so-called green clay beneath theVildtejn Formation and by the study of Neogene relicts inthe ChebDomalice Graben and in the surroundings ofSchirnding in Germany (Kopeck and Vcl 1999).

Altogether more than 1,200 moldavites were found atthe locality near Cheb. The moldavites are flat in shape, al-most rounded, with shallow and rarely deep sculpture. Theweight is 26 g on average but the weight of the largest

moldavite is 36 g. Chemical analyses and colours of theCheb moldavites correspond to those of the moldavitesfrom the South Bohemian area (Bouka et al. 1995).

Western Moravia

The Moravian substrewn field is the second most ex-tensive area of moldavite occurrences after the South Bo-hemian moldavite area. The total amount of recoveredpieces is estimated at about 20,000. The occurrences ofmoldavites are located, with some exceptions (e.g. Mo-ravsk Brnice, Houzar and rein 1998), in an area delim-ited by Teb, Moravsk Budjovice, Znojmo, Hruovany

nad Jeviovkou, Ivanice and Nm nad Oslavou.The northern and northeastern limits of the area are de-

fined by tectonic structures (Rejl1980). The western boundary isprobably controlled by denuda-tion, lying at the altitude of around500 m above sea level. The south-ern and the eastern boundaries aredetermined by the possibility offluvial transport of the moldavitesfrom the places of their dropping.The majority of Moravian mol-davite deposits have a narrow spa-tial connection with streams of theOslava, Jihlava, Rokytn, Jeviov-ka and Dyje rivers. The distribu-tion of moldavite deposits inMoravia is shown in Fig. 5.

Moldavite occurrences can besubdivided into several groups ac-cording to the origin, petrographic

composition and the age of themoldavite-bearing sediments. Theoldest sediments are the Middle-Upper Miocene colluvio-fluvialsand-dominated sediments withgravel admixture (sediments ofthe strewn field). They were dis-covered only at the localities ofSlavice and Teb in the north-

ernmost part of the Moravian moldavite area. The sedimentscontain material transported over short distances, whichoriginated from eluvia of granosyenite (the Teb massif),

and to a lesser degree also from the surrounding rocks(aplites, ferruginous sandstones, cherts and others). Thethickness of the sediments is below 2 metres. The sedimentsfill a local depression in the weathered underlying rocks.Moldavites from these sites have deeply pitted surface.

Fluvial sandy gravels lying farther to the south(tpnovice, Kojetice, Jaromice nad Rokytnou) or tothe southeast near Rouchovany are approximately of thesame age and show signs of longer transport. In all cases,these gravels have a small thickness, are poorly sorted andrepresent sediments of short local streams, which were re-deposited into quaternary loams. The ratio between theprimary and the redeposited sediments is demonstrated by

the ratio of preserved and water-abraded moldavites.Absolutely different are the fluvial terrace gravelly sands

along the Jihlava River from the area between Stropen andDukovany. They include subangular moldavites with deeplycorroded surface. These sediments were deposited in the dis-tal part of a braided river (Fig. 6). Their thickness exceeds 20metres. The sedimentation was rapid and episodic. Two po-pulations of quartz pebbles were distinguished in thepsephitic material. The shape indexes of moldavites are clos-er to less rounded quartz population originated in alluvial en-vironment. The sediments are reddish in colour and highlymature. They are composed of quartz, subordinate quartzite,

granulite, pegmatite, and chert and other rocks from bothnear and faraway surroundings. The heavy mineral assem-

Fig. 5. Sketch map of moldavite localities in western Moravia.


288

Nm nad Oslam nad Oslavou

OslavanyIvanice

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Rybnky

Rouchovany

Slatina

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VacenoviceKojetice

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KoniceSuchohrdly

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Stropentpnovice Mohelno

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JIHLAVA

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blage comprises mostly ilmenite, tourmaline, andalusite andzircon (Nehyba 1992). Sediments are not positively dated,but most often they are considered to be of Miocene age. Wefound sediments of similar character at the localities of Koi-chovice, Mohelno and Nm nad Oslavou. There is a no-ticeable admixture of redeposited marine sedimentsalong the Oslava River (Houzar et al. 1997).

Flat rounded moldavites come from the deposits at Su-chohrdly and Kuchaovice near Znojmo. Their shallowsculpture was formed by widening of fissures on a strong-ly rounded surface. These moldavites are linked with red-dish coloured, fluvial, coarse-grained sandy gravels,which are composed of pebbles of quartz, less of grani-toids, gneiss, quartzite and amphibolite. These sedimentsare considered to be of Pliocene age.

The locality of Konice near Znojmo yielded only one ex-ceptional moldavite. Its colour is bottle-green and its chemi-cal composition is similar to that of the South Bohemian orAustrian moldavites (Houzar et al. 1993). The Konice mol-

davite and a moldavite from Podmol (obes) indicate thatthe moldavite occurrences extend to the southwest of Znoj-mo (Trnka and Houzar 1991, merda 1997, 2000).

The youngest fluvial moldavite-bearing sedimentsare Pleistocene in age, developed along the present-daystreams of the Jihlava River (between Jamolice andIvanice Mrzek and Rejl 1976, Mrzek et al. 1997),the Oslava River (Nm, Kuroslepy), the Rokytn Riv-er (Slatina) and the Jeviovka River (to the east of Znoj-mo Oleksovice, Prosimice, Boice etc., Mrzek1976). The relative height above the present-day streamsis 20100 m. In most instances the moldavite-bearing

sediments are coarse-grained gravels dominated byquartz and local rock fragments. Plentiful pebbles of fos-silized wood have been reported from the localitiesaround Znojmo. All these deposits are characterized bypebble-shaped moldavites with water-abraded or particu-larly slightly corroded surface (enlarged fissures) and bythe general paucity of moldavites.

Quaternary colluvial loams are the youngest moldavite-bearing sediments, in addition to the Pleistocene terracegravels. They developed above older sediments with mol-davites, by the reworking of which they originated.

Lusatia, Germany

Since 1967, more than 300 finds of moldavites havebeen registered in Lusatia (Rost et al. 1979, Lange andWagner 1992, Lange and Suhr 1999). They occur in thearea of about 1300 km2, northeast of Dresden (e.g. locali-ty of Ottendorf-Okrila, Brauna, Gottschdorf, Buchwld-chen, Wiesa, Grossgrabe).

Moldavites have been found in different geologicalunits. Fluvial sandy gravels Older Senftenberg ElbeGravels and/or Rauno Formation indicate characteristicrapid and turbulent sedimentation with clay lenses and rareoccurrences of flora. They consist of quartz, quartzite, ly-dite, agate, silicified wood, and of heavy minerals (tour-maline, staurolite, rutile, sillimanite, andalusite and

zircon). The composition of the gravels, including the flo-ra typical of warm climate, indicates a Miocene rather thana Pliocene age. These observations are supported by thehigh maturity of these sediments compared to the Pliocene

sediments in southern Bohemia.Fluvial sediments of Bautzen Elbe Gravels of LowerPleistocene age consist of higher amounts of unstablecomponents (pebbles: basalts, phonolite, greywacke;heavy minerals: epidote and amphibole) and, together withthe glacio-fluvial moldavite-bearing sediments, showmany features of a cold climate (Lange 1996).

The study of Lusatian moldavites suggests that theirorigin cannot bee explained by fluvial transport from theSouth Bohemian strewn fields. The physical and chemicalcharacteristics of the Lusatian moldavites (e.g. the similar-ity to the Moravian tektites), together with the palaeo-geographical and stratigraphical position of themoldavite-bearing sediments, suggest an independent sub-strewn field for the Lusatian moldavites within the mol-davite strewn fields (Lange 1995, 1996).

Lusatian moldavites are mainly olive green in colour,with rounded appearance and sculpture of shallow pits.They are rather of Moravian type. The shape and the fre-quency of bubbles are halfway between the Bohemian andMoravian moldavites.

Horn area, Austria

Moldavites were also found in northern Austria in theHorn area (localities Eggenburg, Altenburg, Radessen,Mahrersdorf Suess 1914, Koeberl et al. 1988), not far


289

1 m0

1.0

0.5

1.5

2.0

Sp + St

SvSh

Gp + Gt

Sp + St

FmFm

Fm

Gp + Gt

Gm

Sp + St

Gm

Fig. 6. Schematic vertical succession of moldavite-bearing sediments inMoravia (Slavtice sand-pit, Nehyba 1992).Gm massive gravel, Gt trough cross-bedded gravel, Gp planarcross-bedded gravel, Fm massive mud, fine sand, silt, Sp planarcross-bedded sand, St trough cross-bedded sand, Sh horizontallylaminated sand, Sv ripple-bedded sand


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from the moldavite deposits close to Znojmo in Moravia.Yet earlier, Sigmund (1912) reported a moldavite fromStainz near Graz, southeastern Austria. However, Koeberl(1986a) suggested that this one was hauled by prehistorichuman there. Two of the archaeological discoveries werementioned above.

The deposits near Horn yielded about twenty mol-davites. Most of them come from the gravel-pit near Al-tenburg. The moldavite-bearing sediments are fluvialgravels and sands overlying the St. Marein-FreischlingFormation, which probably corresponds to the MydlovaryFormation in southern Bohemia. Two larger moldavitesfrom Radessen were found in a thin gravel bed overlyingthe crystalline basement. These gravels belong to the so-called Irnfritz-Radessen Formation that is most probablyof Miocene age.

All Austrian moldavites, with the exception of onesample, show more or less deep sculpture and are angularin shape. The largest of them come from Eggenburg (104

g) and Radessen (46.4 g). The colour and chemical com-position of most Austrian moldavites appear to be closelyassociated with the South Bohemian moldavites, while theRadessen sample seems to be related to the Moraviangroup. The geological setting, the chemical compositionsand the physical properties of Austrian moldavites indicatethat they belong to an independent substrewn field (Koe-berl et al. 1988). They show a wider range in chemicalcomposition and their colour scale is similar to the mol-davites from the Tebo Basin.

Total mass of moldavites

About 75 years ago, Hanu (1928) estimated the totalamount of fallen moldavites at 20 million pieces and theirweight at 100 metric tons. As indicated by both organizedand illegal mining, the present estimates are 1020 millioncollected pieces with a total weight of 3060 metric tons.

In a similar way, the present-day approximation of thetotal weight of moldavites at the time of their formation andtoday is different. According to Bouka and Rost (1968)about 3,000 metric tons of moldavites fell on the Earth butonly 275 tons remained after the geological windstorm.These values are much lower compared to the estimates forother groups of tektites. If we consider the extent of the mol-

davite-bearing sediments, of the geological progress of thearea and the rate of moldavite destruction, we can make arough estimate that the Ries impact produced at least 106

metric tons of moldavites (splash form and Muong Nongtype), from which only 104 tons have been preserved untiltoday. These values are not compatible with the data forother groups of tektites, because researchers combine theweight of macrotektites and microtektites.

Shape of moldavites

The present-day shape of tektites is not only the resultof their formation in the time of their origin but distinctly

reflects also the effect of later processes. Therefore, Baker(1963) divided the evolution of shapes into three stages.First of all, the separate bodies of tektites were formed andcooled. In the second, so-called ablation phase, the shapeswere changed by melting of front solid parts during theflight of tektites through the atmosphere. This phase, how-ever, did not take place in most of the tektites. In the thirdphase, after falling on Earths surface, various geologicalprocesses finished the morphology of the tektites.

Primary shapes

Depending on the conditions of their origin, differentprimary types of tektites were formed. They are: (i) splash-form tektites, (ii) Muong Nong-type tektites and (iii) mi-crotektites.

The most common group are the splash-form tektites.The initial shapes of all splash-form tektites were drops,which originated in the separation of bigger mass of tek-

tite melt by shearing flow. Other shapes of splash-formtektites are the result of transformation of some parts ofthe original plastic drops by rotation (Trnka and Houzar1991, Trnka 1999).

Most moldavites belong to this type. The most usualshape of splash-form moldavites is a drop (Konta 1980),elongated, flattened, bent or spirally curved to a differentdegree. Lenticular shapes (discs), three-axial ellipsoids,sphere-like bodies and dumb-bells are frequent, too. Discshave sometimes thickened edges.

Statistics from measurements of moldavites show(Konta and Mrz 1969, Konta 1971a, b) that moldavitesfrom southern Bohemia are much more anisometric in

comparison with the moldavites from Moravia. Thismeans that specimens of South Bohemian drops are moreelongated and more flattened than the ones from Moravia,and the same also holds for other shapes.

Layered tektites without total internal stress, whichusually form bigger irregular angular pieces, are designat-ed as Muong Nong type tektites (Lacroix 1935). Theyhave characteristic shimmering structure, higher contentof bubbles and foamy lechatelierite. Some of the bubblesare irregular in shape. A broad range of crystalline phaseshas been found in tektites of this type. In the view ofchemical composition they are rich in volatile components

(H2O, F, B, S, Cl, Cu, Zn, Sb and others for example,Koeberl 1988), slightly acid (Schnetzler 1992) and theyhave higher FeIII/FeII ratio than the splash-form tektites.

Muong Nong type of tektites in typical forms and withall the above mentioned features are common only in Asia(indochinites). According to Rost (1966), only one bigmoldavite found near Lhenice in southern Bohemia is re-garded as a Muong Nong type of tektite, thanks to its mor-phological features and to the absence of total internalstress. Koeberl (1986b) considers the occurrence ofMuong Nong type a rarity among moldavites on the basisof their chemical composition and the absence of crys-talline phases. Lower contents of volatiles and the pres-ence of baddeleyite lead later to the establishment of only


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a few other moldavites of this type by Glass et al. (1989)and Meisel et al. (1989,1993).

On the other hand, Barnes (1969) supposed a more fre-quent occurrence of the Muong Nong type among mol-davites based on the study of their internal structure. Thedifference between Barness conclusions and the conclu-sions of other authors follows from their preference of dif-ferent features for the distinction between the MuongNong type and the splash form and also from their differ-ent understanding of this term. There is no sharp boundarybetween these two types. Izokh and An (1983) andSchnetzler (1992) described tektites with transitionalproperties. Structural features show that these tektitesoccur also among moldavites (Trnka 1997a).

Higher heterogeneity, porosity and cracking of MuongNong type tektites caused their relatively lower chemicaland especially mechanical durability. The known finds ofMuong Nong type moldavites come from localities withvery short transport of material. These facts may indicate

a possible destruction of most of the Muong Nong mol-davites during their geological history.

Essential parts of original shapes of Muong Nong typeof tektites are preserved only very exceptionally in In-dochina. From them it is possible to deduce that these tek-tites had originally the shape of spoon-like bent drops withthe weight attaining 100 kg. It seems evident that theywent through airborne transport together with the splash-forms. The Muong Nong tektites show evident traces offlight through the atmosphere, but their deformations areof primitive character (Futrell 1987, Trnka 1994, 1997a).

Microtektites come close to splash-form tektites be-

cause of their total morphology but their size usually doesnot exceed 1 mm. The small size of microtektites permit-ted substantial use of forces of surface stress of melt,which resulted in the appearance of predominantlyspherule shapes. Drops, dumb-bells and other shapes areless frequent. Microtektites have been found only in ma-rine sediments.

Microtektites are known from all tektite strewn fieldsexcept for the moldavite fields. It is not possible to con-sider microparticles of moldavite glass as microtektites,because the microparticles originated by breaking off fromthe usual moldavites in sediment. In the conditions of con-tinental sediment accumulation, however, there is only a

theoretical possibility that they could be preserved for ap-proximately 15 Ma.

Ablation

Only some splash-form tektites, namely some aus-tralites and javanites, have provable signs of ablation,which originated during their long flight through the at-mosphere at speed exceeding 5 km s-1 (Chapman andLarsson 1963). Melting and evaporation of the glass on thefront side of tektites occurred during the ablation, and themelting glass was ripped off to the edge. This is why theshapes have either characteristic rims along the cores orablation peripheral edges.

Chao (1964), Rost (1972) and Soukenk (1971) havedescribed the impact of ablation on the moldavites. Theyexplained the rarity of ablation features in moldavites bytheir later removal during the transport, chemical corro-sion and so on. The morphological similarity betweentheir samples and some australites is very high. Shapessimilar to ablation shapes can originate also by chipping ofthe flat fragment from the moldavite surface. This happensmainly in bigger pieces with strong internal stress.

According to Soukenk (1971), the majority of mol-davites with presupposed signs of ablation come from lo-calities in the Teb area in Moravia and are very heavy.This is in direct contrast with australites, where the abla-tion shape is well developed just on the smallest pieces.Moreover, the shape of moldavites is much more similar toindochinites and philippinites, which do not show any ab-lation features, than to australites. No other phenomenawere observed in moldavites, which have a direct connec-tion with the origin of ablation. Therefore, it is probable

that no conditions, which would bring about ablation,were present during the flight of moldavites. The cases de-scribed earlier are the consequence of shattering.

Development of shape of moldavites after their fall

Tektites went through a marked change of their shape onthe Earths surface, which was caused by mechanical orchemical effects of various factors: breaking and crackingof tektites was very substantial and lead to the origin of frag-mental or splinter shapes. It happened not only during thedropping and geological transport but also spontaneously insediments due to the influence of internal stress.

Unbroken drops, discs, dumb-bells and other shapesare very rare among moldavites, representing less than 1%of all pieces (Konta 1980).

A specific feature of tektites and in the same way alsoof moldavites is the surface sculpture (sometimes calledsculptation) formed by pits, grooves and other dispari-ties. Because of Suesss accurate study (1900), an opinionsurvived for a long time that the sculpture was formed dueto aerodynamical effect on plastic tektites. The main sup-port for this opinion was the radial or generally regulararrangement of sculpture elements. Later, proofs weregathered on the sculpture formation by chemical corrosion

in sediments (in moldavites e.g. Rost 1972, Trnka 1980).Unevenness of chemical corrosion, leading to the ap-pearance of sculpture elements, is conditioned mainly bythe properties of the moldavites, although the all-roundcharacter of sculpture is influenced by the co-operation ofmany internal and external factors. From this point ofview, Trnka (1988, 1997b) regarded the effect of stressesof various origin the most important.

The influence of stress on chemical corrosion of glass,thoroughly discussed in glass literature (for exampleScholze 1977), is based on the existence of surface mi-crofissures, which originated in tektites mainly duringtheir transport in sedimentary environment and/or duringthe temperature changes in the last phase of their origin.


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Similarly as in other materials, the mechanical strength ofthe tektite glass was reduced by the internal and externalstresses. The glass fatigue leads to gradual growth of fis-sures. The fissures propagated preferentially along thecourse of various inclusions, which substantially decreasethe strength of glass. The orientation of the fissures was re-lated to general shape of the tektite and to its texture. Thereactions between the attacking solution and tektite glasswere the strongest in places where the stress was second-arily concentrated, that means on the peaks of fissures. Atthese places, the structure of glass is more reactive.La Marche et al. (1984) proved experimentally the higherrate of chemical corrosion along the cracks in tektites.

The total appearance of sculpture and the rate of itsformation was influenced also by many other factors: geo-logical history of tektites, duration of corrosion, climaticconditions, the character of sediments and sedimentary so-lutions, and finally by the chemical composition, the tex-ture and the heterogeneity of the tektite etc. Their effects

were commented upon, in addition to the already men-tioned publications, by Barkatt et al. (1984), Glass (1984,1986), Konta (1988) and others.

Rost (1972) estimated that a layer 27 mm in thicknesswas removed by chemical corrosion from the surface ofmoldavites. Knobloch et al. (1980) estimated this at a mini-mum of 2.54.5 mm. Some observations on moldavitesfrom Besednice (southern Bohemia) show even higher ratesof loss of mass. That means that pieces about 1 cm in sizehad only minimum odds to be preserved to the present day.

In addition to chemical corrosion, the moldavite mor-phology was also distinctly influenced by mechanical

abrasion. Abrasion of small particles of moldavite glassduring the sedimentary movement leads to the removal ofthe original surface of moldavites and later also to the re-moval of sculpture. A decrease in the moldavite weightduring transport could distinctly exceed the loss of massdue to chemical corrosion. It could happen that the sculp-ture was totally wiped away during movement on slopeshundreds of metres long.

Physical properties

Mass

The mass of the splash-form tektites comes to the samevalues (hundreds of grams) in all tektite groups and doesnot exceed 1 kg. This value agrees to the calculation ofCentolanzi (1969) of the maximum mass of a tektitesphere, which, during the cooling by radiation, does frag-ment spontaneously. If we take into account the substantialmass losses of moldavites during the time about 15 Ma,we can suppose that the initial mass of the largest mol-davites is close to this value.

Depending on the conditions of origin and on the geo-logical history, both the maximum and the average mass ofmoldavites differ. Relatively largest are the moldavites

from western Moravia today and the smallest are from

southern Bohemia. The largest moldavite comes fromSlavice in Moravia, with its mass being 258.5 g.

Colour

Most tektites are dark to black in incident light. Thebrown or green colour of their glass is usually visible on-

ly in very thin parts. Moldavites are an exception becausethey are more translucent. Higher translucency of mol-davite glass makes it possible to observe the colour differ-ences among them and the relationship between the colourof moldavites and their chemical composition.

The colour scale of moldavites ranges gradually frompale green to brown. The main colouring components areFeII, FeIII and perhaps also MnII. Their content increases inthe direction to brown moldavites (Bouka and Povondra1964, Bouka and Clek 1992), together with the ratio ofFeIII/FeII (Bouka et al. 1982). Deviation from this scale isfound in the so-called poisonous green moldavites

(HCa/Mg moldavites). Their colour can be explained by aslightly higher content of Ni, in combination with a highcontent of alkaline earths (CaO, MgO) and a low contentof K2O (Bouka et al. 1990a).

Some other components, which have low or no colour-ing capability, control the effect of colouring elementsthemselves. The colouring influence of FeIII in glass sub-stantially increases with the increasing content of Ti andMn (Volf 1978). The relatively low content of titanium,nearly half content, in moldavites, rare georgianites, bedi-asites from Muldoon (and also in urengoites) is in all prob-ability the cause of their higher translucency incomparison with other tektites.

In the South Bohemian substrewn field, bottle greenand light green moldavites prevail, with the proportion ofmore than 80%. In the Radomilice area, very pale greenpieces can be also found in addition to these colours.Colour shades of the rare moldavites from the Cheb Basinand from Austria come close to South Bohemian mol-davites (ada et al. 1998, Koeberl at al. 1988). Moravianand Lusatian localities are dominated by olive green tobrown moldavites: over 90% in Moravia, and approxi-mately 70% in the Lusatian region.

Rare specimens of moldavites are composed of twodifferently coloured parts with a sharp boundary. They

originated by collision of molten moldavites before theirimpact on the Earths surface. More than fifty moldavitesof this type were found in the South Bohemian area andabout ten in Moravia (Bouka at al. 1982, Trnka andHouzar 1991).

Stress

All splash-form tektites have strong internal stress. Co-operation of several partial components of various origindetermines the resultant stress. The main component isoverall stress, which was formed by cooling of tektites asa result of temperature differences between surface and in-

ternal parts.


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293

Stress intensity increases with the rate of cooling, withthe size of the tektite and with the temperature of forma-tion. Later cracking of the pieces and/or removal of sur-face parts by mechanical or chemical corrosion lead to asubstantial reduction in stress intensity. Centolanzi (1969)indicates the reduction of the stress intensity by 80% athalf decrease in the diameter of the tektite sphere.

The intensity of overall internal stress is locally in-creased or decreased by stresses that formed on the bound-ary of chemically different parts, through the influence oftheir different thermal expansibility. It may be a boundarybetween lechatelierite particles and the matrix or betweenindividual fibres of fluidal structure.

The strain is connected with the hardening of tektiteson the one side and also with the capability of spontaneousbreaking on the other side. With regularly spaced strain,the tensile strength increased. However, the stress is usu-ally not regular and the centre, sometimes of tensile stress,is close to the surface. In these cases spontaneous cracking

of tectites is frequent.Barnes (1969) surveyed the relative intensity of stress

in moldavites on the basis of distinctiveness of birefrin-gence. He viewed separately the overall stress and stressamong strips of fluidal structure. Moravian moldaviteshave statistically higher intensity of overall stress than Bo-hemian ones as a result of their higher average size. Thestress in fluidal structure increases with SiO2 content,which positively influences the viscosity and therefore de-creases the possibility of diffusion. So, it is lower on aver-age in Moravian moldavites.

Other physical propertiesThe refractive index and the density reflect the chemi-

cal composition of moldavites. It is possible to calculatetheir values from chemical analyses. Both values decreasemainly with the increasing SiO2 content, and increase withthe increasing CaO content. The refractive index and thedensity data for each group of moldavites are shown inTable 2.

Viscosity of tektites is also in close relationship to thechemical composition. Its value, especially at higher tem-peratures, increases mainly with the increasing SiO2 andAl2O3 contents and decreases with the alkalies. The vis-

cosity, together with temperature, undoubtedly influencedthe formation of shape.

Chemical composition

The characteristic feature of all tektites is their specif-ic and rather uniform chemical composition (except forsome microtektites), in which they differ from other glass-es of natural and artificial origin. The affinity with somesediments follows already from the main oxide contents(the high Al2O3/K2O + Na2O ratio, the high content ofCaO and MgO). Bouka (1968) described the affinity be-

tween the sediments and moldavites.

Also, the contents and the ratios of trace elements in-dicate that (i) moldavites differ from meteorites, lunarrocks and terrestrial igneous rocks and that (ii) they areclose to clay- and sand-dominated sediments (for example,the distribution of REE, the ratios of Zr/Hf, K/U, Th/U,K/Rb and others Bouka 1992). No indication of con-tamination by meteoritic material was found whatsoever.

In spite of the generally close chemical composition oftektites coming from various places on the Earth, certaindifferences can be also found between them. The mol-davites form the most acid group of tektites, with SiO2 con-tents about 80 wt%. They are also relatively rich in K2O. Onthe other hand, they have very low average contents ofAl2O3, TiO2, FeO and Na2O. North American tektites arethe nearest to moldavites, especially the georgianites, whichare very close to moldavites from many points of view aswell (Bouka et al. 1990b, Koeberl 1990).

Minor differences exist even among moldavites. Table2 shows the average moldavite compositions from various

parts of strewn fields. The fluctuation of individual oxidecontents can be explained by variable contents of three es-sential mineral components in the parental rock, i.e.quartz, clay minerals and carbonates (Delano et al. 1988 see below). From this point of view, the source material ofthe Bohemian moldavites was relatively rich in carbon-ates, while it was clay minerals in the Moravian mol-davites and quartz in moldavites of the Radomilice group.

Most chemical analyses show the overall compositionof the moldavites. The inhomogeneity of individualpieces, which is obvious also from the existence of fluidaltexture, is caused by local fluctuations in chemical com-

position. The extent of these fluctuations approximatelycorresponds to differences among various moldavites(Clek et al. 1992). Engelhardt et al. (1987) determined thefollowing variation in the chemical composition of a mol-davite from Loenice (southern Bohemia) from 20 pointanalyses on a line 0.27 mm long: SiO2 78.682.7 wt%,CaO 1.82.7 wt%, MgO 1.31.8 wt%, FeO 1.151.67wt%, Na2O 0.300.45 wt%, K2O 3.53.9 wt%.

The extremely low content of most volatile compo-nents is characteristic of all tektites. The results of Beranand Koeberl (1997) show that the tektites have water con-tent ranging from 0.002 to 0.030 wt%, and moldavitesthemselves from 0.0060.010 wt%. These results showthat tektites are in general the driest natural material.

During moldavite formation, vaporization of a highproportion of rare gases, halides, sulphur, carbon and ni-trogen from the source occurred (Moore et al. 1984, Bai-ley 1986, Matsuda et al. 1993, Meisel et al. 1997 andothers.). A distinct decrease also occurred in less volatilecomponents, for example As, Sb, Cu, P.

The valence of elements in tektites indicates reducingconditions of their formation. This is evidenced primarilyby the FeIII/FeII ratio. The FeIII/Fe ratio is 0.13 in the Mo-ravian moldavites, 0.17 in the South Bohemian moldavitesand 0.25 in the Radomilice moldavites. In other tektites,

the FeIII content is even lower, reaching almost zero (for


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294

example, Fudali et al. 1987). On the other hand, this con-tent is higher in tektites of the Muong Nong type (Koeberlet al. 1985).

Substantial reduction of source rocks was also con-firmed by magnetic spherules with a core from pure iron(Kleinmann 1969) or by probable occurrence of metallicsilicon in a moldavite found near Chlum in southernBohemia (Clek 1985). The intensity of reduction reflectsa high temperature of formation and a low partial pressureO2.

The study of isotopes is of essential importance for ourknowledge of tektites. This study enabled not only the de-termination of tektite age but also helped with the prob-lems of their genesis. The most common method for agedetermination is the 40K/40Ar method, which shows thetime of the last remelting. Short-time remelting does notchange the ratios of isotopes of 87Sr/86Sr and 87Rb/86Crand therefore they can lead to the search for the sourcematerial of tektites. The data about ratios of strontium iso-

topes in various types of rocks in the Ries area (Horn et al.1985) show that the only possible source of moldavites aresands of freshwater molasse. The ratio of isotopes 18O/16Ois higher in the moldavites than in the mentioned sedi-ments. This discrepancy was explained by Engelhardt etal. (1987): the oxygen released from water combined withthe oxygen of the moldavites.

When unprotected bodies travel through space, their sur-face is bombarded with cosmic radiation. This leads to theformation of radioactive isotopes 26A1, 10Be and 14C. Theseisotopes cannot be found in moldavites because of their half-life. Neither the estimated values for the Australasian tektites

and Ivory Coast tektites show that they travelled through theouter space (for example Tera et al. 1983).

Internal structure

Tektites are very homogeneous compared to other nat-ural glasses. They differ from the impact glasses, obsidi-ans and fulgurites in almost total absence of unmeltedmineral grains and crystallites. This fact gives evidencenot only for the high temperature of their origin but alsofor a very rapid solidification (Wosinski et al. 1967).

Fluidal arrangement is the basic attribute of the tektitetexture (Fig. 7), and small bubbles, lechatelierite andschlieren are common in tektite glass. Crystalline inclu-sions of various origin are sporadic.

Lechatelierite

Lechatelierite is a common inclusion in tektites.Barnes (1969) and mainly Knobloch et al. (1981, 1983,1988, 1997 and others) studied their abundance, morphol-ogy, arrangement and the chemical composition in mol-davites. Lechatelierite sometimes forms isometricparticles, or has the shape of long flat fibres as a result of

shearing flow. Isometric shapes are not bigger than 1 mmin size. The width and the thickness of flat fibres usually

SouthernBohemia

Radomilicearea

Che

barea

WesternMoravia

Lusatianarea

Austrianarea

n=43

n=3

n=4

n=46

n=15

n=7

range

average

range

average

range

average

range

averag

e

range

average

range

average

SiO2

71.9081.00

78.60

80.0084.70

82.60

78.4679.89

78.97

74.9183.10

79.28

77.2084.10

79.30

78.1085.10

79.73

TiO2

0.230.50

0.31

0.240.29

0.26

0.280.40

0.33

0.300.72

0.42

0.260.42

0.34

0.240.35

0.30

Al2O3

8.9612.70

10.10

7.279.36

8.22

8.3810.13

9.17

9.2713.18

11.01

8.9411.80

10.50

8.10

10.60

9.81

Fe

2O3

0.090.72

0.33(1

7)

Fe

O

1.323.28

2.07(1

7)

Fe

Otot

1.282.86

1.62

1.001.41

1.18

1.341.86

1.54

1.403.50

2.26

1.322.51

1.84

1.021.78

1.54

MnO

0.050.20

0.09

0.050.07

0.06

0.120.25

0.22

0.020.13

0.13(1

8)

0.030.11

0.06

0.030.08

0.06

MgO

1.523.73

2.33

1.602.26

1.91

1.862.75

2.34

0.882.11

1.39

1.062.73

1.75

1.102.03

1.72

Ca

O

2.054.48

2.98

1.802.82

2.29

2.364.56

3.60

0.613.17

1.64

0.933.85

2.00

1.463.30

2.41

Na2O

0.250.60

0.42

0.190.32

0.24

0.310.59

0.41

0.271.08

0.57

0.280.70

0.47

0.190.49

0.39

K2O

2.883.77

3.40

2.202.97

2.53

2.613.85

3.24

2.603.81

3.38

3.063.75

3.46

2.623.90

3.49

P2O5

0.010.03

0.02(4)

refractiveindex

1.4871.502

1.494

1.4811.491

1.486

1.490

1.4851.492

1.489

1.4811.495

1.490

density

2.332.44

2.36

2.322.37

2.34

2.36

2.332.37

2.35

2.312.38

2.35

Re

ferences

Lange(1995)

Lange(1995)

Boukaetal.(1995),

Lange(1995),

Lange(1995)

K

oeberletal.(1988)

Sklaandada(2002)

TrnkaandHouzar(1991)

Table2.Thechemicalcompositions,therefractiv

eindexandthedensityofmoldavitesfromindividualsubstrewnfields.


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reach dimensions from hun-dredths to a few tenths of mil-limetre (the proportion of bothsizes is 1.44.5), their length ofmax. a few centimetres.

The arrangement of elongat-ed lechatelierites is conformablewith the fluidal structure.Longer fibres can have a multi-ple S-shape. The amount oflechatelierite depends on thetemperature conditions of ori-gin. Higher temperature causeda lower abundance of lechate-lierite in Moravian, Radomiliceand Lusatian moldavites(Barnes 1969, Konta 1971a,Lange 1995).

Lechatelierite in moldavites

is formed by very pure silica.Chemical analyses show SiO2contents of above 99.0 wt%(Knobloch 1997). Kinnunen(1990) obtained the same resultsfrom a study of lechatelierite inindochinites.

The origin of lechatelierite in tektites is usually ex-plained by fusion of quartz grains of source rocks. Quartzinclusions in lechatelierite can support this idea. Theywere described from samples of moldavite and bediasite(Barnes 1969, Glass et al. 1986). Conversely, Kinnunen

(1990) supposed the origin of lechatelierite from biogenicCT-opal on the basis of a chemical and a petrographicstudy of their inclusions in indochinites.

Lechatelierite resistance against chemical corrosion issubstantially higher than in the surrounding matrix. There-fore, lechatelierite inclusions often protrude from thesculptured surface. Only rarely were moldavites found inwhich lechatelierite fibres bridge over furrows of sur-face sculpture. A detailed search revealed that lechate-lierite particles separated by etching could be found inadjacent moldavite-bearing sediments.

Schlieren

In addition to lechatelierite, other types of glassy in-clusions also occur in tektites. They used to be marked asschlieren or by specific designations such as fingers,rays or lenticles in some cases. The differences in re-fractive indexes between these bodies and the glass in ma-trix are higher, by one to two orders of magnitude, thantheir fluctuation in the matrix itself. This corresponds withhigher deviations in the chemical composition. If the clas-sification of Rost (1972) is followed, according to whichall glassy inclusions with the refractive index under 1.470(that means with SiO2 content above 92%) are regardedlechatelierite, then schlieren are not very frequent.

In comparison with the glass matrix, schlieren can be

divided into two groups: acid and basic. Acid schlieren areusually similar to lechatelierite in their shape and size.Certain varieties in shape are fingers, which are charac-teristic mainly for australites but were also found in mol-davites (Barnes 1963, Chao 1963). Chemical analysis ofthis inclusion in a moldavite from Mikulovice (western

Moravia) showed an increase in SiO2 against the overallcomposition (+2.7 wt%) and a decrease in Al2O3 (1.4wt%), FeO (0.7 wt%) and K2O (0.5 wt%). Basicschlieren are less frequent and may have a finger-likeshape, too. An analysis of a dark schlier from a moldavitefrom Nchov (southern Bohemia) proved a lower contentof SiO2 (6.4 wt%) relative to the matrix, while the con-tents of Al2O3, FeO and CaO were higher (Barnes 1969).

Long, narrow basic inclusions, forming radiating clus-ters around bubbles, were found in sporadic cases by Barnes(1969) that marked them as rayed bubbles. These inclu-sions in a moldavite from Lhenice (southern Bohemia) con-tain less SiO2 (9.2 wt%) and K2O (1.0 wt%) but more ofCaO (+10.2 wt%), Al2O3 (+0.9 wt%) and FeO (+0.7 wt%)than the matrix. There is an idea that these inclusions origi-nated during an explosion of a zeolite or calcareous nodulein parent sediment during the moldavite origin.

Bubbles

All tektites contain bubbles. The bubbles are associatedboth with the glass in matrix and with lechatelierite. Theirnumber in splash-form tektites represents usually about0.1% of the overall volume. The volume of bubbles inMuong Nong type tektites is several times higher (usually

0.52.0% in indochinites). The most usual bubble dimen-sions are 0.0X to 0.X mm and very rarely exceed 1 cm. On


295

0 2 cm

Fig. 7. Arrangement of fluidal texture in various shapes of moldavites and tektites.


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296

1. Fissures on the surface of a moldavite widened by chemical corrosion, Besednice (S Bohemia); 2. A moldavite enclosed in a concretion of ferruginoussandstone, Slve near Trhov Sviny (S Bohemia); 3. A spirally curved moldavite from Chlum nad Mal (S Bohemia); 4. A deeply sculptured moldavitefrom Besednice (S Bohemia); 5. A pitted sculptured moldavite from the Koroseky gravels and sands, Vrbe sand-pit (S Bohemia); 6. An opened bubblein a moldavite, Zlu (S Bohemia); 7. A spontaneously in situ fractured moldavite under influence of internal stress. The fractured surface is also slight-ly corroded, Chlum nad Mal (S Bohemia); 8. A Muong Nong type moldavite from strewn-field sediments, Jankov (S Bohemia); 9. A dark brown MuongNong type moldavite from Slve near Trhov Sviny (S Bohemia); 10. Moldavites from Denice, Cheb Basin (W Bohemia); 11. A pebble-shaped mol-

davite with a typical sculpture of slightly widened fissures, Litobratice (W Moravia); 12. Angular moldavites from strewn-field sediments, Slavice (WMoravia); 13. A moldavite with two types of sculpture (thin part pitted, thick part strong cuts), Slavice (W Moravia). All photos scale 1 : 1.

12

34

5

9

7

6

8

10

11

12

13

Plate I


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the other hand, the surface of some moldavites reveals no-ticeable parts of open bubbles up to a few centimetres large.

The shape of the bubbles in the matrix is either spher-

ical or elongated and flattened at the same time. The elon-gation corresponds to the direction of flow of tektite meltsat high viscosity. The extreme cases of bubble elongationare the so-called channels, which occur mainly in theSouth Bohemian moldavites. Irregular, angular to spinybubbles are common only in some Muong Nong type tek-tites (Barnes 1963).

A variable amount of comparatively small sphericalbubbles occurs in lechatelierites. In the Muong Nong typetektites, they can accumulate to such a degree that lechate-lierite particles acquire a foamy character. Some South Bo-hemian moldavites contain foamy lechatelierite, while nofoamy lechatelierite was found in the Moravian moldavites.

All bubbles in tektites need not be of the same origin.The sources of most of them were gases, which were in-cluded in the parent rock. This is evident from the fact thatthe number of bubbles decreased when the temperature oforigin rose (Barnes 1969, Konta and Mrz 1969). Addi-tional possible evidence is (i) the absence of a relationshipbetween the size of the bubbles and the size of the tektites,(ii) the abundance of bubbles in lechatelierite and (iii) theirdispersal in the whole tektite volume.

Chao (1963) and Dolgov et al. (1969) regarded thebubbles a result of internal stress effective in the coolingperiod. Only some of the central bubbles in large tektites

can belong to these so-called vacuum bubbles.Angular bubbles in the Muong Nong type tektites are

considered closed pores of the original rocks. The highviscosity of melt prevented their rounding (Barnes 1963).Angular bubbles are rare in normal tektites. They originat-

ed in the final stages of tektite formation, for example,when two parts of a tektite joined together. These cases arecommon among the moldavites in southern Bohemia. Wecan also find a specimen among the Moravian moldavitesfrom Slavice, described probably incorrectly as aMuong Nong type moldavite by Barnes (1969).

Statistical differences in the shape and the abundanceof bubbles exist among moldavites from individual sub-strewn fields. Moravian moldavites and the ones from theRadomilice area are poorer in bubbles when comparedwith other South Bohemian moldavites. The former alsohave less elongated bubbles, in accordance with more iso-metric shapes.

Crystalline inclusions

Crystalline inclusions form only rare components in tek-tites but have a substantial significance for the resolution ofgenetic problems. According to their origin, we can dividethem into (i) unmelted rests of parental rocks (zircon, rutile,chromite, monazite, quartz), (ii) products of recrystalliza-tion and dissociation of original minerals (coesite, badde-leyite, corundum, cristobalite), (iii) products of exsolutionformed by melting and solidification (magnetite, Fe-Nispherules with a mixture of schreibersite and troilite,

hematite, crystals of iron-nickel alloys, chalcopyrite) and fi-nally (iv) devitrified parts (cristobalite).


297

14. A drop-like moldavite from Slve near Trhov Sviny (S Bohemia); 15. A dumb-bell moldavite from Slve near Trhov Sviny (S Bohemia); 16.An ellipsoid-like moldavite from Chlum nad Mal (S Bohemia); 17. Typical moldavites from W Moravia (left Teb-Tervky, middle

tpnovice, right Koichovice). All photos scale 1 : 1.

14

15

16

17

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The most varied range of crystalline inclusions wasfound in the Moung Nong type indochinites, which con-firms that the source rock melted at a relatively low tem-perature. Data about the finds of crystalline phases inmoldavites are rare and were obtained only from the SouthBohemian moldavites. Kleinnman (1969) described mi-croscopic magnetite spherules, which sometimes have anative iron core or an admixture of wstite. The averageamount of these spherules is 25 in one moldavite. Theorigin of magnetite spherules is explained by the crystalli-sation in melt under conditions of strong reduction. Mi-neeva et al. (1984) found small hematite particles(X 10-9 m). Quartz has been found only in one case in amoldavite from Radomilice (Barnes 1969). The quartzforms irregular broken grains of a size of several hun-dredths of millimetre, which are surrounded by lechate-lierite. The occurrence of coesite (Weiskirchner 1962) andbaddeleyite (Glass et al. 1989) in South Bohemian mol-davites is also an exception.

The origin of moldavites

Today, most researchers accept the theory that mol-davites, and in the same way also other tektites, are theproduct of terrestrial impact and represent melted targetrocks ejected during crater formation. The first to presentthis theory was Spencer (1933). His theory was stronglysupported by Cohen (1963), who assigned the Ries craterin Germany to moldavites and the Bosumtwi crater inGhana to Ivory Coast tektites. The credibility of impacttheory was then confirmed by the identical age of the tek-

tites in question and of the respective craters. Later on, theparent crater to North American tektite strewn field wasfound. At present, the only source place still unknown isthat for the Australasian tektites.

Not all impacts result in the formation of tektites. Ac-cording to David (1972) and Stffler et al. (2002), tektitesappear only with the oblique impact of the cosmic body.This is supported by the fact that tektites are today foundin only one direction from the parent crater. Another in-dispensable condition is the effect of strongly compressedair in front of the impacting body on the surface horizons(Remo and Sforza 1977, Delano and Lindsley 1982). The

compressed atmosphere ejected material immediately be-fore the crater-forming explosion. This condition makes itimpossible for tektites to appear on the Moon and on oth-er atmosphere-poor planets.

Dietz (1984) stressed that tektites are formed only dur-ing strong impacts, which create craters at least 10 km indiameter. The chemical composition of target rocks prob-ably plays a certain role, because it strongly influences thecharacter of the melted product and the possibility of glassformation. The proof of this is the relatively small vari-ability of the chemical composition of tektites.

We may sum up the claims of many authors and pres-ent a probable explanation of the origin of tektites. A rela-

tively large meteorite, at least hundreds of metres in

diameter, penetrated the Earths atmosphere at almost theoriginal speed (11 to 72 km s-1) and struck the Earths sur-face at a sharp angle. An extreme compression of air oc-curred on the front part of the meteorite. The compressedair flung away non-solid surface rocks in one direction.The following explosion formed a crater and impactites.

According to Kalenda and Pecina (1997, 1999), the to-tal melting of the ejected material occurred during the firstphase of the impact, as a consequence of friction in the at-mosphere. The adiabatic process distinctly influenced theorigin of tektites from fluid phase (David 1973, 1988).

The origin of various primary tektite forms (the MuongNong type, splash-form tektites and microtektites) is theresult of the same process and it reflects strong temporaland spatial changes in the physical conditions. The originof the Muong Nong type of tektites required low initialspeed of ejection, and the end of the adiabatic process attemperatures about the point of softening or lower. Con-versely, with microtektites this moment happened at tem-

peratures at least by 1000 K higher (Trnka 1992).Solidification of splash-form tektites took place accord-

ing to gas pressure in bubbles at the height of at least 20 to40 km above Earths surface (Rost 1972, Matsuda et al.1993, 2001), with the Muong Nong type tektites solidifica-tion occurred at a lower height. The rate of cooling of tek-tite melt depended on the type of cooling. This rate was thehighest in the period dominated by adiabatic expansion;Feldman et al. (1983) estimated this rate at 70100 Ks-1.With Muong Nong type tektites, the adiabatic cooling wascompleted at temperatures near to the annealing range. Dur-ing the solidification of splash-form tektites, their surface

was cooled by radiation at a rate of about 10 Ks-1. The in-side of the tektites was cooled by conduction and its ratewas about 10 times lower (Wilding et al. 1996). The tektitesreached the Earths surface already in a solid state.

The Ries impact crater, which is genetically associatedwith moldavites, is located about 120 km east of Stuttgart inGermany. It forms a morphologically marked depressionwith the outer diameter of 25 km. Its age is identical withthe age of the moldavites (e.g. Storzer et al. 1995). Accord-ing to several independent determinations (K/Ar, 40Ar/39Ar,fission-track), this age most often ranges from 14.2 to 15.2Ma. The latest 40Ar/39Ar data (Staudacher et al. 1982, Lau-

renzi, Bigazzi 2001, Schwarz, Lippolt 2002) determine themoldavite age in a narrow interval of 14.3 to 14.5 Ma. Thisage is slightly younger (by about 0.30.4 Ma) than the agesuggested by earlier K/Ar determinations (Storzer et al.1995). Relatively oldest age was obtained from fission-trackdata (Bigazzi, De Michele 1996, Bouka et al. 2000).

The cause of the origin of the Ries crater was the im-pact of a stone meteorite. Morgan et al. (1977) consideredthat it was aubrite because of the very low enrichment ofimpact glasses by Ir, Os and Ni. On the other hand, ElGoresy and Chao (1977) supposed that it was the impactof a carbonaceous chondrite because of the chemical com-position of metallic veinlets in the impact glasses. Perni-

ka et al. (1987) and Schmidt and Pernicka (1994) inclined


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to the opinion that it was a chondrite. Preuss (1964) esti-mated the diameter of the meteorite at 500 to 1000 m, En-gelhardt and Graup (1984) at 1000 m.

According to the Stffler et al. (2002), the meteorite atRies impacted the Earth surface at the velocity of 20 km s-1 and at an angle of 30 to 45. The angle of dispersion of

the moldavite matter was 50, which corresponds to thepresent distribution of moldavites.Cohens application of the impact theory on the mol-

davites led to the search for chemically similar rocks in theRies area. It was soon realized that there was a distinct dif-ference in the composition of the crystalline rocks and ofthe Mesozoic sediments on one side and of the moldaviteson the other side. According to Engelhardt (1967), theonly possible source of moldavites in the Ries area are theTertiary sands of the Upper Freshwater Molasse UFM(Fig. 8). This opinion was supported by a detailed geo-chemical study (Luft 1983, Engelhardt et al. 1987,Koeberl et al. 1985, Delano et al. 1988 and others).

Delano et al. (1988) obtained very convincing resultsabout the petrographic composition of the source sedi-ments. They separated the elements contained in the mol-davites on the basis of correlative relationships into threegroups, which correspond to three mineral components ofsand-dominated sediments (Fig. 9). These groups are: a) Si(quartz), b) Al, Fe, K, Na, Ti (clay minerals), c) Ca, Mg,Mn (carbonates). Meisel et al. (1997) also explained thebasic chemical differences between moldavites by petro-graphic variation of these components in source material.

Moldavites and the sands of UFM are chemically verysimilar but not identical. Some of the differences (for ex-

ample, lower contents of Ca, Mg, Sr and higher contents ofFe, Ti and P in sands) can be explained by the variability ofsands or by later influence of weathering processes. How-ever, other differences make us think about a partial changein the sand chemistry during its change to moldavites.

Selective volatilization is the only evident mechanismof change in the chemical composition of the sourcematerial. This process led to a distinct lowering of themost volatile components (H2O, N, S, B, P, halides andnoble gases, As, Sb, Cu and so on Fig. 10) as was alreadymentioned in the chapter about chemical composition.Some studies on indochinites and australites show a par-ticular loss of alkalies. Taylor (1961) compared the chem-ical composition of rims and of the cores of buttonaustralites and found that ablation caused lowering ofalkalies content at their edges for 520 wt%. Ridenour(1986) supposed a loss of alkalies from the positive cor-relation between their contents and the weight of wholepieces of indochinites.

It is generally valid that the loss of volatiles from meltdepends on the boiling point of the applicative component,on the external pressure, on the size of free surface and onthe surface temperature. In the opinion of Knobloch andKuera (1992), the main degassing came about under rel-atively low temperatures in the early stage of tektite de-

velopment.


299

48 50N

10 30E

Wrnitz

Nrdlingen

Harburg

Eger

N

0 10 km

OSM

OMM

Donau

10 40E

Factor1

Factor 2

(Rb, Cs, Ba, Sc,Cr, Co, Zr, Hf,

REE, Ta, Th)

sand

carbonate

clay+heavyminera

ls

CaMg (Mn)

Al

K FeNa, Ti

Si

1

0.01

0.001

0.1

LaFe Al

U

Ca

Lu

Na

MnRb

Ba

H2O

Br

Co Cr

Th

W

YbTa

ZrSe

NiGa

Sb

TiMg

As

components

P2O5

SIO2

K2O

Qv

Fig. 8. Former extension of UFM sediments (moldavite source material)within the Ries Crater, Germany (modified according to Httner andSchmidt-Kaler 1999); (OSM = UFM = Upper Freshwater Molasse).

Fig. 9. Factor analysis of moldavites demonstrating the composition ofthe source rock (Bouka 1994).

Fig. 10. A comparison of element concentrations in the moldavites andin the sands of UFM from Ries on the basis of Qv parameter of contents

of individual components (according to Knobloch and Kuera 1992).


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Conclusions

Moldavites represent a group of tektites subjected to anumber of relatively detailed studies. On the other hand,moldavites have been strongly influenced by their geolog-ical development. They are found in several discrete re-gions: southern Bohemia, western Moravia, the ChebBasin, Lusatia in Germany, and Waldviertel in Austria.Only a few moldavites were found outside these regions.

The extent of moldavite occurrences is a result of in-tensive denudation and redeposition of the initial strewnfield by surface streams. All regions of moldavite occur-rences are spatially connected with regional basins and de-pressions. The oldest moldavite-bearing sediments withvery short-transported material are unsorted colluvio-flu-vial gravelly sands and clays of Middle to Upper Mioceneage. They occur especially in the South Bohemian sub-strewn field. In Moravia and perhaps in Austria, they arerare. Fluvial transport of moldavites to more distant places

determined the present distribution of moldavite occur-rences and led to a substantial lowering of their content inthe sediments.

Roughly 106 metric tons of moldavite matter (macrotek-tites) formed at the time of their formation. Only about 1%of this matter has been preserved till the present.

Most moldavites are splash-form moldavites. No abla-tion features were found on their surface. Muong Nong typemoldavites occur sporadically. It is possible that in the timeof origin their amount was higher. Micromoldavites werenot found. Their preservation in the conditions of continen-tal sediments over about 15 million years is not probable. It

is a question whether they were formed or not.Moldavites represent the most acid group of tektites,with silica content around 80 wt%. They are relatively richin K2O, too. On the other hand, they are characterized bylow average contents of Al2O3, TiO2, FeO and Na2O.These low contents of TiO2 and FeO are responsible fortheir higher translucency, similarly as in georgianites.

In the same way as with other tektites, moldavites orig-inated by the fusion and ejection of surface rocks duringan oblique impact of a large meteorite. The impact body inthe case of moldavite formation was probably a chondrite5001000 m in diameter. Its impact, dating to 14.415.1Ma, created also the Ries crater.

A c k n ow l e d g e m e n t s . The authors thank prof. C. Koeberl for hiscomments on the manuscript and J. Cemprek for technical assistance.This work was supported by the research project MKOCEZ 00F2402.

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