Clays and Clay Minerals, Vol. 48, No. 3, 317-321, 2000.

AUTHIGENESIS OF TRIOCTAHEDRAL SMECTITE IN M A G N E S I U M - R I C H CARBONATE SPELEOTHEMS IN CARLSBAD CAVERN AND OTHER

CAVES OF THE G U A D A L U P E MOUNTAINS, N E W MEXICO

VICTOR JAMES POLYAK 1 AND NECIP GOVEN 2

1Depamnent of Earth and Planetary Sciences, University of New Mexico, Northrop Hall, 200 Yale Blvd. NE, Albuquerque, New Mexico 87131, USA

2Department of Geosciences, Texas Tech University, RO. Box 41053, Lubbock, Texas 79409-1053, USA

Abstract--Trioctahedral smectite is a constituent of Mg-rich carbonate crusts and moonmilks (pasty deposits) in caves of the Guadalupe Mountains of southeastern New Mexico. Energy dispersive X-ray microanalysis of individual crystallites and their aggregates along with the X-ray diffraction analysis indicates that the smectite is probably stevensite. Saponite is likely present in some samples also. The smectite is intimately associated with dolomite crusts and huntite moonmilks in Carlsbad Cavern, Lechu- guilla Cave, and other dolostone caves. Clay particles appear as fibers and films, with aggregates com- prising decimicron-sized filamentous masses that envelop crystals of dolomite, huntite, and magnesite. The occurrence of smectite is related to the genesis of the Mg-rich carbonate minerals. In water films, progressive evaporation and carbon dioxide loss results in the sequential precipitation of Mg-rich calcite, aragonite, dolomite, huntite, and magnesite. This sequence of carbonate precipitation removes Ca and greatly increases the Mg/Ca ratio in the solutions. Silica is commonly available probably because of high pH conditions, and consequently, smectite forms in the Mg-rich alkaline environment. Along with the Mg-rich carbonate minerals, opal, quartz, and uranyl vanadates may precipitate with the smectite.

Key Words----Authigenesis, Carlsbad Cavern, Caves, Dolomite, Huntite, Moonmilk, Stevensite, Triocta- hedral Smectite.

I N T R O D U C T I O N

Caves such as Car l sbad C a v e r n and Lechugu i l l a Cave in the G u a d a l u p e M o u n t a i n s of southeas te rn New M e x i c o are wel l k n o w n for thei r natural beau ty and the i r minera logy . These caves have fo rmed in a thick, do lomi te - r i ch ca rbona te rock sequence and con- ta in abundan t chemica l sed iments k n o w n as speleo- thems, wh ich con ta in a var ie ty of M g - b e a r i n g miner- als. See J a g n o w (1977), Hill (1987), and Po lyak et al. (1998) for a deta i led geologic h i s to ry o f Car l sbad Cav- ern and for the genera l loca t ion of the caves, and Hill and Fort i (1997) for descr ip t ions of spe leothems. We repor t the au th igenes is of t r ioc tahedra l smect i te in Mg- r ich ca rbona te spe leo thems, such as crusts and m o o n - mi lks f rom Car l sbad C a v e r n and o ther caves of the G u a d a l u p e Mounta ins . To our knowledge , au th igenes is o f t r ioc tahedra l smect i te (saponi te and sauconi te) in caves has been repor ted on ly in Bulgar ia , Roman ia , and Turkmen i s t an (Hill and Forti , 1997).

M A T E R I A L S A N D M E T H O D S

Sample descriptions and cave setting

Samples f rom six caves (Table 1) were selected for this study. Ca rbona te crusts and m o o n m i l k s are com- m o n in caves o f the G u a d a l u p e Mounta ins . They occur mos t of ten on the cave walls and floor. Ca rbona te crusts are th in incrus ta t ions of ca rbona te minera l s that have a rough " s t u c c o " or " p l a s t e r " appearance . M o o n m i l k s are thin, pas ty to p o w d e r y deposi ts tha t

Copyright �9 2000, The Clay Minerals Society

m a y r e semble " co t t age c h e e s e " w h e n moist . In these caves, some crusts cons is t p r imar i ly of dolomi te , whereas m o o n m i l k s c o m m o n l y cons is t p r imar i ly of hyd romagnes i t e or hunti te . Do lomi t e crusts exh ib i t des icca t ion and curl ing. Mg- r i ch crust and m o o n m i l k spe leo thems occur t h roughou t these caves, bu t more abundan t ly deep wi th in the zone of total darkness . Ar- eas where samples 90031, 94012, 94027, and 94044 (crusts and m o o n m i l k ) were col lec ted had re la t ive hu- midi t ies (RH) at 9 0 - 9 5 % . M e a s u r e d R H in areas whe re samples 88014, 89037, and 90018 ( r imstone and s ta lagmi tes) were col lec ted var ied more wide ly ( 7 0 - 9 5 % ) because they were located in the zone o f ind i rec t l ight near the cave entrances . Tempera tu res m e a s u r e d in these areas were 10-17~

Methods

Samples were co l lec ted by pe rmis s ion f rom the Car l sbad Caverns Nat iona l Pa rk Service , L inco ln Na- t ional Fores t Service , and the Bureau o f L a n d M a n - agement . The ca rbona te minera l s in crust and m o o n - mi lk samples were r e m o v e d by the sod ium acetate m e t h o d o f J ackson (1974). We used 5% hydroch lo r i c acid to extract inso lub le mater ia l s f r o m a s ta lagmi te ( sample 89037). Inso lub le res idues were dr ied in an oven at 40~ for 24 h, l ight ly ground, and the powde r s were ana lyzed us ing X- ray d i f f rac t ion (XRD) and t r ansmiss ion e lec t ron mic roscopy (TEM). X R D of ran- d o m powder s and or iented c lay aggregate moun t s were pe r fo rmed us ing a Phi l ips Nore lco d i f f rac tomete r

317

318 Polyak and Gtiven

Table 1. General information on cave samples.

Clays and Clay Minerals

Sample Cave Speleothem Mineralogy Remarks

88014 Cottonwood rimstone cc, as 1% ir 89037 Hidden calcitic stalagmite cc, ar, as 1% ir 90018 Gunsight aragonitic stalagmite at, cc, as 1% ir 90031 Hell Below crust ar, as, ps 1% ir 94012 Spider floor crust do, ts, as nd 94027 Hell Below moonmilk hu, do, ma, ts 6% ir 94044 Carlsbad wall crust do, ts, qz 2% ir

ar = aragonite, as = amorphous silica, cc = calcite, do = dolomite, hu = huntite, ir = non-carbonate insoluble residue, ma = magnesite, nd = not determined, ps = poorly crystallized silicate (kerolite-like), qz = quartz, ts = trioctabedral smectite.

operated at 40 kV and 20 m A with CuKc~ radiation. Oriented clay mounts were dried at 25~ and 50% RH. Samples were treated with ethylene glycol vapor for 2 4 - 4 8 h. Characterist ic X-ray spectra were obtained on films and aggregates o f clays using energy disper- sive X-ray (EDX) microanalysis interfaced to a J E O L J E M - 1 0 0 C X analytical electron microscope. Micro- graphs of clay particles were obtained in T E M mode. Small fragments of freshly fractured samples were mounted, coated with gold, and examined with scan- ning electron microscopy (SEM) on the same micro- scope.

RESULTS

Trioctahedral smectite

Filamentous materials compose a small fraction (1 - 10%) of Mg-r ich carbonate speleothems in these caves. These materials are intrinsically associated with dolomite, huntite, magnesite, opal, quartz, and uranyl vanadates in crusts and moonmilks . The fi lamentous materials consisted o f fiber-like particles and films o f trioctahedral smectite.

The crusts (samples 94012, 94044) consist of mi- cron-sized rhombohedral -shaped dolomite, and the moonmi lk (sample 94027) is composed of micron- sized huntite platelets. The dolomite and huntite crys- tals are intertwined with fi lamentous smectite. Figure 1 shows SEM and T E M images of smecti te f rom crusts (Figure l a and lb) and a moonmi lk (Figure l c and ld). In places, the filaments are relat ively large and complex. In moonmilk , they are composed of smaller fiber-like particles as shown in Figure l c with platelets of huntite (sample 94027). In these samples, the smecti te particles are predominant ly fibrous in ap- pearance, al though some particles are films. E D X mi- croanalysis of crystals in sample 94027 indicates that the smecti te lacks A1. Microanalysis of sample 94044 shows a mixture of Al-bear ing and Al-free Mg-r ich smectite. In all cases AI is absent or minor and the d(060) value ranges f rom 1.516 to 1.528 ,~, which indicates that the smecti te is trioctahedral (probably stevensite). Minor to trace amounts of fibrous Mg-r ich silicates such as sepiolite may be present, but we could not identify them because of the small sample sizes.

Figure 2 shows the X R D patterns for the insoluble residues of samples 94027 and 94044.

Silicates in other speleothem types

Other carbonate speleothems were examined for the presence of smectite. A sample of aragonite crust (sample 90031) was digested and found to contain 1% insoluble residue consist ing of a mixture o f amorphous silica and a poorly crystal l ized Mg-r ich keroli te-l ike silicate (Brindley et al., 1977). A fragment o f stalag- mite (sample 89037) was also digested and found to contain 1% insoluble residue consist ing of amorphous silica, preserved mites and arthropod parts, and trace amounts of detrital silt-sized quartz, mica, and dickite grains. The stalagmite is composed of Mg-r ich calci te and aragonite. Amorphous silica, which was especial ly abundant along thin layers of aragonite, fo rmed molds of the aragonite crystals. X-ray microanalysis of the arthropod parts revealed amorphous silica incorporated in the chitinous remains. No authigenic clays were found in the insoluble residue of the stalagmite sam- ple. Samples of an aragonite stalagmite (sample 90018) and a calcite r imstone (sample 88014) were digested and found to contain - -1% insoluble residue that also consisted most ly of amorphous silica. The r imstone fragments consisted o f low magnesian calcite with minor aragonite.

The mineralogy in these carbonate speleothems in- dicates that silica is precipitating f rom cave waters. Amorphous silica is the main precipitate phase in the insoluble residues o f calcite and aragonite stalagmites. Both amorphous silica and a poorly formed Mg-r ich silicate are major components of insoluble residues in an aragonite crust, whereas trioctahedral smecti te is the main constituent of insoluble residues in the Mg- rich carbonate crusts and moonmilks . Opal or quartz, and tyuyamunite are found associated with the car- bonate minerals and trioctahedral smectite.

D I S C U S S I O N

Authigenesis o f smectite and origin o f Mg-rich carbonates

The formation of trioctahedral smecti tes in caves is obviously related to carbonate precipitation, particu-

Vol. 48, No. 3, 2000 Authigenesis of trioctahedral smectite in caves 319

Figure 1. Scanning and transmission electron micrographs of trioctahedral smectite from caves. (a) Fibrous-like smectite particles (sample 94044). (b) Filamentous smectite intertwined with dolomite crystals (sample 94044). (c) Filament associated with platelets of huntite. Note that the filament fragment is comprised of fiber-like smectite (sample 94027). (d) Filamentous smectite intertwined with huntite platelets (sample 94027).

larly the precipitation of Mg-rich carbonates. The se- quence of carbonate deposition in caves is predicted from the phase diagram of the CaCO3-MgCO3-H20 system at 25~ (Lippmann, 1973). Ford and Williams (1992) and Hill and Forti (1997) modified this diagram to show the expected sequence of carbonate precipi- tation/neoformation in caves as water evaporates and loses carbon dioxide (Figure 3). From petrographic and field observations, we suggest that the sequence of anhydrous carbonate precipitation in these caves is Mg-rich calcite ~ aragonite ~ dolomite --~ huntite --~ magnesite. Precipitation of calcite depletes the thin cave water film on the speleothems of Ca 2§ and in- creases the Mg2+/Ca 2§ ratio. Aragonite then precipi- tates, which further depletes the water film of Ca 2§ and further increases the Mg2§ 2§ ratio. There is a cor- responding increase in alkalinity and pH of the cave water. The carbonate-mineral phase diagram suggests the precipitation of magnesite and brucite under con- ditions of extreme evaporation and carbon dioxide loss. Magnesite is a minor constituent of the huntite moonmilk (sample 94027); however, brucite has not been reported in caves (Hill and Forti, 1997). The smectite has formed with the Mg-bearing carbonate

minerals, indicating that the fluids have undergone sig- nificant evaporation and carbon dioxide loss. The rate of evaporation and loss of carbon dioxide is probably very slow during formation of the Mg-rich carbonate and smectite.

The silicates associated with the carbonates in these speleothems indicate that a sequence of silicate pre- cipitation coexists with a sequence of carbonate pre- cipitation with progressive evaporation and carbon di- oxide loss. This path of sequential precipitation is il- lustrated in the carbonate phase diagram in Figure 3. The succession from amorphous silica in stalagmites to smectite in crusts and rnoonmilks is indicated by the XRD patterns of insoluble materials from these speleothems (Figure 4).

Two depositional settings

Two locations of Mg-rich smectite are areas: (1) where condensate forms and (2) near dripping water. In the first setting, condensate forms by mixing of in- coming cool and dry air with outgoing humid, warm, and carbon dioxide-charged air. The condensate forms a water film that moves downward along the walls dissolving the dolostone bedrock. On the lower cave

320 Polyak and Gfiven Clays and Clay Minerals

oo =< gylcolated ' r

~ / , - o - - , - ~ ,_ , - . r ^ / < q

: ".~ 05 ~ < "r"

�9 ' - ~ . . . . o ,- - / i . . ~ I ~ o inl z 5

-= a 94027 M..._. . . . jv~./ , , =: b 94044 ' ~

2

e,,

_=

4 6 8 10 12 14 2 4 6 8 10 12 14 Degrees theta Degrees theta

0< < ~ co 1.528 A

060 ~o ~ < - " ~< ~ < �9 . . , . , ~ ~ O" ~ Oq CO 0 3 n

- I z ~ . 1 r / % ~ ~. I |

94027 - powder pattern i �9 | �9 i i m | �9 J | |

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 Degrees theta

Figure 2. XRD patterns of oriented (a and b) and random (c) preparations of smectite fractions dissolved from Mg-rich carbonate speleothems (samples 94027 and 94044).

wall and floor, the envi ronment is conducive for pre- cipitation o f carbonates and eventual ly Mg-r ich sili- cates. In the second setting, drip water charged with carbon dioxide and the appropriate chemist ry for car-

4

% Z O

o - 0

O -1 o . 2 r => -3

~-4

, | , ,

CALCITE . . . . . . . calcite Mg-ne~ ca~ctre ,~

D O L O M I T ~ " ' ' ' " ~

H Y D R O M A G N ~ ~ ~ 1 ~ | MAGNESITE

I I I I II II I I I ~ I -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0

(carbon dioxide loss)Log PCO 2

Figure 3. Carbonate phase diagram modified after Lipp- mann (1973), Ford and Williams (1992), and Hill and Forti (1997). The diagram shows the evolutionary path of carbon- ate precipitation from a hypothetical cave water. The evolu- tion of silicates follows this path because of the presence of silica, extraction of Ca z+, and progressive increase of Mg2+/ Ca z+ as the cave water evaporates and degasses COz.

bonate precipitation, produces splashes, aerosols, and thin films, which are sensitive to evaporat ion and car- bon dioxide loss. Mg-r ich carbonates and smecti te pre- cipitate along the margin of the dripstones. In both settings (by s low evaporation, carbon dioxide loss), the Ca 2+ in solution is r emoved by the precipitat ion of calcite (Mg-calci te) and, with a subsequent increase in M g 2+ content, by the precipitation of aragonite. Then dolomite, huntite, magnesite, and/or hydromagnesi te precipitate util izing the remaining Ca 2+ and much of the Mg 2+. The pH of these solutions is high (i.e., pH > 8). The resultant solution is concentrated in Mg 2+ and silica. Smect i te forms in crusts and moonmi lks with occasional opal or quartz.

Possible origin o f trioctahedral smectite

Detrital clays probably have not been altered to smectites in these settings. The fibrous-like morphol- ogy of the smectite, and the way in which they anas- tomose around the dolomite crystals is consistent with this conclusion. It is also not l ikely that sepiolite formed first and later converted to stevensite i f tem- peratures needed for such a convers ion approach 150~ (GiJven and Carney, 1979). Presence of sepio-

Vol. 48, No. 3, 2000 Authigenesis of trioctahedral smectite in caves 321

r

b ,~ ~ Q II o

== /~, g AI ~ > c t

I L I I I

0 5 10 15 20 25 30 Degrees theta

Figure 4. Powder XRD patterns for insoluble fractions of huntite moonmilk, aragonite crust, and stalagmites show an apparent progression of silicates with increasing Mg2+/Ca 2§ of the carbonate speleothems. (a) Insoluble residue from hun- tite moonmilk consists of trioctahedral smectite and minor quartz. (b) Insoluble residue from aragonite crust consists of amorphous silica and poorly crystalline Mg-rich silicate. (c) Insoluble residue from calcite and aragonite stalagmites con- sists of amorphous silica and minor to trace amounts of quartz and other detrital components. Q = quartz, O = amorphous silica.

lite is not conf i rmed in these caves, and the cave tem- peratures probably never exceeded 30~ Smect i te may have formed by the convers ion of hydromagne- site similar to that synthes ized by Takahashi et al. (1997). Such an origin of the smect i te is reasonable, but hydromagnes i te is platy and most o f the triocta- hedral smecti te particles axe fibrous-like. The smecti te may fo rm by neoformat ion of brucite as d iscussed by Baird et al. (1971, 1973) and Harder (1972) for syn- thetic smecti te. Al though a brucite precursor is engag- ing, brucite is not k n o w n f rom caves. Stevensi te prob- ably forms in cave envi ronments by the maturat ion of Mg-bear ing amorphous silicate materials (Farmer et al., 1991), or by direct precipi tat ion f rom cave waters. For the samples s tudied here, the progress ion o f the complex i ty of silicate mineral izat ion f rom amorphous sil ica to tr ioctahedral smecti te obse rved in calcite, ara- gonite, dolomite , and huntite spe leo thems supports ei- ther mode of formation.

A C K N O W L E D G M E N T S

We thank R. Turner and the Lincoln National Forest Ser- vice, D. Pate and J. Richards of the Carlsbad Caverns Na- tional Park, and J. Goodbar and the Carlsbad District Bureau of Land Management for field assistance and permissions to collect samples. We are grateful to S- Attaner, S. Guggen- helm, H. Roberson, and an anonymous reviewer for their con- strnctive comments.

R E F E R E N C E S

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Baird, T., Cairns-Smith, A.G., and MacKenzie, D.W. (1973) An electron microscopic study of magnesium smectite syn- thesis. Clay Minerals, 18, 17-26.

Brindley, G.W., Bish, D.L., and Wan, H. (1977) The nature of kerolite, its relation to talc and stevensite. Mineralogical Magazine, 41, 443-452.

Farmer, V.C., McHardy, W.J., Palmieri, E, Violante, A., and Violante, P. (1991) Synthetic allophanes formed in calcar- eous environments: Nature, conditions of formation, and transformation. Soil Science Society o f America Journal, 55, 1162-1166.

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Giiven, N. and Carney, L.L. (1979) The hydrothermal trans- formation of sepiolite to stevensite and the effect of added chlorides and hydroxides. Clays and Clay Minerals, 27, 253-260.

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Hill, C.A. (1987) Geology o f Carlsbad Cavern and Other Caves in the Guadalupe Mountains, New Mexico and Tex- as. New Mexico Bureau of Mines and Mineral Resources, Bulletin 117, 150 pp.

Hill, C.A. and Forti, R (1997) Cave Minerals o f the World. National Speleological Society, Huntsville, Alabama, 238 pP.

Jackson, M.L. (1974) Soil Chemical Analysis---Advanced Course, 2nd edition. Published by author, University of Wisconsin, Madison, Wisconsin, 895 pp.

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Lippmann, E (1973) Sedimentary Carbonate Minerals. Springer-Verlag, Berlin, 228 pp.

Polyak V.J., McIntosh, W.C., Gtiven, N., and Provencio, R (1998) Age and origin of Carlsbad Cavern and related caves from 4~ of alunite. Science, 279, 1919-1922.

Takahashi, N., Tanaka, M., Satoh, T., Endo, T., and Shimada, M. (1997) The study of synthetic clay-minerals. 4. Synthe- sis of microcrystalline stevensite from hydromagnesite and sodium-silicate. Microporous Materials, 9, 35-42.

E-mail of corresponding author: polyak@unm.edu (Received 28 September 1999; accepted 6 January 2000;

Ms. 381; A.E. Stephen Altaner)