clay formation

8/9/2019 Clay Formation

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FORMATION AND OCCURRENCE OF CLAY MINERALS

BY P AUL F . KERR *

INTRODUCTION

Clay minerals may be si


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20 CLAYS AXD CLAY TECHNOLOGY [Bull. 169

stated that kaolinite must form between pH 4.3 and 5.2,the best range being 4.8 to 5.0. In later experiments,Schwarz (1933) prepared kaolinite through the alteration of feldspar bv treating with 0.5N to l.OX HCl orH,S04 at 300°C for 250 hours. According to Noll (1936),kaolinite forms below 400°C in a neutral solution free ofalkali metals or in an acidic solution containing ions of

alkali metals. The mineral was produced under acidic,neutral, or slightly alkaline conditions at 300°C and at apressure of 87 atmospheres.

Folk (1947, p. 393), who has studied the al terat ion offeldspars and their products in the laboratory to gaininformation on the origin of kaolinite, writes ". . . .kaolin forms in acid solutions up to about 350°C if Alis rather high and K is low; muscovite forms from as lowa temperature as 200°C, through 525°C in slightly basicand rather acid solutions if K and Al are high; andpyrophyllite forms from about 300°C up to 550°C if Aland K are both low. It appears thus that the Al: Si ratioand the K concentrations are very important in determining which mineral will form, since, if the temperature and acidity are proper, that mineral will be stable

whose formula most closely resembles the compositionof the solution around it."Gruner (1944) reported that in about 30 experiments,

largely with feldspars, using HCl solutions at 300°Cto 400°C and in one case at 525°C, museovite formedabove 350°C and kaolinite below; and that pyrophyllitecommonly formed throughout the range of experiments.

The experiments by Ewell and Insley (1935) in whichkaolinite was formed from Al20:3-Si02 gels in a bomb at310°C are in accord with other recent syntheses.

Experiments in synthesis, therefore, indicate that kaolinite may form under pressure at about 300°C. Underacid conditions and given an abundance of time—bothof which nature provides—kaolinite may form at normalsurface temperature and atmospheric pressure. The de

hydration curve for kaolinite indicates that a temperature of formation greatly in excess of 400°C is unlikely.Thus, kaolinite probably does not form at the extremeupper limit of temperature prevailing in fumaroles.

Montmorillonite was synthesized by Noll (1935) at300°C and 87 atmospheres; he used ratios of alkali: abi-mina : silica of 0.02 : 1 : 4 to 1:1:4. The alkalis used wereNaOH, KOH, and Ca(0H)2, under conditions slightlymore alkaline than required for kaolinite. Noll concludedthat montmorillonite forms when solutions are alkalineand alkali-earth metals are present. If an excess ofMg(0H)2 is present, the magnesium enters into thecomposition of the montmorillonite up to 15.3 percentMgO; this is thought by Noll to correspond to fuller'searth.

Beidellite has been formed by Norton (1939) a t 275-325°C by the action of COo-eharged water on albite; in1937 he developed sericite by the same method, usingorthoclase, anorthite, and albite. Finely ground ortho-clase was altered with HoO and CO2 in 33 days at 280°Cand 1800 psi pressure, and in 150 days at 320°C and2950 psi pressure. Noll (1932) found that sericite formsin greatest amount at 250°C, the lowest temperature offormation being 225°C.

Ewell and Insley (1935) synthesized dickite fromhydrous alumina-silica gels at 350° and 365°C. Differ

ential thermal curves indicate that dickite is stable athigher temperatures than kaolinite.

Thugutt (1894) reported that hydrated nepheline gavea product similar to nacrite.

Xontronite was produced in a bomb at 350°C by Ewelland Insley (1935) from copreeipitated gels using Fe203.2Si02.

Ilalloysite, according to Noll (1936), is unstable above50°C; however, complete dehydration is obtained only byheating to about 4:00°C. Partial dehydration occursreadily at temperatures as low as about 75°C, or in a dryatmosphere, or under pressure. The intermediate statesof dehydration appear to be quite stable because manyspecimens occur in nature in this form.

Adularia is not a clay mineral, but it may be foundunder similar conditions and may be an anhydrousassociate of clay minera ls. Morey and Ingerso n (1937)indicate that the temperature needed for the synthesisof adul aria is in the range 245-300°C. Gruner (1936)heated montmorillonite in gold-lined pressure bombs inaqueous solutions of KIICO3 (10 percent). Seven daysat 300 °C produced very good orthoclase t ha t gave anX-ray powder photograph identical with that of adularia.At 272 °C the feldspar pa ttern became distinct after 10days. At 245°C the stronger lines of orthoclase appearedafter 42 days of heating. No changes were noticeableaft?r 4 weeks at 200°C.

Alunite is another mineral frequently associated withclay. Leonard (1927) formed 60 to 90 percent alunite in7 days in sealed pyrex tubes with O.IM solutions ofH2SO4, K2SO4, and Na2S04 with ammonium sulphate ata temperat ure of 200°C. Alunite (50 to 99 percen t) wasformed at 100°C in 0.05M solutions with 50g of aluminum sulphate by heating for 100 days. With similarsolutions, 40 to 60 percent alunite resulted in 60 daysat 22°C.

Zeolites are reported by Noll (1936) to form undermore alkaline conditions than either montmorillonite orsericite. In the same type of experiment which producedmontmorillonite, analcime was formed with an excess ofNaOH, Ca(0H)2, and Mg(0H)2.

Gaseous Emanations

The vapors from fumaroles and fissure emanations orhot springs are composed essentially of steam that contains some acidic materials. Zies (1929) has given thepercentages of acid constituents for the vapors of theValley of Ten Thousand Smokes as follows: HCl—0.117percent; H2S—0.029 percent; HP—0.032 percent.

Byers and Brannock (1949) repor t the const ituen ts ofthe fumarole gases on northeastern Umnak Island as

water vapor, carbon dioxide, and sulphur dioxide, andthe water from thermal springs to contain as high as 159ppm of boron and a few ppm of arsenic and antimony.

Residual gases in freshly erupted volcanic rock werestudied at the time of the eruption of Mt. Pelee byShepherd and Merwin (1927). Samples were heated invacuo and the gases identified were thought to be characteristic of the eruption. The chief volatiles found were :II2O—80 percent; CO2—9 percent; and CI, S, and Pin smaller amounts.


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Par t I ] GEOLOGY AND MINERALOGY 21

T h e r m a l W a t e r s

Character of Hydrotherma l Solutions The character

of the solut ions responsible for the al terat ion of a rock

to clay depends upon the original source material and

the effects of wall-rock contamination. Both are impor

tant .

Though fluids from the magmatic source may be acidic,

containing the vapors of HC l, H P , and H 2S, react ions

with ground-water and wall rocks often produce a neu

tral or alkal ine environment for the formation of clay

minerals . On the other hand, there is good evidence that

clay minerals may be formed under acid condit ions.

S in gewald (1932), in s tudying the al terat ion of dia

base in igneous intrusions at Loveland Mountain, Park

County, Colorado, concluded that the agents causing the

al terat ion were contained in the magma i tself and fol

lowed the same paths as the in t rud ed porp hyry . Al tera

tion was an end of the intrusion c.ycle of each porphyry.

Mineral izers within the si l ls were augmented by mineral-

izers from more deep-seated magma.

Acc eleration of alter atio n as a result of acid cond itions

has been noted in volcanic regions. Payne and Man

(1946) call attention to two types of chemical decom

posi t ion in basal t around Kilauea crater. At s team vents

where no sulph ur dioxide is prese nt , s i lica and soluble

bases are leached away, leaving the hydrated oxides of

a luminum and i ron . At the so l fa tara , where su lphur d i

oxide is present , al terat ion is accelerated, and alkal i and

alkal ine-earth elements are removed, leaving a yel low

siliceous residue.

MacDonald (1944) has reported the development of

opal and a lesser amount of kaolinite as the result of the

act ion of therm al solut ions weak in carbonic, sulph urou s,

and sulphuric acids. Many cavit ies were l ined with sul

phur crystals , and the other minerals deposi ted include

alum, gypsum, mirabi l i te, kieseri te, epsomite, and aph-

thi tal i te.

Allen (1935 ), in s tudyin g the geyser basins and ig

neous emanations of Yellowstone Park, found i t neces

sary to account for bicarbonates of the alkal is in the

thermal water. He concluded that there must have been

an at tack by CO2 upon the wall rocks at depth.

Pen ne r (1936) found two different processes causing

al terat ion to clay at Yellowstone Park. Near the surface,

by the at tack of acid sulphate on the feldspars , kaolini-

zat ion resul ts . At depth, where an at tack of CO2 was

probably involved, the al terat ion produced beidel l i te. An

interest ing feature described by Penner is the close as

sociat ion of pyri te with the beidel l i te. Casts of feldspars

replaced by clay contain br ight l i t t le cubes of pyr i te,

which were probably introduced by alkal ine solut ions re

sponsible for the clay.

Acids of sulphur on the outlet s ide of an al terat ion

zone, as pointed out by Lovering (1949, p. 53), would

yield a quartz-alunite rock. At a greater distance from

the source of the solut ions, or nearer the surface, aluni te

would be expected to give way to diaspore. This se

quence was described long ago by W hitm an Cross (1896)

in the al terat ion of rhyoli te on Mount Robinson, Colo

rado.

In d iscussing the Ba st Tin t ic district , Lovering (1949,

p.

49) suggested that aluni te forms in a less acid en

vironmen t than th at in which the kaolin mine rals form.

He postulated precipi tat ion from cooling, nearly spent

acid solut ions, with the pH increased to a l i t t le less than

7 by contact with bases in the rocks t raversed.

Sulphate-bearing solut ions, act ing on kaolini t ic clay,

are bel ieved by Ross and Ke rr (1934) to form hal loysi te.

Halloysi te is often formed within or near masses of car

bonate rocks. The mineral is also characteris t ic of vein

deposi ts formed und er acid condit ions (see Love ring,

1949). Halloysi te deposi ts at Liege, Tint ie, and Park

City may be the resul t of precipi tat ion brought about by

acid solut ions act ing on a carbonate rock. At low levels ,

in siliceous or argillaceous-siliceous wall rocks, kaolinite

resul ts from the act ion of acid solut ions; but in the car

bonate rocks higher up, hal loysi te forms.

In solfataric act ion described by MacDonald (1944),

in which kaolini te was formed, the vapors responsible

for the a l t era t ion contained su lphur d ioxide, su lphur

vapor, and a t race of hydrochloric acid.

Nu t t ing (1945) repor ted th at so il mineral s , par t i cu

larly those of the montmori l loni te group, are known to

dissolve or disperse readi ly in water solut ions containing

0.01 to 0.04 percent acid.

Hy dro therma l epo s i t s

Clay Minerals in Active Thermal Regions

Knowl

edge of clay minerals in active thermal localities is frag

mentary. The solfataric act ion reported by MacDonald

(1944) has resul ted in the formation of kaolini te at the

expense of ol ivine basal t lava and ash. As early as 1851,

Bunsen (1851) observed that the t ransformation of a

basal t to clay was no simple soaking process, but that

certain const i tuents might be added, others subtracted.

The al terat ion usual ly s tarts in the glassy or microcrys-

tal l ine groundmass, leaving the phenoerysts intact . The

rock eventual ly becomes soft . Ba rth (1950, p. 49) found

the rocks in Iceland to be pro fou ndl y a l tered by hot -

spr ing act ion; mos t in tense and rap id i s the a l t era t ion

caused by acid springs. The end product consists of opal

with small amounts of clay minerals . The mechanical

act ion of boi l ing and gas exhalat ion breaks up the rock

into a finely divided elaylike substance. In areas of in

tense acid-spring act ivi ty such sl ippery warm clay de

posi ts cover large areas. . . . The basal t ic lava is at tacked

bj - acid steam and vapors emanating from cracks and

fissures in the lava. Around such cracks incrustat ions

usually occur, and the pores and vesicles of the rock are

fi l led with secondary minerals . Simultaneously the pri

mary minerals are al tered by metasomatic act ion, in

ducing changes in the bulk chemistry of the rock.

The dome aroun d Geys i r , Ice land , he descr ibed (B ar th

1950, p. 97) as havin g be en buil t up by prec ipi tat ion of

silica from the hot-spring water as it flowed out of the

basin. It is s t rat i fied (consist ing chiefly of a l ter na t ing

layers of clay an d si lica s inter.) . . . The clay (laye rs)

are identical (in composi t ion) with the decomposit ion

product formed by the act ion of acid hot-spring water

on basal t ic rocks at other places in Iceland, and i t ap

pears that only acid waters can produce this kind of

clay. On the other hand, only alkal ine waters deposi t

s i l ica s inter. . . . The hot spring act ivi ty in this place

has changed many t imes from alkal ine to acid and vice

v e r s a . ' '

Rock al terat ions in the Kilauea region of Hawaii have

been described by MacDonald (1944) and by Payne and

http://ba.se/http://ba.se/


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Man (1946). The resultant rock is composed largely ofopal, with smaller amounts of kaolinitc or related clayminerals, and relict magnetite and ilmenite. It has approximately the same volume as the unaltered rook, andoriginal structures and te:stures are surprisingly wellpreserved.

Anderson (1935) has described and given many im

portant details of the alteration of the lavas surroundingthe hot springs in Lassen Volcanic National Park. Opal,accompanied by minor amounts of kaolin and alunite, isthe chief product in the altered exteriors of all lavafragments collected whore active decomposition is takingplace. The muds from the liot-spring basins arc similarin composition except that they usually contain morekaolin; the mud pots and the sediments from BoilingIjake consist largely of kaolin. Day and Allen (1925)suggested that two types of lava decomposition are inprogress: ". . . The one producing kaolin and some silicawithout aluminum sulphate, the other prodixcing silicawith aluminum sulphate. Tn the fact that kaolin is decomposed by strong sulphuric acid into silica and aluminum sulphate, the key to the difEerence is doubtless to

be found. If the acid forms in a place whore sufficientwater is percolating, its concentration is kept down tosu-ch a valiie that the decomposition of feldspars, volcanic glass, and possibly other minerals is incomplete.The intermediate and comparatively stable compoundkaolin results, and this generally occiirs in the springs,together with very dilute acid.

"On the other hand, if sulphuric acid forms in nearlydry ground, it will accumulate by progressive oxidationof the sulphur gases and the concentration may reachcomparatively high values—probably in the form ofsirupy films."

Anderson (1935) believes there is considerable evidence in support of this conclusion, namely the highalumina content of the sediments from Boiling Lake

where one might expect the greatest diUition of sulphuric acid. Moreover, opal is being formed from lavasthat are being attacked by steam where the acid may bemore concentrated.

Clay Minerals as Cavity Fillings in Pegmatites. Presumably cavities in pegmat ite dikes, signifying gaseousconditions, may provide channels along which thermalfluids permeate and alter the feldspars. They representa late, probably hydrothermal, stage in pcgmatitic activity. The alteration could be largely either the result ofcompressed vapors or thermal solutions. Silva and Neiva(1948) attribute montmorillonite formed in granite pegmatites to hj'drothermal solutions.

Clay Minerals in Metalliferous Deposits. Tn recentyears the importance of clay minerals as indicators ofmode of origin of metal-bearing dejiosits has come to begenerally recognized. Many of the metal-bearing depositsassociated with late Mesozoic or Tertiary volcanism, particularly in the Cordilleran region of the continent, haveassociated with them prominent zones containing clayminerals. Deposits of tungsten, molybdenum, zinc, lead,copper, and uranium have been observed in a numberof instances to occur in sites where the wall rocks haveundergone partial or complete replacement by clay minerals. In particular, montmorillonite, kaolinite, halloy-

site, hydromica (il lit e), and dickite are coming to berecognized as significant by-products of the ore-formingprocess.

The initial stages of hydrothermal alteration may notinvolve elaj^ minerals, as in the stages described byLovering et al. (1949) at Tintic, Utah, where the ferro-magnesian minerals of igneous rocks are converted to

chlorite, and limestone is dolomitized. However, Salesand Meyer (1949) consider the montmorillonite at Butteone of the earliest indicators of hydrothermal alteration.Kerr (1951) reports the formation of chlorite at SilverBell prior to the clay-mineral stage, and also at the copper deposit at Santa Rita (Kerr ct al., 1950).

The initial stage represents the greatest penetrationof the solutions and vapors from the magma into thefresh wall rock. It is here also that the alkali and alkaline-earth ingredients of the wall rock react most stronglywith acidic fluids from the magma to produce alkalineor approximately neutral conditions. Sah>s and Meyer(1949) have given a summary of numerous analyses ofthe rocks of this stage, indicating particularly the release of alkaline earth elements from quartz monzonite

bordering the Butte veins. The outer reactive zone inliydrothermal alteration encloses a zone of somewhatloAver pIT in which hydromica (illit e) rat her tha n montmorillonite becomes the dominant clay mineral. On theinner boundary of the hydromica zone, kaolinite becomes the dominant clay mineral. Tjocated concentrically within the kaolinite zone is an inner zone inwhich sericite predominates.

The chemical and physical conditions giving rise to theformation of the concentric zones of alteration are thesame conditions involved in the processes of metallicmineralization. The evidence of the mineral constituentsobserved indicates an increase in temperature toward thecenter; pressure conditions which are more or less uniform and characteristic of open fractures from a few

hundred to a few thousand feet below the surface; andvariations in pH from a slightly alkaline periphery, toneutral, to slightly acid, to slightly alkaline, from theedge to the center.

Montmorillonite is found in the border zone along themargins of hydrothermal veins, cutting intrusive igneousmasses in the Cordillera of the United States. Sales andMeyer (1949) report that at Butte it is the first claymineral to develop in the hydrothermal alteration of thequartz monzonite.

Field studies in the Marysvale district, Utah, havedemonstrated that montmorillonite and hydromica (illite) may be formed by the hydrothermal alteration ofglass}- volcanic dikes. In the vicinity of Lovelock, Nevada, solid masses of volcanic glass have been found al

tered to montmorillonite, apparently by hydrothermalsolutions. Volcanic ash is believed to be even more susceptible to alteration in areas of hydrothermal activity.

Among the clay minerals, dickite seems likely to formunder conditions of highest temperature. The breaks inthe static dehydration curve are slightly higher, andfield relationships suggest an extension to deeper-seatedconditions of occurrence. Exceptions exist, and althoughis has been suggested that dickite may be formed undersupergene conditions, most, if not all of it is hydrothermal ; it is ordinarily the high-temperature kaolin mineral.


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Part

I]

GEOLOGY

ND

MINER LOGY

3

The original dicki te of Anglesey, dicki te from three

localities near Columbia, Missouri, dickite in the lead

ores of southe astern Missouri (T ar r an d Keller, 1936),

the dicki te at San Juanito, Mexico, dicki te formed in the

al terat ion of the Oilman porphyry at Gilman, Colorado,

and dicki te reported (Frankel , 1949) from the gold-

bear ing rocks of Wi twaters rand are a l l hydro thermal in

origin. Dickite in the veins of Cerro de Pasco coats

coarse crystals of enargi te; at Ouray, Colorado, dicki te

coats sulphide minera ls . Sales and Meyer (1949) found

dicki te in the higher temperature or central zone in the

deepest levels of the Butte, Montana, mines. In the Dag-

gafontein mine on the Witwatersrand (Frankel , 1949)

the dickite was associated with a greenish-gray clay, a

ehlori t ic mineral , pyrrhoti te, gersdorffi te, and a lustrous

hydrocarbon, identified by Davidson and Bowie (1951)

as thucholi te, probably also of hydr othe rm al origin.

Where dicki te and kaolini te are found together in

hydrothermal veins, evidence points to dicki te as the

earl ier of the two. It is a less abundant clay mineral ,

and only in a few places such as San Juanito and Ouray

have masses of any considerable size been found.

l ioness and Will iams (1935) have reported dicki te

from the Pine Knot Coll iery in Schuylki l l County, Penn

sylvania. The material is well crystal l ized and occurs,

for the most part , as a white, gl is tening powder, or in

small lumps composed of t ransparent tabular monoclinic

crystals , many of exceptional beauty, ranging in s ize

from 0.1 to 0.5 mm along the major axis. Of the asso

ciated species, clear t ransparent quartz is most common,

al though a small amount of pyri te and magnetic mate

rial is present . The quartz crystals are everywhere com

pletely coated with dicki te that impregnates the crystal

surfaces in a manner suggest ive of replacement .

The association of dickite with sulphides, such as in

the southeas tern M issouri lead ores, ma y indicate neu tral

or s l ight ly alkal ine condit ions of precipi tat ion. At Butte,

serici te is an associated mineral ; at San Juanito, pyri te

may have been, prec ipi tated with dicki te.

In the Tint ic district , Utah, Lovering (1949) found

an abundance of supergene kaolin in the oxidized zone

of ore shoots. Argillie alteration is found below the lava

near channels fol lowed by later mineral izing solut ions.

In origin, nacri te is probably more closely associated

with dicki te than with kaolini te. At St . George, Utah,

and at San Juanito, Mexico, nacri te and dicki te are

closely associated. Both occurrences are believed to be

hydrothermal , as is the well-known occurrence at Brand,

Saxony.

The sequence of clay minerals appears most s ignifi

cant in hydrothermal veins . Indiv idual mineral s may

occur under a variety of condit ions, but the progressive

al terat ion from fresh rock on the wall through a se

quence such as montmorillonite, illite, kaolinite, sericite,

and dicki te is probably characteris t ic of hydrothermal

vems.

S U P E R G E N E PROCESSES

The processes by which clays are formed on or near

the surface have received much less experimental at ten

t ion than have the processes of hypogene clay formation.

It seems clear, however, that atmospheric condit ions are

responsible for the formation of clay minerals under a

variety of conditions.

Clay-mineral development under normal surface con

di t ions may require periods that are long even in terms

of geologic time unless some accelerating conditions

prevai l . Under even sl ight ly favorable condit ions, how

ever, leaching and deposi t ion, together with weathering

and soi l formation, do take place. The chemistry of

supergene processes is complex. It is general ly thought ,

however, that even weak concentrat ions of acid or alkal i

solijtions together with small amounts of alkali or alka

l ine-earth elements, alumina, s i l ica, and other chemical

constituents, given sufficient time, may produce clay

deposi ts , some of which are very large. Special condi

t ions, such as a more concentrated chemical act ion or

moderate increase in t emperature , may great ly decrease

the time factor.

In sal ine or brackish bodies of water, clay minerals

are thought to be often subject to diagenesis . In regions

of heavy rainfal l and high temperature, organic acids

are apt to accumulate in unusual concentrat ion, accel

erat ing the formation of clay. Even organic concentra

t ions in bogs where the temperature is not extreme may

form clay minerals .

The formation of clay minerals original ly, by ei ther

hypogene or supergene act ion, fol lowed by removal ,

t ransportat ion, and redeposi t ion at a new si te, is s imple

in general concept but involves many features that

should receive fur ther scru t iny—part icu lar ly the ex

tent to which the al terat ion or direct precipi tat ion of

clay minerals may take place in the cycle.

N o r m a l S u r f a c e A c t i o n

Normal surface act ion in clay formation involves

leaching and deposi t ion, and weathering and soi l for

mation. Leaching and deposi t ion may take place as a

resul t of special chemical condit ions exist ing in frac

tures in rock masses considerably below the zone of

weathering and soi l formation. The dist inguishing of

clay minerals formed by leaching and deposi t ion from

those formed by low-temperature hy drothe rma l processes

may not always be possible, but the at tempt is worth

while. In clay accumulat ions formed by mechanical

transport , the presence or absence of chemical precipi

tates and the origin of the introduced clay minerals

pr ior to t ranspor t are the mos t in teres t ing features .

Leaching and Deposition In the formation of ben-

tonite, for example, supergene leaching and deposi t ion

are probably involved. As defined by Ross and Shannon

(1926) bentonite is formed by the chemical al terat ion

of glassy igneous material usually tufl* or volcanic ash.

One large group of bentonite deposi ts found in the lower

Mississippi Valley and Gulf Coast region (H agn er,

1939) has probably been formed under supergene con

di t ions. Even in many volcanic areas of the western

United States, the actual formation of clay may have

resul ted from supergene act ion. In recent years , as a

result of the search for bentonite to be used as an ad-

sorpt ive clay, thousands of other bentonite occurrences

have been discovered.

N utt i ng (1945) has pointed out that the complete

replacement of shel ls by montmori l loni te at Pontotoc,

Mississippi , indicates that alumino-si l icate sols may be

deposi ted under condit ions that al low removal of the

original bases present in the shel l-forming material .


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24 CLAYS AND CLAY TECHNOLOGY' [Bull. 169

Allen (1944) emphasizes the importance of sedimentary processes in elay-mineral foriftation in certain Pacific Coast areas where volcanic materials predominate.Sedimentary processes are more important than volcanic processes at Tone, Cal ifornia; Castle Rock, Wash-injiton ; Whiteware, Montana; Ilobart Butte and Molalla,Orei>on. Clays derived directl.v from volcanic materials

are composed dominantly of niontmorillonite; but theseclays, with the exception of the beidellite-nontronitevarieties, have relatively low percentages of availablealumina. In contrast, clays derived through leaching ofvarious aluminous rocks in thoroughly drained areas arecomposed dominantly of kaolinite; furthermoi'e, the dep-ositional structure of these sedimentary clays favors theformation of gibbsite by weathering or the formation ofdickite or kaolinite by hydrothermal action.

Large accumulations of fuller's earth that containattapulgite as the eifective clay mineral have beenworked for many years in the Georgia-Florida region(Kerr, 1937). The sediments which contain the clayshave been derived from the erosion of the crystallinerocks of the highlands. On the other hand, no such accumulation of attapulgite as is found in the sedimentarylayers has been reported in the source area. It seemslikely that the accumulation of attapulgite may represent more than simple transport and deposition; perhaps waters unusually high in magnesium, or long-continued action of waters low in magnesium, contributedto the formation of this unusually high-magnesia clay.

The unique clay mineral lieetorite described by Foshagand Woodford (1936) is found in the Mojave Desert,California, in folded sediments of Tertiary age. Relictextures are abundant in the clay matrix; they includeminute collapsed cavities lined with layers of the claymineral, and curved structures which may be alteredpumice shards. The rock is probably a greatly altereddaeite t\iff. The clay mineral of which the rock is now

chiefly composed, is chemicall.v and optically almostident ical with the magnesian clay minera l, but differsfrom it by swelling greatly in water. In origin, therefore, this bentonitic magnesian clay is not essentiallydifiPerent from most normal bentonites. Chemical analyses of this clay show that it is essentially a magnesiumsilicate with only an insignificant amount of alumina.Noteworthy are the high soda content (3 perc ent) andthe lithia content (1.12 percent).

According to Tarr and Keller (1937), kaolinite foundin several localities in Missouri gives evidence of havingbeen deposited from solution. Kaolinite occurs in theOrongp Circle mine as thin veins in nearly vertical jointsof the Pennsylvanian shales, as well as in cavities andas a replacement mineral. The kaolinite at the NationalPigment barite mine was deposited in joints in dolomitethat had been enlarged by solution. At the Reavis mine,kaolinite occurs dominantly in solution cavities and along joints in the Jefferson City dolomite. This mine and theNational Pigment mine are in sink holes. Kaolinite in theKeokuk, Iowa, area occurs in quartz geodes and in smallsolution cavities in siltstone.

Allen (1937) has described kaolinite derived fromlimestone by supergene processes. The Cheltenham clayof Missouri had its source in an earthy limestone subjectto the action of carbonic-acid waters. Erosion into a

choked sink hole or closed basin has resulted in the accumulation of a sedimentary clay deposit.

MacKenzie (1949) has described a clay mineral ofpossible supergene origin, which differs somewhat fromillite but belongs to the same groi;p. The material occursat Ballater, Aberdeenshire, in a crush-band in the altered rock along a vein in a granite outcrop exposed inroad-widening. The alteration is apparently a result ofthe action of water, but since the band was still highlyaltered at the base of the exposure (about 25 feet fromthe surface), it is not certain from the field relationshipswhether the water was of hydrothermal origin orwhether it percolated from the surface.

As pointed out by Ross and Ke rr (1934) halloysiteforms many times bj' supergene processes. Callaghan(1948) has described halloys ite 4H2O from Bedford,Indiana, which represents a supergene reorganization ofdeposited material. Other halloysite deposits were appar ent ly at one time a resid ual soil. Halloysite fromWagon Wheel Gap, Colorado (Cross, 1896), formed asan alteration product of rhyolite, and may have beendeposited from solution or suspension by downward-

moving waters.Lovering (1949) has described kaolinite, halloysite,

and allophane in the East Tintie district, Eureka, tltah,which formed when weathering of sulphide ores produced acid solutions. Away from the sulphide bodies,montmorillonite and beidellite are the common productsof weathering.

Bates et al. (1950) have made a number of interestingobservations on the morphology of halloysite. Halloysite4II2O (endellite) is believed to consist of well-developedtubes which split upon dehydration to halloysite 2H2O.Study with an electron microscope has revealed thathalloysite 2H2O crystals consist of hollow tubes whichhave collapsed, or have split and partially or completelyunrolled. It is suggested that this change in morphology

explains the irreversibility of the dehydration process.There is no appare nt morphological trans ition fromthese crystals to the pseudo-hexagonal plates characteristic of kaolinite, dickite, and nacrite.

Stringham and Taylor (1950) ascribe the formationof nontronite occurring in a contact metamorphic zoneto the action of slightly acid solutions. This clay is probably a weathering product of diopside, tremolite, andpyrophyllite. Nontronite (Main, 1950) occurs within fissures and fills cracks and spaces between polygonal jointsin basalt in the vicinity of Garfield and Manito, Washington. The mineral occurs chiefly as a filling, althoughin some cases the basalt is altered to nontronite. Thenontronite contains a small amount of sericite, pyrite,and quartz. Presumably neutral or alkaline solutions at

normal temperature and supergene in origin, acted onthe basalt and deposited nontronite in openings.

Weathering and Soil Formation. Clay minerals areproduced in various areas on the earth's surface wherethe normal temperature is low; at such a temperature,however, special physical or chemical conditions areessential, and small differences may be significant. Theclay minerals form slowly by rock decomposition. Varveclays deposited during Pleistocene glaciation along theHudson Valley are essentially rock flour with a minorclay-mineral content; working near Newburgh, New


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Part I] GEOLOGY AND MINBEALOGY 25

York, Allen (1947) found a relatively small clay-mineralcontent in the glacial clay—a feature found elsewhereby other investigators.

In contrast, weathering in Brazil under tropical orsemitropieal conditions has produced kaolinitic masseswhich may extend for hundreds of feet in depth. Kerr(1942) has described granite at Fazenda, Pacii, which

decomposed in place, forming kaolinite; the texture ofthe granite was preserved in the weathered material.

Residual clays formed by weathering of granite andpegmatite in the southern Appalachian region furnishmixtures of kaolinite and halloysite in varying proportions. According to Sand and Bates (1952) halloysite isformed only from the weathering of feldspars and, underconditions of intense leaching, is formed from both plagi-oelase and potash feldspars. Primary mica, however,always alters to vermicular kaolinite. Potash feldspars,where leaching is not intense, pass through an intermediate stage of secondary mica, changing finally tovermicular kaolinite. On the other hand, halloysite formsfrom plagioclase, apparently without passing through anintermediate stage.

Rocks can be decomposed by action of normal surfacewaters when sufficient time exists for the completion ofthe process. Water in soil may occur in cavities andcapillaries as adhering water, hygroscopieally combinedwater, chemically combined water, and even watervapor. The total water content of a soil changes constantly through additions and subtractions. (Mohr 1944,p. 41) has pointed out in his discussion of tropical soilsthat the pH may vary from a figure lower than 3 tohigher than 9. Eve n quartz (Mohr, 1944, p. 75), whichis resistant at less than pH 7, dissolves in time at a pHgreater than 7.5.

In tropical weathering, acid water may be responsiblefor the formation of extensive kaolinite deposits. Mohrconcluded that kaolinite would form in tropical soils asa result of weathering of feldspar, under the influence ofpure water or water containing carbonic, sulphuric, orhumic acid. Under the influence of alkali or OH" ions,montmorillonite is more likely to form. In the same climate, if the rocks undergoing weathering are highlycalcic, montmorillonite develops; alkali-rich rocks yieldkaolinite. "Wherever weathering takes place the possibility of an accelerating chemical action always exists.

Correns and Engelhardt (1938) have investigated themechanism of weathering of potash feldspar and conclude that potash, alumina, and silica are all at firstremoved in ionic form with the relative proportions ofthe three constituents dependent to some extent on theacidity of the extracting medium. In time, a goodly number of rock-forming silicates not particularly soluble may,where present in soil and subjected to solutions of suitable pH, decompose to form clay.

An alkaline environment which gives a sufScient supply of magnesium or ferric or ferrous iron in addition tosilica and alumina, is apt to lead to the development ofmontmorillonite. In wet and warm climates, whereweathering and oxidizing conditions are comparativelyrapid, kaolinite and lateritic soil are more likely to formthan montmorillonite.

Frederiekson (1951) proposes, largely on theoreticalgrounds, that the mechanism of weathering is essentially

a base-exchange process between two similar crystalstructures with different degrees of order. The mechanism is visualized as a process whereby the hydrogen ionsof crystalline water are base-exchanged for the sodiumions of albite (which is used as a model). The small sizeof the hydrogen ion and the fact that its introductioninto a crystal system is an exothermic reaction makes the

process possible. The mechanical effect of this base-exchange reaction is a net expansion in the reacting layerof the crystal which causes the rock to exfoliate and become more chemically reactive. The minerals will breakdown into colloids or small clumps of insoluble silica depend ing on the Al : Si ratio in the crys tal latti ce.

Wiklander (1950) has studied the fixation of potassium by clays sa tur ate d with different cations . A micaceous clay (Clarence soil, Illinois) was treated for 21days with HCl, NaCl, KCl, NH4CI, and CaClg in orderto obtain maximum replacement of K+ by the other ions.The subsequent fixation of K by the K+-depleted clayswas then studied by the use of radioactive K*; fixationwas found to be low for H+-, K+-, and NHj^ -saturatedclays but high for Ca+*-, and more especially, Na*-

saturated clays. Similar experiments on ground biotiteshowed low fixation values for H+- and NH4+-saturatedbiotite, but high values for Mg*+- and Ba*+-saturatedbiotite.

Marshall (1948) has reported on ionization of calciumfrom soil colloids. Dilute suspensions of beidellite, montmorillonite, and kaolinite have been titrated with NaOHand with Ca(0II)2 and the activities of H% Na% andCa** in the solution (p H, pNa, and p Ca) determin edat frequent intervals during the titration. Kaolinite,which probably binds the bases on the surface of theparticles onlj'', gives highly ionized "salts," whereasmuch of the sodium or calcium taken up by beidellite ormontmorillonite is held in an un-ionized state. This effectis more marked with calcium than with sodium.

It was Nut tin g's (1945) conclusion tha t montmorillonite, given sufficient time, will go into solution in slightlyacid water, but that only the bases will be removed instrong acids, and that only free silica will be removedin pure water.

Kelley (1939) has pointed out that the most conspicuous effect produced on clays and argillaceous sedimentsby base exchange is reflected in their permeability. Generally speaking, calcium-saturated clays tend to be granular and comparatively pervious, whereas sodium claysare highly dispersed and relatively impervious.

Kelley indicates that the pH of a clay may be influenced significantly by base exchange. Calcium clays,if free from calcium carbonate, are approximately neu

tral, whereas sodium clays may be highly alkaline, a result of the hydrolytic property of sodium clay by whichsodium ions are replaced by hydrogen ions. Ordinarywater always contains a low concentration of hydrogenions and rain water is commonly acidic because of dissolved carbon dioxide, which substantially increases itshydrogen ion concentration. The hydrogen ions, havingstrong replacing power, tend to replace sodium, with theconsequent formation of dilute solutions of sodium bicarbonate, sodium carbonate, or sodium hydroxide, resulting in increased alkalinity of the sodium clays. Ifsodium clay is mixed with calcium carbonate, it may be-


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come highly alkaline upon leaching, owiii" to the factthat calcium ions, furnished by calcium carbonate, willreplace significant amounts of sodium from the clay withthe consequent formation of eorres])onding amounts ofsodium carbonate.

According to Kelley, magnesium, although present insea water in lower concentration than soclium, takes a

more active part in the base exchange of sediments withwhich it comes in contact. Consequently, sediments incontact with sea water will inevitably contain practicallyas much replaceable magnesium as replaceable sodium.

The decomposition of biotite in soil has been discussedby Walker (1949) with special reference to soils innortheastern Scotland. The weathering of biotite causesan increase in the optic axial angle, a decrease in refractive indices, a decrease in specific gravity, the disappearance of pleochroism, and a marked color change. This isaccompanied by a loss of iron, preceded by oxidation offerroTis to ferric, loss of magnesium, potassium, andsodium, and a gain in water. The products of the weathering are hydrobiotite, chlorite, a vermiculite, and ultimately kaolinite.

In biotite at Glen Buchat, weathered biotite flakesalter to a vermiculite, the alteration proceeding throughthe crystal layer by layer. The properties of the vermiculite differ somewhat from those of the hj^drothermalvermiculite previously described.

In a study of Australian soils derived from graniticand basaltic parent material, Hosking (1940) has separated colloidal fractions from the subsoils and hasidentified the clay mineral s present by X-ray diffraction.Kaolinitic clay was found in several granitic soils ofdifferent conditions of origin. In basaltic soils, however,the type of clay mineral present is a reflection of the soilmoisture conditions: kaolin-grouf) clays alone characterize the red loams, montmorillonite-group clays prevail, together with a little kaolinite, in the red-brown

earth, and the montmorillonite group alone is typical ofthe black earths. Montmorillonite may form in areas ofrestricted drainage over basaltic rocks, where climaticconditions would lead to the formation of kaolinite ifthe area were readily drained.

Graham (1941) looks upon an acid clay as an agentin chemical weathering. He believes that hydrogen claymay become an active agent in mineral and rock weathering processes. His conclusion, however, is based uponlimited data on the transport of calcium from anorthite,a not too common feldspar, to colloidal clay.

Grim et al. (1939) conclude, on the basis of data relating to the climatic conditions under which variousclay minerals in Illinois sediments are formed, that the

join t presence of ill ite and kaolinite in a formation indicates a source area, some parts of which had a warmclimate and other parts a cool climate.

Studies of the clay minerals contained in the fine fraction of the insoluble residues from 35 Illinois limestoneand dolomite samples representing the major strati-graphic units of the Illinois geologic column reveal thepresence of illite in all samples, kaolinite in twenty samples, and beidellite (?) in three samples. Much of theillite may be authig enic and probably was derived frombeidellite; the kaolinite is probably detrital. Kaolinitecharacterizes all the Pennsylvanian and Silurian samplesstudied. Some Mississippian and some Devonian samples

contain kaolinite, others do not. The Ordovician samplescontained only illite.

Transportation and Accumulation. Deposits of clayminerals resulting from transportation and depositionprobably constitute some of the most extensive clay-mineral deposits. The minerals most frequently encoun

tered among these clays are illite (hydromica), kaolinite,montmorillonite, and attapulgite.In some basin areas of the earth's surface, the clay-

mineral constituents of sediments derived from widelyseparated areas appear to be remarkably similar. Thisformation of similar products from widely divergentoriginal materials is a matter of considerable interest.Such convergence in claj^ sedimentation, although entirely supergene, finds a parallel in the clay-mineral convergence in hydrothermal metamorphism where variousrock types alter to the same olay sequence.

In a study of flint clays, Galpin (1912) reported amixture of kaolinite and hydromica, probably essentiallythe same as illite. These clays were formed by the re-crystallization of fine sediments purified by the action

of carbon dioxide in the waters transporting them, andby leaching action caused by plant roots. The increasein hydromica is thought to be a result of heat andpressure.

According to Bat es (1947) illite (hydr omica) formsthe bulk of the fine micaceous material in the slates fromthe Lehigh-Northampton district of northeastern Pennsylvania. Although similar to the type illite from Fit hia n,Illinois, in all other respects, the mineral in the slate,studied under the electron-microscope, is seen to occurin flakes which morphologically are larger and muchbetter defined than the fuzzy aggregates of minute shredsor fibers which characterize electron micrographs of illiteobtained from shales. X-ray studies show that the flakesare arranged in parallel orientation in the slate and thatthe amount of alignment is related to the degree of metamorphism. In the specimens studied, the developmentand alignment of illite plates by metamorphic processeswas thought to be the cause of slaty cleavage.

The illite of the underclays of Illinois described byGrim and Bradley (1939) is presumably of sedimentaryorigin. The source of the illite-bearing clay is presumably the weathered surface material from the area enclosing the region of accumulation. The clay was evidently transported and deposited as illite.

Grim (1933) has identified the constituents of theso-called fuller's earth from the Eocene Porter's Creekformation near Olmstead, Illinois. Montmorillonite isthe most abundant mineral present and the only claymineral listed. Grim ascribes near-shore marine originto the Olmstead deposits . Po rter (1907) states tha t clays

that contain montmorillonite originated in basic rockswhere augite and hornblende were prevalent. Rocks highin magnesia probably constitute the source of sedimentary accumulations of montmorillonite.

Grim and Allen (1938) record the alteration of feldspar in gumbotil in situ to montmorillonite. This hasbeen confirmed in the case of the subtropical tur fs,sometimes called black turf soils, of the Bushveld igneouscomplex.

The sedimentary kaolinite of Georgia was derived(Henry and Vaughan, 1937) from the feldspathic rocks


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Part I] GEOLOGY AND MINERALOGY 27

of the Piedmont Plateau. Prior to the Cretaceous, theseroeks were weathered to great depths, and elay ^vasformed. At the bei>'inniiio: of the Cretaceous, tilting ofthe land surface and the rearrangement of the drainagesystem allowed the claj" to be eroded and to be carriedalong with other sediments to the sea. The deposits ofclay accumulated in fan-like freshwater deltas. That

some recrystallization took place after deposition is indicated by the presence of fragile vermicular growths ofkaolinite which can be observed with the microscope.

Kaolinite from Mesa Alta, New Mexico (Main, 1950),represents a sedimentar.y accumulation, probably havingbeen subject to transportation more than once. Microscopic study of the textures in the clay indicates thatin parts of the deposit clay pellet conglomerates existwhich suggest kaolinitic alteration, both before andafter final deposition.

Diagenesis in Saline Waters

It is believed by some that clay minerals may formunder conditions of marine diagenesis. If this theorybe correct, small flakes of mica, kaolinite, or other minerals deposited on the ocean floor Tindergo progressivechange to illite and montmorillonite. Clay deposited askaolinite in brackish water or near shore may disintegrate if deposited at depth. Tt is even suggested that,in a general way, the clay-mineral assemblage and textures of crystallization may indicate the depth at whichthe sediments were deposited.

Accord ing to Dietz (1941) , montmorilloni te may bethe original clay mineral deposited in some places onthe ocean floor, and subsequently alter to illite.

Grim et al. (1949) in their discussin of recent sediments of the Pacific Ocean near California and in theGulf of California attribute broad-range depth characteristics to kaolinite, montmorillonite, and illite.Kaolinite is thought to be gradually eliminated by adiagenetic process with increase in depth. The samplesstudied cover a depth range from near shore to 13,386feet. While the process suggested is of considerableinterest, too little data are available for precise application.

Glass (1951) has applied diagenesis in the interpretation of the conditions of origin of the clay minerals inthe Cretaceous and Tertiary sediments of New Jersey.The minerals involved are kaolinite, montmorillonite,hydromuscovite, illite, a chloritic mineral, and glau-conite. Trans ition s may be traced, using differentialthermal curves from light brown to dark brown montmorillonite, from light green dispersed glauconite,through light green glauconite grains, to dark greenglauconite grains.

Well-crystallized kaolinite in the New Jersey sediments is attributed to a fresh-water lake in Triassicshale hollows. The kaolinite in coastal sediments showsa decrease in crystallinity because the waters are notentirely fresh, and brackish water deposits show a decrease in crystallinity of kaolinite and hydromuscovite,and an increase in the amount of chlorite formed largelyat the expense of hydromuscovite. In marine sediments,illite is formed. When marine conditions are non-reducing, an increase in salinity is considered to bringabout a decrease in the amount of kaolinite and chloriteformed, and an increase in the amount of illite formed.

Alteration Aided by Organic Acids

Bog Deposits. Clay deposits have been repoi'ted anumber of times from bog or swamp accumulations,either of prcsent-daj- or more ancient origin. The claysformed in this way may owe their purity to the leachingaction of waters containing carbonic and various humicor organic acids that are derived from the decay of or

ganic matter and organic processes, as a result of theleaching action of living plants.

A residual kaolinite deposit formed by the decomposition in situ of granodiorite, probably as a result of thecorrosive action of organic acids acting in old peat bogs,has been described by Kerr (1930 ). The deposit wasencountered in a subway tunnel excavated in Brooklvn,New York.

Laferife. Such a close relat ionsh ip exists between theformation of clay minerals and laterite or bauxite thatdiscussion of the origin of clay minerals would be incomplete without at least a brief consideration of thesefrequently associated materials.

The essential mineralogical constituents of laterite aresuch aluminian minerals as cliachite, gibbsite, boehmite,or diaspore, and such ferrian minerals as limonite,goethite, or lepidocroeite. These hydrous aluminian orferrian oxides are ordinarily impure, and the extent oftheir development is an index of the amount of latcriti-zation involved. The tropical weathering of magnesianand ferrian rocks in certain areas suggests that perhapssecondary serpentine or other hydrous magnesian minerals may form in a similar manner.

Many divergent solutions have been proposed for theproblem of the nature and mode of origin of laterite(Robinson, 1949). More than a hundred years ago, H.Buch anan (1807) proposed the name laterite for a typeof red soil frequently found in southern India. The termhas since been applied to red soils both in tropical regionsand elsewhere.

Mar tin and Doyne (1927), on the basis of chemicalanalysis, consider laterite a material in which the molecular ratio of silica to alumina is less than 1.33:1, whilein lateritie soils the ratio is 1.35 to 2.0:1. This definitionis inadequate as it neglects iron as a elay mineral constituen t and groups toge ther soils of different profilecharacteristics and history.

Pendleton (1936) restricted the term laterite to specificsoil layers rich in sesquioxides of iron and aluminum.Many such laj^ers are characterized by the presence ofpisolitic material and colloform crusts overlying mottledor vesicular zones. Campbell (1917) considers lateriteof western Australia and Africa to have been developedby ground-water. Pendleton (1936) considers laterites toresult from pseudo-illuvial deposits in the zone of a

fluctuating wate r table und er peneplanic conditions .Later uplift may cause these ground-water conditions todisappear, inducing the development of a new profile.Profiles which maj- be explained in this way occur inmany parts of the world.

Ilarrassowitz (1926) lists four horizons for a typicallateritie profile : (1) the parent rock at the base ; (2) thehorizon representing the material of initial weathering,Avhich appears to be kaol init ic; (3) the laterit ie hori zon;and (4) a surface horizon with ferruginous incrustationsor pisolites. The physical characteristics are fairly well


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28 CLAYS AND CLAY TECHNOLOGY [Bull. 169

recognized. The lateritic horizon is colorful, consisting ofred material mottled with yellow or violet, and is generally argillaceous. When derived from quartzose rocks,it may be granular in texture, vesicular or cellular, withpores frequently filled with white or grayish material.When moist, the laterite can be dug, but it hardens ondrying and can be used as a building material. The sur

face crust may develop to such an extent that it preventsthe growth of vegetation.

Pendleton (1936) considers laterites in Siam to be aresult of ground-w'ater action. He distinguishes lateriteoccurring in deep horizons, w'here parent rocks aresaturated with water and iron compoiinds go into solution, from laterite formed near the surface where thepores are filled with air and the ferrous compounds become oxidized to ferric.

The typical lateritic profile exhibits concretionary orcrust-like material und erla in bĵ mottled clay, but thereare profiles in which, although concretionary materialoccurs, the mottled horizons are absent. Hardy andFollett-Smith (1931) describe variations in the lateriticprofile. The source material apparently influences the

development of laterite. Hardy and FoUett-Smith statethat acidic rocks siach as granite do not yield laterites.This may be because of the substitution of a kaoliniticweathering product formed from the potash and potash-soda feldspars occurring in such rocks.

Various opinions have been given concerning themechanism of laterite formation. Harrison (1934) considers that two main processes are involved: (1) primarylateritic formation; and (2) resilicification of the lateriticproduct by deposition of silica from ascending solutions.The latter process occurs in localities affected by a highwater-table.

Campbell (1917) considers Australian and Africanlaterites to have been formed by changes which occurin the zones of permanent a nd interm itten t saturat ion

in the vicinity of the water-table. Oxygen required forthe oxidation of ferrous to ferric iron implies a nearsurface formation.

Hydrated sesquioxides are a result of hydrolysis ofsilicate minerals. Alkaline ground-water soUitions convert the iron silicates to ferrous hydrogen carbonate andthe alumina silicates to alkaline aluminates. Duringperiods of drought, dissolved material from the watertable migrates upward; the ferrous hydrogen carbonateis oxidized to amorphous hydrated ferric oxide, and thealuminates to amorphous hydrargillate ALO., • 3H2O.Campbell (1917) considers further changes to involvethe formation of something he called "turgite" 2Fe203 •3H2O, and possibly hematite, Fe20,-i, from amorphousferric oxide hydr ate , and c rysta lline gibbsite AI2O3 •

3H2O from hydrargillite. The greater mobility of ferricoxide in colloidal solution causes lateritic horizons tobecome progressively more aluminous as laterizationcontinues.

Woolnough (1918), in his studies of western Australia,concurs with Campbell. He limits laterization to thosesituations in w-hieh free drainage does not exist, as in apeneplain near sea level. Laterites at higher elevationsare products of processes which took place prior touplift, and of a change in hydrological conditions.

Whether alluvial deposition of sesquioxidic materialmay or may not have occurred in the presence of a high

ground-water table, it seems likely that the principalclimatic circumstances are high temperature and thealternation of aerobic and anaerobic conditions in thesoil profile.

The mobility of sesquioxidic constituents in the soilprofile has been attributed by Campbell (1917) to thealkalinity of the ground-water, by Harrassowitz (1926)

to the protective effect of silicic acid soils. Bach of theseexplanations may be applicable in the wide range oflateritic profiles which exists.

Kaolinite is an intermediate product in the formationof Arkansas bauxite. Gordon, Tracey, and Ellis (1949)have recognized residual and colluvial bauxite depositsas the chief source for commercial production. Kaoliniticclay separates the residual bauxite from fresh nephelinesyenite, the initial source of the alumina. The colluvialdeposits grade into a surrounding kaolinite envelope.

Harder (1949) agrees with other students of the sub ject th at bauxite may originate directly by the al tera tionof primary aluminum-bearing rocks, lie also points outthat both field and microscopic evidence show thatkaolinite may be an intermediate product in the forma

tion of bauxite. He does not think that kaolinite foundin association with bauxite represents resilication of thebauxite.

Bauxite deposits (Harder, 1949) may consist mainlyof the trihydrate, gibbsite, or the monohydrate, boehmite,mixed locally with diaspore. Pressure and possibly heatfirst change the gibbsite to boehmite, and eventuallychange the boehmite to diaspore.

According to Hard er (1949), the weathering of anyone of many rock types or their weathered derivativesmay produce bauxite or laterite, although more thannormal alumina content and the presence of soluble constituents accelerate the process. The rock type, composition of ground-water, topographic relief, temperature,and rainfall are all significant factors in bauxite forma

tion or lateritization. A warm humid climate with alternating wet and dry seasons hastens the process. Aland surface of low relief, permitting rainfall to penetrate the ground, is considered essential. The chemicalcomposition of the ground-water and the solvents presentare important. In many cases, the original constituentsof the rock determine whether bauxite, iron ore, or manganese ore results.

Rankama (1950) emphasized the role of carbonatedwaters in the formation of aluminum hydroxide frombasic igneous rocks. He also attributes the formation ofsome calcareous bauxite to the action of carbon dioxide-bearing waters in a warm climate. The argillaceous constituents of limestone are converted into bauxite and thecalcium carbonate is removed as bicarbonate.

Goldman (1949 ), in a detailed petrogra phic stu dy ofspecimens from Arkansas, has described the alteration offeldspars to finely crystalline gibbsite, thence to seemingly amorphous pisolitic bauxite, followed by kaoliniza-tion of the aluminous hydrates by silica derived from theunaltered core. It is not clear how often this process maytake place.

Allen (1948) describes three processes in the formationof bauxite: (1) desilication; (2) migration; and (3)resilication. Desilication of the original rock and themigration of the clay and aluminous material have longbeen recognized. Resilication, or the union of silica with


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gibbsite to form clay, however, has not been generallyrecognized. Allen states that the occurrence of cellularkaolin in Georgia suggests formation by addition of silicato gibbsite, rather than by migration of kaolinite. Thisconclusion he strongly supports in a more recent publication (Allen, 1952).

Bates (1942, 1945) has described a kaolinitic clay, the

Edwin clay, from near lone, California. Field work, microscopic study, and evidence secured from firing testsshow that the Edwin clay is residual, derived directlyfrom laterite. Transitions observed in the field, similartextures, and other evidence suggest that the laterite isintimately connected with greenstone (Jurassic meta-andesite), common in the foothills of the Sierra Nevada.

Goldich (1948) concludes that in regions favorablefor the formation of laterite and bauxite, the position ofthe water table may determine whether "transitional"clay will form, or bauxite will be produced directly. Below the water table, clay minerals are most likely to bethe end product of weathering; but above the watertable, the alteration product will be bauxite or laterite.Below the water table in tropical regions, endellite, hal-

loysite, and kaolinite develop. Why halloysite 2H2O andhalloysite 4H2O both develop below the water table is notclear.

Lapparent (1936) reports that the bauxitic clays ofAyrshire contain boehmite, diaspore, and kaoliniteformed from basaltic rock. He believes that the alteration began as an alumina-silica gel, resulting from thehydrolysis of silicates in a tropical climate.

The evolution of the gel first took place in the presenceof humic waters that favor the crystallization of boehmite; subsequently, it evolved at a lower depth, wherehigher temperature favored the crystallization of diaspore. Kaolinite was formed later than the hydroxidesof aluminum.

CONCLUSION

In view of the complex chemical conditions involvedand the wide variety of possible origins, it is remarkablethat the clay minerals fall into a comparatively smallnumber of groups.

Although great improvement in the nomenclature ofclay minerals has been made in recent years, there isstill considerable progress to be made. It is hoped that amore general agreement on the nomenclature of severalimportant groups may be possible before long. Suchclarification would facilitate discussion.

Major contributions remain to be made, particularlyin accumulating experimental data on clay-formingprocesses. Further, a closer correlation of field andlaboratory studies and studies of supergene and hypo-

gene clay deposits on the part of workers in this field isto be encouraged. Much laboratory work is to be checkedin the light of field conditions.

DISCUSSION

R. A. Rowland:

Professor Kerr mentioned some of Dr. Grim's work in the Gulfof California which indicated the disappearance of kaolinite andthe presence of degraded illite. It has been noted that flakes ofmica larger than most clay particles are common in the soils inthe United States. S. B. Hendricks refers to these as "strippedmicas" because most of the potash has been removed. Below theupper soil zone where the stripped micas are found, there is oftena gumbotil composed largely of a clay mineral which has been

called either beidellite or montmorillonite. The materi al from thegumbo horizon is simil ar to tha t in the shales of the Gulf Coastregion. It is a stripped illite which now behaves like montmorillonite. These shales also contain larger flakes of potash micas whichcould be called illite, and a variety of chlorites. I t is though t th atthe soil material has been thoroughly weathered and broken downso that only montmorillonite remains to be transported to theGulf of Mexico. Once in the Gulf the montmorillonite may changeto either illite or chlorite. What is this process of diagenesis in

clays? Is there any evidence to indicate that isomorphous substitution takes place in the process of diagenesis or whether the substitution takes place long before the material has become a mineral?In other words, do these clays undergo diagenesis by high-energy-level processes or do they undergo diagenesis by low-energy-levelsolution processes?

R. E. Grim:

The American Petroleum Institute is investigating recent sediments in the Gulf of Mexico and a number of samples have beencollected off the south part of the Texas coast in deep water. Thesesamples are being studied in my laboratory. The water-solublesalts have been washed out of the samples and the amounts ofcalcium, sodium, potassium, and magnesium determined. The determinations did not yield absolute values but ratios of sodium toeach of the elements. It was found that material deposited on theGulf floor is largely montmorillonite, with a little illite and kaolinite. A plot of the ratio of sodium to potassium in the solublesalts against distance from shore shows a relative decrease inpotassium close to shore. There is a correlative increase in theproportion of illite-like material close to shore. The sodium-magnesium ratios remain constant until some distance from shore,where the magnesium seems to disappear.

P. F. Kerr:

I referred to the large accumulation of clay minerals in the GulfCoast region which has been derived from the erosion of a veryheterogeneous assemblage of material. Dr. Grim has pointed outthat many of these materials come from a variety of sources in abasin at least 1,000 miles across, and yet ultimately are reducedchiefly to montmorillonite. There is a parallel to this in connectionwith hydrothermal alteration. At Marysvale, Utah, similar clay-miner al assemblages are produced in surp risi ngly different rocktypes.

Isaac Barshad:

In order to answer Dr. Rowland's question as to the possiblediagenesis of the clay minerals upon being deposited in the Gulf bythe rivers, it is first necessary to consider the changes which themica minerals undergo during the course of weathering into thehydrous mica minerals—or if you like—the "stripped micas." Twoof these changes are significant in the present discu ssio n: (1) thereplacement, through exchange, of the interlayer K* by H+, Ca++,Mg++, and possibly Na* ; and (2) the resu lta nt hydr atio n a ndinterlayer expansion of the Crystal lattice. Both of these changesbring about a reduction in the density of the weathered mica.It is also important to remember that the changes indicated arereadily reversible. With these facts in mind, the process of diagenesis may be pictu red to involve (1) a replacem ent of the readilyexchangeable cation with K*, particularly in those mica clay mineral s which have underg one the smallest degree of cha nge ; (2) adehydration and contraction of the crystal lat tic e; and (3) anincrease in density of the crystal lattice. The increase in densityis imp ort ant because it causes thi s fracti on of clay to settl e outfirst; th at may be the explanat ion of the fact th at the clay mater ialnearest the shore line contains the most mica minerals. The fraction

of the clay which does not undergo the changes described will settlefarther from the shore line and change into a chlorite-like mineral.

As to whether isomorphous substitution plays a role in diagenesis—I would say that it probably does not. It is unnecessaryto postulate isomorphous changes to explain the results.

W. F, Bradley:

One difference between kaolin minerals and montmorillonite minerals is that those of the montmorillonite group are stable inextremely minute flakes, while those of the kaolin group have alarge inherent particle size. Alteration resulting in kaolinite usuallyends at that point. When alteration products pass through adegraded mica or montmorillonite stage they are highly mobile.They lose alkalis and magnesium and the particles are then rela-


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32 CLAYS AND CLAY TECHNOLOGY [Bull. 169

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clay formation

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