Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks

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SMECTITE CROUP

675

of the slurry and D is the hydraulic diffusivity, a property that

depends on the compressibility and permeability of the mixture

and the viscosity of the fluid. Values of D are influenced by the

concentration and size distribution of solids in a slurry, and

can range over several orders of magnitude. Concentrated

suspensions of sand that contain at least a few percent silt and

clay have diffusivities on the order of 10~'m per second or

smaller, whereas dilute suspensions of sand have diffusivities

on the order of 0.1 m^ per second. In the context of mixture

transport, the wide range in diffusivity values illustrates

why the mixture dimension (/;), as well as its solids and fluid

properties, influences whether it remains coherent over

relevant transport timescales.

Applications

Although naturally occurring slurries, such as debris flows,

subaqueous slumps, and snow-rich slushflows, commonly pose

signiflcant environmental hazards, artificial slurries are used

for a number of agricultural, industrial, and geotechnical

purposes. For example, fluvial and estuarine bed sediments

have been dredged, mixed into slurries, and pumped to

facilities for bioremediation treatment to remove contaminants

(e.g., Glaser, 1997); by-products of coal treatment and other

industrial wastes commonly are mixed into slurries and

pumped into impoundment ponds for storage or disposal;

agricultural and animal wastes are converted into slurries and

spread across the land surface or injected underground for

disposal; and clay-slurry barrier walls have been developed and

used for industrial waste containment and other geotechnical

applications (e.g., Guy, 1995).

Jon J. Major

Bibliography

Glaser, J.A., 1997. Utilization of slurry bioreactors for contaminated

solids treatment; an overview. In Proceedingsofthe4thlnternational

In Situ and On-Site

Bioretywdiation

Symposium. New Orleans, April

28-May 1, 1997, 5, pp. 123-130.

Guy, D.G., 1995. A slurry wall groundwater barrier used to isolate a

major trunk road from an exposed aquifer. Soil and Environment,

1235-1236.

Hampton, M.A., 1972. The role of subaqueous debris flow in

generating turbidity currents. Journal of Sedimentary Petrology,

42:

775-793.

Iverson, R.M., and Vallance, J.W., 2001. New views of granular mass

flows. Geology, 29: 115-118.

Major, J.J., 2000. Gravity-driven consolidation of granular slurries:

Implications for debris-flow deposition and deposit characteristics.

Journal of Sedimentary Research, 70:

64-83.

Cross-references

Atterberg Limits

Autosuspension

Coal Balls

Colloidal Properties of Sediments

Debris Flow

Earth Flows

Grain Settling

Gravity-Driven Mass Flows

Hindered Settling

Liquefaction and Fluidization

Smectite Group

SMECTITE GROUP

Smectite, also known as "swelling clay", is a diverse group of

clay minerals with a 2:1 layer silicate structure that can expand

and contract upon wetting and drying. All 2:1 layer silicates

(which also include illite/mica, chlorite, and vermiculite

groups) have 2 tetrahedral sheets surrounding a central

octahedral sheet. Smectite 2:1 layers have a negative charge

which is balanced by weakly bonded, exchangeable cations,

located in the interlayer region (between 2:1 layers). Smectite

interlayers also contain water, which results in little to no

stacking order between 2:1 layers, a property known as

turbostratic stacking.

Chemical composition, morphology, and

identification

The smectite group (derived from the Greek, smectos, which

means soap) shows large chemical variability, resulting in

many specific mineral names (Guven, 1988). The most

common smectite mineral is montmorillonite, which has the

following representative chemical formula: X

nH2O

[(Al|.5Fe^-*;2Mg.3)Si40io(OH)2]" \ where X = exchangeable ca-

tions such as Na""" and Ca^"*"; n = number of interlayer water

molecules, which varies but commonly is ~4-5; Al, Fe, and

Mg = octahedral cations; and Si = tetrahedral cation. Tetra-

hedral substitutions of Al^"*" for Si'*'*" are also common and

develop negative 2:1 layer charge, which can vary from

0.25 to-0.6. Smectite has a crystal structure similar to

vermiculite, except that vermiculite has a larger negative

charge on 2:1 layers. There are two subgroups of smectite

minerals: dioctahedral smectite, which has 2 octahedral cations

per Oio(OH)2 and trioetahedral smectite, which has 3 octahe-

dral cations per Oio(OH)2. Fe-rich dioctahedral smectite is

nontronite and Mg-rich trioetahedral smectite has several

specific mineral names including saponite (with a tetrahedral

charge) and stevensite (with an octahedral charge). Much rarer

examples include Co-rich, Cr-rich, Cu-rich,

Mn-rich, Ni-rich, V-rich, and Zn-rich smectites. Many of the

above elements, and others such as Li and F, occur in small

amounts in common smectite minerals.

Smectite morphology, which depends on conditions of

formation, includes subhedral flakes, euhedral elongate laths

and ribbons, and less commonly, euhedral rhombohedra and

hexagons (Guven, 1988). Because natural smectite is nearly

always very fine-grained (typically <2nm), identification is

normally by X-ray diffraction. Upon exposure to ethylene

glycol, smectite has a diagnostic 1.7nm (17A) basal spacing,

which represents the thickness of the 2:1 layer plus interlayer.

Sedimentary petrology

Smectite is relatively common in soil, sediment, and Cenozoic

to Mesozoic sedimentary rock and paleosol. tt forms in a

variety of surficial environments associated with waters that

have relatively high cation (Na, Ca, +K)/H activity ratios and

SiO2(aq) activities. It is thermodynamically metastable com-

pared to other more ordered silicates; it forms readily,

however, because of fast reaction kinetics (Essene and Peacor,

1995).

676

SMECTITE CROUP

Smectite forms commonly in soils, particularly in areas with

moderate to relatively low rainfall, poor drainage, and neutral

to basic soil waters (Chamley, 1989, pp. 21-50). The relatively

weak leaching capacity of such soil waters allows retention of

soluble elements (Na, Ca, Mg) and silica necessary for smectite

formation. Smectite forms by alteration of volcanic glass,

feldspars, mafic silicates, and other aluminosilicate minerals as

well as by direct precipitation from solution. Soil smectite also

forms from clay mineral precursors such as illite, chlorite, and

vermiculite as well as tfie disordered phases imogolite and

allophane. Most commonly dioctahedral Al-rich smectite

forms in soils; however, dioctahedral Fe-rich smectite (non-

tronite) forms from parent rocks rich in mafic silicates.

Smectite formed in a soil environment is notoriously difficult

to characterize chemically because of its heterogeneity and the

presence of other minerals in the clay size-fraction, making

chemical characterization difficult. Soil smectite commonly

occurs with an incomplete Al- or Fe-octahedral sheet in the

interlayer, making it intermediate between smectite and

chlorite. Soil smectite can also have a relatively high nega-

tive charge on the 2:1 layers, making it transitional to

vermiculite. Such minerals are called hydroxy-interlayered

smectite, hydroxy-interlayered vermiculite, or simply hydroxy-

interlayered expandable. Finally, soil smectite occurs com-

monly as a mixed-layer mineral (i.e., 2 different structures

present within a single crystallite) with chlorite, illite, or

vermiculite. The presence of smectite in a paleosol can suggest

an arid paleoclimate.

Smectite has a detrital origin in many other surficial

depositionai environments such as glacial, fluvial, desert,

freshwater lacustrine, and deltaic (Chamley, 1989, pp. 53-

116).

Study of minerals in such settings can give insight into

provenance and paleoenvironmental conditions in the sedi-

ment source area. Evaporative (saline and alkaline) lakes can

produce trioetahedral Mg-rich smectite, which forms by either

alteration of a precursor dioctahedral smectite or direct

precipitation. Smectite tends to stay in suspension and pass

directly into the marine environment rather than settle in an

estuary because it is not affected as much as other clay

minerals by differential settling resulting from fiocculation.

Floccuiation (q.v.) involves aggregation of fine-grained miner-

als into larger masses commonly because of their interaction

with saline water. As smectite is transported from continental

to marine environments there is little change other than

interlayer cation exchange, which commonly involves a change

from C^a-rich smectite to Na-rich smectite. Authigenic Fe-rich

smectite forms as a precursor to Fe-rich illite (glauconite) in

shallow, low-latitude marine environments near rivers that

supply abundant dissolved iron (Chamley, 1989, 213-234).

Smectite in a deep marine environment commonly has a mixed

origin; much of it is detrital and Al-rich but some of it can be

authigenic and Fe-rich (Chamley, 1989, 259-289).

Al-rich smectite forms during low-temperature (<~50°C)

burial diagenesis or hydrothermal alteration of silicic volcanic

glass and feldspar (Chamley, 1989, 333-358). Bentonites

(^.v.)

are soft, plastic, light-colored clay layers comprised predomi-

nantly of smectite that typically form by alteration of volcanic

ash. With increasing temperature of burial or hydrothermal

alteration as well as geologic age, smectite reacts to mixed-

layer illite-smectite and eventually to pure illite (Chamley,

1989,

pp. 359-389). Interlayer water lost during this reaction is

considered important to geopressure development and petro-

leum migration out of source rocks (Eslinger and Pevear, 1988,

Section 5, pp. 33-42). Low-temperature burial diagenesis or

hydrothermal alteration of mafic volcanic glass and mafic

silicates results in formation of palagonite, which consists ofa

variety of authigenic minerals including abundant smectite

that ranges from Fe-rich to Mg-rich (Chamley, 1989, pp. 291-

329).

High-temperature hydrothermal alteration of basalt

produces mixed-layer ehlorite-smectite.

Physical properties and environmental applications

The very fine grain size of smectite and presence of a negative

2:1 layer charge as well as exchangeable cations and water in

the interlayer result in environmentally important physical

properties including high cation exchange capacity (ability to

exchange cations between the interlayer and pore water),

catalytic action (ability to increase the rate of a chemical

reaction), swelling (ability to incorporate large amounts of

interlayer water causing expansion of 2:1 layers from each

other),

and low permeability. The high cation exchange

capacity of smectite can influence the chemistry of natural

waters and mobility of pollutants in groundwater. Smectite can

catalyze chemical reactions because of its extremely large and

chemically reactive surface area, including the interlayer

region. The catalytic activity of smectite along with its ability

to adsorb polar organic molecules, has lead some to propose

that smectite was important to the origin of life and that

smectite minerals can even be considered crystal genes because

of their ability to grow (replicate) and introduce defects

(evolve) (Cairns-Smith and Hartman, 1986, pp. 130-158). In

dilute water, smectite with interlayer Na can expand to up to

10-15 times the volume of the dry clay, exerting a great

upward pressure. As it dries, it shrinks back to its original

volume. This shrink-swell action can damage walls, founda-

tions,

and roads. Average annual damage from swelling clays

in USA is several billion dollars, making it one of the most

costly natural hazards (Keller, 2000, pp. 68-69). Problematic

areas in USA are in Montana, Wyoming, South and North

Dakota, Colorado, Texas, and Louisiana. The swelling of Na-

smectite along with its relatively low engineering (shear)

strength can also greatly increase damage due to liquefaction

(transformation from a solid to liquid state) during an

earthquake and landsliding. Swelling of Na-smectite in an oil

reservoir rock can greatly reduce production potential

(Eslinger and Pevear, 1988, Section 8, pp. 6-10). The extremely

low permeability of smectite-rich earth materials make them

effective aquitards and very useful as liners in waste disposal

systems. Conversely, smectite-rich soils (known as vertisols)

are not particularly productive because of their impermeable

nature and their tendency to be very soft and plastic when wet

and very hard when dry, which makes plowing difficult (Grim

and Guven, 1978, pp. 245-246).

Commercial deposits and uses

Commercial deposits of smectite are mainly from bentonite

(defined above) and fuller's earth (any natural material that

has a high absorptive capacity and can decolorize oil; smectite

is commonly abundant in fuller's earth). Large deposits of

bentonite and fuller's earth are located in the USA (Wyoming,

Mississippi, Texas, Arizona, and California), England,

Germany, Russia, Japan, and many other countries (Grim

and Guven, 1978, pp. 13-126). The major commercial uses of

smectite are for foundry molding sands and drilling mud.

SOLUTION BRECCIAS

677

Many other minor commercial uses of smectite are listed by

Grim and Guven, (1978, pp. 217-248). Molding sands, which

consist of sand and most commonly smectite clay, are used by

foundries to receive molten metal during the casting process.

The function of smectite is to bind the sand and maintain the

mold shape after hot metal is poured into it. Smectite is a

major component in mud used in drilling of oil and water

wells.

The function of smectite is to create a viscous fluid with

water that cools and lubricates the drill bit and removes rock

chips by circulating drilling mud down through the hollow drill

stem and up through the area between drill stem and hole.

Smectite in drilling mud gives it the desirable property of thix-

otropy, which is the ability to go from a liquid state when

stirred to a semi-solid state upon standing. Smectite thixo-

tropy, which is related to its abiiity to undergo swelling, allows

the drilling mud to stiffen and prevent rock chips from settling

back into the hole when pumping of drilling mud is stopped

temporarily. There are many other uses of smectite including

decolorization of oil, catalysis in petroleum refining, ceramics,

animal bedding, adhesives, cement, clarification of wine and

water, animal feed, grease, ink, pharmaceutical products,

paint, paper, pelletizing iron ore, pesticides, rubber, and soap.

Summary

Smectite represents a chemically diverse group of clay minerals

with a 2:1 layer silicate structure that swells and shrinks with

wetting and drying. It occurs commonly in soil, sediment, and

post-Paleozoic sedimentary rock associated with waters that

have relatively high cation (Na, Ca, -|-K)/H activity ratios and

SiO2 (;,q) activities. It forms most commonly in soils from

subhumid to arid climates, however, it also forms in saline and

alkaline lacustrine, deep marine, and low-temperature burial

diagenetic and hydrothermal environments. Aluminosilicates

and silicic volcanic glass alter to Al-smectite whereas mafic

silicates and mafic volcanic glass alter to Fe- or Mg-smectite.

Environmentally important physical properties include high

cation exchange capacity, catalytic action, swelling, and low

permeability. Commercial uses are mainly as drilling mud and

foundry molding sands.

Stephen P. Altaner

Bibliography

Cairns-Smith, A.G., and Harttnan, H. (eds.), 1986. Clay Mitieralsand

the

Origin

of Life. Cambridge University.

Chamley, H., 1989. Clay Sedimentology. Springer-Verlag.

Eslinger, E., and Pevear, D., 1988. Clay Minerals for Petroleutn Geolo-

gists and

Engirieers.

Society Economic Paleontologists and Miner-

alogists Short Course 22.

Essene, E.J., and Peacor, D.R., 1995. Clay mineral thermometry: a

critical perspective. Clays and Clay Minerals, 43: 540-553.

Grim, R.E., and Giiven, N., 1978. Bentonites: Geology, Mineralogy

Properties,

and

Uses.

Developments in Sedimentology 24, Elsevier.

Giiven, N., 1988. Smectites. In Bailey, S.W. (ed.). Hydrous

Phyllosili-

cates (Exclusive of

Micas).

Reviews in Mineralogy 19. Washington:

Mineralogical Society of America, pp. 497-559.

Keller, E.A., 2000. Environmental

Geology.

Prentice-Hall.

Cross-references

Bentonites and Tonsteins

Cation Exchange

Chlorite in Sediments

Diagenesis

Illite Group Clay Minerals

Kaolin Group Minerals

Mixed-Layer Clays

Mudrocks

Vermiculite

Weathering, Soils and Paleosols

SOLUTION BRECCIAS

Solution breccias are common and widespread in the

geological record. Brecciation may occur entirely within one

soluble rock (usually limestone or dolostone) or where more

soluble evaporites are removed in mixed evaporite-carbonate

and evaporite-redbed sequences. It may be extended upward

by mechanical failure (stoping) into overlying insoluble strata,

e.g., dissolution of ~180m of salt has propagated through

400 m of carbonates overlain by ~650m of clays, mudstones,

and sandstones at a site in Saskatchewan (Christiansen and

Sauer, 2001).

Brecciation can occur during early diagenesis when evapor-

ites are dissolved in supratidal carbonate sequences (Choquette

and James, 1988) or where caves in case-hardened carbonate

sand dunes collapse. It is common at inter- and intra-stratal

sites during burial and later diagenesis, caused by expulsion of

basinal fiuids, or by hydrothermal or meteoric waters; it may

extend to pressure solution (stylolite) depths. It is also

common in superficial karst terrains, where meteoric ground-

water is known to brecciate carbonate rocks at depths as great

km.

Breccia fabrics and matrix

Three principal fabrics are recognized (Stanton, 1966):

(1) crackle breccia, where beds sag apart and crack upon

dissolutional removal of support but there is little displace-

ment; (2) mosaic pack breccia, where there is clast support.

The clasts have usually dropped and vary from partly rotated

to completely disorganized in orientation. Globally, clast sizes

range from small pebbles to "cyclopean breccias" with blocks

of hundreds of cubic meters but are normally more limited

within any given breccia; and (3) rubble fioat breccia, which is

matrix-supported. A genetic association of these fabrics is

often found, consisting of crackle breccias at the top and

around the perimeter of a body, pack breccia within it where

beds are thicker, and float breccias where beds are thin or at

the base. In some Mississippi Valley Type (MVT) deposits,

these may be underlain by a basal "trash" zone of insoluble

residua (Sangster, 1988).

Clast-supported breccias may be comprised of clasts and

voids only, with little or no matrix or cement. This is most

common in breccias in sinkholes and caves that are actively

developing today. Matrix is usually present in early diagenetic,

inter- and intrastratal breccias, consisting of carbonate fines

and insoluble residua that have filtered down or (more rarely)

allochthonous sands and clays. Older breccias of all types will

normally display partial or complete cementation. Calcite is

the principal cement but aragonite, dolomite and gypsum are

also common. In MVT the chief cements are dolomite, FeS2,

PbS,

and ZnS.

678

Si'ELEOTHEMS

Figure S35

dolomite tind dcdolomite solution brerci.!

reatetl

prelerfnlijl dissolutiun

gypsum

in a

sabkha sequence,

the

Be^ir

Rock Formation iLower-Middie Devonian!

Bear Rock near

Fort Norman, Northwest Territories, Canada. The section

is ^

in height.

Form

and

location

the principal types

of breccia bodies

As noted, solution breccias iirc common

on the

surfiice

and

underground

modern karst terrains, chietly where sinkholes,

river ciiffs,

shaft.s

and

chambers

caves, have collapsed.

Individual collapses

may be

millions

cubic meters

volume. Where surficial collapses

are

preserved

buried

paleokarsts. Choquette

and

James (1988) term them "mantle

breccias".

Extensive brecciation

usually present where

the

proximal

margins

buried salt bodies

on the

continents

are

subject

dissolution

groundwaters penetrating

the

cover rocks,

creating

receding "salt slope'" (Ford

and

Williams,

1989.

p.46O).

Manitoba

and

Saskatchewan. Canada,

the

salt slope

Devonian deposit

-800km

length

and now at a

mean depth

200 m below

the

surface.

It has

been subroded

westward

average

130 km since bnrial

in the

Devonian.

Breccia pipes ("geological organs", "breakthroughs", "pris-

matic bodies"

different authors)

are the

most widely

reported breccia bodies, thousands

examples being

described

in the

world literature (Bosak

ci al.,

1989).

Diameters range from <lOm

>l()km, with

majority

being 20-250 m. Most

are

plumb vertical, with reported

heights ranging from 20

more than 1000 m. Higher

examples have usually stoped upward through

one or

cover formations, which

may

include siliciclastics. coal

measures, extrusive volcanics,

etc. The

upward termination

may

be in

undisturbed rock,

downfaulted

but not

brecciated

strata,

in a

closed depression

at the

surface,

even upstanding

lirmly cemented

and

resistant "castille" above

erosional plain (e.g.. Hill, 1996,

in New

Mexico). Most breccia

pipes originate

point dissolution

salt (occasionally,

gypsum

anhydrite) above

fracture junction, anticlinal crest

or buried reef that channels groundwater. They

may be

targets

for

the

later precipitation

economic

ore,

such

as the

well-known uranium pipes

of the

Grand Canyon, Arizona

(Huntoon. 1996).

Other forms

are

described

most detail from MVT deposits

(e.g., Sangster. 1988). Breccia domes typically 30-IOOm

diameter

are

similar

breakdown rooms

modern eaves

transitional

breccia pipes. Tabular

sinuous forms

are

wide

but

shallow breccias extending

for one km or

more, often

along paleoreef margins. Straight linear features ("runs'")

and

reticulate ma:^es include individual breccia-tilled corridors

.10m

high.

150m or

more wide,

and one or

km in

length. Most

these features occur

relatively shallow

depths beneath long-exposure paleokarst surfaces

and are

believed

to be

geneiically associated with them. Some

are

attributed

brecciation

and

subsequent sulfide

ore

(cement)

deposition from invading hydrothermal solutions (Dzulynski

and Sass-Gutkiewicz, 1989).

Derek Ford

Bibliography

Bosak,

P.,

Kord,

D.C.

Gla/ek,

,1.,

and

Horiicck,

(eds.), 19S9. Paico-

kiirM:

SysU'imaic

and

Ref^Umal

Review. Prague: .^aidL-mia Pritha/

Elsevier.

Choquette, P.W..

and

James. N.P.. 1988. Inlroduclion.

J;tmes, N.P.,

and Choquette.

P.W.

(eds,). Pcileokarsi.

New

York: .Sprinacr

Verlag.

pp, 1 21.

Christiansen,

H.A., and

Sauer, E.K.., 2001. Stratigraphy iind structure

Late Wisconsin salt collapse

in the

Saskatoon

Low.

south

Saskatoon, Saskatchewan. Canada:

update. CiiiuHliaiiJotinuihil

Ecir/li

Sciences.

38: 1601 1613.

DzLilynski.

S., and

Sass-Gutkiewiez,

M..

19^9. Pb-Zn ores.

Bosak

(•/'(//.

(op. eit). pp.

.177-.3y8.

Ford.

D.C, and

Williams.

P.W., 1989,

Kar.si Gconiorpluilogy

ami

Hyilrolo<^y.

London: Chapman

and

Hall.

Hill, C".A.,

1996.

GeiilofiyofilwDelaware

Ba.sin.

(iitaihilu/ic. Apaeheand

Glass MiHiniains.

New

Mexico

and

Te.ya.*^.

Society

Economic

Paleontologists

and

Mineralogists. Pnhlicalion.

.19, 4S()pp.

Huntoon.

P.W,. 1996.

Large-basin gronnd water circulation

and

paleo-reconstrnclion

circnlation leading

uranium mineraliza-

tion

Grand Canyon breccia pipes. Arizona.

Ihe

Maiinliiin

Geohgisl.

33 (.1):

71-84.

Sangster,

D.F,,

l'')88, Breecia-hosted Lead-Zint,'deposits

carbonate

rocks.

James,

N.P. and

Choquette,

P.W.

(eds.),

PaienkaiM.

New York: Springer Verlag.

pp. 102 116.

Stanton,

R.J.. 1966. The

solution breceiation process. Geological

Sociciyof

Aim-iicii.

Bidleiiii.

77: 843 848.

Cross-references

Anhydrite

and

Gypsum

Ca\e Sediments

Dedolomiti/ation

Micritization

Pressure Solution

Red Beds

Stylolites

Snlhde Minerals

Sediments

SPELEOTHEMS

Introduction

"Speleothem" (Greek; "spele"—cave, "them" make)—a

general term

for

secondary minerals deposited

natural caves

(Moore. 1952). More than

250

different minerals

are

recorded

SPtLEOTHEMS

679

(Hill and Forti. l';97l. At the global seale ealcite precipitates in

solutional caves in carbonate rtieks are overwhelmingly

predominant, both in IVeqneney of individual deposits and in

aggregate volume. They accumulate in air-tilled (vadose) and

water-tilled (phreatic) eaves. Arag(tnite is second in abundance

but rare as a subaqueous deposit. "Traverline"" and "sinter"

are alternative general terms lor these two precipitates, or

•"tLila" when they are precipitated partially or entirely on

organic tVameworks. usually vegetal and in the illuminated

entrance zones of caves. Gypsum is third in abundance, in

gypsum caves but also in limestone caves with gypsum

interbeds or inclusions, or where H^SOj can be created by

oxidation and react with the rock. Hydrated carbonates and

sullates (e.g.. hydromagnesite, epsomite) arc also quite

common, usually as pastes or powders of crystallites that are

small in amount. Other secondary minerals arc more localized,

associated with particular source conditions in the bedrock

or clastic sedimentary fillings, and quite rare at the global

scale. They include native sulfur, many oxides and hydroxides.

halides, nitrates, phosphates, silicates, vanadates. and a lew

organic minerals (Hill and Forti. 1986. 1997). Perennial ice is

\'oimi\ in many cold caves.

Calcite and aragontte speleothems

Crystallography and diagenesis

Calcite (CaCOO is trigonal, with rhombohedral or scaleno-

hedra! habit. In most speleothems it is present in niicrocrysial-

line torm uith the c axis oriented roughly across the direction

of growth ("length slow" of Folk and Assercto. 1976) or as

coarser palisade, columnar or equant ("coconut meat") crystal

aggregates with the c axes oriented to growth ("length fast";

see Railsback, 2000). Large monocrystalline speleolhems occur

but are rare. Density and size of fluid inclusions are very

variable. Many vadose speleothems display alternations oi

coarser and (Iner crystalline growth, often with temporary

cessations due to drying or dissolution, zones with dust, flood

mud or organic grains: existing crystals may grow through

these or new crystals form upon them. Subaqueous deposits

arc more homogeneous. Pure calcite is translucent, or opaque

white due to fluid inclusions. Strong color banding {yellow,

brown, red-brown) is common, however, due chielly to

incorporation of fulvic acid chromophores from soil waters

above the cave (Van Bcynen eial.. 21)01), Metals such as iron

(red),

copper (blue), and nickel (bright green) also provide

color but rarely in visible concentrations (see Cabro! and

Maiigin, 2()0()).

Aragonite (CaCO,) is orthorhombic. In caves it almost

always occurs in aeicular or tabular crystal habit, as clusters

of small needles radiating from the rock or calcite speleothems.

or as massive botryoidat aggregates forming larger deposits

such as stalagmites. Twinned tabular habit is occasionally

louiid. The deposits are more homogeneous in crystal size and

more uniformly white than catcite. lacking strong color

banding from organic chromophores. There are terminations

due to drying, dissolution, or flood mud deposition.

Deposition of aragonite rather than ealcite is usually

attributed to enrichment of Mg"* ions in the feedwater. This

may be due to presence of dolostone or be part of an

evaporative depositional sequence beginning with a core of

calcite, proceeding to a surround of aragonite when the Ca"'

ions are sufficiently depleted and concluding with an aureole of

hydromagnesite (4MgCOrMg(OH.)-4H.O) when they are

exhausted. Because evaporation is important aragonile spe-

leothems are more common in warmer climates. Railsback

c'Uii (1994) report wet-dry seasonal alternations of calcite and

aragonite in a Botswana stalagmite. Thicker, nonrhythmic

aragonite layers are also found in some calcite speleothems.

Diagenesis is common in aragonite speleothems. Smaller

botryoids are replaced by larger ones or by equant or

columnar calcite. It is unusual to find aragonite older than

- lOOkyr that does not display at least some calcite replace-

ment. In purely calcite deposits there may be partial or

complete replacement of microcrystalline layers by columnar

calcite. Vugs become infilled with zoned cements.

Vadose morphology and size

Shapes of speleothems in vadose caves are created by gravity,

or by crystal and capillary forces. Gravity forms arc over-

whelmingly predominant in volume. Principal types are

stalactites, stalagmites, and sheets of "llowstone" on floors

and/or walls (Figure S36). A "column" is a stalactite-

stalagmite pair grown together.

The fundamental form is the soda straw stalactite, a thin

sheath of calcite enclosing the feedwater canal. The r-axis is

oriented downward. New growth occurs only at the tip.

Diameter is determined by droplet size and ranges 2 9 mm.

Calcite straws can extend for several meters before they fall

nnder their own weight. Aragonite straws are rare, taking a

clumped form with narrower necks that may represent

temporary halts in deposition. Leakage or obstruction of the

canal overplates the sheath, creating tapered (carrot-like)

ealeite stalactites up to one meter in diameter and several in

length. There may be accelerated deposition on protruber-

ances.

creating a myriad of subsidiary forms such as

crenulations, corbels, drapes and lesser stalactites. "Draperies"

develop where feedwater trickles down an inclined roof and

grow dounuard with c-axes perpendicular to growth edges. If

initiated on rough surfaces they may be quite sinuous, like a

drawn household drape.

Shapes of calcite stalagmites are similarly varied. The most

simple is the "candlestick'", which adds all new growth at the

top under a nearly constant drip feed. Minimum diameters are

-3 cm. Varying drip or greater fall height causes terraced or

corbelled thickening: at the extreme the form is like a pile o\'

soup plates. More common arc conical t^- tapered forms,

broadening into domes with flowstone sheets around them.

Some corbelled stalagmites are >30m high. Domes may be

50 ni or more in basal diameter. Aragonite stalagmites are

much smaiier. with narrow taper.

Calcite flowstones arc deposited from film flow and accrete

roughly parallel to the host surface, even where this is vertical.

They may extend tens to hundreds of meters downstream of a

single source and accumulate to thicknesses of several meters.

Aragonites ;ire again smaller and thinner. Flowstone layers

interbedded with sands and clays arc common in river caves.

"Gours" or "rimstones" are calcite dams building upward

from irregularities in small channels or on flowstone and

stalagmite surfaces. The greatest impound water to depths of

several meters. Rims are often strikingly crenulated.

Calcite "helictitcs" or "excentrics" grow where crystal or

capillary forces predominate, skewing (-axis orientation to

create narrow, curvilinear tubes extending out. up and down

from rock or parent staiaclitcs. etc. Most arc short. < 10 2()cm

680

SPELCOTHEMS

..iSJtLt.

.±JL

''7^

Soil waiar dissoiv

Soluiion ol CmCOn ii bedroci tuili

J-t

••;!•...-i^i:..!.

Figure S3G Model lo show vadose .ind sub.iqueous speleothem pre( ipit<ites; from Hill and Forti (1986) with permission.

in length. Dense clustering may form tangled masses like the

Medusa's hair. "Anemolitcs" grow upwind into prevailing

drafts.

Clusters

needles fanning outward ("frostwork"*)

are the most common type of aragonite deposit encountered

in caves.

Globular forms are also common. "Cave pearls"

are

spheroidal accretions about

nucleus such as grit agitated

by water dripping into a pool. "Popcorn" or "cave coral" are

semi spherical accretions on host flowstone or other surfaces,

of\en in dense, multi-layered elusters.

Subaqueous speleothems

Calcite may precipitate from thermal and meteoric waters. The

principal thermal form is

spar lining on all surfaces, e.g..

most

the 150 + km

passages

Jewel Cave. South

Dakota. Deposition extends from the watertable to a limiting

depth determined

pressure and Ca'' saturation state.

Aggregate thicknesses

one meter more are known. Spar

linings are also eommon in meteoric caves. A wealth of more

complex, branching crystal structures and rounded micro-

crystalline "clouds" can form in static pools. Water surfaces

are marked by growth

shelfstone around the edges and

floating rafts of microcrystalline calcite accreting around dust

particles.

Distribution and abutidance

Stalactites, stalagmites and flowstones may occur as isolated

individuals in

eave, in clusters or aligned along feedwater

fractures, or broadcast. Density may increase until all surfaces

are covered.

Vadose speleothems grow most readily

shallow depths

beneath soils rich

CO2

humid tropical and temperate

eonditions. These conditions permit year-round deposition,

which may be accelerated by evaporative elTects during dry

periods. The largest individuals and greatest densities are

found in these settings. Accumulations generally diminish

drier and cooler climates. Large stalagmites found

some

desert

arctic caves tend

be relicts from earlier, more

favorable conditions.

Many caves have several levels. Speleothems are often fewer

and smaller in the lower levels, which are usually younger. In

any setting speleothem deposition is largely prohibited where

there are impermeable beds, e.g., shales, above

cave.

Growth rates, age and environmental studies

Under optimal conditions (large thermodynamic excess of Ca""*"

ions,

drip rate and evaporation) soda straws may extend several

in a

year. Normal accretion rates (increase

thickness

of a growth layer) in other speleothems probably range between

•^I.Omm/10"' years

cold climate flowstones

>i.Om/

lo'year

warm cave entrances. The lowest rate reliably

measured (by

series dating)

-0.001 mm/lO-'year

in a

thermal water spar close to the pressure limit for deposition.

Some speleothems appear to have accreted at constant rates but

others vary by factors often or more. Many contain hiatuses

caused by drought, cold or ehange of groundwater routing.

Many speleothems can be dated accurately (±1 percent

error) by the -'"Th/-'''U method if they arc less than

500 kyr

in age. Variations

'^O/"'O isotope ratios along the dated

growth trend may indicate paleotemperature changes at the site

and "C/'"C ratios suggest correlative changes of vegetation

SPICULITES AND SPONCOLITES

681

amount or type. Where present, annual or event banding

revealed by UV fluorescenee and other techniques now permits

very high resolution reconstructions of past conditions above a

cave (see Hill and Forti, 1997, pp. 271-284).

Gypsum speleothems

Crystallography and morphology

Gypsum occurs as aeicular, curved, equant, prismatic or

tabular crystals as contact or penetration twins and as long

fibers. Individual prismatics >1.0m in length are known.

Gypsum is deposited in three principal modes in caves: (1) as

evaporitic inclusions of coarse, fibrous crystals growing as

lenses up to several cm in thickness in bedrocks or cave sedi-

ments, which they rupture—"evapoturbation". (2) as scattered

tiny patches, thicker encrustations or excentric extrusions, on

rock, sediment or calcite speleothems. Most frequent are

"fiowers", extruded, twisting bundles of fibrous crystals up to

cm in length. Epsomite and mirabolite, the most common

Mg, and Na sulfates, also display this form. Clusters of

gypsum needles grow from sulfate-rich sediments and fibers of

"hair" from roofs. Much larger, bifurcating stalactites are

reported from a few caves, e.g., "The Chandeliers", Lechu-

guilla Cave, New Mexico. (3) as regularly bedded floor

deposits or wall encrustations in evaporating lakes or pools:

thicknesses of several meters occur in Carlsbad Caverns, New

Mexico, where much is reprecipitated from alteration crusts

formed by H2SO4 reacting with the limestone walls.

Derek Ford

Bibliography

Cabrol, P., and Mangin, A., 2000. Fleurs de Pierre. Lausanne:

Delachaux et Niestle.

Folk, R.L., and Assereto, R., 1976. Comparative fabrics of length-slow

and length-fast calcite and calcitized aragonite in a Holocene

speleothem, Carlsbad Caverns, New Mexico. Journal of Sedimen-

tary Petrology, 46 (3): 486-496.

Hill, d, and Forti, P., 1986. CaveMineralsoftheWorld. Huntsville, AL:

National Speleological Society of America.

Hill, C, and Forti, P. (eds.), 1997.

Cave

Minerals oftheWorld, 2nd edn.

Huntsville, AL: National Speleological Society of America.

Moore, G.W., 1952. Speleothem—a new cave term. National Speleolo-

gical Societyof America, News, 10 (6): 2.

Railsback, L.B., 2000. An Atlas of Speleothem Microfabrics. Athens:U-

niversity of Georgia, GA. http:// www.gly.uga.edu/railsback/

speleoatlas/SAindexl.html

Railsback, L.B., Brook, G.A., Chen, J., Kalin, R., and Fleisher, C.J.,

1994.

Environmental controls on the petrology of a late Holocene

speleothem from Botswana with annual layers of aragonite and

ealcite. Journal of Sedimentary Research, 64: 147-155.

Van Beynen, P.E., Bourbonniere, R., Ford, D.C , and Schwarcz, H.P.,

2001.

Causes of color and fluorescence in speleothems. Chemical

Geology, 175 (3-4):

319-341.

Cross-references

Anhydrite and Gypsum

Caliche-Calcrete

Cave Sediments

Geodes

Hydroxides and Oxyhydroxide Minerals

Neomorphism and Recrystallization

Siliceous Sediments

Sulfide Minerals in Sediments

Travertine and Other Spring Deposits

SPICULITES AND SPONGOLITES

Introduction

Spongolites and spieulites are common in the rock record, and

represent environments that were dominated by sponges, a

phylum of organisms that inhabit practically all modern

aquatic environments (Reitner and Keupp, 1991). Sponges use

silica and/or carbonate and/or organic tissue to build skeletons

composed of discrete spicules or amalgamated spieules that

form a rigid skeleton. A sediment predominantly composed of

discrete spicules is termed a spiculite, whilst one predominantly

composed of rigid bodied sponges is termed a spongolite.

Taxonomy

Phylum Porifera contains an estimated 15,000 living sponges

making it one of the most diverse and successful groups of

aquatic organisms. Fossil and extant sponges are divided into

two subphyia: Cellularia (classes Demospongia, Calcarea,

Archaeocyatha); and Symplasia (class Hexactinellida; Hooper,

2000).

Most demosponges and hexactinellids form their

skeletal material from siliceous spicules and/or spongin

(organic tissue), with some calcareous "corraline" demo-

sponges. Calcarea and Archeocyatha have calcareous skeletal

components. Most sponges contain discrete spicules within a

mass of cells ("soft sponges"), with relatively few taxa

containing rigid skeletons formed by articulation, secondary

silicification/calcification, or fusion of spicules. Spicule

morphologies are extremely diverse, with taxanomic iden-

tification determined from the complete spicule complement,

and arrangement. Relatively few spicules are diagnostic,

making identification from isolated sponge spicules generally

only possible to a high taxonomic level. This has resulted in

paleontological concentration on rigid skeleton sponges.

Accurate identification from isolated spicules is one of the

most pressing problems in sponge paleontology. Rigid

skeleton sponges traditionally include: lithistids (siliceous),

stromatoporoids (calcareous), sphinetozoans (calcareous),

chaetitids (calcareous), and sclerosponges (calcareous basal

skeleton, siliceous spicules). However, it is now generally

accepted that these "fossil groups" are primarily optimal

sponge body plans rather than monophyletic lineages, and that

these body plans have evolved convergently from various

demosponge and calcarea lineages during the Phanerozoic

(e.g., Debrenne, 1999; Pisera, 1999). The current paleontolo-

gical revision will hopefully clarify the many problems with

fossil sponge identification. Hexactinellid (siliceous) and

archaeocyathid (calcareous) are apparently the sole mono-

phyletic groups amongst rigid bodied sponge fossils (see

figure 6 of Debrenne, 1999, Hooper and van Soest, in

preparation).

Paleoecoiogy

Sponges are sessile benthie invertebrates that gather food by

filtering particulate, colloidal and dissolved nutrients from

water actively pumped through internal canals and chambers

(Hooper, 2000; Rigby and Stearn, 1983). In general they are

slow growing, contain bacterial symbionts, and have a cell-

colony metabolic organization. Sponges reproduce both

sexually and asexually, with sponge larvae motile for only a

682

SPICULITES AND SPONCOLITES

short period before encrusting a firm substrate. They generally

have low growth rates and long lifespans (Reitner and Keupp,

1991),

and are some of the most evoltjtionally conservative

animals known (e.g., Pisera, 1999), Freshwater sponges are

restricted to a few lineages, and fossil spieulites and spongolites

are generally interpreted as indicators of marine environments

(Hooper, 2000). Sponges are most common on temperate

carbonate shelves, within reef and rocky substrate environ-

ments, and within muddy, low-energy environments (van Soest

etal., 1994). Their general preference is for oligotrophic, low

energy, fine-grained, low sedimentation-rate environments.

However, sponges exploit many different habitats and such

a diverse and evolutionally conservative group of animals will

have developed species-dependent environmental controls

(e.g., Maldonado etal., 1999). Paleoecological issues deserve

considerable further investigation.

Sponge-dominated deposits have previously been inter-

preted as indicating deep and/or cold-water environments

(Beauchamp and Desrochers, 1997) due to modern hexacti-

nellid spiculite and spongolite accumulations in arctic and

antarctic regions (Conway et al., 1991). However, fossil

spieulites and spongolites, including hexactinellid commu-

nities,

have been interpreted as shallow and warm (Gammon

etal., 2000), and the presence of hexactinellid and demosponge

faunas throughout modern oceans, including tropical assem-

blages, suggests modern sponge habitats may not be repre-

sentative of the past because of the inability of modern

sponges to compete with more highly evolved organisms such-

as photosymbiotic corals (e.g., Levi, 1991; Tabachnick, 1991).

The presence of shallow-marine deposits composed primarily

of deep-marine sponges also suggests that water-depth is not a

major control factor for sponges (Gammon etal., 2000).

The modern ocean is undersaturated with respect to silica,

leading to rapid sea-floor dissolution of sponge skeletal

remains. Additionally, carbonates and silica buffer solutions

to substantially differing pH's. Opaline biogenic silica is

therefore generally unstable within predominantly calcareous

sediments, and vice versa (Johnson, 1976). Siliceous sponge

assemblages are probably significantly impoverished by

diagenesis and assemblage reconstruction is particularly

challenging.

Spiculite and spongolite

Modern Arctic and Antarctic spiculite and spongolite deposits

contain matted, interlocked spicule-mat textures developed

on glacial moraine substrates in deep shelf localities (Conway

etal., 1991). The very high length:width ratio of interlocked

spicules produces a cohesive sediment structure that resists

erosive currents and provides greater substrate aeration which

promotes microbial populations. The resulting muddy, micro-

bially-infiuenced matrix may be a good analogue for ancient

spicular mudmounds. Ancient spieulites are occasionally cross-

bedded, but more commonly massive, possibly due to such

spicule networks. Detailed textural work on spieulites is a

promising avenue for further sponge community research.

Modern lithistid localities are muddy, deep shelf localities,

which is consistent with muddy spongolite deposits in the rock

record (e.g., Wiedenmayer, 1994). Such fossil sponge faunas

represent in situ assemblages, but are often poorly preserved

due to diagenesis. There are no modern analogues for ancient

calcareous or siliceous sponge reef/bioherm spongolites (e.g.,

Reitner and Keupp, 1991). These have coarse-grained whole

and fragmented sponges with or without copious synsedimen-

tary cements. Interpretations exist for both deep and shallow

waters reefs (Vacelet, 1988), consistent with the generally large

depth ranges and nonphotosymbiotic metabolisms of modern

sponges.

Spieulites and spongolites have been formed repeatedly

throughout the Phanerozoic. The earliest known sponge fossils

are Late Neoproterozoic, but sponge-dominated environments

started with early Cambrian archaeocyathids, the world's first

metazoan reef builders (Debrenne, 1999). Sphinetozoans and

stromatoporoids were common reef builders during the

Paleozoic (mainly Ordovician—Devonian), whilst siliceous

sponges tended to form spongolite bioherms and spiculitic

mudmounds.

Siliceous sponges particularly enjoyed Mesozoic tropical

Tethyan environments, where they arose to dominance

repeatedly through the Jurassic and Cretaceous (mainly

lithistids with some hexactinellids in relatively deep shelf

environments; Pisera, 1999). Calcareous stromatoporoids and

sphinetozoans made brief appearances as reef-builders, but

were not as prolific as Paleozoic forms (Debrenne, 1999).

The general lack of Tertiary spieulites and spongolites has

been correlated to the diatom radiation that is inferred to have

decreased oceanic dissolved silica, a key nutrient for siliceous

sponges (Maldonado efo/., 1999). However, sponge-dominated

sediments occur through the Eocene (Europe and Australia;

Gammon and James, 2001; Pisera, 1999). These deposits are

primarily shallow-marine deposits in unusual basinal settings,

emphasizing the Phanerozoic trend of sponge displacement to

more marginal habitats by more evolved taxa.

Paul R. Gammon

Bibliography

Beauchamp, B., and Desrochers, A., 1997. Permian warm-to very cold-

water carbonates and cherts in northwest Pangea. In James, N.P.,

and Clarke, J.D.A. (eds.), Cool-Water Carbonates. SEPM (Society

for Sedimentary Geology), Special Volume 56, pp. 327-347.

Conway, K.W., Barrie, J.V., Austin, W.C, and Luternauer, J.L., 1991.

Holocene sponge bioberms on the western Canadian continental

shelf.

Continental Shelf Research, 11: 111-190.

Debrenne, F., 1999. The past of sponges—sponges of the past.

In Hooper, J.N.A. (ed.). Proceedings of the 5th International

Sponge Symposium: Origin and Outlook. Brisbane: Memoirs of the

Queensland Museum, 707pp.

Gammon, P.R., and James, N.P., 2001. Palaeogeographical influence

on Late Eocene biosiliceous sponge-rich sedimentation, southern

Western Australia. Sedimentology, 48: 559-584.

Gammon, P.R., James, N.P., and Pisera, A., 2000. Eocene spieulites

and spongolites in southwestern Australia: not deep, not polar, but

shallow and warm. Geology, 28 (9): 855-858.

Hooper, J.N.A., 2000. Sponge Guide: Guide to Sponge Collection and

Identification. Brisbane: Queensland Museum, 141pp.

Hooper, J.N.A., and van Soest, R. (eds.), 2002. Systema Porifera (in

preparation—a complete revision of fossil and extant sponge taxa).

Johnson, T.C., 1976. Controls on the preservation of biogenic opal in

sediments of the eastern tropical Pacific. Science, 192: 887-890.

Levi, C, 1991. Lithistid sponges from the Norfolk Rise-Recent and

Mesozoic genera. In Reitner, J., and Keupp, H. (eds.).

Fossil

and

Recent Sponges. Berlin: Springer-Verlag, pp. 72-82.

Maldonado, M., Carmona, M.C., Uriz, M., and Cruzado, A., 1999.

Decline in Mesozoic reef-building sponges explained by silicon

limitation. Nature, 401: 785-788.

Pisera, A., 1999. Postpaleozoic history of the siliceous sponges with

rigid skeleton. In Hooper, J.N.A. (ed.). Sponge Guide: Guide to

SpongeCollectionandldentification. Brisbane: Queensland Museum,

pp.

463-472.

STAINS FOR CARBONATE MINERALS

683

Reitner, J., and Keupp, H. (eds.), 1991. Fossil and Recent Sponges.

Springer-Verlag.

Rigby, J.K., and Stearn, CW,, 1983. Sponges and Spongiomorphs.

Short course of The Paleontological Society. Indianapolis:

University of Tennessee, 220pp.

Tabachnick, K.R., 1991. Adaptation of the hexactinellid sponges to

deep-sea life. In Reitner, J., and Keupp, H. (eds.). Fossil and Recent

Sponges. Springer-Verlag, pp. 378-386.

Vacelet, J., 1988. Indications de profondeur donnees par les

Spongiaires dans les milieux benthiques actuels. Geologic

Mediterraneenne, 15: 13-26.

van Soest, R.W.M., van Kempen, T.M.G., and Braekman, J.-C, 1994.

Sponges in Time and Space: Biology, Chemistry, Paleontology.

Rotterdam: A.A. Balkema.

Wiedenmayer, F., 1994. Contributions

the Knowledge of Post-Palaeo-

zoic Neritic and Archibenthal Sponges (Porifera). Schweizerische

Paliiontologische Abhandlungen 116. Basel: Kommission der

Schweizerischen Paliiontologischen Abhandlungen.

STAINS FOR CARBONATE MINERALS

Introduction

Stains have been devised for the rapid identification of many

common minerals; Reid (1969) provides an excellent compen-

dium. The identification of minerals, however, can now be

achieved with much greater certainty using modern analytical

techniques (SEM, microprobe and so on). Stains have been

devised that display intracrystalline variations in chemical

composition although techniques such as cathodolumines-

cence, fluorescence and backscatter electron imaging can

display this type of information more comprehensively and

reliably than staining. Some stains, however, are very versatile,

they can be used in the field, on rock slabs or thin sections;

they are inexpensive, easy to use and rapid to apply.

Staining carbonate minerals

The staining of carbonate minerals has a long history

(Lemberg, 1887) and involves a plethora of different methods

(Freidman, 1959, 1971; Reid, 1969).

General carbonate stain

The most widely used stain for carbonates employs a mixture of

Alizarin red S and potassium ferricyanide dissolved in a dilute

hydrochloric acid solution. Set procedures are published for this

dual stain (Diekson, 1965, 1966; Evamy, 1963, 1969) but the

stain can, within certain limits, be modified to suit the needs of

the observer and the type of material being stained. The organic

dye Alizarin red S (ARS) will produce a pink to red stain on any

carbonate that will react with dilute acid. The reaction between

carbonates and acid is usually controlled (1-2 minutes at 25°C

for thin sections) so that the more reactive minerals, such as

calcite and aragonite, stain red but the less reactive ones, such as

dolomite and siderite, remain unstained. Calcite is however an

anisotropic mineral that is more soluble in a direction parallel to

its c-axis than normal to the c-axis. The ARS stain (at HCI

concentrations between 0.2 percent and 1.0 percent, vol./vol.)

can differentiate calcite sections that are normal to the c-axis

(stained pink) from those that are parallel to the c-axis (stained

red).

This appears counterintuitive—the c-axis normal section

reacts more vigorously with the acid yet stains less intensely, and

viceversa. This is due to greater evolution of CO2 bubbles from

the c-axis normal surface preventing the stain from settling. The

c-axis parallel sections produce fewer bubbles and stain

precipitation is unimpeded. This differentiation is only effective

at acid concentrations between 0.2 percent and 1.0 percent (vol./

vol.) given stain times of between I minute and 2 minutes.

The intensity of the ARS stain is affected by HCI

concentration. At 1.5 percent the stain is faint, vigorous

evolution of CO2 bubbles occurs, and the section is etched

thinner (from 30 |im to ~

|im) after being treated for 1

minute. At 0.1 percent HCI concentration and 1 minute

staining time the calcite c-axis orientation is weakly differ-

entiated, and the intensity of the stain masks the underlying

fabric. Staining for more than 2 minutes at 0.1 percent HCI

causes the layer of stain to crack and peel off the stained

surface. Reaction between calcite and acid in the 1-2 minutes

recommended for staining is very sensitive to acid concentra-

tion, which can be adjusted to develop the desired stain

intensity. Dolomite does not stain using these acid concentra-

tions but as iron is substituted into the dolomite lattice it

becomes more reactive. Ferroan dolomite and ankerite react

with dilute HCI, effervescing and staining with ARS. The ARS

stain color of ankerite (mauve) can be distinguished from

calcite (red) but in the dual stain the mauve color is surpressed

due to intense staining by potassium ferricyanide.

Potassium ferricyanide (PF) produces a precipitate of

Turnbull's blue when ferrous iron is released to the staining

solution. It might be expected that siderite (ferrous carbonate)

would react with this stain but siderite does not react with

dilute cold HCI, iron is not released to the staining solution

and consequently no stain is precipitated. The rate of reaction

of the PF stain, as with ARS, is controlled by reaction rate

between carbonate and HCI. Caleites and dolomites contain-

ing ferrous iron do react with the stain but dolomite reacts less

vigorously than calcite so the intensity of PF stain is not

proportional to the iron concentration of the two minerals.

The PF stain is very sensitive to ferrous iron concentrations in

calcite and will distinguish differences in concentration in the

order of a few 100 ppm. Lindholm and Finkelman (1972)

correlated iron concentration of calcite with stain color, but to

match their color range their staining procedure must be

reproduced exactly. The relationship between iron concentra-

tion and stain color is non-linear as the reactivity of carbonate

minerals increases with rising iron concentration (Reeder,

1983).

Ferroan calcite stains blue and ferroan dolomite stains

green to turquoise. As iron concentration increases in these

two minerals the colors converge. The difficulty in distinguish-

ing such iron-rich carbonates can be overcome by staining with

ARS alone.

When iron concentrations are low the dual staining

procedure can be modified to increase iron differentiation.

Extending the staining time in the dual stain beyond 90

seconds is inadvisable as the stain cracks and thick deposits of

ARS obscure the section. Material with low iron concentra-

tions can be stained first in a PF/HCl solution for 60 seconds

before staining with the dual (ARS + PF) stain. The distribu-

tion of ferrous iron (resolvable to

|im) in thin section or rock

slab can be differentiated in a few minutes using this stain

(Figure S37*).

' Figure S37 appears on Plate II, facing page 115.

684

STATISTICAL ANALYSIS OF SEDIMENTS AND SEDIMENTARY ROCKS

Distinguishing aragonite from calcite

Feigl's stain (Feigl, 1937), a solution of silver and manganese

sulfates to which sodium hydroxide is added (for preparation

see Friedman, 1959), stains aragonite black. Other orthor-

hombic carbonates also react positively with Feigl's stain. The

black color is due to a precipitate of manganese oxide and

metallic silver. Calcite remains unstained during limited

exposure to Feigl's solution. The stain differentiates aragonite

from calcite due to their different solubilities and also causes

differential etching between the two minerals. Schneidermann

and Sandberg (1971) have used this selective etching (imaged

by SEM) as an additional feature of the stain to help

distinguish between the two minerals. The latter authors also

recommend using MnSO4H2O, rather than MnS04 7H2O, to

prepare the stain, diluting the original recipe with water, and

warming the solution. Variations in crystal size and orientation

have led to differing results using Feigl's stain so that many

regard the stain as unreliable.

Distinguishing Mg calcite from calcite

The Titan-yellow (Clayton yellow) stain for assessing the Mg

content of calcite was described by Friedman (1959) modified

by Winland (1971) and again improved by Choquette and

Trusell (1978). Choquette and Trusell give the staining and

fixing procedure for this stain which barely stains calcite with

up to 3 percent MgCO3 stains Mg calcite with 5-8 percent

MgC03 pink to pale red, and Mg calcite with >8 percent

MgC03 a deep red color. This indication of Mg content is

qualitative because crystal orientation and size affect the stain

in addition to Mg content. The stain is cheap and simple to use

but has not gained wide acceptance.

Conclusions

Stains have been used to determine mineralogy and intracrys-

talline chemical variations although both these properties can

now be more reliably and quantitatively determined using

analytical techniques. Stains can however be applied to

outcrop, rock slabs or thin sections in a few minutes at

negligible expense. Their application requires little training

other than some basic safety training in the use of chemical

substances and an understanding of how the stain operates.

J.A.D. Diekson

Bibliography

Choquette, P.W,, and Trusell, F.C., 1978. A procedure for making the

Titan-yellow stain for Mg-calcite permanent. Journal of Sedimen-

tary

Petrology,

48:

639-641.

Diekson, J.A.D., 1965. A modified staining technique for carbonates in

thin section. Nature, 105: 587.

Diekson, J.A.D., 1966. Carbonate identification and genesis as

revealed by staining. Journal of Sedimentary Petrology, 36: 491-

505.

Evamy, B.D., 1963. The application ofa chemical staining technique

to a study of dedolomitization. Sedimentology, 2: 164-170.

Evamy, B.D., 1969. The precipitational environment and correlation

of some calcite cements deduced from artificial staining. Journatof

Sedimentary

Petrology,

39:

787-821.

Feigl, F., 1937. Qualitative Analysis by Spot Test. New York:

Nordemann Publications Company.

Friedman, G.M., 1959. Identification of carbonate minerals by

staining methods. Journatof Sedimentary Petrology, 29: 87-97.

Friedman, G.M., 1971. Staining. In Procedures in Sedimentary

Petrol-

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Lemberg. J., 1887. Zur mierochemisehen Untersuchung von Calcit,

Dolomit und Predazzit. Zeitschrift der Deutschen Geologischen

Geseltschaft, 40: 357-359.

Lindholm, R.C., and Finkelman, R.B., 1972. Calcite staining:

semiquantitative determination of ferrous iron. Journal of Sedi-

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Reeder, R.J., 1983. Crystal chemistry of the rhombohedral earbonates.

In Carbonates: Mineralogy and Chemistry. Reviews in mineralogy

11,

Mineralogical Society of America, pp. 1-47.

Reid, W.P., 1969. Mineral staining tests. Colorado School of Mines,

Mineral Industries Bulletin, 12: 1-20.

Schneidermann, N., and Sandberg, P.A., 1971. Calcite-aragonite

differentiation by selective staining and electron microscopy. Gulf

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As.sociation

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Winland, H.D., 1971. Non-skeletal deposition of high-Mg calcite in

the marine environment and its role in the retention of textures. In

Carbonate Cements. John Hopkins University Studies in Geology,

19,

278-284.

Cross-references

Carbonate Mineralogy and Geochemistry

Diagenesis

Dolomite Textures

Dolomites and Dolomitization

STATISTICAL ANALYSIS OF SEDIMENTS AND

SEDIMENTARY ROCKS

Statistical analysis is a standard tool used by sedimentologists,

who are concerned with sampling and describing properties of

populations of particles. They use summary statistics such as

the range, mean, mode, median, standard deviation, variance,

skewness, and kurtosis to transmit information about sedi-

ments. Univariate, bivariate, and multivariate statistical tests

are among their basic statistical tools. Descriptive statistics in

geology date back to the 19th Century when Charles Lyell used

the ratio of living to extinct invertebrate fossils to subdivide

the Tertiary into epochs as reported in his Principlesof Geology

(1833) (see Merriam, 1981).

Because much of the historical data in the earth sciences are

qualitative, nonparametric statistics are used for their analysis.

Nonparametric tests do not require assumptions about the

populations from which the samples were taken as do

parametric tests. There also is an inherent amount of

uncertainty in geological properties because of irregularly

spaced or missing data. Methods for determining the spatial

relations and display of sediment properties, therefore, are

especially important. Dieter Marsal's (1987) book is an

excellent introduction to statistics for the geoseientist; John

Davis'

(1986) book on statistics and data analysis is an

introductory to immediate level text with many examples and a

computer program, STAT, on disk; and Nick Rock's (1988)

numerical geology is an intermediate text for university courses

on the subject with an extensive reference list and glossary of

terms.

With introduction of quantitative methods, the subject of

sedimentation gradually transformed into sedimentology. One

of the first papers in this transition was the classic On the