Rafferty J.P. (ed.) Minerals

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Minerals 7

The crystal structure of calcite. This schematic diagram shows both (left)

the true unit cell (the acute rhombohedron, which contains 2[CaCo

]) and

(right) an alternative cell based on the cleavage rhombohedron. Copyright

Encyclopædia Britannica, Inc.; rendering for this edition by Rosen

Educational Services

frequently used bases are the true rhombohedral unit cell,

which is the acute rhombohedron, and the cleavage rhom-

bohedron setup. The true unit cell includes 2 CaCO

with

calcium ions at the corners of the rhombohedrons and

groups, each of which consists of a carbon ion at the

centre of a planar group of oxygen atoms whose centres

deﬁne an equilateral triangle. The conﬁguration can be

considered another way: the structure consists of alternat-

ing sheets of hexagonally arranged calcium ions and CO

complex anions. This array is in the hexagonal (trigonal)

crystal system. The threefold symmetry is quite obvious in

both crystals and cleavage rhombohedrons. The crystals

occur most commonly in cavities in rocks—e.g., in vugs

253

Calcite crystals. Some of the many fairly common crystal habits represented by nat-

ural calcite crystals are illustrated here. Copyright Encyclopædia Britannica,

Inc.; rendering for this edition by Rosen Educational Services

(including druses), in vesicles in igneous rocks, and lining

partially ﬁlled ﬁssures. More than 300 forms of calcite

have been recognized.

Physical Properties

Calcite is colourless or white when pure but may be of

almost any colour—reddish, pink, yellow, greenish, blu-

ish, lavender, black, or brown—owing to the presence of

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7 Minerals 7

254

diverse impurities. It may be transparent, translucent, or

opaque. Its lustre ranges from vitreous to dull; many crys-

tals, especially the colourless ones, are vitreous, whereas

granular masses, especially those that are ﬁne-grained,

tend to be dull. Calcite is number 3 on the Mohs hardness

scale; thus, it can be scratched readily by a knife blade or

geologic pick. It has a speciﬁc gravity of 2.71. Three per-

fect cleavages give calcite its six-sided polyhedrons with

diamond-shaped faces; the angles deﬁning the faces are

78° and 102°. The three important crystal habits (distinc-

tive shapes of the mineral) of calcite are: (1) prismatic

(both short and long), (2) rhombohedral, and (3) scaleno-

hedral. Twinning is very common and may be of secondary

origin in crystalline limestones. Some calcites ﬂuoresce

under ultraviolet light; some are also triboluminescent

(luminescent when scratched). When light passes through

some minerals, it is split into two rays that travel at differ-

ent speeds and in different directions. This phenomenon

is known as birefringence. The difference between the

velocities is especially notable in calcite, and consequently

crystals of a colourless variety—sometimes called Iceland

spar—exhibit double refraction that can be observed with

the naked eye.

The previously described fact that calcite effervesces

vigorously with dilute hydrochloric (muriatic) acid distin-

guishes calcite from dolomite, which it tends to resemble

in both appearance and occurrence. (Dolomite effervesces

only when powdered and with only a slow, smoldering

action.)

In rocks made up predominantly of calcite, the min-

eral is typically granular, with grains ranging from those

discernible only under a microscope to those that are a

few millimetres in greatest dimension. The colour of most

of these rocks is gray or tan, but some calcite marbles are

white and a few are multicoloured.

255

origin and occurrence

A large percentage of the calcite in rocks was deposited in

sedimentary environments; consequently, calcite is a con-

stituent of several diverse sediments, sedimentary rocks,

and their metamorphosed products. A minor amount of

the Earth’s calcite is of magmatic (i.e., igneous) origin; it is

the chief constituent of the rare rock called carbonatite.

Calcite also occurs widely in veins: some of the veins are

wholly or largely calcite; others contain valuable ore min-

erals and are usually described as ore veins, even though

calcite is the predominant constituent.

The sedimentary rocks composed largely of calcite

include limestones of chemical and biochemical origin and

also limestones usually referred to as clastic because they

consist of transported fragments of previously deposited,

typically biogenetic materials. Travertine (also known

as tufa), chalk, and micrite, respectively, are examples of

these kinds of limestones.

Many limestones have gained their mineralogical

makeups and textures during diagenesis. Aragonite, the

orthorhombic polymorph of CaCO

, was deposited and

subsequently transformed into the calcite of some lime-

stones; magnesian calcites that constitute some organic

skeletal parts and cements of marine sediments were the

precursors of the calcite of many other limestones. During

diagenesis, most of the magnesian calcites were trans-

formed into stable assemblages of rather pure calcite,

often along with scattered grains of dolomite.

When sedimentary and diagenetic limestones

undergo metamorphism, the calcite is frequently recrys-

tallized and tends to become coarsely crystalline. The

resulting rocks are calcite marbles. Some calcite mar-

bles, however, appear to have had dolostone rather than

limestone precursors; i.e., the dolostone underwent

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7 Minerals 7

256

dedolomitization during metamorphism. The calcite

grains in some marbles have cleavage planes that are

curved; this is usually interpreted to reﬂect recrystalli-

zation during deformation or plastic ﬂowage associated

with dynamic metamorphism.

The calcite of carbonatites is generally thought to have

formed from dense H

O-CO

ﬂuids that in many ways are

more like the volatile-rich ﬂuids from which pegmatites

are believed to have been deposited than the more “nor-

mal” magmas from which igneous rocks such as granites

and basalt have consolidated. These rocks closely resemble

marbles, and some masses of both marble and carbonatite

were originally misidentiﬁed, one as the other. In most

cases, the identities of the accessory minerals serve as

ready criteria for differentiating between an igneous and

metamorphic origin.

Calcite is deposited by solutions, either ordinary

groundwater solutions or hydrothermal solutions associ-

ated with magmatic activities. Such calcite constitutes

the cement for many clastic sediments—e.g., some

sandstones—and also the calcite or aragonite of speleo-

thems and of both calcite and calcitic ore-bearing veins.

Calcite breaks down in most areas where chemical

weathering takes place. It is dissolved and its products are

carried in surface-water and groundwater solutions. The

excavation of caves is a subsurface manifestation of these

processes, just as their subsequent ﬁlling-in with speleo-

thems is a manifestation of one of the modes whereby

calcite is deposited. As might be suspected, most karst

topography, which is characterized by sinkholes and under-

ground drainage, occurs in areas underlain by limestone.

uses

Cal

cite has many uses. Since ancient times, limestone

has been burned to quicklime (CaO), slaked to hydrated

257

lime [Ca(OH)

], and mixed with sand to make mortar.

Limestone is one of the ingredients used in the manufac-

ture of portland cement and is often employed as a ﬂux

in metallurgical processes, such as the smelting of iron

ores. Crushed limestone is utilized widely as riprap, as

aggregate for both concrete and asphalt mixes, as agri-

cultural lime, and as an inert ingredient of medicines.

Marble is used for statuary and carvings, and as pol-

ished slabs it is a popular facing stone. The term marble

is used differently in the marketplace than in geology;

in the marketplace, it is applied to any coarse-grained

carbonate rock that will take a good polish rather than

to metamorphic carbonate-rich rocks exclusively. Some

coarsely crystalline diagenetic limestones are among

the most widely used commercial “marbles.” Travertine

and onyx marble (banded calcite) are also popular fac-

ing stones, usually for interior use. Iceland spar has been

employed for optical purposes for nearly two centuries.

These uses constitute only a few of the many applica-

tions of calcite.

dolomite

Dolomite is a type of limestone, the carbonate fraction

of which is dominated by the mineral dolomite, calcium

magnesium carbonate [CaMg(CO

)

Along with calcite and aragonite, dolomite makes up

approximately 2 percent of the Earth’s crust. The bulk of

the dolomite constitutes dolostone formations that occur

as thick units of great areal extent in many sequences of

chieﬂy marine strata. (The rock dolostone is referred

to by only the mineral name—i.e., dolomite—by many

geologists.) The Dolomite Alps of northern Italy are a

well-known example. Other relatively common occur-

rences of the mineral dolomite are in dolomite marble

7 carbonates and other Minerals 7

7 Minerals 7

258

and dolomite-rich veins. It also occurs in the rare igneous

rock known as dolomite carbonatite.

From the standpoint of its origin, the dolomite of

dolostones is one of the most interesting of all the major

rock-forming minerals. As discussed below, a large per-

centage of the dolomite in thick marine dolostone units is

thought by many geologists and geochemists to have been

formed by replacement of CaCO

sediment rather than by

direct precipitation.

chemical composition

Ferrous iron commonly substitutes for some of the magne-

sium in dolomite, and a complete series very likely extends

between dolomite and ankerite [∼CaFe(CO

)

]. Manganese

also substitutes for magnesium, but typically only to the

extent of a few percent and in most cases only along with

iron. Other cations known to substitute—albeit in only

relatively minor amounts—within the dolomite structure

are barium and lead for calcium and zinc and cobalt for

magnesium.

Nearly all the natural elements have been recorded as

present in at least trace quantities in dolostones. It is, how-

ever, unclear which ones actually occur in the dolomite;

some of them may occur within other mineral constitu-

ents of the analyzed rocks. Indeed, only a few of these

elements—e.g., strontium, rubidium, boron, and uranium

(U)—are known deﬁnitely to occur within the dolomite

structure.

Dolomite effervesces with dilute hydrochloric acid,

but slowly rather than vigorously as calcite does; in gen-

eral, it appears to smolder slowly, and in some cases it

does so only after the rock has been powdered or the acid

warmed, or both. This difference in the character of the

effervescence serves as the test usually used to distin-

guish dolomite from calcite in the ﬁeld. In the laboratory,

259

staining techniques, also based on chemical properties or

typical compositions, may be used to distinguish between

these minerals. The stains generally employed are espe-

cially valuable for investigating rocks made up of alternate

lamellae of dolostone and limestone composition.

crystal Structure

In a somewhat simpliﬁed way, the dolomite structure can

be described as resembling the calcite structure but with

magnesium ions substituted for calcium ions in every other

cation layer. Thus, the dolomite structure can be viewed

as ideally comprising a calcium layer, a CO

layer, a mag-

nesium layer, another CO

layer, and so forth. However, as

described for the potassium feldspars, dolomites—unlike

calcites—may also exhibit order-disorder relationships.

This results because the purity of some of the cation layers

may be less than ideal—i.e., some of the “calcium layers”

may contain magnesium, and some of the “magnesium

layers” may contain some calcium. The term protodolo-

mite is frequently applied to Holocene dolomites (those

formed during approximately the last 11,700 years) that

have less than ideal dolomite structures. Most dolomites

of ancient dolostones, however, appear to be well ordered.

Modiﬁcations that may reﬂect diverse calcium-versus-

magnesium layering aberrations are treated extensively in

professional literature.

Physical Properties

Dolomite crystals are colourless, white, buff-coloured,

pinkish, or bluish. Granular dolomite in rocks tends to be

light to dark gray, tan, or white. Dolomite crystals range

from transparent to translucent, but dolomite grains in

rocks are typically translucent or nearly opaque. The lus-

tre ranges from subvitreous to dull. Dolomite, like calcite,

cleaves into six-sided polyhedrons with diamond-shaped

7 carbonates and other Minerals 7

260

Minerals 7

Relations between lamellar twinning and cleavage planes in dolomite and

calcite. This difference can be discerned best when thin sections of the miner-

als are viewed under a microscope. Copyright Encyclopædia Britannica,

Inc.; rendering for this edition by Rosen Educational Services

faces. Relations between lamellar twinning and cleavage

planes of dolomite, however, differ from those of calcite,

and this difference may be used to distinguish the two min-

erals in coarse-grained rocks such as marbles. Dolomite

has a Mohs hardness of 3½ to 4 and a speciﬁc gravity of

2.85 ± 0.01. Some dolomites are triboluminescent.

The dolomite of most dolostones is granular, with

the individual grains ranging in size from microscopic up

to a few millimetres across. Most dolomite marbles are

coarsely granular with individual grains ranging between

2 and 6 mm (0.079 and 0.24 inch) in greatest dimension.

Vein dolomite grains may be up to several centimetres

across. Saddle-shaped groups of dolomite crystals, most

of which occur on fracture surfaces, measure from 0.5 to 2

cm (0.20 to 0.79 inch) across.

origin and occurrence

Dolomite

occurs widely as the major constituent of dolos-

tones and dolomite marbles. As mentioned above, the

origin of dolomite-rich rocks in marine sequences remains

an unresolved problem of petrogenesis.

261

Dolomite—actually protodolomite—is known to have

formed fairly recently in restricted environments such as

on supratidal ﬂats that occur in the Bahamas and Florida

Keys. Also, no dolomite has been synthesized in an envi-

ronment comparable to natural conditions. Thus, the

explanation for the formation of dolomite in these marine

units remains in question. It is now thought that dolos-

tones may be of various origins. Indeed, several different

models have been suggested for dolomite formation, each

based on diverse considerations, combined with empirical

and/or experimental data.

Except for models invoking formation of dolomite

by direct precipitation, a process thought by most geolo-

gists to apply to only a small percentage of all dolostones,

each model is based on the assumption that the dolomite

of dolostones has been formed by conversion of CaCO

sediment or sedimentary rocks to dolostone. Thus, the

models have been formulated to account for this conver-

sion, which is known as dolomitization.

The most widely discussed models for dolomiti-

zation, either partial or complete, involve four chief

variables: time, location with respect to the sediment-

seawater interface, composition and derivation of the

solutions involved, and ﬂuxing mechanisms. The time

ranges from dolomitization that occurs penecontem-

poraneously with deposition to that which takes place

subsequent to relatively deep burial of the precursor

sediments. The location ranges from at or very near

the sediment-seawater interface to well beneath some

overlying sediments that were deposited at a later time.

The solutions supply the magnesium needed and must

have the appropriate pH and concentrations of other

necessary ions; these solutions are generally consid-

ered to be seawater (either “normal” seawater or brines

concentrated by evaporation), connate water, meteoric

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