Rafferty J.P. (ed.) Minerals

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7 Minerals 7

272

must occur for precipitation of the borates, such basin

deposits usually occur in desert regions, as for example

the Kramer district of the Mojave Desert and Death

Valley in California, where enormous beds of stratiﬁed

kernite, borax, colemanite, and ulexite are recovered, pri-

marily by stripping away the overburden and mining the

borates by classical open-pit techniques. Other notewor-

thy evaporite deposits occur in the Inderborsky district of

Kazakhstan and in Tuscany, Italy. The sequence of precipi-

tating alkali borates can be duplicated in the laboratory

because the temperatures and pressures of their forma-

tion are low and easily accessible. Solutions of the alkali

borates and the addition of metal ions such as calcium

and magnesium result in the precipitation of yet other

borate compounds. Among the borates commonly found

in evaporite deposits are borax, colemanite, inyoite, kern-

ite, and tincalconite.

The second geologic setting for borate minerals is a

metamorphic carbonate-rich environment, where they are

formed as a result of alteration of the surrounding rocks

by heat and pressure; similar borates also occur as nodules

in some deeply buried sediments. These compounds were

formed at relatively high temperatures and usually consist

of densely packed BO

triangles associated with such small

metal ions as magnesium, manganese, aluminum, or iron.

The origin of these borates is not as obvious as that of the

evaporite varieties. Some were produced by the reaction

of boron-bearing vapour derived from hot intruding gran-

ites during metamorphism; others are the recrystallization

products of evaporite borates. Numerous borosilicates

(e.g., dumortierite and tourmaline) were formed under

these conditions. Compounds of this type contain both

triangular units and SiO

tetrahedral units. Among

the borate minerals associated with metamorphosed envi-

ronments are boracite, ludwigite, sussexite, and kotoite.

273

Arsenate Minerals

An arsenate mineral is a compound of arsenic, oxygen, and

various metals. Most arsenate minerals are rare, having

crystallized under very restricted conditions. At the min-

eralogically famous Långban iron and manganese mines

in central Sweden, more than 50 species of arsenate min-

erals have been described, many peculiar to the locality.

Such compounds occur in open cavities and resulted from

the reaction of arsenic acid (H

AsO

) upon pyrochroite

[manganese hydroxide; Mn(OH)

] at moderate to low

temperature. Arsenates at other localities are often oxi-

dation products of arsenide ores and are deposited at low

temperatures in late-stage veins and open cavities.

Only a few arsenate minerals have economic impor-

tance. Because the transition metals (e.g., cobalt, copper,

nickel) give brilliant colour to some of the arsenates, these

can be used to advantage in prospecting; such arsenate

oxidation products, or blooms, as erythrite (bright pink)

and annabergite (green) are indicators of nickel and cobalt

arsenide ores. Many of the nickel and cobalt deposits at

Sudbury and Cobalt, Ont., were located in this manner.

The arsenate minerals, which are salts of arsenic acid,

contain arsenate groups (AsO

) in which four oxygen (O)

atoms are arranged at the corners of a tetrahedron about

a central arsenic (As) atom. Each arsenate tetrahedron has

a net electric charge of -3, which is neutralized by large,

positively charged metal ions (e.g., calcium, manganese,

or ferrous iron) outside the tetrahedron. Unlike the simi-

lar silicate tetrahedra, which link to form chains, sheets,

rings, or frameworks, arsenate tetrahedra are insular.

The crystal structure of the arsenate minerals is very

similar to that of the phosphate and vanadate minerals;

indeed, many arsenate minerals form solid solutions with

both the phosphates and the vanadates.

7 carbonates and other Minerals 7

7 Minerals 7

274

Halide Minerals

Halide minerals make up a group of naturally occurring

inorganic compounds that are salts of the halogen acids

(e.g., hydrochloric acid). Such compounds, with the

notable exceptions of halite (rock salt), sylvite, and ﬂuo-

rite, are rare and of very local occurrence. Compositionally

and structurally, three broad categories of halide minerals

are recognized; these categories, which are also distin-

guishable in their modes of occurrence, include the simple

halides, the halide complexes, and the oxyhydroxy-halides.

The simple halides are salts of the alkali, alkaline

earth, and transition metals. Most are soluble in water;

the transition-metal halides are unstable under exposure

to air. Halite, sodium chloride (NaCl), is the most familiar

example; it often occurs with other evaporite minerals in

enormous beds resulting from the accumulation of brines

and trapped oceanic water in impermeable basins and

their evaporation. Minor amounts of sylvite, potassium

chloride (KCl), also are present in such beds.

Fluorite, or calcium ﬂuoride (CaF

), another simple

halide, is found in limestones that have been perme-

ated by aqueous solutions containing the ﬂuoride anion.

Noteworthy deposits of ﬂuorite occur in Mexico;

Cumberland, Eng.; and Illinois, Missouri, Kentucky, and

Colorado in the United States.

Other simple halides such as sal-ammoniac, ammonium

chloride (NH

Cl); lawrencite, ferrous chloride (FeCl

); and

molysite, ferric chloride (FeCl

) occur in fumarolic vents

and are highly unstable in air. A few hydrothermal vein min-

erals in silver deposits, such as chlorargyrite and calomel,

serve as minor and occasional ores of silver and mercury,

respectively. A few double salts (e.g., carnallite and tachy-

hydrite) included among the simple halides have formed

under conditions similar to the formation of halite.

275

hAlIDe MINerAlS

name colour lustre M

ohs

hard-

ness

specific

gravity

atacamite various bright green

shades; dark emerald-

green to blackish

adamantine 3–3½ 3.8

calomel colourless, white,

grayish, yellowish,

brown

adamantine 1½ 7.15

carnallite milk-white; some-

times reddish (from

included hematite)

greasy, dull

to shining

2½ 1.6

cerargy-

rite

colourless when pure

and fresh; usually

gray; becomes purple

or violet-brown on

exposure to light

(cerargyrite)

hornlike 2½ 5.6 (AgCl)

to 6.5

(AgBr)

7 carbonates and other Minerals 7

In the halide complexes, halide anions are tightly

bound to a cation, usually aluminum; the resulting unit

behaves as a single negative ion. The most common exam-

ples are the ﬂuoroaluminates cryolite, cryolithionite,

thomsenolite, and weberite. Enormous quantities of cryo-

lite formerly were mined at Ivigtut, Greenland, to be used

for ﬂux in the recovery of aluminum from bauxite.

Most oxyhydroxy-halides are rare and highly insoluble

compounds. Many have formed by the action of halide-

bearing waters upon the oxidation products of previously

existing sulﬁdes; atacamite, matlockite, nadorite, and

diaboleite are examples. A few compounds such as a

ﬁedlerite, laurionite, and penﬁeldite have formed through

the action of seawater upon ancient lead slags from the

historic deposits at Laurium, Greece.

7 Minerals 7

276

hAlIDe MINerAlS

name habit or form

frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

ataca-

mite

brittle, transpar-

ent to translucent

tabular to slender

prismatic crystals

one perfect

cleavage

alpha = 1.831

beta = 1.861

gamma =

1.880

ortho-

rhombic

calomel tabular crystals;

drusy crusts; earthy

masses

one good

cleavage

omega =

1.956–1.991

epsilon =

2.601–2.713

tetragonal

carnall-

ite

granular, massive conchoidal

fracture

alpha =

1.465–1.466

beta =

1.474–1.455

gamma =

1.444–1.446

ortho-

rhombic

name colour lustre Mohs

hard-

ness

specific

gravity

cryolite colourless to white,

brownish, reddish,

brick red

vitreous to

greasy

2½ 3.0

ﬂuorite variable vitreous 4 3.2

halite colourless when

pure, often splotched

blue or purple

vitreous 2 2.2

sal

ammoniac

colourless, white,

grayish, yellow

vitreous 1–2 1.5

sylvite colourless, white,

grayish, bluish, or

red (from included

hematite)

vitreous 2 2.0

277

nitrate and Iodate Minerals

This small group of naturally occurring inorganic com-

pounds is practically conﬁned to the Atacama Desert

of northern Chile; the principal locality is Antofagasta.

These minerals occur under the loose soil as beds of gray-

ish caliche (a hard cemented mixture of nitrates, sulfates,

halides, and sand) 2–3 m (7–10 feet) thick. The much rarer

name habit or form frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

cerargy-

rite

crusts; waxy coat-

ings; hornlike

masses

uneven to

subcon-

choidal

fracture

n =

2.071–2.253

isometric

cryolite coarsely granular

masses

cleavage

alpha = 1.338

beta = 1.338

gamma =

1.339

mono-

clinic

ﬂuorite brittle, transpar-

ent or translucent

cubes and two-

cube penetration

twins

perfect

octahedral

cleavage

n =

1.432–1.437

isometric

halite transparent cubic

(often cavernous or

stepped) crystals;

granular masses

perfect

cubic

cleavage

n = 1.544 isometric

sal

ammo-

niac

skeletal aggregates conchoidal

fracture

n = 1.639 isometric

sylvite transparent cubes

or granular masses

perfect

cubic

cleavage

n = 1.490 isometric

7 carbonates and other Minerals 7

278

Minerals 7

iodate minerals occur sporadically, intermixed with the

nitrates, and are distinguished from the former by their

yellow colour. The caliche has accumulated as a result of

drainage. Because most of these compounds are soluble

and unstable, they are practically conﬁned to arid regions

and soils, which possess a paucity of microorganisms.

Before World War I, Chile possessed a near monop-

oly on nitrates, with as many as 100 plants in operation.

The introduction of practical methods for the ﬁxation of

nitrogen early in the 20th century resulted in a decline of

the marketing of natural nitrates.

The nitrate and iodate minerals are structurally related

to the carbonate minerals. The most important nitrates

are soda nitre, nitre (saltpetre), darapskite, and humber-

stonite. Among the iodates are lautarite and dietzeite.

oxide Minerals

Any naturally occurring inorganic compound with a struc-

ture based on close-packed oxygen atoms in which smaller,

positively charged metal or other ions occur in interstices

is considered an oxide. Oxides are distinguished from

other oxygen-bearing compounds such as the silicates,

borates, and carbonates, which have a readily deﬁnable

group containing oxygen atoms covalently bonded to an

atom of another element.

The oxide minerals can be grouped as simple oxides and

multiple oxides. Simple oxides are a combination of one

metal or semimetal and oxygen, whereas multiple oxides

have two nonequivalent metal sites. The oxide structures

are usually based on cubic or hexagonal close-packing of

oxygen atoms with the octahedral or tetrahedral sites (or

both) occupied by metal ions; symmetry is typically iso-

metric, hexagonal, tetragonal, or orthorhombic.

279

A sample of the oxide mineral cuprite from Morenci, Ariz. U.S. Geological

Survey (Bureau of Mines, Mineral Specimens C\01786)

7 carbonates and other Minerals 7

The simple oxides can be subdivided on the basis

of the ratio of the numbers of atoms of metal (or other

elements) and oxygen, giving general formulas of the

type. In such formulas A represents a metal atom,

and x and y represent integers. Chemical compositions

then fall into categories such as those designated AO,

O, A

, AO

. Speciﬁc simple oxide minerals include

periclase (MgO), cuprite (Cu

O), hematite (Fe

), and

uraninite (UO

Complex oxides show a more varied chemistry, often

with extensive solid solution. Most common is the spi-

nel group, with the general formula AB

, in which A

and B are ions of different metals, the same metal with

different oxidation states, or a combination of the two;

A (with oxidation state +2), B (with oxidation state +3) is

the commonest, as, for example, in spinel itself, MgAl

Frequently occurring doubly charged ions include

7 Minerals 7

280

magnesium, iron, zinc, and manganese, while common

triply charged ions are aluminum, iron, manganese, and

chromium.

Oxide minerals occur as decomposition products of

sulﬁde minerals, in pegmatites, early crystallizing miner-

als in ultrabasic rocks, and as accessory minerals in many

igneous rocks.

OxIDe MINerAlS

name colour lustre mohs

ardness

specific

gravity

anatase brown to indigo

blue and black;

also variable

adamantine

to metallic

adamantine

5½–6 3.8–4.0

boehmite white, when

pure

3 3.0–3.1

brookite various browns metallic

adaman-

tine to

submetallic

5½–6 4.1–4.2

brucite white to pale

green, gray, or

blue

waxy to

vitreous

2½ 2.4

cassiterite reddish or yel-

lowish brown

to brownish

black

adamantine

to metallic

adaman-

tine, usually

splendent

6–7 7.0

chromite black metallic 5½ 4.5–4.8

chryso-

beryl

variable vitreous 8½ 3.6–3.8

columbite iron black to

brownish black;

often with iri-

descent tarnish

6–6½ 5.2 (colum-

bite) to 8.0

(tantalite)

281

name colour lustre mohs

hardness

specific

gravity

corundum red (ruby); blue

(sapphire); also

variable

adamantine

to vitreous

9 (a hard-

ness

standard)

4.0–4.1

cuprite various shades

of red

adamantine

to earthy

3½–4 6.1

delafossite black metallic 5½ 5.4–5.5

diaspore white, grayish

white, colour-

less; variable

brilliant

vitreous

6½–7 3.2–3.5

euxenite black brilliant

submetallic

to greasy or

vitreous

5½–6½ 5.3–5.9

franklinite brownish black

to black

metallic to

semimetal-

lic

5½–6½ 5.1–5.2

gibbsite white; grayish,

greenish, red-

dish white

vitreous 2½–3½ 2.3–2.4

goethite blackish brown

(crystals);

yellowish or

reddish brown

adaman-

tine-metal-

lic

5–5½ 3.3–4.3

hausman-

nite

brownish black submetallic 5½ 4.8

hematite steel gray; dull

to bright red

metallic or

submetallic

to dull

5–6 5.3

ilmenite iron black metallic to

submetallic

5–6 4.7–4.8

lepido-

crocite

ruby red to

reddish brown

submetallic 5 4.0–4.1

7 carbonates and other Minerals 7