Rafferty J.P. (ed.) Minerals

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7 Minerals 7

302

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

anhydrite granular or

ﬁbrous massive;

concretionary

(tripestone)

two

perfect,

one good

cleavage

alpha =

1.567–1.580

beta =

1.572–1.586

gamma =

1.610–1.625

ortho-

rhombic

antlerite thick tabular

crystals

one perfect

cleavage

alpha = 1.726

beta = 1.738

gamma =

1.789

ortho-

rhombic

barite usually in

tabular crystals;

rosettes (desert

roses); massive

one

perfect,

one good

cleavage

alpha =

1.633–1.648

beta =

1.634–1.649

gamma =

1.645–1.661

ortho-

rhombic

botryo-

gen

reniform,

botryoidal,

or globular

aggregates

one

perfect,

one good

cleavage

alpha = 1.523

beta = 1.530

gamma =

1.582

mono-

clinic

brochan-

tite

prismatic to

hairlike crys-

tal and crystal

aggregates;

granular mas-

sive; crusts

one perfect

cleavage

alpha = 1.728

beta = 1.771

gamma =

1.800

mono-

clinic

cale-

donite

coating of

small elongated

crystals

one perfect

cleavage

alpha =

1.815–1.821

beta =

1.863–1.869

gamma =

1.906–1.912

ortho-

rhombic

303

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

celestite tabular crystals;

ﬁbrous massive

one

perfect,

one good

cleavage

alpha =

1.618–1.632

beta =

1.620–1.634

gamma =

1.627–1.642

ortho-

rhombic

chal-

canthite

short pris-

matic crystals;

granular masses;

stalactites and

reniform masses

conchoidal

fracture

alpha = 1.514

beta = 1.537

gamma =

1.543

triclinic

coquim-

bite

prismatic and

pyramidal crys-

tals; granular

massive

omega = 1.536

epsilon =

1.572

hexagonal

epsomite ﬁbrous or

hairlike

crusts; woolly

efﬂorescences

one perfect

cleavage

alpha =

1.430–1.440

beta =

1.452–1.462

gamma =

1.457–1.469

ortho-

rhombic

glauberite tabular, dipy-

ramidal, or

prismatic

crystals

one perfect

cleavage

alpha = 1.515

beta = 1.535

gamma =

1.536

mono-

clinic

gypsum elongated

tabular crystals

(some 5 ft long;

others twisted

or bent); granu-

lar or ﬁbrous

masses; rosettes

one perfect

cleavage

alpha =

1.515–1.523

beta =

1.516–1.526

gamma =

1.524–1.532

mono-

clinic

7 carbonates and other Minerals 7

7 Minerals 7

304

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

halo-

trichite

aggregates of

hairlike crystals

conchoidal

fracture

alpha =

1.475–1.480

beta =

1.480–1.486

gamma =

1.483–1.490

mono-

clinic

jarosite minute crystals;

crusts; granu-

lar or ﬁbrous

massive

one

distinct

cleavage

omega = 1.82

epsilon = 1.715

hexagonal

kainite granular mas-

sive; crystalline

coatings

one perfect

cleavage

alpha = 1.494

beta = 1.505

gamma =

1.516

mono-

clinic

kieserite granular mas-

sive, intergrown

with other salts

two perfect

cleavages

alpha = 1.520

beta = 1.533

gamma =

1.584

mono-

clinic

linarite elongated

tabular crystals,

either singly or

in groups

one perfect

cleavage;

conchoidal

fracture

alpha = 1.809

beta = 1.839

gamma =

1.859

mono-

clinic

mirabilite short prisms;

lathlike or

tabular crystals;

crusts or ﬁbrous

masses; granular

massive

one perfect

cleavage

alpha =

1.391–1.397

beta =

1.393–1.410

gamma =

1.395–1.411

mono-

clinic

plumbo-

jarosite

crusts, lumps,

compact masses

of microscopic

hexagonal plates

one fair

cleavage

omega = 1.875

epsilon =

1.786

hexagonal

305

7 carbonates and other Minerals 7

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

polyhalite ﬁbrous to foli-

ated massive

one perfect

cleavage

alpha = 1.547

beta = 1.560

gamma = 1.567

triclinic

thenard-

ite

rather large

crystals; crusts,

efﬂorescences

one

perfect,

one fair

cleavage

alpha =

1.464–1.471

beta =

1.473–1.477

gamma =

1.481–1.485

ortho-

rhombic

VANADATE MiNERAlS

Vanadate minerals are made up of naturally occurring compounds

of vanadium (V), oxygen (O), and various metals. Most of these

minerals are rare, having crystallized under very restricted condi-

tions. Although vanadinite occasionally is mined as a vanadium ore

and carnotite as a uranium ore, most vanadates have no economic

importance; they are prized by mineral collectors, however, for their

brilliant colours.

The structures of the vanadate minerals are complex. Some

vanadate minerals contain vanadate tetrahedra (VO

), in which

four oxygen atoms occupy the corners of a tetrahedron surround-

ing a central vanadium atom. Each vanadate tetrahedron has a net

charge of -3, which is neutralized by large, positively charged metal

ions (e.g., calcium, manganese, or ferrous iron) outside the tetra-

hedron. Unlike the similar silicate tetrahedra, which link to form

chains, sheets, rings, or frameworks, vanadate tetrahedra are insu-

lar. The vanadates containing these tetrahedra are structurally and

chemically similar to the phosphate and arsenate minerals; indeed,

some vanadium in many of these vanadates often is replaced by

phosphorus or arsenic, forming solid-solution series with both the

phosphates and the arsenates. Like the phosphate and sulfate miner-

als, many vanadates are complexes of transition metals, particularly

of ferrous iron, manganese, and copper.

306

Minerals 7

Other vanadates, particularly those that contain uranium, con-

tain V

ions, in which two atoms of vanadium are surrounded by

eight atoms of oxygen arranged in two square pyramids that share one

edge. Very complex clusters also exist but are usually classed with the

complex oxide minerals rather than with the vanadate minerals.

name colour lustre Mohs

hard-

ness

specific

gravity

carnotite bright yellow to

lemon or greenish

yellow

dull or

earthy

soft 4–5

descloizite brownish red to

blackish brown;

various shades from

orange-red to black

and green

greasy 3–3½ 5.9–6.2

tyuyamu-

nite

canary yellow;

lemon to greenish

yellow

waxy; also

pearly

about 2 variable

with water

content

vanadinite various shades of

yellow, orange, red,

and brown

subresinous

to subada-

mantine

about 3 6.5–7.1

name habit or

form

frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

carnotite powder of micro-

scopic platy or

lathlike crystals

one perfect

cleavage

alpha =

1.750

beta = 1.925

gamma =

1.950

monoclinic

307

carbonates and other Minerals 7

name habit or

form

frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

des-

cloizite

crusts of inter-

grown crystals;

rounded ﬁbrous

masses

no cleav-

age; uneven

fracture

desc mott

alpha =

2.18–2.21

beta =

2.25–2.31

gamma =

2.34–2.33

ortho-

rhombic

tyuy-

amunite

compact to crypto-

crystalline massive;

scales and lathlike

crystals; radiating

crystal aggregates

one perfect,

micalike

cleavage

ortho-

rhombic

vanadi-

nite

hairlike or barrel-

shaped (frequently

hollow) prismatic

crystals

uneven to

conchoidal

fracture

omega =

2.628–2.370

epsilon =

2.505–2.313

hexagonal

Sulﬁde Minerals

Sulﬁde minerals make up a group of compounds that join sul-

fur with one or more metals. Most of the sulﬁdes are simple

structurally, exhibit high symmetry in their crystal forms,

and have many of the properties of metals, including metal-

lic lustre and electrical conductivity. They often are strikingly

coloured and have a low hardness and a high speciﬁc gravity.

The composition of the sulﬁde minerals can be

expressed with the general chemical formula AmSn, in

which A is a metal, S is sulfur, and m and n are integers, giv-

ing A

S, AS, A

and AS

stoichiometries. The metals that

occur most commonly in sulﬁdes are iron, copper, nickel,

7 Minerals 7

308

lead, cobalt, silver, and zinc, though about ﬁfteen others

enter sulﬁde structures.

Almost all sulﬁde minerals have structural arrange-

ments that belong to six basic types, four of which are

important. These arrangements are close-packing combina-

tions of metal and sulfur, governed by ionic size and charge.

The simplest and most symmetrical of the four impor-

tant structural types is the sodium chloride structure, in

which each ion occupies a position within an octahedron

consisting of six oppositely charged neighbours. The most

common sulﬁde crystalling in this manner is galena (PbS),

the ore mineral of lead. A type of packing that involves

two sulﬁde ions in each of the octahedral positions in the

sodium chloride structure is the pyrite structure. This is a

high-symmetry structure characteristic of the iron sulﬁde,

pyrite (FeS

O). The second distinct structural type is that

of sphalerite (ZnS), in which each metal ion is surrounded

by six oppositely charged ions arranged tetrahedrally. The

third signiﬁcant structural type is that of ﬂuorite, in which

the metal cation is surrounded by eight anions; each anion,

in turn, is surrounded by four metal cations. The reverse of

this structure—the metal cation surrounded by four anions

and each anion surrounded by eight metal cations—is called

the antiﬂuorite structure. It is the arrangement of some of

the more valuable precious metal tellurides and selenides

among which is hessite (Ag

Te), the ore mineral of silver.

In virtually all of the sulﬁdes, bonding is covalent, but

some have metallic properties. The covalent property of

sulfur allows sulfur-sulfur bonds and the incorporation of

pairs in some sulﬁdes such as pyrite. Several sulﬁdes,

including molybdenite (MoS

) and covellite (CuS), have

layer structures. Several rare sulﬁde varieties have the

spinel (q.v.) structure.

Phase relations of sulﬁdes are particularly complex,

and many solid state reactions occur at relatively low

309

temperatures (100–300° C [212–572° F]), producing com-

plex intergrowths. Particular emphasis has been placed on

the experimental investigation of the iron-nickel-copper

sulﬁdes because they are by far the most common. They

also are important geologic indicators for locating pos-

sible ore bodies and provide low-temperature reactions

for geothermometry.

Sulﬁdes occur in all rock types. Except for dissemina-

tion in certain sedimentary rocks, these minerals tend to

occur in isolated concentrations which make up mineral

bodies such as veins and fracture ﬁllings or which comprise

replacements of preexisting rocks in the shape of blan-

kets. Sulﬁde mineral deposits originate in two principal

processes, both of which have reducing conditions: (1) sep-

aration of an immiscible sulﬁde melt during the early stages

of crystallization of basic magmas; and (2) deposition from

aqueous brine solutions at temperatures in the 300–600 °C

(572–1,112 °F) range and at relatively high pressure, such as at

the seaﬂoor or several kilometres beneath Earth’s surface.

The sulﬁde deposits formed as a result of the ﬁrst process

include mainly pyrrhotites, pyrites, pentlandites, and chal-

copyrites. Most others occur because of the latter process.

Weathering may act to concentrate dispersed sulﬁdes.

Pyrite (FeS

) and pyrrhotite (Fe

1 -

xS) are the most com-

mon sulﬁde minerals. Brassy yellow pyrite, often called

“fool’s gold,” occurs variously as an accessory mineral in

many rocks, in veins, and even as a chief component of

some fossils. Pyrrhotite, which typically has a bronze-

like appearance and is slightly magnetic, is a common

accessory mineral in maﬁc igneous rocks. Both of these

minerals, which are associated with each other in some

deposits, have yielded large quantities of sulfur recovered

for uses such as the production of sulfuric acid.

Sulﬁde minerals are the source of various precious

metals, most notably gold, silver, and platinum. They also

7 carbonates and other Minerals 7

7 Minerals 7

310

are the ore minerals of most metals used by industry, as for

example antimony, bismuth, copper, lead, nickel, and zinc.

Other industrially important metals such as cadmium and

selenium occur in trace amounts in numerous common

sulﬁdes and are recovered in reﬁning processes.

SulfIDe MINerAlS

name colour lustre M

ohs

hard-

ness

spe-

cific

gravity

argentite blackish lead-gray metallic 2–2 1/2 7.2–7.4

arsenopy-

rite

silver-white to

steel-gray

metallic 5 1/2–6 6.1

bornite copper-red to

pinchbeck-brown,

tarnishing quickly to

iridescent purple

metallic 3 5.1

chalcocite blackish lead-gray metallic 2 1/2–3 5.5–5.8

chalcopy-

rite

brass-yellow, often

tarnished and

iridescent

metallic 3 1/2–4 4.1–4.3

cinnabar cochineal-red to

brownish or lead-gray

adamantine

to metallic

2–2 1/2 8.1

cobaltite silver-white to red;

steel-gray or grayish

black

metallic 5 1/2 6.3

covellite indigo-blue; highly

iridescent; brass-

yellow or deep red

submetallic

to resinous

(crystals);

subresin-

ous to dull

(massive)

1 1/2–2 4.6–4.8

cubanite brass- to

bronze-yellow

metallic 3 1/2 4.0–4.2

311

name colour lustre Mohs

hard-

ness

spe-

cific

gravity

domey-

kite

tin-white to steel-

gray; tarnishes

yellowish brown,

becoming iridescent

metallic 3–3 1/2 7.2–7.9

galena lead-gray metallic 2 1/2–3 7.6

gree-

nockite

various shades of yel-

low and orange

adamantine

to resinous

3–3 1/2 4.9

krenne-

rite

silver-white to light

brass-yellow

metallic 2–3 8.6

linnaeite light gray to steel- or

violet-gray, tarnish-

ing to copper-red or

violet-gray

brilliant

metal-

lic (when

fresh)

4 1/2–5

1/2

4.5–4.8

loellingite silver-white to

steel-gray

metallic 5–5 1/2 7.4–7.5

marcasite tin-white, deepen-

ing with exposure to

bronze-yellow

metallic 6–6 1/2 4.9

mauch-

erite

reddish platinum-

gray, tarnishing

copper-red

metallic 5 8.0

metacin-

nabar

grayish black metallic 3 7.65

millerite pale brass-yellow,

tarnishing iridescent

gray

metallic 3–3 1/2 5.3–5.7

molybde-

nite

lead-gray metallic 1–1 1/2 4.6–4.7

niccolite pale copper-red,

tarnishing gray to

blackish

metallic 5–5 1/2 7.8

7 carbonates and other Minerals 7