Rafferty J.P. (ed.) Minerals

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7 Minerals 7

312

name colour lustre Mohs

hard-

ness

spe-

cific

gravity

orpiment lemon-yellow,

golden-yellow,

brownish yellow

resinous;

pearly on

cleavages

1 1/2–2 3.5

pentland-

ite

light bronze-yellow metallic 3 1/2–4 4.6–5.0

pyrite pale brass-yellow splendent

to glistening

metallic

6–6 1/2 5.0

pyrrhotite bronze-yellow to

pinchbeck-brown,

tarnishing quickly

metallic 3 1/2–4

1/2

4.6–4.7

4.8

(troilite)

realgar aurora-red to

orange-yellow

resinous to

greasy

1 1/2–2 3.5–3.6

rickardite purple-red metallic 3 1/2 7.5

sphalerite brown, black, yellow;

also variable

resinous to

adamantine

3 1/2–4 3.9–4.1

stannite steel-gray to

iron-black

metallic 4 4.3–4.5

stibnite lead- to steel-gray,

tarnishing blackish

metallic 2 4.6

stromey-

erite

dark steel-gray, tar-

nishing blue

metallic 2 1/2–3 6.2–6.3

sylvanite steel-gray to

silver-white

brilliant

metallic

1 1/2–2 8.1–8.2

tetrady-

mite

pale steel-gray,

tarnishing dull or

iridescent

metallic 1 1/2–2 7.1–7.5

umangite dark cherry-red,

tarnishing quickly to

violet-blue

metallic 3 5.6

wurtzite brownish black resinous 3 1/2–4 4.0–4.1

313

carbonates and other Minerals 7

name habit or

form

frac-

ture or

cleavage

refractive

indices or

polished

section

data

crys-

tal

system

argen-

tite

cubic or octa-

hedral crystals,

often in groups;

arborescent or

hairlike massive

subcon-

choidal

fracture

faintly

anisotropic

isomet-

ric

arseno-

pyrite

cubic or dodeca-

hedral crystals

having rough

or curved faces;

granular or com-

pact massive

one

distinct

cleavage

strongly

anisotropic

mono-

clinic

bornite prismatic crys-

tals; columnar,

granular, or com-

pact massive

uneven

fracture

isotropic in

part; pinkish

brown

isomet-

ric

chal-

cocite

short prismatic

or thick tabular

crystals; massive

conchoidal

fracture

weakly

anisotropic

ortho-

rhombic

chalco-

pyrite

compact mas-

sive; tetragonal

crystals

uneven

fracture

weakly anisotro-

pic; often shows

lamellar and

polysynthetic

twinning

tetrago-

nal

cinnabar rhombohedral,

tabular, or pris-

matic crystals;

massive; earthy

coatings

one

perfect

cleavage

omega =

2.756–2.905

epsilon =

3.065–3.256

hexago-

nal

7 Minerals 7

314

name habit or

form

frac-

ture or

cleavage

refractive

indices or

polished

section

data

crys-

tal

system

cobaltite cubic or pyrito-

hedral crystals

with striated

faces

one

perfect

cleavage

isomet-

ric

covellite massive; rarely in

hexagonal plates

one highly

perfect

cleavage

strongly

anisotropic

hexago-

nal

cubanite thick tabular

crystals; massive

conchoidal

fracture

anisotropic ortho-

rhombic

domey-

kite

reniform or bot-

ryoidal masses

uneven

fracture

isotropic isomet-

ric

galena cubic crystals;

cleavable masses

one

perfect

cleavage

isotropic isomet-

ric

gree-

nockite

earthy coating conchoidal

fracture

omega = 2.431–

2.506 epsilon =

2.456–2.529

hexago-

nal

krenne-

rite

short prismatic

crystals

one

perfect

cleavage

strongly aniso-

tropic; creamy

white

ortho-

rhombic

lin-

naeite

octahedral crys-

tals granular to

compact masses

uneven to

subcon-

choidal

fracture

isotropic isomet-

ric

loellin-

gite

prismatic or

pyramidal crys-

tals; massive

uneven

fracture

very strongly

anisotropic

ortho-

rhombic

315

name habit or

form

frac-

ture or

cleavage

refractive

indices or

polished

section

data

crys-

tal

system

marca-

site

tabular or pyra-

midal crystals;

spear-shaped or

cockscomb-like

crystal groups

one

distinct

cleavage

strongly aniso-

tropic and

pleochroic;

creamy white,

light yellowish

white, and rosy

white

ortho-

rhombic

mauch-

erite

tabular crystals uneven

fracture

weakly anisotro-

pic; pinkish gray

tetrago-

nal

meta-

cinnabar

tetrahedral crys-

tals; massive

subcon-

choidal

to uneven

fracture

isotropic;

grayish white;

shows lamellar

twinning

isomet-

ric

millerite very slender

to capillary

crystals in

radiating groups,

sometimes

interwoven

two

perfect

cleavages

strongly

anisotropic

hexago-

nal

molyb-

denite

hexagonal

tablets; foliated

massive, in scales

one

perfect

cleavage

very strongly

anisotropic and

pleochroic; white

hexago-

nal

nicco-

lite

reniform

massive; also

branching

cleavage

strongly

anisotropic

hexago-

nal

orpi-

ment

foliated, ﬁbrous,

or columnar mas-

sive; reniform

or botryoidal

masses; granular

one

perfect

cleavage

alpha = 2.4

beta = 2.81

gamma = 3.02

mono-

clinic

7 carbonates and other Minerals 7

7 Minerals 7

316

name habit or

form

frac-

ture or

cleavage

refractive

indices or

polished

section

data

crys-

tal

system

pent-

landite

granular

aggregates

conchoidal

fracture

isotropic isomet-

ric

pyrite cubic, pyri-

tohedral, or

octahedral crys-

tals with striated

faces; massive

conchoidal

to uneven

fracture

isotropic;

creamy white

isomet-

ric

pyrrho-

tite

granular mas-

sive; sometimes

platy or tabular

crystals

uneven to

subcon-

choidal

fracture

strongly

anisotropic

hexago-

nal

realgar short, stri-

ated prismatic

crystals; granu-

lar or compact

massive;

incrustations

one good

cleavage,

three less

alpha =

2.486–2.538

beta =

2.602–2.684

gamma =

2.620–2.704

mono-

clinic

rickard-

ite

massive irregular

fracture

strongly aniso-

tropic and

pleochroic

ortho-

rhombic

sphaler-

ite

tetrahedral or

dodecahedral

crystals, often

with curved

faces; cleavable

masses

one

perfect

cleavage

n = 2.320–2.517 isomet-

ric

stannite granular massive uneven

fracture

anisotropic tetrago-

nal

317

name habit or

form

frac-

ture or

cleavage

refractive

indices or

polished

section

data

crys-

tal

system

stibnite aggregates of

needle-like

crystals; crystals

are easily bent or

twisted

one

perfect

cleavage

alpha =

3.184–3.204

beta =

4.036–4.056

gamma =

4.293–4.313

white; strongly

anisotropic

ortho-

rhombic

stro-

meyer-

ite

pseudohex-

agonal prisms;

compact massive

subcon-

choidal to

conchoidal

fracture

strongly

anisotropic

ortho-

rhombic

sylva-

nite

short prismatic,

thick tabular, or

bladed crystals

one

perfect

cleavage

strongly aniso-

tropic and

pleochroic;

creamy white;

shows polysyn-

thetic twinning

mono-

clinic

tetrady-

mite

foliated to

granular massive;

bladed crystals

one

perfect

cleavage

weakly aniso-

tropic; white;

sometimes

shows a graph-

like intergrowth

hexago-

nal

umang-

ite

small grains;

ﬁne-grained

aggregates

uneven

fracture

strongly aniso-

tropic; dark

red-violet;

apparently

uniaxial

tetrago-

nal

7 carbonates and other Minerals 7

7 Minerals 7

318

Sulfosalts

Sulfosalts constitute an extensive group of minerals, mostly

rare species, marked by some of the most complicated

atomic and crystal structures known to inorganic chemis-

try. They conform to the general composition AmBnXp, in

which m, n, and p are integers; A may be lead, silver, thal-

lium, or copper; B may be antimony, arsenic, bismuth, tin,

or germanium; and X may be sulfur or selenium. Formerly

it was believed that the sulfosalts were salts of complex

hypothetical thioantimonic or thioarsenic acids (e.g.,

HSbS

, H

AsS

), but X-ray diffraction analyses

indicate that the atomic structures of many sulfosalts are

based on structural fragments of simpler compounds such

as galena (lead sulﬁde; PbS) blocks and stibnite (antimony

trisulﬁde; Sb

) sheets. No encompassing theory has been

evolved to rationalize many of these curious compounds.

The complexity of many of the structures evidently results

from their having crystallized at low temperatures and the

consequent high degree of ordering of the metal atoms.

Syntheses of such compositions at higher temperature

usually result in structures simpler than the complicated

low-temperature forms.

Although sulfosalts are much rarer than the sulﬁde

minerals with which they are often associated, some

name habit or

form

frac-

ture or

cleavage

refractive

indices or

polished

section

data

crys-

tal

system

wurtzite pyramidal crys-

tals; ﬁbrous or

columnar massive;

concentrically

banded crusts

one easy

cleavage

omega =

2.330–2.356

epsilon =

2.350–2.378

hexago-

nal

319

7 carbonates and other Minerals 7

localities are truly remarkable for the variety of species

encountered. At the Lengenbach Mine in Switzerland,

for example, more than 30 distinct species have been

recognized, 15 of which are not found elsewhere. Most sul-

fosalts have formed at low temperature in open cavities,

usually in association with copper–zinc–arsenic sulﬁde

ores. Very often they occur in cavities of calcite and dolo-

mite, as at the Lengenbach Mine. Most are lead gray in

MOlyBDATE AND

TUNGSTATE MiNERAlS

These naturally occurring inorganic compounds are salts of molybdic

acid, H

MoO

, and tungstic acid, H

. Minerals in these groups

often are valuable ores.

The structural unit of these minerals is a tetrahedral group

formed by four oxygen atoms at the corners of a tetrahedron sur-

rounding a molybdenum or tungsten atom. Each MoO

or WO

tetrahedron has a net charge of -2, which is neutralized by metal ions

outside the tetrahedron. Unlike the silicate or borate minerals, which

form chains, rings, sheets, or framework structures by sharing oxy-

gen atoms between adjacent tetrahedra, the molybdate and tungstate

minerals share none; they are similar in this respect to the phosphate,

vanadate, arsenate, and chromate minerals. Because the molybdenum

ion and the tungsten ion have similar radii, they may substitute for

one another within the structure of any naturally occurring example;

thus, they tend to form solid solution series.

Among the molybdate and tungstate minerals, only the powellite-

scheelite series (calcium-bearing molybdate/tungstates) and wulfenite

(lead molybdate) are noteworthy. Scheelite is a valuable tungsten ore;

wulfenite is a minor ore of lead.

One other series of tungstates is important. Wolframite, another

name for the hübnerite-ferberite series of manganese/iron tung-

states, is perhaps the most important ore of tungsten. These minerals

have a structure, unlike that of the other tungstates, based on WO

octahedra—i.e., each tungsten atom is surrounded by six oxygen atoms

arranged at the corners of an octahedron. These minerals are classed

with the complex oxides and are related to the niobates and tantalates.

7 Minerals 7

320

SulfOSAlTS

name colour lustre M

ohs

hardness

specific

gravity

argyrodite bluish to pur-

plish black; steel

gray when fresh

metallic 2½ 6.1–6.3

bourno-

nite

steel gray to

iron black

metallic 2½–3 5.8–5.9

enargite gray black to

iron black

metallic 3 4.4–4.5

polybasite iron black metallic 2–3 6.0–6.2

proustite scarlet

vermillion

adaman-

tine

2–2½ 5.6

pyrargyrite deep red adaman-

tine

2½ 5.8

stephanite iron black metallic 2–2½ 6.2–6.3

tetrahe-

drite

ﬂint gray to iron

or dull black

metallic 3–4½ 4.6–5.1

colour with a metallic lustre, brittle (rarely malleable),

crystalline, and difﬁcult to tell apart without recourse to

X-ray diffraction and electron microprobe analyses. The

thallium-bearing sulfosalts often are deep red and trans-

parent, as sometimes are the sulfosalts of silver.

Although under exceptional circumstances some

sulfosalts may constitute silver ores (i.e., proustite, pyrar-

gyrite, and stephanite), and other species have constituted

ores of silver (in minor amounts), mercury, arsenic, and

antimony (i.e., boulangerite, livingstonite, enargite, and

tennantite-tetrahedrite), their economic importance is

trivial. Aside from mineralogical curiosities, the sulfo-

salts are of interest because their electronic properties are

related to those of semiconductors.

321

carbonates and other Minerals 7

SulfOSAlTS

name habit frac

ture or

cleavage

refractive

indices or

polished

section

data

crystal

system

argy-

rodite

crystals and

crystal aggre-

gates; crusts;

compact

masses

conchoidal

to uneven

fracture

violet gray

white; isotro-

pic (canﬁeldite)

or weakly pleo-

chroic

(argyrodite)

isometric

bour-

nonite

prismatic to

tabular crystals;

crystal aggre-

gates; granular

to compact

masses

subcon-

choidal

to uneven

fracture

white; weakly

anisotropic and

very weakly

pleochroic

ortho-

rhombic

enar-

gite

tabular crys-

tals; granular

masses

one perfect

cleavage

gray to light

rose brown;

strongly aniso-

tropic; weakly

pleochroic

ortho-

rhombic

poly-

basite

tabular crys-

tals; massive

uneven

fracture

gray white;

moderately

anisotro-

pic; weakly

pleochroic

monoclinic

proust-

ite

prismatic crys-

tals; compact

masses

one distinct

cleavage

omega =

2.979–3.088

epsilon =

2.711–2.792

hexagonal

pyrar-

gyrite

prismatic crys-

tals; compact

masses

one distinct

cleavage

omega = 3.084

epsilon = 2.881

hexagonal