Rafferty J.P. (ed.) Minerals

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7 Minerals 7

112

determination of crystal structure possible. Prior to this, the

classiﬁcation of silicates was based on chemical and physical

similarities, which often proved to be ambiguous. Although

many properties of a silicate mineral group are determined

by tetrahedral linkage, an equally important factor is the

type and location of other atoms in the structure.

Silicate minerals can be thought of as three-dimensional

arrays of oxygen atoms that contain void spaces (i.e.,

crystallographic sites) where various cations can enter.

Besides the tetrahedral (4-fold coordination) sites, 6-fold,

8-fold, and 12-fold sites are common. A correlation exists

between the size of a cation (a positively charged ion) and

the type of site it can occupy: the larger the cation, the

greater the coordination, because large cations have more

surface area with which the oxygen atoms can make con-

tact. Tetrahedral sites are generally occupied by silicon

and aluminum; 6-fold sites by aluminum, iron, titanium,

magnesium, lithium, manganese, and sodium; 8-fold sites

by sodium, calcium, and potassium; and 12-fold sites by

potassium. Elements of similar ionic size often substitute

for one another. An aluminum ion, for example, is only

slightly larger than a silicon ion, allowing substitution for

silicon in both tetrahedral and 6-fold sites.

SIlICATe MINerAlS

name colour lustre M

ohs

hard-

ness

specific

gravity

Tectosilicates (three-dimensional networks)

feldspar (for other examples, see feldspar)

orthoclase ﬂesh-red, white

to pale yellow,

red, green

vitreous 6–6½ 2.6

113

the Silicates 7

name colour lustre Mohs

hard-

ness

specific

gravity

feldspathoid (for other examples, see feldspathoid)

nepheline light-coloured;

reddish, green-

ish, brownish

vitreous to

greasy

5½–6 2.6–2.7

silica (for other examples, see silica mineral)

quartz variable vitreous

to greasy

(coarse-

grained);

waxy to

dull (ﬁne-

grained)

7 (a hard-

ness

standard)

2.65

zeolite (for other examples, see zeolite)

chabazite white; ﬂesh-red vitreous 4½ 2.0–2.1

Phyllosilicates (sheet structures)

clay (for other examples, see clay mineral)

chlorite green vitreous or

pearly

2–3 2.6–3.0

smectite 2.2–2.7

mica (for other examples, see mica)

apophyllite colourless,

white, pink, pale

yellow, or green

pearly

iridescent

4½–5 2.3–2.4

muscovite commonly white

or colourless;

light shades of

green, red, or

brown

vitreous

to silky or

pearly

2–2½ 2.8–3.0

7 Minerals 7

114

name colour lustre Mohs

hard-

ness

specific

gravity

prehnite pale green to

gray, white, or

yellow

vitreous 6–6½ 2.9–3.0

pyrophyl-

lite

white and vari-

ous pale colours

dull and

glistening

1–2 2.6–2.9

talc colourless;

white; pale or

dark green;

brown

pearly 1 (a hard-

ness

standard)

2.6–2.8

Inosilicates (chain structures)

amphibole (for other examples, see amphibole)

common

horn-

blende

pale to dark

green

glassy 5–6 3.0–3.4

mullite white 3.0

pyroxene (for other examples, see pyroxene)

augite brown, green,

black

vitreous 5½–6 3.2–3.5

rhodonite pink to brown-

ish red

vitreous 5½–6½ 3.6–3.8

wollas-

tonite

white; also

colourless, gray,

or very pale

green

vitreous 4½–5 2.9–3.1

Cyclosilicates (ring structures)

axinite clove- or

lilac-brown;

pearl-gray;

yellowish

highly

glassy

6½–7 3.3–3.4

115

name colour lustre Mohs

hard-

ness

specific

gravity

beryl various greens;

variable, includ-

ing deep-green

(emerald),

blue-green (aqua-

marine), pink

(morganite), yel-

low (heliodore)

vitreous 7½–8 2.7–2.8

cordierite various blues vitreous 7 2.5–2.8

tourmaline extremely

variable

vitreous to

resinous

7–7½ 3.0–3.2

Sorosilicates (double tetrahedral structures)

hemimor-

phite

white, some-

times tinted

bluish or green-

ish; yellow to

brown

vitreous 5 3.4–3.5

melilite colourless; gray-

ish green; brown

vitreous to

resinous

5–6

gehlenite 3.1

åkerman-

ite

2.9

Nesosilicates (independent tetrahedral structures)

andalusite pink, white, or

rose-red; also

variable

vitreous 6½–7½ 3.1–3.2

chryso-

colla

green, bluish

green

vitreous 2–4 2.0–2.8

datolite colourless or

white; also vari-

ous pale tints

vitreous 5–5½ 2.9–3.0

7 the Silicates 7

7 Minerals 7

116

name colour lustre Mohs

hard-

ness

specific

gravity

epidote yellowish green

to dark green

vitreous 6–7 3.3–3.5

garnet variable vitreous to

resinous

6–7½

almandine 4.3

andradite 3.9

grossula-

rite

3.6

pyrope 3.6

spessartite 4.2

uvarovite 3.9

kyanite blue; white; also

variable

vitreous to

pearly

4–7

(variable)

3.5–3.7

olivine (for other examples, see olivines)

forsterite-

fayalite

series

various greens

and yellows

vitreous 6½–7 3.2 (forster-

ite) to 4.4

(fayalite)

phenacite colourless; also

wine-yellow, pale

rose, brown

vitreous 7½–8 3.0

sillimanite colourless or

white; also vari-

ous browns and

greens

vitreous

to subada-

mantine

6½–7½ 3.2–3.3

sphene colourless,

yellow, green,

brown, black

adaman-

tine to

resinous

5 3.4–3.6

staurolite dark red-brown;

yellow-brown;

brown-black

subvitreous

to resinous

7–7½ 3.7–3.8

117

name colour lustre Mohs

hard-

ness

specific

gravity

thorite black; also

orange-yellow

(orangite)

4½–5 4.5–5.0;

5.2–5.4

(orangite)

topaz straw- or wine-

yellow; white;

grayish, greenish,

bluish, reddish

vitreous 8 (a hard-

ness

standard)

3.5–3.6

vesuvianite yellow, green,

brown

vitreous 6–7 3.3–3.4

willemite white or green-

ish yellow

vitreous to

resinous

5½ 3.9–4.2

zircon reddish brown,

yellow, gray,

green, or

colourless

adamantine 7½ 4.6–4.7

zoisite white; gray;

green-brown;

pink (thulite)

vitreous 6–6½ 3.2–3.4

7 the Silicates 7

SIlICATe MINerAlS

name habit frac

ture or

cleavage

refrac-

tive

indices

crystal

system

Tectosilicates (three-dimensional networks)

feldspar (for other examples, see feldspar)

orthoclase twinned crystals two good

cleav-

ages of 90

degrees

alpha =

1.518–1.529

beta =

1.522–1.533

gamma =

1.522–1.539

monoclinic

7 Minerals 7

118

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

feldspathoid (for other examples, see feldspathoid)

nepheline small glassy crys-

tals or grains

poor

cleavage

omega =

1.529–1.546

epsilon =

1.526–1.542

hexagonal

silica (for other examples, see silica mineral)

quartz prismatic and

rhombohedral

crystals; massive

conchoidal

fracture

omega =

1.544

epsilon =

1.553

hexagonal

zeolite (for other examples, see zeolite)

chabazite single, cubelike

rhombohedrons

poor

cleavage

omega =

1.470–1.494

epsilon =

1.470–1.494

hexagonal

Phyllosilicates (sheet structures)

clay (for other examples, see clay mineral)

chlorite large crystalline

blocks; ﬁne-

grained, ﬂaky

aggregates

platy

cleavage

alpha =

1.57–1.64

gamma =

1.575–1.645

monoclinic

or triclinic

smectite broad undu-

lating mosaic

sheets that

break into

irregular ﬂuffy

masses of min-

ute particles

alpha =

1.480–1.590

gamma =

1.515–1.630

119

the Silicates 7

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

mica (for other examples, see mica)

apophyllite tabular, pris-

matic, or

granular crys-

tals; prisms and

bipyramids when

well-formed

one per-

fect, one

poor

cleavage

omega =

1.534–1.535

epsilon =

1.535–1.537

tetragonal

muscovite large tabular

blocks (called

books); pseu-

dohexagonal

crystals; ﬁne-

grained

aggregates

one per-

fect, platy

cleavage

alpha =

1.552–1.574

beta =

1.582–1.610

gamma =

1.587–1.616

prehnite rosettes of small

radiating crystals;

tabular or pris-

matic crystals;

lamellar or botry-

oidal massive

one good

cleavage

alpha =

1.611–1.632

beta =

1.615–1.642

gamma =

1.632–1.665

ortho-

rhombic

pyrophyl-

lite

lamellar massive;

granular to com-

pact massive

one perfect

cleavage

alpha =

1.534–1.556

beta =

1.586–1.589

gamma =

1.596–1.601

monoclinic

talc compact foliated

masses

one perfect

cleavage

alpha =

1.539–1.553

beta =

1.589–1.594

gamma =

1.589–1.600

monoclinic

7 Minerals 7

120

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

Inosilicates (chain structures)

amphibole (for other examples, see amphibole)

common

horn-

blende

massive one good

cleavage of

56 degrees

alpha =

1.615–1.705

beta =

1.618–1.714

gamma =

1.632–1.730

monoclinic

mullite elongated pris-

matic crystals;

melts

one dis-

tinct

cleavage

alpha =

1.642–1.653

beta = 1.644

gamma =

1.654–1.679

ortho-

rhombic

pyroxene (for other examples, see pyroxene)

augite short, thick,

tabular crystals

one good

cleavage of

87 degrees

alpha =

1.671–1.735

beta =

1.672–1.741

gamma =

1.703–1.761

monoclinic

rhodonite rounded tabular

crystals; cleav-

able to compact

massive; embed-

ded grains

two perfect

cleavages

alpha =

1.711–1.738

beta =

1.715–1.741

gamma =

1.724–1.751

triclinic

wollas-

tonite

cleavable,

ﬁbrous, or com-

pact massive;

tabular crystals

one per-

fect, two

good

cleavages

alpha =

1.616–1.640

beta =

1.628–1.650

gamma =

1.631–1.653

triclinic

121

the Silicates 7

name habit frac-

ture or

cleavage

refrac-

tive

indices

crystal

system

Cyclosilicates (ring structures)

axinite broad,

sharp-edged,

wedge-shaped

crystals; lamellar

massive

one good

cleavage

alpha =

1.674–1.693

beta =

1.681–1.701

gamma =

1.684–1.704

triclinic

beryl long hexagonal

crystals

conchoidal

to uneven

fracture

omega =

1.569–1.598

epsilon =

1.565–1.590

hexagonal

cordierite short prismatic

crystals; embed-

ded grains;

compact massive

one dis-

tinct

cleavage

alpha =

1.522–1.558

beta =

1.524–1.574

gamma =

1.527–1.578

ortho-

rhombic

tourmaline parallel or radi-

ating groups of

striated, elon-

gated hexagonal

prisms, often

rounded or

barrel-shaped;

massive

subcon-

choidal

to uneven

fracture

omega =

1.635–1.675

epsilon =

1.610–1.650

hexagonal

Sorosilicates (double tetrahedral structures)

hemimor-

phite

sheaﬂike crystal

aggregates

one perfect

cleavage

alpha =

1.614

beta = 1.617

gamma =

1.636

ortho-

rhombic