Rafferty J.P. (ed.) Minerals

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7 Minerals 7

valence electrons of its constituent atoms are shared to

form bonds and therefore are not mobile.

Metallic Bonds

Bonding in metals is distinct from that in their salts, as

reﬂected in the signiﬁcant differences between the proper-

ties of the two groups. In contrast to salts, metals display

high plasticity, tenacity, ductility, and conductivity. Many

are characterized by lower hardness and have higher melt-

ing and boiling points than, for example, covalently bonded

materials. All these properties result from a metallic bond-

ing mechanism that can be envisioned as a collection of

positively charged ions immersed in a cloud of valence elec-

trons. The attraction between the cations and the electrons

holds a crystal together. The electrons are not bound to any

particular cation and are thus free to move throughout the

structure. In fact, in the metals sodium, cesium, rubidium,

and potassium, the radiant energy of light can cause elec-

trons to be removed from their surfaces entirely. (This

result, known as the photoelectric effect, is utilized in light

meters.) Electron mobility is responsible for the ability of

metals to conduct heat and electricity. The native metals

are the only minerals to exhibit pure metallic bonding.

Van der Waals Bonds

Neutral molecules may be held together by a weak elec-

tric force known as the van der Waals bond. It results

from the distortion of a molecule so that a small positive

charge develops on one end and a corresponding negative

charge develops on the other. A similar effect is induced

in neighbouring molecules, and this dipole effect propa-

gates throughout the entire structure. An attractive force

is then formed between oppositely charged ends of the

dipoles. Van der Waals bonding is common in gases and

organic liquids and solids, but it is rare in minerals. Its

presence in a mineral deﬁnes a weak area with good cleav-

age and low hardness. In graphite, carbon atoms lie in

covalently bonded sheets with van der Waals forces acting

between the layers.

Hydrogen Bonds

In addition to the four major bond types described above,

there is an interaction called hydrogen bonding. This

takes place when a hydrogen atom, bonded to an electro-

negative atom such as oxygen, ﬂuorine, or nitrogen, is also

attracted to the negative end of a neighbouring molecule.

A strong dipole-dipole interaction is produced, forming

a bond between the two molecules. Hydrogen bonding is

common in hydroxides and in many of the layer silicates—

e.g., micas and clay minerals.

Physical Properties

The physical properties of minerals are the direct result

of the structural and chemical characteristics of the min-

erals. Some properties can be determined by inspection

of a hand specimen or by relatively simple tests on such

a specimen. Others, such as those determined by opti-

cal and X-ray diffraction techniques, require special and

often sophisticated equipment and may involve elabo-

rate sample preparation. In the discussion that follows,

emphasis is placed on those properties that can be most

easily evaluated with only simple tests.

crystal Habit and crystal Aggregation

The external shape (habit) of well-developed crystals can

be visually studied and classiﬁed according to the crystal

systems and crystal classes they belong to. The majority of

crystal occurrences, however, are not part of well-formed

single crystals but are found as crystals grown together

7 the nature of Minerals 7

7 Minerals 7

in aggregates. Examples of some descriptive terms for

such aggregations are given here: granular, an intergrowth

of mineral grains of approximately the same size; lamel-

lar, ﬂat, platelike individuals arranged in layers; bladed,

elongated crystals ﬂattened like a knife blade; ﬁbrous, an

aggregate of slender ﬁbres, parallel or radiating; acicular,

slender, needlelike crystals; radiating, individuals forming

starlike or circular groups; globular, radiating individuals

forming small spherical or hemispherical groups; den-

dritic, in slender divergent branches, somewhat plantlike;

mammillary, large smoothly rounded, masses resembling

mammae, formed by radiating crystals; botryoidal, globu-

lar forms resembling a bunch of grapes; colloform, spherical

forms composed of radiating individuals without regard to

size (this includes botryoidal, reniform, and mammillary

forms); stalactitic, pendent cylinders or cones resembling

icicles; concentric, roughly spherical layers arranged about

a common centre, as in agate and in geodes; geode, a par-

tially ﬁlled rock cavity lined by mineral material (geodes

may be banded as in agate owing to successive depositions

of material, and the inner surface is often covered with

projecting crystals); and oolitic, an assemblage consisting

of small spheres resembling ﬁsh roe.

cleavage and Fracture

Both these properties represent the reaction of a mineral

to an external force. Cleavage is breakage along planar sur-

faces, which are parallel to possible external faces on the

crystal. It results from the tendency of some minerals to

split in certain directions that are structurally weaker than

others. Some crystals exhibit well-developed cleavage, as

seen by the planar cleavage in mica; perfect cleavage of this

sort is characterized by smooth, shiny surfaces. In other

minerals, such as quartz, cleavage is absent. Quality and

Common crystal aggregations and habits. Copyright Encyclopædia

Britannica, Inc.; rendering for this edition by Rosen Educational Services

7 the nature of Minerals 7

7 Minerals 7

direction are the general characteristics used to describe

cleavage. Quality is expressed as perfect, good, fair, and so

forth; cleavage directions of a crystal are consistent with

its overall symmetry.

Some crystals do not usually break in any particular

direction, reﬂecting roughly equal bond strengths through-

out the crystal structure. Breakage in such minerals is

known as fracture. The term conchoidal is used to describe

fracture with smooth, curved surfaces that resemble the

interior of a seashell; it is commonly observed in quartz

and glass. Splintery fracture is breakage into elongated

fragments like splinters of wood, while hackly fracture is

breakage along jagged surfaces.

Lustre

The term lustre refers to the general appearance of a

mineral surface in reﬂected light. The main types of lus-

tre, metallic and nonmetallic, are distinguished easily by

the human eye after some practice, but the difference

between them cannot be quantiﬁed and is rather difﬁcult

to describe. Metallic refers to the lustre of an untarnished

metallic surface such as gold, silver, copper, or steel. These

materials are opaque to light; none passes through even at

thin edges. Pyrite (FeS

), chalcopyrite (CuFeS

), and galena

(PbS) are common minerals that have metallic lustre.

Nonmetallic lustre is generally exhibited by light-coloured

minerals that transmit light, either through thick por-

tions or at least through their edges. The following terms

are used to distinguish the lustre of nonmetallic miner-

als: vitreous, having the lustre of a piece of broken glass

(this is commonly seen in quartz and many other nonme-

tallic minerals); resinous, having the lustre of a piece of

resin (this is common in sphalerite [ZnS]); pearly, having

the lustre of mother-of-pearl (i.e., an iridescent pearl-like

lustre characteristic of mineral surfaces that are parallel

to well-developed cleavage planes; the cleavage surface of

talc [Mg

(OH)

] may show pearly lustre); greasy, hav-

ing the appearance of being covered with a thin layer of oil

(such lustre results from the scattering of light by a micro-

scopically rough surface; some nepheline [(Na, K)AlSiO

]

and milky quartz may exhibit this); silky, descriptive of the

lustre of a skein of silk or a piece of satin and characteris-

tic of some minerals in ﬁbrous aggregates (examples are

ﬁbrous gypsum [CaSO

∙ 2H

O], known as satin spar, and

chrysotile asbestos [Mg

(OH)

]); and adamantine,

having the brilliant lustre of diamond, exhibited by min-

erals with a high refractive index comparable to diamond

and which as such refract light as strongly as the latter

(examples are cerussite [PbCO

] and anglesite [PbSO

]).

colour

Minerals occur in a great variety of colours. Because colour

varies not only from one mineral to another but also

within the same mineral (or mineral group), the observer

must learn in which minerals it is a constant property and

can thus be relied on as a distinguishing criterion. Most

minerals that have a metallic lustre vary little in colour,

but nonmetallic minerals can demonstrate wide variance.

Although the colour of a freshly broken surface of a metal-

lic mineral is often highly diagnostic, this same mineral

may become tarnished with time. Such a tarnish may dull

minerals such as galena (PbS), which has a bright bluish

lead-gray colour on a fresh surface but may become dull

upon long exposure to air. Bornite (Cu

FeS

), which on a

freshly broken surface has a brownish bronze colour, may

be so highly tarnished on an older surface that it shows var-

iegated purples and blues; hence, it is called peacock ore. In

other words, in the identiﬁcation of minerals with a metal-

lic lustre, it is important for the observer to have a freshly

broken surface for accurate determination of colour.

7 the nature of Minerals 7

7 Minerals 7

A few minerals with nonmetallic lustre display a

constant colour that can be used as a truly diagnostic

property. Examples are malachite, which is green; azurite,

which is blue; rhodonite, which is pink; turquoise, which

gives its name to the colour turquoise, a greenish blue to

blue-green; and sulfur, which is yellow. Many nonmetal-

lic minerals have a relatively narrow range of colours,

although some have an unusually wide range. Members

of the plagioclase feldspar series range from almost pure

white in albite through light gray to darker gray toward

the anorthite end-member. Most common garnets show

various shades of red to red-brown to brown. Members of

the monoclinic pyroxene group range from almost white

in pure diopside to light green in diopside containing a

small amount of iron as a substitute for magnesium in the

structure through dark green in hedenbergite to almost

black in many augites. Members of the orthopyroxene

series (enstatite to orthoferrosilite) range from light beige

to darker brown. On the other hand, tourmaline may

show many colours (red, blue, green, brown, and black) as

well as distinct colour zonation, from colourless through

pink to green, within a single crystal. Similarly, numerous

gem minerals such as corundum, beryl, and quartz occur

in many colours; the gemstones cut from them are given

varietal names. In short, in nonmetallic minerals of vari-

ous kinds, colour is a helpful, though not a truly diagnostic

(and therefore unique), property.

Hardness

Hardness (H) is the resistance of a mineral to scratching.

It is a property by which minerals may be described rela-

tive to a standard scale of 10 minerals known as the Mohs

scale of hardness. The degree of hardness is determined by

observing the comparative ease or difﬁculty with which

one mineral is scratched by another or by a steel tool.

the nature of Minerals 7

MOh’S hArDNeSS SCAle AND

ObServATIONS ON hArDNeSS Of

SOMe ADDITIONAl MATerIAlS

min

eral Mohs

hardness

other

materials

observations on

the minerals

talc 1 very easily scratched by

the ﬁngernail; has a greasy

feel

gypsum 2 ~2.2 ﬁngernail can be scratched by the

ﬁngernail

calcite 3 ~3.2 copper

penny

very easily scratched with

a knife and just scratched

with a copper coin

ﬂuorite 4 very easily scratched with

a knife but not as easily as

calcite

apatite 5 ~5.1

pocketknife

~5.5 glass plate

scratched with a knife

with difﬁculty

orthoclase 6 ~6.5 steel

needle

cannot be scratched with

a knife, but scratches glass

with difﬁculty

quartz 7 ~7.0 streak

plate

scratches glass easily

topaz 8 scratches glass very easily

corundum 9 cuts glass

diamond 10 used as a glass cutter

Source: Modiﬁed from C. Klein, Minerals and Rocks: Exercises in

Crystallography, Mineralogy, and Hand Specimen Petrology. Copyright

1989 John Wiley & Sons. Reprinted by permission of John Wiley &

Sons, Inc.

7 Minerals 7

For measuring the hardness of a mineral, several com-

mon objects that can be used for scratching are helpful,

such as a ﬁngernail, a copper coin, a steel pocketknife,

glass plate or window glass, the steel of a needle, and a

streak plate.

Because there is a general link between hardness and

chemical composition, these generalizations can be made:

1. Most hydrous minerals are relatively soft (H < 5).

2. Halides, carbonates, sulfates, and phosphates

also are relatively soft (H < 5½).

3. Most sulﬁdes are relatively soft (H < 5), with

marcasite and pyrite being examples of excep-

tions (H < 6 to 6½).

4. Most anhydrous oxides and silicates are hard

(H > 5½).

Because hardness is a highly diagnostic property in

mineral identiﬁcation, most determinative tables use rela-

tive hardness as a sorting parameter.

tenacity

Sev

eral mineral properties that depend on the cohesive

force between atoms (and ions) in mineral structures

are grouped under tenacity. A mineral’s tenacity can be

described by the following terms: malleable, capable of

being ﬂattened under the blows of a hammer into thin

sheets without breaking or crumbling into fragments (most

of the native elements show various degrees of malleability,

but particularly gold, silver, and copper); sectile, capable of

being severed by the smooth cut of a knife (copper, silver,

and gold are sectile); ductile, capable of being drawn into

the form of a wire (gold, silver, and copper exhibit this

property); ﬂexible, bending easily and staying bent after

the pressure is removed (talc is ﬂexible); brittle, showing

little or no resistance to breakage, and as such separating

into fragments under the blow of a hammer or when cut

by a knife (most silicate minerals are brittle); and elastic,

capable of being bent or pulled out of shape but returning

to the original form when relieved (mica is elastic).

Specific Gravity

Speciﬁc gravity (G) is deﬁned as the ratio between the

weight of a substance and the weight of an equal volume

of water at 4 °C (39 °F). Thus a mineral with a speciﬁc grav-

ity of 2 weighs twice as much as the same volume of water.

Since it is a ratio, speciﬁc gravity has no units.

The speciﬁc gravity of a mineral depends on the

atomic weights of all its constituent elements and the

manner in which the atoms (and ions) are packed together.

In mineral series whose species have essentially identi-

cal structures, those composed of elements with higher

atomic weight have higher speciﬁc gravities. If two miner-

als (as in the two polymorphs of carbon, namely graphite

and diamond) have the same chemical composition, the

difference in speciﬁc gravity reﬂects variation in internal

packing of the atoms or ions (diamond, with a G of 3.51,

has a more densely packed structure than graphite, with

a G of 2.23).

Measurement of the speciﬁc gravity of a mineral

specimen requires the use of a special apparatus. An esti-

mate of the value, however, can be obtained by simply

testing how heavy a specimen feels. Most people, from

everyday experience, have developed a sense of relative

weights for even such objects as nonmetallic and metal-

lic minerals. For example, borax (G = 1.7) seems light

for a nonmetallic mineral, whereas anglesite (G = 6.4)

appears heavy. Average speciﬁc gravity reﬂects what a

nonmetallic or metallic mineral of a given size should

weigh. The average speciﬁc gravity for nonmetallic min-

erals falls between 2.65 and 2.75, which is seen in the

7 the nature of Minerals 7