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magnesium and aluminium. Some Fe replaces Mg and associated with serpentine. Flakes are flexible but not
gives colour to the chlorite. Like the micas, they have a elastic, and are easily scratched by the finger-nail. H = I.
perfect cleavage, due to the atomic sheet structure
(Fig. 4.26). Different kinds of chlorite are given Kaolin (china clay)
distinctive names (e.g. penninite, clinochlore); these are This substance is largely made up of the mineral
not distinguished in the following general description. kaolinite, Al 4 Si 4 O 10 (OH) 8 , one of the group of Clay
Crystals: Monoclinic, frequently 6-sided in shape, Minerals which, like the micas, are built up of silicon-
with a perfect cleavage parallel to the basal plane; the oxygen sheets (Fig. 4.26).
mineral splits into hexagonal flakes which are flexible Kaolin is derived from the breakdown of feldspar by
but not elastic (cf. mica). H = 2 to 2\ (often soft enough the action of water and carbon dioxide; the chemical
to be scratched by the finger-nail). G = 2.65 to 3.0. equation for the change is given and the kaolinization of
Chlorite is found in igneous rocks, as described granite masses is described on p. 138. It is white or grey,
below, and in metamorphic rocks such as chlorite-schist, soft, and floury to the touch, with a clayey smell when
and in some clays. damp. G = 2.6. In thin section it is seen as a
In thin section: Chlorite occurs as an alteration decomposition product of feldspar (Fig. 4.29), which
product of biotite, augite, or hornblende; it may replace when altered appears clouded and looks white by top
these minerals completely, forming a pseudomorph light (i.e. by light reflected from the surface of the slice
( = 'false form') in which the aggregate of chlorite flakes and not transmitted through it).
and fibres retains the shape of the original mineral.
Together with other minerals such as calcite, chlorite Epidote, Ca 2 (AlFe) 3 (SiOJ 3 (OH)
also forms an infilling to cavities in basalts (q.v.). The monoclinic crystals of this mineral are typically
Colour: Shades of bluish-green and yellowish-green, of a yellowish-green colour. Often in radiating clusters;
sometimes very pale; noticeably pleochroic; cleavage vitreous lustre. H = 6 to 7. G = 3.4.
often seen. Epidote occurs as an alteration product of calcic
Mean RJ. = about 1.58. plagioclases or of augite; also as infillings to vesicles in
Biref: Weak grey. basalts, and as pale green veins traversing igneous and
metamorphoc rocks.
Serpentine, Mg 6 Si 4 O 10 (OH) 8 , some Fe replaces Mg, in
Zeolites
part
Serpentine is an alteration product of olivine, of These form a group of hydrous aluminous silicates of
orthorhombic pyroxene, or of hornblende. This reaction calcium, sodium, or potassium; they contain molecular
takes place in an igneous rock while it is still moderately water which is readily driven off on heating, a property
hot (hydrothermal action), the source of the hot water to which the name refers (Greek zein, to boil). They
being magmatic; it is thought that the change from occur as white or glassy crystals clusters, filling or lining
olivine to serpentine may also be brought about by the the cavities left by escaping gases (amygdales, p. 100) in
action of water and silica. basic lavas, or filling open joints, and are derived from
feldspars or feldspathoids by hydration.
Serpentine grows as a mass of green fibres or plates,
Two commonly occurring natural zeolites are:
which replace the original mineral as a pseudomorph. A
fibrous variety is called chrysotile, and is worked in veins
for commercial asbestos. In the mass, serpentine is Analcite, NaAlSi 2 O 6 H 2 O
rather soapy to the touch, and may be coloured red if Cubic; crystallized as trapezohedra (as in Fig. 4.35b),
iron oxide is present. H = 3 to 4. G = 2.6. Serpentine is white in colour. G = 2.25. Occurs in the amygdales of
found in basic and ultrabasic rocks (p. 101), and in basalts.
serpentine-marble.
In this section: as a pseudomorph after olivine, Natrolite, Na 2 Al 2 Si 3 O 10 2H 2 O
serpentine appears as a matte of pale green fibres, Forms white, acicular orthorhombic crystals,
weakly birefringent, and having a low RJ. (1.57). generally in radiating clusters. G = 2.2.
Specks of black magnetite, the oxidized by-product
from iron in the original olivine, are often present. The
change to serpentine involves an increase in volume, and Clay minerals
this expansion may fracture the surrounding minerals in
the rock, fine threads of serpentine being developed in Clays can form as either primary or secondary minerals.
the cracks so formed. Here they are grouped under one heading because of
their economic importance, their presence in most
profiles of weathering and their influence upon the
JaZcMg 3 Si 4 O 10 (OH) 2 mechanical character of rocks and less well consolidated
A soft, flaky mineral, white or greenish in colour, sediments. Being minute they can be seen using only an
which occurs as a secondary product in basic and electron microscope (Fig. 4.37), but their presence may
ultrabasic rocks, and in talc-schist (p. 140). It is often be revealed by placing a few particles of material that is
Fig. 4.37 Clay minerals, (a) Kaolinite, (b) montmorillonite and (c) illite, as seen at high magnification with an electron
microscope (provided by L. Dobereiner).

suspected to contain clay, on the teeth. The clay tetrahedral and octahedral-layers; each pair,
component, if present, will feel smooth and the non-clay Si 2 O 3 (OH) 2 +Al 2 (OH) 6 , with loss of water becomes
component will be gritty. The dominant clay mineral Al 2 Si 2 O 5 (OH) 4 .Kaolinite occurs in hexagonal flakes of
present can be identified inexpensively by measuring its minute size, and forms the greater part of kaolin (china
activity, as described on p. 195. clay) deposits; it is also found in soils and sedimentary
In common with other flaky minerals such as the clays, of which it forms a variable and often small
micas, chlorites, and talc, clay minerals are built up of proportion. It is the main constituent of fire-clays (q.v.).
two-dimensional atomic layers or sheets (p. 74) which The mineral dick it e has the same composition as
are stacked one upon another in the ^-direction. The kaolinite, but the layers in the structure have a differ-
layers are of two kinds: ent arrangement relative to one another. Halloysite,
(0 silicon-oxygen sheets, each formed by the linking Al 2 Si 2 O 5 (OH) 4 -2H 2 O, may be included in a group with
together of tetrahedral SiO4-groups as described on dickite and kaolinite; it occurs as minute tubes, the
p. 70, and generally referred to as a tetrahedral layer. rolled-up 'sheets' of silicon-oxygen and Al-hydroxyl
The composition of this layer is a multiple OfSi 2 O 5 , or composition. Certain clays having a high content of
with attached hydrogen, Si 2 O 3 (OH) 2 . halloysite possess special properties with regard to
(H) octahedral layers, in which a metal ion (Al or Mg) porosity and water content, which are discussed in
lies within a group of six hydroxyls which are arranged Chapter 18 (p. 289).
at (b) Montmorillonite, which has important base-
the corners of an octahedron (Fig. 4.38). Adjoining octa- exchange properties, is built up of 3-layer units
hedra are linked by sharing hydroxyls. Such an octa- comprising two tetrahedral layers separated by an
hedral layer has the composition Al 2 (OH) 6 or octahedral layer, and has the ideal formula
Mg 3 (OH) 6 (the minerals gibbsite and brucite Al 4 Si 8 O 20 (OH) 4 . Some aluminium is usually replaced
respectively); in addition, some substitution may take by magnesium or iron, and small amounts of sodium or
pace for the metal ion, for example Fe 3 + for Al 3 + , or calcium are then attached, as ions lying between the 3-
F e 2 + f o r Mg 2 + layer units or around the edges of the minute crystals.
Different arrangements of the above layers build up These alkali ions are exchangeable, and give rise to the
the units of which the clay minerals are composed, the high base-exchange capacity of the mineral. In addition,
flat surfaces of the minute crystals being parallel to the layers of molecular water may occur between the
layers. Some clay minerals have two-layer units, as in 3-layer units. A typical Ca-montmorillonite would
Fig. 4.38 (kaolinite); others (e.g. montmorillonite) have be represented by the formula:
three-layer units in which an octahedral layer lies Ca 05 (MgAl 3 )Si 8 O 20 (OH) 4 JcH 2 O; the calcium is
between two tetrahedral layers, one of which is inverted replaced by sodium in Na-montmorillonite. The
relative to the other so that apexes of the tetrahedra proportion of water is variable, and the absorption of
point inwards in the unit (cf. Fig. 4.26). Layers of water between the 3-layer units gives rise to the
molecular water may lie between these units, as in considerable swelling properties possessed by clays
montmorillonite. containing much montmorillonite.
The commoner clay minerals include: The mineral occurs sparsely in soils along with
(a) Kaolinite, Al 4 Si 4 O 10 (OH) 8 , made up of alternate kaolinite; it is the chief component of clays such as
Fig. 4.38 Kaolinite. Upper. Electron micrograph ( x 35000). Lower. Atomic structure of kaolinite (open circles represent
oxygen atoms), (a) Tetrahedral group; (b) octahedral group; (c) unit of structure, consisting of a silicon-oxygen layer of
linked tetrahedral groups, combined with an aluminium-hydroxyl, layer. The c-axis of the mineral is vertical.

fuller's earth and bentonite, which are described briefly Treasure Island, San Francisco. The material used for
on p. 122, with a note on their uses. the lining was a sandy clay, having a small content of
(c) Illite (named after Illinois by R. E. Grim, 1937) is calcium which was probably attached as ions to
similar in many respects to white mica, but has less aggregates of colloidal particles, by virtue of which the
potassium and more water in its composition. It has a clay was 'crumbly' and to some extent permeable. By
much lower base-exchange capacity than mont filling the lake with salt water the clay was enabled to
morillonite. Illite is built up of units comprising two take up sodium in exchange for the calcium; this
tetrahedral layers separated by an octahedral layer, resulted in a considerable decrease in its permeability
and forms minute flaky crystals in a similar way to and a 90% reduction in the seepage losses. The colloidal
montmorillonite. Some of the silicon is replaced by aggregates were dispersed by the exchange of bases, thus
aluminium, and atoms of potassium are attached, changing the physical properties of the clay and filling
giving a general formula of the type: the voids with a sticky gel which rendered it largely
K x Al 4 (Si 8 _xAlx)O20(OH)4, the value of x varying impervious to water. This treatment is the reverse of the
between 1.0 and 1.5. The OH-content may exceed 4, common agricultural process of adding calcium (in the
out of a total of 24 for O + OH. form of lime) to a heavy, sticky soil in order to improve
Sedimentary clays are mostly mixtures of illite and its working qualities.
kaolinite, with some montmorillonite, and shales have
illite as the dominant clay mineral. Illite is probably the
most widely distributed clay mineral in marine
argillaceous sediments. Non-silicate minerals
An early use (1940) in engineering of the property of
base-exchange in clays was to render impervious a leakv The more common, or economically important non-
clay lining to the artificial freshwater lake constructed at silicate minerals are listed in Table 4.9, each with
Table 4.9 Examples of important non-silicate minerals

Class Example H G Colour Streak Lustre Notes

NATIVE ELEMENTS

Copper Cu 2.5 8.9 red red metallic malleable

Silver Ag 2.5 10.5 grey-white white
GoIdAu 2.5 19.3 yellow yellow
Antimony Sb 3.5 6.7 grey-white grey sectile
Diamond C* 10 3.5 variable greasy sparkles
Graphite C# 1 2.2 black black metallic marks paper
Sulphur S 2 2.1 yellow white-yellow resinous odour on burning

Chalcocite Cu2S 2.5 5.7 grey-black black metallic blue-green tarnish

Bornite Cu5 Fe S4 3 5.1 brown grey iridescent
SULPHIDES

Galena PbS* 2.5 7.5 lead grey " (dull) cubic cleavage
Sphalerite ZnS* 3.5 4 dark brown brown resinous translucent
Chalcopyrite CuFeS2* 3.5 4.2 brass yellow black metallic iridescent
Covellite CuS 1.5 4.7 dark blue grey-black purple tarnish
Cinnabar HgS* 2.5 8.1 red red vitreous sectile
Pyrite FeS2* 6.5 5.1 brass yellow black metallic strikes fire with steel
Molybdenite MoS2* 1 4.6 grey grey marks paper
OXIDES & HYDROXIDES HALIDES

Halite NaCI* 2.5 2.2 variable white vitreous saline taste

Fluorite CaF2* 4 3.2 translucent

Cuprite Cu2O 3.5 6.1 red red metallic soluble in HCI

Corundum AI2O3 9 4 brown-blue vitreous v. hard
Haematite Fe2O3* 6 5.3 brown-black red metallic magnetic when heated
llmenite FeTiO3* 5.5 4.7 black black non-magnetic
Magnetite Fe3O4* 6 5.2 magnetic
Chromite FeCr2O4* 5.5 4.6 brown no cleavage
Cassiterite SnO2* 6.5 7 brown-black white-brown vitreous translucent
Goethite FeO(OH)* 5 4.4 brown brown dull crystalline
Bauxite AI2O3(2H2O)* 2 2.1 red-brown earthy, concretionary
Limonite2Fe2O3(3H2O)* 5 3.8 brown yellow-browr earthy

Calcite CaCO3* 3 2.7 colourless white vitreous effervesces in cold HCI

CARBONATES

Siderite Fe2CO3* 3.5 3.9 brown

Dolomite CaMg(CO3)2 3.5 2.9 white-brown effervesces in warm HCI
Aragonite CaCO3* 3.5 2.9 colourless stained with MnSO4
Witherite BaCO3 3.5 4.3 white-grey resinous rough striations common
Cerussite PbCO3 3 6.6 vireous effervesces in warm HNO3
Azurite Cu3(OH)2 (C0 3 ) 2 3.5 3.8 dark blue blue
Malachite Cu2(OH)2CO3* 3.5 4 bright green green silky-dull

Wolframite (FeMn)WO4* 4.5 7.3 black red-brown submetallic dull on fractures

Apatite Ca 5 (POJ 3 (FXlOH)* 5 3.2 green white vitreous sub-resinous also
SULPHATES

Barytes BaSO4* 3 4.5 colourless-white white vitreous tabular crystals common

Celestine SrSO4* 3.5 4 white-blue v. brittle
Anglesite PbSO4 3 6.3 white often with galena
Anhydrite CaSO4* 3 3 white-brown pearly splintery fracture
Gypsum CaSO42H2O* 2 2.3 colourless vitreous flexible, inelastic

T & P = Tungstates and phosphates.

H = Hardness in Mohs' scale.
G = Specific gravity.
* See text.
diagnostic characters that can be defined easily. A Sulphides
selection is briefly described here.
Compounds of the large atom sulphur, are important
Native elements ore minerals and often concentrated to form economic
accumulations of Cu, Zn and Pb. As shown in Table
Minerals in this class have one type of atom in their 4.9, they are relatively soft, heavy and dark. In thin
structure and can be metals (Cu, Ag, Au, Pt, Fe), semi- section they are opaque.
metals (As, Sb, Bi) and non-metals (C, S).
Galena
Diamond and graphite Generally with some contant of silver sulphide; when
The element carbon (C) is found in two crystalline enough silver is present to be worth extracting the term
forms: diamond (cubic), and graphite (hexagonal). 'argentiferous galena' is used. Galena occurs in lodes in
Amorphous carbon also occurs, as charcoal and soot. association with sphalerite (q.v.), calcite, quartz, and
Diamond and graphite have entirely different atomic other gangue minerals (p. 88), often filling fracture
structures (Fig. 4.39) which account for their different zones; or replacing limestone along joints as in the small
physical properties. The diamond structure permits occurrence in Derbyshire, Cumbria, and Cornwall,
carbon atoms to be linked in tetrahedral groups and the England. The well-known lode at Broken Hill, New
strength of this structure is reflected in the hardness of South Wales, yields argentiferous galena. At Leadville,
the mineral. Colorado, the mineral replaces limestone. Galena is the
chief ore of lead and an important source of silver.
Crystals are cubic with faces of the cube and octahedra:
perfect cleavage parallel to the cube faces.

Sphalerite (also called Zincblende, or Blende)

Blende is the principal ore of zinc, and often occurs in
lodes with galena and gangue minerals. Apart from the
small British examples (see galena), rich deposits are
found at Broken Hill, New South Wales; and at the
large Sullivan Mine, British Columbia, where the lead-
zinc ore-bodies are replacements in Precambrian
quartzites. The United States is the chief zinc-producing
country. Crystals are cubic, commonly as tetrahedra,
Fig. 4.39 The atomic structure of diamond (a) and graphite
(b). Carbon atoms in diamond are shown clear and those at
with perfect cleavage.
the centre of the tetrahedra are omitted for clarity. Carbon
atoms in graphite are shown black and the layers formed by Chalcopyrite (also called Copper pyrites)
them are shaded. Chalcopyrite may contain 34% Cu and is the principal
commercial source of copper. It is mainly formed in
association with igneous rocks especially during the late
stages of intrusion when Cu rich hydrothermal fluids
Diamond occurs as octahedral crystals in ultrabasic permeate the cooling magma and surrounding country-
rocks (e.g. serpentinite), as in volcanic pipes at rock (see p. 109). Important deposits occur world-wide
Kimberley, South Africa, and also in related rocks in but especially in fold belts, e.g. Norway (Caledonian),
Brazil and Russia. It is also found in alluvial deposits Cornwall, Rio Tinto in Spain (Hercynian), western
along with other hard minerals of high specific gravity. America (from Alaska to California) and in Japan.
Diamonds that are colourless or nearly so are valuable Crystals are tetragonal but cleavage is poor and the
as gems. In the form of granular aggregates (bort) they mineral normally occurs in a massive state.
are much used as abrasives, and for the cutting edges of
diamond drills, and in emery wheels. (Natural abrasives, Pyrite (also called Tool's gold')
in the order of their hardness, are diamond, corundum Pyrite occurs massive, and in ore-veins, and as
(Al2O3), emery (a grey-black variety of corundum nodules with a radiating structure; sometimes replacing
containing magnetite and haematite, and garnet.) calcite in fossil shells. Deposits of massive pyrite may
In graphite the carbon atoms are in layers and there is contain a small percentage of copper and gold. The
only a weak bonding between the layers; in consequence important deposits at Rio Tinto, Spain, are worked for
graphite is a very soft mineral and has important uses as sulphur and copper. Others in Norway, Sweden, Italy,
a lubricant. The mineral occurs in veins (as at Russia, and Japan are also worked on a large scale. By
Borrowdale, Lake District), or in lenticular patches in decomposition and oxidation, pyrite gives rise to
certain metamorphic rocks such as schists and gneisses sulphuric acid (H2SO4). The iron released during
(Sri Lanka); and in some crystalline limestones (e.g. in natural decomposition helps to form limonitic coatings
eastern Canada). on rock surfaces (p. 86). Crystals are cubic in the form
 bodies of salt water (see p. 127). Deposits are worked in
the Triassic beds of Cheshire; at Stassfurt, Germany; in
Ontario and Michigan, and in other countries.
Occurrences in salt-domes are discussed on p. 151.
Crystals are cubic with perfect cleavage parallel to the
faces of the cube (Fig. 4.19). Commonly colourless but
may be white, yellow or brown.

Fluorite (also called Fluorspar)

Fig. 4.40 (a) Pyrite (pyritohedron), note striations on faces The mineral occurs in hydrothermal veins, often
of the crystal, (b) Marcasite (orthorhombic form of FeS2), associated with blende and galena, or with tinstone
composition as for pyrite. (cassiterite, p. 86). Fluorspar is used in the manufacture
of hydrofluoric acid. A massive purple or blue variety
of cubes or pyritohedron (Fig. 4.40), normally with from Derbyshire is called Blue John. Crystals are cubic,
striated surfaces. commonly as cubes, sometimes zoned from green at
their centre through white to purple at their outermost
Pyrrhotite, describes the variety Fe n S n+1? where n is zone. Cleavage perfect and parallel to octahedron faces
between 6 and 11. Colour brownish or coppery; often (see Fig. 4,4a).
occurs massive, tarnishes on exposure. Streak, dark
greyish-black; lustre metallic. H = 3.5 to 4.5, G = 4.6 (for
Fe7S8). Magnetic (a distinction from pyrite), and Oxides and Hydroxides
soluble in HCl.
In the important pyrrhotite ore-deposits at Sudbury, These occur mainly as alteration products of sulphide
Canada, the mineral is accompanied by the nickel ores (see above) and as accessory minerals in igneous
sulphide pentlandite (Fe,Ni)S. These deposits yield the rocks (p. 100). The most remarkable oxide, SiO2 (silica
greater part of the world's nickel supply; the ore-bodies dioxide), is not included here as it is the fundamental
occur at the margins of a basic igneous mass (gabbro or component of silicate minerals and described on p. 77.
norite).
Haematite (also spelled Hematite)
Cinnabar An important ore of iron. The great haematite
The chief ore of mercury, usually the product of deposits of the Lake Superior area (the Mesabi and
volcanism as in the orogenic belts of California, Peru Marquette iron-ranges) contain 50-60% Fe, and are an
and at Almada in Spain. Commonly associated with extensive source of iron ore. At Hamersley, western
chalcopyrite, pyrite, quartz, opal and calcite. Crystals Australia, banded haematite and chert form an
are hexagonal but the mineral normally occurs in important economic deposit. Residual haematite
massive or granular form. deposits are found in Cuba; and in Brazil large
accumulations are metamophosed sedimentary ores. It
Molybdenite is also found in large or small 'pockets' in limestone, as
The chief ore of molybdenum in which two sheets of a replacement, as at Ulverston in Cumbria, England.
sulphur atoms sandwich a single sheet of molybdenum Haematite is the cementing material in many
atoms to produce a layered atomic structure similar to sandstones (p. 120), and is a very common cause of the
that of graphite (Fig. 4.39). This gives the mineral red staining seen in many rocks.
important lubricant properties. Found near igneous Crystals are Trigonal rhombohedral in form, often in
intrusions of granite and often associated with thin tabular forms, with brilliant metallic lustre; this
tourmaline. Important occurrences have been located in variety of haematite is called specular iron. Kidney ore is
Colorado, New Mexico, Norway, Queensland and New a massive, reniform variety (Fig. AAb) with an internal
South Wales. Crystals are hexagonal often in the form structure of radial fibres, and is the common form of red
of shiny scales. haematite.

Halides Hmenite
The chief ore of titanium. Occurs as an accessory
This class has many members, the most important mineral in basic igneous rocks; large, massive
often occurring as chemically deposited sediments segregations of the mineral are found in association with
formed by evaporation (p. 127) and as vein minerals in such rocks, as in Norway and Canada (Quebec and
igneous rocks. Ontario). Hmenite grains are found in many beach-
sands; important deposits of this type are worked in
Halite (also called Rock-salt) India (Travancore), Australia, Tasmania, Florida, and
Rock-salt occurs, together with gypsum and other elsewhere. Alters to white leucoxene, the presence of
salts, as a deposit from the evaporation of enclosed which distinguishes ilmenite from magnetite.
Magnetite Goethite produces the 'iron-hat', or gossan that mantles
An important iron ore. Occurs in small amount in a weathered exposure of sulphide ores (see p. 89). It may
many igneous rocks; when segregated into large masses form the iron-pan in bogs and tropical soils.
it forms a valuable ore of iron, as at Kiruna and
Gellivaare in north Sweden, and in the Urals. Lenses of Carbonates
magnetite are found in schists in the Adirondacks of
eastern United States, and elsewhere. Magnetite grains In this group, atoms of carbon are surrounded by
are commonly found in the heavy residues obtained three atoms of oxygen to form a planar triangle, the
from sands. Crystals are cubic (Fig. 4.4a) but magnetite resulting structure being arranged in sheets. Calcite
often occurs in massive form. consists of these carbonate sheets interlayered with
sheets of calcium which facilitate perfect cleavage. In
Chromite dolomite the calcium sheets also contain magnesium.
The only commercial source of chrome. Occurs as a Many hydrous carbonates are the weathered products
primary mineral in basic igneous rocks accumulating of other minerals.
during the early stages of magma crystallization. Being
extremely resistant to weathering it also accumulates as Calcite
a secondary mineral in sediments derived from the The principal source of carbonate of lime, invaluable
erosion of chromite-rich rocks. Principal sources are in to manufacturing and chemical industries. Limestones
the Ural Mountains, Zimbabwe, the Bush veld (S. are essentially composed of calcium carbonate, of which
Africa), Kiruna (Sweden), Turkey, Cuba and India. crystalline calcite may form a large part. Calcite also
Crystals are cubic but the mineral is normally massive. occurs as a secondary mineral in many igneous rocks,
e.g. in the amygdales of basalts. Open fractures in rocks
Cassiterite are often filled by calcite veins, which are recognized by
The most important source of tin. Found in acid their colour and distinguished from quartz veins by the
igneous rocks; in quartz veins and pegmatites (q.v.), much lower hardness. Calcite is commonly associated
often in association with tourmaline, topaz, and with sulphide ores such as blende and galena in mineral
fluorspar in granite areas, as in Cornwall, Saxony and veins (p. 109); it forms the material of stalactites and
Tasmania. Occurs also in alluvial deposits, as water- stalagmites in caverns (q.v.).
worn grains which can be recovered from many stream- Crystals are Trigonal (Figs. 4.4b and 4.7), often as
sands ('placer' deposits) in granite areas. Deposits of well-formed crystals. Generally colourless or white, but
this kind were worked extensively in Malaya for may have various tints. Cleavage is perfect, parallel to
commercial supplies of tin, but are now becoming the rhombohedral faces, and twinning is common on
exhausted. Similar deposits have been located at rhombohedral planes. H = 3; the ease with which the
Yunnan (China), Nigeria, Congo and Bolivia. Crystals mineral can be scratched with a knife affords a useful
are tetragonal and frequently twinned into 'knee- index to identification. Calcite dissolves in dilute acids,
shaped' or 'L-shaped' forms. including acid groundwater.
In thin section: Crystals in rocks often interlocking,
Bauxite irregular in shape, colourless, the rhombohedral
An important source of aluminium being a mixture of cleavage seen in some sections.
three minerals, gibbsite Al(OH3), diaspore AlO(OH) and R.I. = 1.658 (max.), 1.486 (min.); these values are
boehmite AlO(OH) which is the major constituent of respectively above and below the R.I. of balsam. In
bauxite. Generally produced by intense chemical consequence, the mineral shows changes in relief and
weathering of Al-rich silicate minerals in tropical strength of outline as it is rotated on the microscope
climates and may occur as residual deposits capping less stage (a twinkling effect).
weathered parent rock at depth, or as sedimentary Biref. = 0.172 (max.); polarization colours are whites
deposits derived from the erosion of such weathered or delicate pinks and greens. Lamellar twinning appears
profiles. Important reserves occur in Guiana, Jamaica, as bands of colour along the diagnonals of rhomb-
the southern states of N. America, southern Europe shaped sections.
especially France, Russia, India and Australia. Aragonite, orthorhombic, is another crystalline form
of calcium carbonate; it is less common and less stable
Limonite than calcite, and is often associated with gypsum, as in
The name limonite is used as afieldterm to denote eyaporites (p. 127).
hydrated oxides of iron which are poorly crystallized.
They are brown in colour, yellowish brown to yellow Siderite (also called Chalybite)
when earthy, and form coatings on rock joints and Siderite occurs in clay-ironstone beds and nodules in
weathered surfaces, and concretions. The principal the Coal Measures; formerly worked as a source of iron
constituent is goethite which results from the alteration in British coalfields. The mineral is also found in rocks
of Fe-bearing minerals, and in residual deposits may be where Fe-carbonate has replaced the original calcium
mixed with clay and other materials (cf. laterite, p. 38). carbonate of a limestone (p. 126) and occurs in many of
the marine sedimentary oolitic iron ores of Mesozoic the Z>-axis (Fig. 4M). Perfect cleavage is developed
age in Britain and continental Europe. In bog-iron ores parallel to the prism faces and basal plane, so that
siderite was precipitated direct from the water in lakes crystals break into flat diamond-shaped cleavage
(p. 129). Found as brown rhombhedral crystals fragments. Also occurs massive and in granular form.
(trigonal), and also massive.
Celestine
Malachite Principal source of strontium. Often occurs as an
A valuable ore of copper and an ornamental stone. evaporite, as at Yate in Bristol, and associated with
Produced by weathering of copper-rich minerals, usually halite and gypsum; found in salt domes (p. 151). Also
sulphides, by percolating ground-water. Famous occurs with sulphur deposits as in Sicily. Crystals are
deposits exist in the Urals, southern Zaire and northern orthorhombic, tabular (like those of barytes, Fig. 4.4d):
Zambia. Crystals are monoclinic but malachite normally often occurs in massive form.
occurs in botryoidal form (Fig. 4.1 a).
Gypsum
Tungstates and Phosphates Gypsum is formed chiefly by the evaporation of salt
water in shallow inland seas, the calcium sulphate in
Molybdates, Arsenates, and Vanadates may also be solution being precipitated, as at the southern end of the
considered with these classes. All generally occur in Dead Sea; extensive deposits of Permian age, hundreds
hydrothermalfluidsthat develop late in the of metres thick, are worked at Stassfurt in Germany.
crystallization of a granite magma and permeate the Gypsum is also formed by the decomposition of pyrite
cooling igneous intrusion. (FeS2) in the presence of calcium carbonate, e.g. crystals
of selenite found in the London Clay are due to this
Wolframite reaction. Gypsum is much used in the building industry
The principal ore of tungsten. Occurs in veins in the manufacture of plasters and plasterboard, and as
associated with the intrusion of granite and found a retarder of cement. Selenite is the transparent variety
world-wide, e.g. in Bolivia, Portugal and Britain (in St of gypsum. Alabaster is white or pink massive gypsum;
Austell, Cornwall, and Carrock in Cumbria). and the form known as satin-spar is composed of silky
Frequently associated with cassiterite, and like that fibres, occurring in veins.
mineral, resists weathering: often found concentrated in Crystals are monoclinicflattenedparallel to the side
alluvial deposits, as in Burma. Crystals are monoclinic pinacoid and often twinned (Fig. 4.8b). Perfect cleavage
and prismatic. Often occurs as massive or bladed forms. parallel to side pinacoid.
Apatite
Anhydrite
An important source of phosphorus. Apatite occurs Frequently associated with gypsum in evaporites
as a common accessory mineral in many igneous rocks, (p. 127), and also occurs in the cap-rock of salt domes
the small crystals being usually visible only with a (p. 151). Crystals are orthorhombic, tabular, white or
microscope. Large crystals are found in coarse-grained greyish in colour; Anhydrite expands on hydration.
veins (pegmatites, q.v.), which yield commercial supplies
from which phosphate is obtained for use as fertilizers,
as in Canada, Norway, and the Kola peninsula.
Mineral accumulations
Sulphates
The outer part of the lithosphere, the crust, is largely
Most sulphates form at low temperatures either during composed from only 13 of the 100 or so elements known,
weathering, especially of mineral veins, or by namely O, Si, Al, Fe, Ca, Na, K, Mg, Ti, P, H, C, and
evaporation in arid and semi-arid climates. Barytes is an Mn. Many of the elements of value to industry exist in a
exception to this general occurrence and forms in diffuse state within the crust and must be concentrated by
hydrothermal veins, often in association with galena, the natural processes of mineralization before they form
fluorite and quartz. a resource that can be worked economically. The product
of such concentrating processes reflects the chemical af-
Barytes (also called 'Heavy-spar') finities of the minerals involved and certain minerals typi-
Barytes is of common occurrence in ore-veins of lead cally occur together to form 'mineral associations' (e.g.
and zinc in association with galena, blende, fluorspar gold and silver, or copper, lead, zinc, gold and silver, or
and quartz, and such veins are worked in the north of nickel and copper). Such associations can be of value
England, in Germany, the United States, and in many when exploring for minerals, as a mineral difficult to find
other countries. The mineral also occurs alone (or with may be located by tracing its more easily identified associ-
calcite) in veins traversing limestones; and as a cement ates. The natural processes that concentrate minerals of
in certain sandstones (p. 119). economic value are here described after certain com-
Crystals are orthorhombic and elongated parallel to monly used terms are defined.
Ore mineral
A desired metal generally bound with other elements,
e.g. chalcopyrite, Cu Fe S2 (80% metal).

Gangue mineral
A non-metallic mineral e.g. quartz, calcite mixed with
ore minerals. Many gangue minerals are valuable re-
sources and are mined, e.g. fluorite, apatite, arytes and
gypsum.

Ore
All ore mineral and gangue that has to be mined.
Hypogene mineral Lead-zinc
One formed with the ore and not from the later alter- Lead
ation of existing ore: may be considered as primary. Copper
Tin
Supergene mineral
Formed by the alteration of existing minerals: may be Diamond
considered as secondary. Gold

Chrome
Tenor
The metal content of an ore (also called the grade)
expressed either as a percentage or a weight of ore (g per Fig. 4.41 Distribution of some major mineral deposits in
tonne). The higher the tenor, the richer the ore. Africa. Stippled areas have been stable for the past 1.5 x 109
years. Other areas have been deformed during the past 1.2 x
109 years. (Data from Clifford 1971.)
The concentration of minerals
Deposits of ore and other valuable non-metals are formed specific gravity (such accumulations are called placer de-
by the processes associated with igneous activity, sedi- posits); by the precipitation of minerals in solution during
mentation and metamorphism, and examples of them are the evaporation of water (to produce evaporites) and
described in Chapters 5, 6 and 7. Igneous rocks such as through precipitation promoted by the activity of aerobic
basalt, whose origin is closely related to mantle magmas and anaerobic bacteria. The former live in the presence
at the base of the lithosphere, may bring to the near of oxygen and are thought to be responsible for many
surface metals rare in the crust. Other igneous rocks, sedimentary deposits of iron; the latter live in environ-
especially granites, form at depth in the lithosphere. They ments devoid of oxygen and precipitate sulphides and
are produced by heat and pressure which create chemical sulphur. Sedimentary processes are also responsible for
gradients that may affect hundreds of cubic kilometres of the burial of organic matter from which are derived coal,
country rock. Such gradients encourage the mobilization oil and petroleum gas. Many sedimentary deposits extend
of elements and their reassembly in concentrated form as laterally for considerable distances unless geometrically
new minerals, some of which are valuable. Such migration condensed by later folding and faulting.
from surrounding ground to particular centres creates In all these processes the migration and concentration
metallogenic provinces, e.g. the Canadian Shield gold belt of valuable substances is intimately associated with the
extending 3200 km from Quebec to the Great Slave Lake; movement of water in the crust, as a hot hydrothermal
the Chilean copper belt; the lead-zinc-tin province of fluid, or as a low temperature brine, or as a vapour. The
Cornwall; the pyrite belt of Portugal, Spain and Central position and shape of many valuable deposits has been
Europe (p. 23); the silver-lead-zinc belt of Broken Hill in governed by the former pathways available forfluidflow,
Australia. Numerous provinces can be identified in Africa especially faults, joints and other fractures, and the pres-
and are illustrated in Fig. 4.41. The location of many such ence of impermeable rock that acted as a barrier to such
provinces appears to be related to the position of plate movement (see Figs 5.27 and 5.28).
boundaries, and the crustal deformation that has oc-
curred or is occurring along them (see Chapters 1 and 2). The search for minerals
Many ores of igneous origin extend to considerable
depth (p. 109 and p. 110). Ores so formed are enriched by Most near surface deposits reveal their presence by the
chemical weathering and some of the richest deposits colour of soil and nature of vegetation developed upon
have developed in this way (Fig. 4.42). them (e.g. the iron-hat of sulphide veins, Fig. 4.42), and
Sedimentary processes concentrate valuable metals have been located by prospectors. Nowadays the deeper
and non-metals by the mechanical sorting of minerals reserves must be found that are hidden from sight. To
weathered from existing deposits, according to their locate these large areas of country, or sea, are systemati-
 Gossan HoO Rainwater

H 2 SO 4 Pyrite (FeS 2 )
Leached zone
Fe 2 (SO 4 J 3

OXIDATION
(= solvents)

Cuprite
Secondary oxide
enrichment Malachite

WATER TABLE Au. Ag.

precipitants
Chalcocite

Secondary sulphide Bornite

enrichment
Ag.
COUNTRY
ROCK
Pyrite, Chalcopynte,
Unaltered primary
sulphide ore Galena , Silver, Gold,
Cassiterite , Sphalerite

Fig. 4.42 Secondary enrichment of primary sulphide vein by weathering. Rainwater reacts with oxidized pyrite and creates
ferric sulphate and sulphuric acid which leach the vein leaving insoluble iron hydroxides (limonite) as a gossan, or iron-hat.
Botryoidal precipitates accumulate in the zone of oxide enrichment filling fractures and other voids. This zone is terminated
by the water table. Below the water table primary sulphides are progressively replaced in order of solubility. Examples of
precipitants and primary ore minerals are shown: many other minerals also exist in such veins. The insets illustrate diagram-
matically the microscopic appearance of samples from the zones.

cally searched to identify areas which exhibit a noticeable Stream Stream

change in some geophysical or geochemical character.
For example, the magnetic field of the Earth follows a Vein beneath
smooth path along the lines of longitude from pole to weathered
pole unless deflected from this path by the presence of profile

other magnetic sources such as metallic ores. Hence an- ORE AT

omalies in the magnetic field of a land area may indicate DEPTH
the presence of ore at depth. Much initial surveying for
land resources can be conducted from the air, including
magnetic surveying, and utilizes air photographs and re-
motely sensed images.
Areas where anomalies have been identified or where,
Stream
for other reasons, ores are suspected to exist, are searched
in detail using ground surveys. These have the following Position of
objectives. stream sample
Bedrock
(0 To map the geology of the area.
(U) To improve the map with results from detailed geo-
physical surveys (see Table 10.3).
(Hi) To sample the area, collecting specimens for miner- Fig. 4.43 Airborne regional geophysical surveys identify an
alogical analyses. Panning stream deposits in the anomaly in the area of two streams. Stream sampling
traditional manner of the old prospector is one reveals appropriate minerals ( • = minerals present x =
method of sampling and remains an essential part of absent) and locates the northern limit (L) of a possible vein
(V) coming from the ore body. Sampling on the A-A' traverse
many surveys. Alluvium, colluvium and soil profiles locates the vein. Deep drilling follows.
on hillsides will reveal the presence of thin veins of
ore that are hidden beneath a weathered profile and
whose location may indicate richer reserves at onto mineral surfaces in the soil profile, absorbed
greater depths (Fig. 4.43). into plants or lost to the atmosphere. The analyses
(iv) To collect samples for chemical analyses. This is of soil samples, of leaves and of gases in soil can
geochemical prospecting and is an exploration tech- reveal the presence of ore at depth.
nique of equal importance as geophysical prospect- Off-shore a different approach is required and surveys
ing. Many elements concentrated in ores at depth rely heavily upon seismic geophysical surveys and sam-
can migrate to the surface as gas, to be adsorbed pling with dredgers.
 Eventually promising areas are drilled to confirm pred- Dixon, C J . (1979). Atlas of Economic Mineral Deposits.
ictions. Once a deposit has been located it must be eval- Chapman Hall, London.
uated before being accepted as an economically workable Bates, R. L. (1969). Geology of the Industrial Rocks and
resource. Minerals. Dover, New York.
Evans, A.M. (1980). An Introduction to Ore Geology.
Blackwell Scientific Publishers, Oxford.
Peters, W. C. (1978). Exploration and Mining Geology.
Selected bibliography John Wiley and Sons, New York.
Reedman, J.H. (1979). Techniques in Mineral Explora-
Battey, M. H. (1972). Mineralogy for Students. Oliver and tion. Applied Science Publishers, London.
Boyd, Edinburgh. Parasnis, D.S. (1973). Mining Geophysics. Elsevier Pub-
Mackenzie, W.S. and Guilford, C. (1980). Atlas of Rock lishing Company, Amsterdam.
Forming Minerals in Thin Section. Longman, Lon- Read, H. H. (1970). Rutley 's Elements of Mineralogy, 26th
don. edition. George Allen and Unwin, London.