Rafferty J.P. (ed.) Minerals

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7 Minerals 7

frequently applied to all minerals included in a rock but

more appropriately should be used for those minerals

that are in equilibrium (and are known more speciﬁcally

as the equilibrium assemblage). The granite discussed

above may display surﬁcial cavities that are lined by sev-

eral clay minerals and limonite (a hydrous iron oxide). The

original high-temperature granite was altered to form

the low-temperature clay minerals and limonite; there

are consequently two distinct assemblages present in

the rock: the high-temperature orthoclase-albite-quartz-

biotite assemblage and the low-temperature assemblage

of clay minerals and limonite.

Metamorphic rocks also may contain separate assem-

blages. A shale that at low temperatures was composed

of a sericite-kaolin-dolomite-quartz-feldspar assemblage

can become metamorphosed at higher temperatures to

produce a garnet-sillimanite-biotite-feldspar assemblage.

An assemblage thus consists of minerals that formed

under the same or quite comparable conditions of pres-

sure and temperature. In practice, minerals that physically

touch one another with no reaction rims or alteration

products are included in the assemblage. It is likely that

the minerals satisfying these conditions are in equilibrium,

but additional chemical tests are commonly necessary to

deﬁne the equilibrium assemblage without ambiguity.

Phase systems are governed by a phase rule, which

deﬁnes the number of minerals that may coexist in equi-

librium: F = C - P + 2, where F is the variance, or number

of degrees of freedom, C is the number of independent

components, and P is the number of phases. Applying

this rule to a three-phase, three-component system, F

is 2. This indicates that two parameters—e.g., pressure

and temperature—may be varied independently of one

another without altering the number of phases.

Phase diagrams

Phase (or stability) diagrams are used to illustrate the

conditions under which certain minerals are stable. The

following are examples of phase diagrams employed in the

study of igneous, metamorphic, and sedimentary rocks.

use in Igneous Petrology

In the ﬁeld of igneous petrology, the researcher com-

monly employs a phase equilibrium approach to

compare the mineral assemblages found in naturally

occurring and synthetic rocks. Much can be learned

from studying the melting of an igneous rock and the

reverse process, the crystallization of minerals from a

melt (liquid phase). Graphic representations of systems

with a liquid phase are called liquidus diagrams. The

dashed contours on such diagrams, which are called

isotherms, represent temperatures at which a mineral

melts. They deﬁne what is known as a liquidus surface.

As temperatures decrease, the minerals will crystallize

in the manner deﬁned by the arrows on the boundaries

separating the different mineral phases. A careful study

of the crystalline products formed upon the cooling of

melts of speciﬁc compositions allows the igneous petrol-

ogist to compare such results with minerals observed in

natural igneous rocks.

use in Metamorphic Petrology

ressure-temperature (P-T) phase diagrams are applied

in the study of the conditions under which metamorphic

rocks originate. They illustrate the equilibrium relation-

ships among various mineral phases in terms of pressure

and temperature. The minerals that are separated by a

reaction curve may exist in equilibrium at the conditions

7 Mineral classification and Associations 7

7 Minerals 7

occurring along the line. The reaction curves for Al

SiO

and for muscovite + quartz ←→ potassium-feldspar + sil-

limanite + H

O are signiﬁcant in metamorphic rocks that

have a high aluminum oxide (Al

) content as compared

to other components (e.g., calcium oxide [CaO], mag-

nesium oxide [MgO], and ferrous oxide [FeO]). Shales

enriched in clay minerals contain a rather large amount of

aluminum oxide, and during metamorphism of the shale

mineral reactions and recrystallization occur. In their

metamorphic form, shales appear as pelitic schists, and

these may include signiﬁcant amounts of sillimanite, mus-

covite, and quartz.

Theoretical calculations are combined with experi-

mental observations to arrive at phase diagrams. In

laboratory experiments conducted on the three poly-

morphs of Al

SiO

, chemicals of high purity are most

often used as the starting materials, but extremely pure

minerals may be substituted. A specimen of gem-grade

kyanite that does not contain any inclusions may be

reacted at high temperatures to form sillimanite. The

placement of the reaction curve between the kyanite and

sillimanite ﬁelds is determined by the ﬁrst instance of sil-

limanite formation from kyanite and also the initial stage

of the reverse reaction. X-ray powder diffraction and opti-

cal microscopic techniques are employed to estimate the

conditions under which these reactions commence, but

the experimental methods are subject to some degree

of uncertainty. Therefore, the reaction curves, although

commonly drawn as narrow lines, may actually represent

wider reaction zones. Also, the naturally occurring sys-

tem is more complex than the simpliﬁed version devised

in the laboratory, and so difﬁculty arises when attempting

to relate the two. A P-T diagram serves mainly as a tool in

evaluating the conditions of metamorphism, such as pres-

sure and temperature.

EH–PH DiAGRAMS

Eh–pH diagrams illustrate the ﬁelds of stability of mineral or

chemical species in terms of the activity of hydrogen ions (pH)

and the activity of electrons (Eh). Consequently, the reactions

illustrated on Eh–pH diagrams involve either proton transfer (e.g.,

hydrolysis) or electron transfer (oxidation or reduction) or both.

In natural environments, pH values extend from 1 to 9.5, and Eh

values from -500 to +800 millivolts. Rarely are temperatures and

pressures other than those normally encountered on the Earth’s

surface considered.

The area on an Eh–pH diagram that represents the range of these

variables within which a particular mineral is stable is called the stabil-

ity ﬁeld of that mineral. Such a representation enables a geochemist

to determine whether a mineral is in equilibrium with its surround-

ings or subject to chemical transformation.

use in Sedimentary Petrology

Phase diagrams can also be helpful in the assessment of

physical and chemical conditions that prevailed dur-

ing the deposition of a chemical sedimentary sequence.

Atmospheric conditions are characterized by low temper-

atures and pressures, and under such conditions stability

ﬁelds of minerals can often conveniently be expressed in

terms of Eh (oxidation potential) and pH (the negative

logarithm of the hydrogen ion concentration [H

]; a pH of

0–7 indicates acidity, a pH of 7–14 indicates basicity, and

neutral solutions have a pH of 7).

A high Eh value corresponds to a compound stable

under oxidizing conditions, while a low Eh value indi-

cates a mineral that occurs in reducing environments.

For example, pyrite and pyrrhotite, two sulﬁde miner-

als, occur at low Eh values and at pH values of 4–9. Lines

separating the ﬁelds of an Eh-pH diagram represent

conditions under which the two minerals may exist in

equilibrium. Hematite and magnetite, for example, are

7 Mineral classification and Associations 7

7 Minerals 7

often found together in iron-bearing sediments. Eh-pH

diagrams are valuable in providing information regard-

ing the chemical and physical environments that existed

during atmospheric weathering and during chemical

sedimentation and diagenesis of sediments deposited by

water at temperatures of 25 to about 100 °C (77 to 212 °F)

and approximately one atmosphere pressure. The coexis-

tence of hematite and magnetite common in Precambrian

iron-bearing rocks (those formed from 4.6 billion to 542

million years ago) may enable investigators to estimate

variables such as Eh and pH that prevailed in the original

ancient sedimentary basin.

mineral deposit is the name for an aggregate of a

mineral in an unusually high concentration. About

half of the known chemical elements possess some metal-

lic properties. The term metal, however, is reserved for

those chemical elements that possess two or more of the

characteristic physical properties of metals (opacity, duc-

tility, malleability, fusibility) and are also good conductors

of heat and electricity. Approximately 40 metals are made

available through the mining and smelting of the minerals

in which they occur.

Certain kinds of mineral can be smelted more read-

ily than others; these are commonly referred to as ore

minerals. Ore minerals tend to be concentrated in small,

localized rock masses that form as a result of special geo-

logic processes, and such local concentrations are called

mineral deposits. Mineral deposits are what prospec-

tors seek. The terms ore mineral and mineral deposit were

originally applied only to minerals and deposits from

which metals are recovered, but present usage includes

a few nonmetallic minerals, such as barite and ﬂuorite,

that are found in the same kinds of deposit as metallic

minerals.

No deposit consists entirely of a single ore mineral.

There are always admixtures of valueless minerals, col-

lectively called gangue. The more concentrated an ore

mineral, the more valuable the mineral deposit. For

every mineral deposit there is a set of conditions, such

as the level of concentration and the size of the deposit,

that must be reached if the deposit is to be worked at a

proﬁt. A mineral deposit that is sufﬁciently rich to be

chapter 3

MIneral deposIts

7 Minerals 7

worked at a proﬁt is called an ore deposit, and in an ore

deposit the assemblage of ore minerals plus gangue is

called the ore.

All ore deposits are mineral deposits, but the reverse

is not true. Ore deposit is an economic term, while mineral

deposit is a geologic term. Whether a given mineral deposit

is also an ore deposit depends on many factors other than

the level of concentration and the size of the deposit; all

factors that affect the mining, processing, and transport-

ing of the ore must be considered as well. Among such

factors are the shape of a deposit, its depth below the sur-

face, its geographic remoteness, access to transportation,

the political stability of the region, and market factors such

as the price of the metal in world trade and the costs of

borrowing the money needed to develop a mine. Because

market factors change continually, a given mineral deposit

may sometimes be an ore deposit, but at other times it may

be uneconomic and hence not an ore deposit.

Mineral deposits have been found both in rocks that lie

beneath the oceans and in rocks that form the continents,

although the only deposits that actually have been mined

are in the continental rocks. (The mining of ocean depos-

its lies in the future.) The continental crust averages 35–40

km (20–25 miles) in thickness, and below the crust lies the

mantle. Mineral deposits may occur in the mantle, but with

present technology it is not possible to discover them.

GEOCHEMiCAlly A BUNDANT

AND SCARCE META lS

Metals used in industrial and technological applications

can be divided into two classes on the basis of their abun-

dance in the Earth’s crust. The geochemically abundant

metals, of which there are ﬁve (aluminum, iron, magne-

sium, manganese, and titanium), constitute more than

0.1 percent by weight of the Earth’s crust, while the geo-

chemically scarce metals, which embrace all other metals

(including such familiar ones as copper, lead, zinc, gold,

and silver), constitute less than 0.1 percent. In almost

every rock, at least tiny amounts of all metals can be

detected by sensitive chemical analysis. However, there

are important differences in the way the abundant and

scarce metals occur in common rocks. Geochemically

abundant metals tend to be present as essential con-

stituents in minerals. For example, basalt, a common

igneous rock, consists largely of the minerals olivine

and pyroxene (both magnesium-iron silicates), feldspar

(sodium-calcium-aluminum silicate), and ilmenite (iron-

titanium oxide). Careful chemical analysis of a basalt will

reveal the presence of most of the geochemically scarce

metals too, but no amount of searching will reveal min-

erals in which one or more of the scarce metals is an

essential constituent.

Geochemically scarce metals rarely form minerals in

common rocks. Instead, they are carried in the structures

of common rock-forming minerals (most of them silicates)

through the process of atomic substitution. This process

involves the random replacement of an atom in a min-

eral by a foreign atom of similar ionic radius and valence,

without changing the atomic packing of the host min-

eral. Atoms of copper, zinc, and nickel, for example, can

substitute for iron and magnesium atoms in olivine and

pyroxene. However, since substitution of foreign atoms

produces strains in an atomic packing, there are limits

to this process, as determined by temperature, pressure,

and various chemical parameters. Indeed, the substitution

limits for most scarce metals in common silicate minerals

are low—in many cases only a few hundred substituting

atoms for every million host atoms—but even these limits

are rarely exceeded in common rocks.

7 Mineral deposits 7

7 Minerals 7

One important consequence that derives from the

way abundant and scarce metals occur in common rocks

is that ore minerals of abundant metals can be found in

many common rocks, while ore minerals of scarce metals

can be found only where some special, restricted geologic

process has formed localized enrichments that exceed the

limits of atomic substitution.

ORE MiNERAlS

Two factors determine whether a given mineral is suitable

to be an ore mineral. The ﬁrst is the ease with which a min-

eral can be separated from the gangue and concentrated

for smelting. Concentrating processes, which are based on

the physical properties of the mineral, include magnetic

separation, gravity separation, and ﬂotation. The second

factor is smelting—that is, releasing the metal from the

other elements to which it is chemically bonded in the

mineral. Smelting processes are discussed below, but of

primary importance in this consideration of the suitability

of an ore mineral is the amount of energy needed to break

the chemical bonds and release the metal. In general, less

energy is needed to smelt sulﬁde, oxide, or hydroxide min-

erals than is required to smelt a silicate mineral. For this

reason, few silicate minerals are ore minerals. Because the

great bulk of the Earth’s crust (about 95 percent) is com-

posed of silicate minerals, sulﬁde, oxide, and hydroxide ore

minerals are at best only minor constituents of the Earth’s

crust—and in many cases they are very rare constituents.

The preferred ore minerals of both geochemically abun-

dant and geochemically scarce metals are native metals,

sulﬁdes, oxides, hydroxides, or carbonates. In a few cases,

silicate minerals have to be used as ore minerals because

the metals either do not form more desirable minerals or

form desirable minerals that rarely occur in large deposits.

native Metals

Only two metals, gold and platinum, are found principally

in their native state, and in both cases the native metals

are the primary ore minerals. Silver, copper, iron, osmium,

and several other metals also occur in the native state,

and a few occurrences are large enough—and sufﬁciently

rich—to be ore deposits. One example is the rich deposits

of native copper in the Lake Superior area of Michigan in

the United States. Here the copper occurs in interbedded

conglomerates (a sedimentary rock consisting of pebbles

and boulders) and basaltic lava ﬂows, most of which are

vesicular and have fragmental layers of basaltic rubble

on top of each ﬂow. In the basalt, the native copper and

associated gangue minerals ﬁll the vesicles and cavities in

the rubble; in the conglomerate, the copper ﬁlls spaces

between the pebbles and in part replaces some of the

smaller rock fragments. Originally discovered and worked

by native Americans who manufactured and traded orna-

ments of malleable copper, the great Michigan copper

deposits were ﬁrst mined in 1845 and continued in pro-

duction for more than a century.

Sulﬁdes

The largest group of ore minerals consists of sulﬁdes.

Because the physical and chemical properties of telluride,

selenide, and arsenide minerals are very similar to those

of sulﬁde minerals, and because these minerals tend to

occur together, it is convenient to use the term sulﬁde to

embrace these similar minerals. Copper, lead, zinc, nickel,

molybdenum, silver, arsenic, antimony, bismuth, cobalt,

and mercury all form sulﬁde ore minerals. Gold and silver

form tellurides under certain circumstances, and platinum

forms an important arsenide ore mineral.

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