Rafferty J.P. (ed.) Minerals

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132

Minerals 7

Riebeckite (of the crocidolite variety) from South Africa. © Rodolfo Crespi

in colour. The tremolite-ferroactinolite series ranges from

white to dark green with increasing iron content. The ﬁnely

ﬁbrous and massive variety of actinolite-tremolite known

as nephrite jade ranges from green to black. Common horn-

blende is typically black. Glaucophane and riebeckite are

usually blue. Anthophyllite is gray to various shades of green

and brown. The cummingtonite-grunerite series occurs in

various shades of light brown. Iron-free varieties of tremo-

lite containing manganese can have a lavender colour.

The common crystallographic habit of amphiboles is

acicular or prismatic; however, most of the amphiboles

are also known to crystallize in the asbestiform habit.

The asbestiform variety of riebeckite is called crocido-

lite or blue asbestos. Amosite is a rare asbestiform variety

of grunerite, named from the company Amos (Asbestos

Mines of South Africa). The most important commercial

asbestos material is chrysotile, the asbestiform variety of

serpentine.

In thin sections, amphiboles are distinguished by sev-

eral properties, including two directions of cleavage at

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approximately 56° and 124°, six-sided basal cross sections,

characteristic colour, and pleochroism (colour variance

with the direction of light propagation). Orthorhombic

amphiboles exhibit less intense pleochroism than the

monoclinic amphiboles.

origin and occurrence

Exhibiting an extensive range of possible cation sub-

stitutions, amphiboles crystallize in both igneous and

metamorphic rocks with a broad range of bulk chemical

compositions. Because of their relative instability to chem-

ical weathering at the Earth’s surface, amphiboles make up

only a minor constituent in most sedimentary rocks.

Igneous rocks

Calcic amphiboles

are characteristically contained in

igneous rocks. Igneous amphiboles are intermediate in

composition between tremolite, tschermakite, edenite,

and pargasite end-members. Typically, these amphiboles

of intermediate composition are called hornblende.

Hornblende occurs in various plutonic igneous rocks,

including diorites, quartz diorites, and granodiorites. It

also occurs as phenocrysts in andesite lavas that contained

enough water for amphiboles to form. Hastingsite is found

in granites and alkali-rich intrusives such as syenites. The

alkali amphiboles riebeckite and arfvedsonite are found

most commonly in granites, syenites, nepheline syenites,

and related pegmatites. Richterite occurs as a hydrother-

mal product and in veins in alkaline igneous rocks.

cont

act Metamorphic

rocks

Amp

hiboles occur in contact metamorphic aureoles

around igneous intrusions. (An aureole is the zone sur-

rounding an intrusion, which is a mass of igneous rock

7 the Silicates 7

7 Minerals 7

134

that solidiﬁed between other rocks located within the

Earth.) The contact aureoles produced in siliceous lime-

stones and dolomites, called skarns or calc-silicate rocks,

characteristically contain metamorphic amphiboles

such as tremolite or actinolite. The presence of tremolite

implies a relatively low grade of metamorphism as trem-

olite breaks down to form the pyroxene diopside in the

presence of calcite and quartz at elevated temperatures.

Richterite-winchite occurs in hydrothermally metamor-

phosed limestones. Magnesium-rich anthophyllites are

found along contact zones of granitic dikes intruding

ultramaﬁc rocks (those rich in iron and magnesium).

regional Metamorphic rocks

Man

y different amphiboles may be contained in regional

metamorphic rocks. Commonly several amphiboles may

coexist with one another in the same sample, depending

on the bulk chemistry of the rock and on the pressure and

temperature of metamorphism. The amphiboles typically

occur with plagioclase feldspar, quartz, and biotite, as

well as with chlorite and oxide minerals. In magnesium-

rich rocks, tremolite, anthophyllite, and hornblende may

exist together. Gedrite and cummingtonite coexist with

garnet in rocks enriched in aluminum and iron. Rocks

containing cummingtonite or grunerite are characteris-

tic of metamorphosed iron formations associated with

iron oxides, iron-rich sheet silicates, carbonates, and

quartz. Glaucophane occurs only in such metamorphic

rocks as schist, eclogite, and marble. Glaucophane asso-

ciated with jadeite, lawsonite, and calcite or aragonite

is the characteristic assemblage found in high-pressure,

low-temperature metamorphic rocks called blueschists,

which have a blue colour imparted by the glaucophane.

Blueschists have basaltic bulk compositions and may

also contain riebeckite. The latter also may occur in

135

regional metamorphic schists. Tremolite-actinolite and

the sheet-silicate chlorite are the principal minerals in

the low-to-moderate temperature and pressure green-

schist metamorphic rocks. Hornblende is characteristic

of some medium-grade metamorphic rocks known as

amphibolites, in which hornblende and plagioclase are

the major constituents.

Dehydration of amphiboles in the lower crust or

mantle may be an important source of water that aids in

the generation of magmas from partial melting processes.

FElDSPARS

Feldspars constitute a group of aluminosilicate minerals

that contain calcium, sodium, or potassium. Feldspars

make up more than half the Earth’s crust, and professional

literature about them constitutes a large percentage of the

literature of mineralogy.

Of the more than 3,000 known mineral species, less

than 0.1 percent make up the bulk of the Earth’s crust and

mantle. These and an additional score of minerals serve

as the basis for naming most of the rocks exposed on the

Earth’s surface.

Each of the common rock-forming minerals can be

identiﬁed on the basis of its chemical composition and

its crystal structure (i.e., the arrangement of its constitu-

ent atoms and ions). The nonopaque minerals can also

be identiﬁed by their optical properties. Fairly expensive

equipment and sophisticated procedures, however, are

required for such determinations. Therefore, it is for-

tunate that macroscopic examination, along with one

or more tests, are sufﬁcient to identify these minerals

as they occur in most rocks. The following descriptions

include basic chemical and structural data and the prop-

erties used in macroscopically based identiﬁcations.

7 the Silicates 7

7 Minerals 7

136

Optical data, which are not included in these descrip-

tions, are available in mineralogy books.

Two important rock-forming materials that are not

minerals are major components of a few rocks. These are

glass and macerals. Glass forms when magma (molten

rock material) is quenched—i.e., cooled so rapidly that

the constituent atoms do not have time to arrange them-

selves into the regular arrays characteristic of minerals.

Natural glass is the major constituent of a few volcanic

rocks—e.g., obsidian. Macerals are macerated bits of

organic matter, primarily plant materials; one or more of

the macerals are the chief original constituents of all the

diverse coals and several other organic-rich rocks such as

oil shales.

In the classiﬁcation of igneous rocks of the

International Union of Geological Sciences (IUGS), the

feldspars are treated as two groups: the alkali feldspars

and the plagioclase feldspars. The alkali feldspars include

orthoclase, microcline, sanidine, anorthoclase, and the

two-phase intermixtures called perthite. The plagioclase

feldspars include members of the albite-anorthite solid-

solution series. Strictly speaking, however, albite is an

alkali feldspar as well as a plagioclase feldspar.

chemical composition

All the rock-forming feldspars are aluminosilicate min-

erals with the general formula AT

in which A =

potassium, sodium, or calcium (Ca); and T = silicon (Si)

and aluminum (Al), with a Si:Al ratio ranging from 3:1 to

1:1. Microcline and orthoclase are potassium feldspars

(KAlSi

), usually designated Or in discussions involving

their end-member composition. Albite (NaAlSi

—

usually designated Ab) and anorthite (CaAl

—An)

are end-members of the plagioclase series. Sanidine,

137

anorthoclase, and the perthites are alkali feldspars

whose chemical compositions lie between Or and Ab.

As is apparent from the preceding statements, solid

solution plays an important role in the rock-making

feldspars. (Members of solid-solution series are single

crystalline phases whose chemical compositions are inter-

mediate to those of two or more end-members.) The

alkali (Or-Ab) series exhibits complete solid solution at

high temperatures but only incomplete solid solution at

low temperatures; substitution of potassium for sodium

is involved. The plagioclase (Ab-An) series exhibits

essentially complete solid solution at both high and low

temperatures; coupled substitution of sodium and silicon

by calcium and aluminum occurs. The An-Or system has

only limited solid-solution tendencies.

The most obvious differences between the high- and

low-temperature diagrams are along the alkali-feldspar

(Or-Ab) join (the boundary line between the phases). As

indicated, sanidine and anorthoclase are high-temperature

alkali feldspars, and perthite is their low-temperature ana-

logue. Sanidine is a single-phase alkali feldspar; although

frequently described chemically by the formula (K, Na)

AlSi

, most analyzed specimens of sanidine range

between Or

and Or

. (This designation is used to specify

the fractions of the constituents. For example, Or

indi-

cates that the mineral is composed of 80 percent KAlSi

and 20 [i.e., 100 - 80] percent NaAlSi

.) Anorthoclase is a

variously used name that is most often applied to apparently

homogeneous alkali feldspar masses, at least some of which

consist of submicroscopic lamellae (layers) of albite and

orthoclase; their bulk compositions typically range between

and Or

. Perthite consists of intimate intermixtures

of a potassium feldspar—either microcline or orthoclase—

and a sodium-rich plagioclase that occurs as microscopic to

macroscopic masses within the potassium feldspar host.

7 the Silicates 7

7 Minerals 7

138

Many perthites are formed when high-temperature

potassium-sodium feldspars of appropriate compositions

are cooled in such a manner that the original solid-solution

phase exsolves (i.e., unmixes, so that a homogeneous min-

eral separates into two or more different minerals) to

form intermixtures—sometimes termed intergrowths—

of two phases.

Some perthites, however, appear to have been formed

as a result of partial replacement of original potassium

feldspars by sodium-bearing ﬂuids. In any case, perthite is

the name properly applied to intimate mixtures in which

the potassium feldspar component predominates over the

plagioclase constituent, whereas antiperthite is the name

given to intimate mixtures in which the plagioclase con-

stituent is predominant. Perthites are common, whereas

antiperthites are relatively rare.

The plagioclase series is essentially continuous at both

high and low temperatures. The names of members of the

series designate relative proportions of the end-members.

Although plagioclase grains in some rocks are essentially

homogeneous, those in many rocks are zoned—i.e., dif-

ferent parts of individual grains have different Ab and

An contents. One explanation for zoning in plagioclases

formed from magmas can be implied from information

known about the Ab-An system. Upon cooling, the ﬁrst

crystals that form from a melt with the composition X (=

) will have the composition y (approximately An

With further cooling, in some cases the ﬁrst and subse-

quently formed crystals will react continuously with the

remaining liquid, thereby maintaining equilibrium; when

the liquid becomes totally crystallized, the system will

consist of homogeneous plagioclase crystals. In cases in

which such equilibrium is not maintained during cooling,

the early and subsequently formed feldspars have differ-

ent An contents. For example, zoned crystals may form

139

with differing An contents arranged one on top of another

so that their margins are relatively sodium-rich as com-

pared to their earlier-formed, more calcium-rich cores.

The resulting zoning may be gradational or well-deﬁned

or may assume some combination of these characteristics.

Many elements other than those required for the Or,

Ab, and An end-member compositions have been recorded

in analyses of feldspars. Those that have been recorded

to occur as substitutions within the feldspar structures

include lithium (Li), rubidium (Rb), cesium (Cs), mag-

nesium (Mg), strontium (Sr), barium (Ba), yttrium (Y),

ferrous iron (Fe

), thallium (Tl), lead (Pb), lanthanum (La)

and other rare earth elements, and ammonium (NH

) in

the A position; and titanium (Ti), ferric (Fe

) and ferrous

(Fe

) iron, boron (B), gallium (Ga), germanium (Ge), and

phosphorus (P) in the T position. Of these, substitution

of some barium for potassium and some titanium or ferric

iron or both for aluminum are especially common in alkali

feldspars. Several other elements also have been recorded

as traces in feldspar analyses; it seems very likely, however,

that some of these elements may reside in impurities—

i.e., within unrecognized microscopic or submicroscopic

inclusions of other minerals.

crystal Structure

Sanidine and orthoclase are monoclinic or nearly so; the

plagioclase feldspars are triclinic. All, however, have the

same fundamental structure: it consists of a continu-

ous, negatively charged, three-dimensional framework

that is made up of corner-sharing SiO

and AlO

tetra-

hedrons (each tetrahedron consists of a central silicon

or aluminum atom bonded to four oxygen atoms) and

positively charged cations (e.g., the potassium, sodium,

and/or calcium) that occupy relatively large interstices

7 the Silicates 7

140

Minerals 7

Schematic diagram showing ordered (left) and disordered (right) arrays

within a structure having two kinds of sites (type 1 and type 2) and two

types of occupants (x atoms and y atoms). in the ordered structure all x

atoms are distributed uniformly in the spaces between the y atoms, whereas

in the disordered structure no regular arrangement obtains. Copyright

Encyclopædia Britannica, Inc.; rendering for this edition by Rosen

Educational Services

within the framework. Although the framework is suf-

ﬁciently elastic to adjust itself to the different sizes of

the A cations, the relatively large potassium cations give

structures that have a monoclinic or only slightly off-

monoclinic symmetry, whereas the smaller sodium and

calcium cations lead to distorted structures that have

triclinic symmetry.

One aspect of the feldspar—especially the potassium

feldspar—structures that is of particular interest is termed

ordering. This phenomenon is indicative of the conditions

under which the feldspar was formed and its subsequent

thermal history. Ordering in feldspars is based on the dis-

tributional pattern of silicon and aluminum within the

different tetrahedrons. It can be characterized as follows:

silicon and aluminum have a random distribution within

the tetrahedrons of sanidine, an arrangement termed

disordered; they have a regular distribution within the

constituent tetrahedrons of microcline, an arrangement

141

termed ordered; and they are distributed within the tet-

rahedrons of orthoclase in a manner usually characterized

as only partly ordered. The disordered structure of sani-

dine reﬂects formation at high temperatures followed by

rapid cooling; the high degree of ordering of microcline

reﬂects either growth at low temperatures or very slow

cooling from higher temperatures; the partial ordering

of orthoclase indicates either formation at intermediate

temperatures or formation at high temperatures followed

by fairly slow cooling. With regard to this phenomenon, it

is also noteworthy that all plagioclase feldspars are more

nearly ordered than their associated potassium feldspars

regardless of the temperatures that prevailed when they

were formed.

Crystals of all the common rock-forming feldspars

tend to look alike; megascopic examination of crystal

form typically cannot be used to distinguish between feld-

spars. The angle between the face that intersects the b axis

7 the Silicates 7

Twinning in feldspars. Copyright Encyclopædia Britannica, Inc.; ren-

dering for this edition by Rosen Educational Services