Rafferty J.P. (ed.) Minerals

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Minerals 7

AlkAli FElDSPARS

Alkali feldspars are common silicate minerals that often occur as vari-

ously coloured, glassy crystals. They are used in the manufacture of

glass and ceramics; transparent, highly coloured, or iridescent varieties

are sometimes used as gemstones. The alkali feldspars are primarily

important as constituents of rocks; they are very widespread and abun-

dant in alkali and acidic igneous rocks (particularly syenites, granites,

and granodiorites), in pegmatites, and in gneisses. The alkali feldspars

may be regarded as mixtures of sodium aluminosilicate (NaAlSi

)

and potassium aluminosilicate (KAlSi

). Both the sodium and potas-

sium aluminosilicates have several distinct forms, each form with a

different structure. The form stable at high temperatures is sanidine

(a sodium aluminosilicate), which has a random distribution of alumi-

num and silicon atoms in its crystal structure. Low-temperature forms

include orthoclase, microcline, and adularia (all potassium aluminosili-

cates); these have an ordered arrangement of such atoms. If specimens

of the high-temperature varieties are rapidly cooled, the random dis-

tribution is preserved. In the Earth’s crust the alkali feldspars display

a range of ordering from the fully random distribution of sanidine and

orthoclase to the fully ordered distribution of microcline.

and is parallel to a and c and the face that intersects the c

axis and is parallel to a and b is 90° for the monoclinic feld-

spars and ranges from about 86° to roughly 89°30′ for the

triclinic feldspars; the deviations from 90° are not read-

ily discernible with the naked eye. In any case, feldspar

crystals are relatively rare; almost all occur in miarolitic

cavities, in pegmatite masses, or as phenocrysts within

porphyries. (A porphyry is an igneous rock containing

conspicuous crystals, called phenocrysts, surrounded by a

matrix of ﬁner-grained minerals or glass or both.) In most

rocks, both alkali and plagioclase feldspars occur as irreg-

ularly shaped grains with only a few or no crystal faces.

This general absence of crystal faces reﬂects the fact that

crystallization of these feldspars was interfered with by

previously formed minerals within the same mass.

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Both crystals and irregularly shaped grains of feld-

spars are commonly twinned. Some individual grains are

twinned in two or more ways. Carlsbad twinning occurs

in both monoclinic and triclinic feldspars; albite twinning

occurs only in triclinic feldspars. Albite twinning, which

is typically polysynthetic (i.e., multiple or repeated), can

be observed as a set of parallel lines on certain crystal or

cleavage surfaces of many plagioclase feldspars.

Physical Properties

It is important to be able to distinguish feldspar group

minerals from other rock-forming minerals and from one

another because their presence (versus absence), along

with their relative quantities, serves as the basis for classi-

fying and naming many rocks, especially those of igneous

origin. In the laboratory, it is relatively easy to identify the

feldspars by determining their chemical compositions,

their structures, or their optical properties. In some cases,

staining techniques are employed. Fortunately, most feld-

spar grains can also be identiﬁed rather easily on the basis

of macroscopic examination in the ﬁeld, using properties

such as those described below.

common Properties of the Group

As might be suspected on the basis of their similar chemi-

cal compositions and structures, all of the rock-forming

feldspars have several similar properties. As indicated by

the fact that they lack inherent colour, feldspars can be

colourless, white, or nearly any colour if impure. In gen-

eral, however, orthoclase and microcline have a reddish

tinge that ranges from a pale, ﬂeshlike pink to brick-red,

whereas typical rock-forming plagioclases are white to

dark gray. As a group, feldspars range from transparent

to nearly opaque, have nonmetallic lustres—typically

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vitreous to subvitreous on fractures and pearly or porcela-

neous on cleavage surfaces, exhibit two cleavages—one

perfect, the other good—at or near 90° to each other, and

have a Mohs hardness of approximately 6.

The presence of two cleavages at or near 90° distin-

guishes the feldspars from all other common rock-forming

minerals except halite and the pyroxenes. The hardness

(2½) and the salty taste of halite make that distinction

clear. The gray to black streak of the common rock-forming

pyroxenes, which contrasts markedly with the white

or slightly tinted hues of the streaks of the feldspars—

including those that are dark-coloured—affords a simple

way to distinguish between these minerals, even those that

are similar in appearance. (Streak is the colour of a min-

eral’s powder, which can be produced readily by pounding

or scratching the mineral with a geologic pick or hammer.)

Identification of Specific Feldspars

Alkali feldspars can often be distinguished from plagio-

clase feldspars because most grains of the latter exhibit

albite twinning, which is manifested by parallel lines on

certain cleavage surfaces, whereas grains of alkali feld-

spars do not. This criterion is not, however, absolute;

some plagioclase feldspars are not polysynthetically

twinned. Furthermore, upon only cursory examination

some perthitic textures may be mistaken for polysyn-

thetic twinning. Fortunately, this resemblance is seldom

confusing once one has thoroughly examined several

examples of both features. The two features differ rather

markedly: the traces of the polysynthetic twinning are

straight, whereas the perthitic textures that are most

likely to be mistaken for polysynthetic twinning have an

interdigitated appearance.

Another property that is sometimes used to distin-

guish between alkali and plagioclase feldspars is their

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different specific gravity values. The ideal value for

the potassium-rich alkali feldspars is 2.56, which is less

than the lowest value for the plagioclases (namely, 2.62

for albite).

Sanidine is usually distinguished rather easily from the

other alkali feldspars because it typically appears glassy—

i.e., it tends to be colourless, and much of it is transparent.

Microcline and orthoclase, by contrast, are characteristi-

cally white, light gray, or ﬂesh- to salmon-coloured and

subtranslucent. Except for its green variety, usually called

amazonstone or amazonite, microcline can seldom be

distinguished from orthoclase by macroscopic means. In

the past, much microcline was misidentiﬁed as orthoclase

because of the incorrect assumption that all microcline is

green. Today, prudent geologists identify potassium feld-

spars other than sanidine simply as alkali, or in some cases

potassium, feldspars when describing rocks on the basis of

macroscopic examination. That is to say, they do not make

a distinction between microcline and orthoclase until they

have proved their identity by determining, for example,

their optical properties. Upon macroscopic examination,

an orthoclase is also generally identiﬁed merely as an

alkali feldspar except by those who are acquainted with

the rocks known to contain anorthoclase.

The rock-forming plagioclases can seldom be identi-

ﬁed as to species by macroscopic means. Nevertheless,

some rules of thumb can be employed: White or off-white

plagioclase feldspars that exhibit a bluish iridescence (the

so-called peristerites) have overall albite compositions,

even though they are submicroscopic intergrowths of 70

percent An

and 30 percent An

; and dark-coloured pla-

gioclases that exhibit iridescence of such hues as blue,

green, yellow, or orange are labradorites. In addition,

the identities of associated minerals tend to indicate the

approximate An-Ab contents of the plagioclase

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146

feldspars—for example, biotite most commonly accom-

panies albite or oligoclase; hornblende commonly occurs

with andesine; and the pyroxenes, augite and/or hyper-

sthene, typically accompany labradorite or bytownite.

Additional characteristics for two of the feldspars are as

follows: Microcline commonly exhibits “grid twinning.”

This combination of two kinds of twinning, although best

seen by means of a microscope equipped to use doubly

polarized light, is sometimes discernible macroscopically.

(Polarized refers to light that vibrates in a single plane.)

Plagioclase feldspars that constitute lamellar masses in

complex pegmatites are albite; this variety is often referred

to by the name cleavelandite.

origin and occurrence

Feldspars occur in all classes of rocks. They are widely dis-

tributed in igneous rocks, which indicates that they have

formed by crystallization from magma. Physical weather-

ing of feldspar-bearing rocks may result in sediments and

sedimentary rocks that contain feldspars; however, this

is a rare occurrence because in most environments the

feldspars tend to be altered to other substances, such as

clay minerals. They also may be found in many metamor-

phic rocks formed from precursor rocks that contained

feldspars and/or the chemical elements required for

their formation. In addition, feldspars occur in veins and

pegmatites, in which they were apparently deposited by

ﬂuids, and within sediments and soils, in which they were

probably deposited by groundwater solutions.

uses

Feldspars are used widely in the glass and ceramics

industries. Alkali feldspars are more commonly used

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commercially than plagioclase feldspars. Albite, or soda

spar as it is known commercially, is used in ceramics. The

feldspar-rich rocks larvikite and a few anorthosites are

employed as both interior and exterior facing slabs.

In addition, several feldspars are used as gemstones.

For example, varieties that show opalescence are sold as

moonstone. Spectrolite is a trade name for labradorite

with strong colour ﬂashes. Sunstone (oligoclase or ortho-

clase) is typically yellow to orange to brown with a golden

sheen; this effect appears to be due to reﬂections from

inclusions of red hematite. Amazonite, a green variety of

microcline, is used as an ornamental material.

Sanidine occurs as phenocrysts (large noticeable crys-

tals) in extrusive felsic igneous rocks such as rhyolite

and trachyte. It indicates that the rocks cooled quickly

after their eruption. Sanidine is also diagnostic of high-

temperature contact metamorphism as an indicator of

sanidinite hornfels or facies.

Orthoclase is a primary constituent of intrusive felsic

igneous rocks such as granite, granodiorite, and syenites.

It may also occur in some metamorphic pelitic schists

and gneisses. Microcline, also found in granitic rocks

and pegmatites, is present in sedimentary rocks such as

sandstones and conglomerates. It can also occur in meta-

morphic rocks.

Albite is found commonly in granites, syenites, rhyo-

lites, and trachytes. Albite is common in pegmatites and

may replace earlier formed microcline as cleavelandite. It

is also common in low-grade metamorphic rocks ranging

from zeolite to greenschist facies.

Oligoclase is characteristic of granodiorites and mon-

zonites. It may also have a sparkle owing to inclusions of

hematite, in which case it is called sunstone. Oligoclase

is found in metamorphic rocks formed under moderate

temperature conditions such as amphibolite facies.

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148

Certain feldspars are somewhat less common.

Anorthite is found only in gabbros, though it is common

in certain high-grade metamorphic rocks such as granu-

lite facies. Many metamorphosed limestones also contain

anorthite. Bytownite is also only found in gabbros, whereas

labradorite is found in gabbros, basalts, and anorthosites.

Labradorite is often iridescent. This quality makes it desir-

able for interior and exterior building slabs. In contrast,

andesine is rare except in andesites and diorites.

FElDSPATHO iDS

Feldspathoids make up a group of alkali aluminosilicate

minerals similar to the feldspars in chemical composition

but either having a lower silica-alkali ratio or containing

chloride, sulﬁde, sulfate, or carbonate. They are consid-

ered to be the speciﬁc minerals of igneous rocks usually

termed alkalic, which is the designation applied to igne-

ous rocks whose alkali content (i.e., amount of sodium

[Na] and/or potassium [K]) exceeds the amount required

by the available silica to form one or more feldspars plus or

minus mica. Minerals of the feldspathoid group whose sil-

ica contents are less than those of their feldspar analogues

include nephelin, leucite, sodalite, and cancrinite.

chemical composition and

cryst

al Structure

The feldspathoid group minerals are sodium, potassium,

and calcium aluminosilicates, many of which resemble

the feldspars in appearance. Like the feldspars, they

have framework structures that consist of silica and alu-

mina tetrahedrons. Unlike the feldspars, however, the

arrangements of the tetrahedrons differ from species

to species, and the interstices may contain water and/

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or other simple or complex anions such as chlorine (Cl

carbonate (CO

⁄

), or sulfate (SO

⁄

), as well as sodium,

potassium, and calcium. Consequently, different feld-

spathoids have somewhat different structures: some

are isometric, others are hexagonal, and still others are

tetragonal.

Physical Properties

The physical properties of the various feldspathoids dif-

fer from those of the feldspars and from one another.

Properties of nepheline, leucite, sodalite, and cancrinite

are summarized below.

Nepheline (hardness [H] 5½–6, speciﬁc gravity [G]

2.56–2.67) typically occurs as irregularly shaped, white,

gray, or brownish, greasy- to waxy-appearing grains that

may exhibit one rather poor cleavage. Simple hexagonal

prisms occur as phenocrysts in some volcanic porphyries.

On weathered exposures, nepheline grains are commonly

weathered more than their associated minerals, thus leav-

ing, for example, feldspar grains of nepheline syenites in

relief. A simple test often used to help identify nepheline

is based on the fact that it, as well as sodalite and cancrin-

ite, reacts with acids to form gelatinous silica.

Leucite (H 5½–6, G 2.47–2.50) commonly occurs

as white to light gray trapezohedrons. The crystal form

represents the formation of leucite as an isometric min-

eral. However, the mineral can be shown to be tetragonal

by methods such as optical studies. The crystal form of

leucite indicates the conditions under which it was crys-

tallized, and the tetragonal structure shows that it has

undergone post-crystallization inversion.

Sodalite (H 5½–6, G 2.27–2.50) consists of a group of

minerals. Different species of this group may exhibit differ-

ent colours. The sodalite of most rocks occurs as irregularly

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150

shaped, translucent, bluish-coloured grains with a vitreous

to greasy lustre.

Cancrinite (H 5–6, G 2.32–2.51) typically occurs as yel-

lowish grains, some of which exhibit one perfect cleavage,

that are closely associated with one or more of the other

feldspathoids, most commonly nepheline.

origin and occurrence

The feldspathoids are relatively rare. As noted previously,

they are considered here primarily because they are the

speciﬁc minerals used in naming alkalic igneous rocks.

In this role, the feldspathoids, along with minerals of the

melilite group, are referred to as foids in the IUGS clas-

siﬁcation of igneous rocks. Feldspathoids may occur along

with feldspars in igneous rocks. They do not occur in igne-

ous rocks containing original free silica—i.e., in rocks that

contain quartz of the same generation. They are, in fact,

incompatible with consanguineous quartz (that derived

from the same parent magma), as is quite apparent from

the following equations:

As can be seen from these equations, a feldspar would

form in lieu of its feldspathoid chemical analogue in any

silica-saturated magma.

Nepheline is the most common feldspathoid; it is a

major constituent of many alkalic igneous rocks such as

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nepheline syenites. Feldspathoids also occur in a few other

kinds of rocks—e.g., contact-metamorphic skarns.

uses

Nepheline is sometimes used as a source of soda, silica, and

alumina in the manufacture of glass and ceramics. Leucite

was formerly used in Italy for fertilizer. Sodalite, once

treasured as the basic ingredient of ultramarine pigment,

is still used as a gemstone and as the desired constituent of

many of the blue rocks used as facing stones.

GARNETS

Garnets, favoured by lapidaries since ancient times and

used widely as an abrasive, occur in rocks of each of the

major classes. In most rocks, however, garnets occur in

only minor amounts—i.e., they are accessory miner-

als. Nevertheless, as a consequence of their distinctive

appearances, they are frequently recognized in hand

specimens and become part of the name of the rock

in which they are contained—e.g., garnet mica schist.

Garnets may be colourless, black, and many shades of red

and green.

chemical composition

Garnets comprise a group of silicates with the general

formula A

(SiO

)

in which A = Ca, Fe

, Mg, Mn

;

B = Al, Cr, Fe

, Mn

, Si, Ti, V, Zr; and Si may be replaced

partly by Al, Ti, and/or Fe

. In addition, many analyses

indicate the presence of trace to minor amounts of Na,

beryllium (Be), Sr, scandium (Sc), Y, La, hafnium (Hf),

niobium (Nb), molybdenum (Mo), cobalt (Co), nickel

(Ni), copper (Cu), silver (Ag), Zn, cadmium (Cd), B, Ga,

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