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 (NaAlSi3O8) and potassium aluminosilicate (KAlSi3O8). 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
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.
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
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.
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 An2 and 30 percent An25; 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
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
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.
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.
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 crystal 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
or other simple or complex anions such as chlorine (Cl-),
carbonate (CO2-⁄3), or sulfate (SO2-⁄4), 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
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 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, 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 A3B2(SiO4)3 in which A = Ca, Fe2+, Mg, Mn2+;
B = Al, Cr, Fe3+, Mn3+, Si, Ti, V, Zr; and Si may be replaced
partly by Al, Ti, and/or Fe3+. 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,
indium (In), Ge, tin (Sn), P, arsenic (As), F, and rare earth elements. Grossular is often recorded as having a compo- sition containing water, but the true substitution seems to involve 4 H+ for Si4+; and a complete series appears to
exist between grossular [Ca3Al2(SiO4)3] and hydrogros-
sular [Ca3Al2(SiO4)3 - x(H4O4)x]. Other hydrogarnets have been reported—e.g., hydroandradite and hydrospes- sartine; the general formula for hydrogarnets would be
A3B2(SiO4)3 - x(H4O4)x, and the general formula for an
end-member hydrogarnet would be A3B2(H4O4)3.
Nearly all natural garnets exhibit extensive substitu- tion; solid-solution series—some complete, others only partial—exist between several pairs of the group. In prac- tice, the name of the end-member that makes up the largest percentage of any given specimen is usually applied—e.g., a garnet with the composition Al45Py25Sp15Gr9An6 would be called almandine.
Analyses of natural specimens suggest that the following solid-solution series exist: in the pyralspite subgroup, a complete series between almandine and both pyrope and spessartine; in the ugrandite sub- group, a continuous series between grossular and both andradite and uvarovite; less than a complete series between any member of the pyralspite subgroup and any member of the ugrandite subgroup; and an additional series between pyrope and andradite and one or more of the less common garnets (e.g., pyrope with knorr- ingite [Mg3Cr2(SiO4)3] and andradite with schorlomite [Ca3Ti2(Fe2, Si)O12]).
A few well-studied garnets from metamorphic rocks have been shown to be chemically zoned with layers of differing compositions. Most of the differences thus far described appear to reflect, for the most part, differences in occupants of the A structural positions.
The structure of garnet. This schematic diagram of part of the garnet struc- ture shows the distorted silicon-oxygen tetrahedrons and BO6 octahedrons
and the distorted cubes with central A cations. Copyright Encyclopædia
Britannica, Inc.; rendering for this edition by Rosen Educational Services
crystal Structure
Garnets consist of groups of independent, distorted SiO4
tetrahedrons, each of which is linked, by sharing corners, to distorted BO6 (e.g., aluminum- and/or iron-centred)
octahedrons, thus forming a three-dimensional frame- work. The interstices are occupied by A divalent metal ions (e.g., Ca, Fe2+, Mg, and Mn), so that each one is sur-
rounded by eight oxygen atoms that are at the corners of a distorted cube. Therefore, each oxygen is coordinated by two A, one B, and one silicon cation. The configuration of the array is such that garnets are isometric (cubic).
Garnets commonly occur as well-developed crys- tals. The typical forms of the crystals have 12 or 24 sides
and are called dodecahe- drons and trapezohedrons, respectively, or they are combinations of such forms. All tend to be nearly equant. A few studies have led to the suggestion that these crystal habits can be correlated with chemi- cal composition—i.e., that dodecahedrons are most likely to be grossular-rich; that trapezohedrons tend to be pyrope-, almandine-, or spessartine-rich; and that combinations are generally andradite-rich. In any case, many garnets have individual faces that are not well developed, and thus the crystals are roughly spherical. Garnet also occurs in fine to coarse granular masses.
Physical Properties
The diverse garnets can be distinguished from other common rock-forming minerals rather easily since they do not physi- cally resemble any of them. All garnets have vitreous to resinous lustres. Most are translucent, although they may range from trans- parent to nearly opaque. Dodecahedron, a common crystal form
of garnet. Miller Museum of Geology