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 continuous, negatively charged, three-dimensional framework that is made up of corner-sharing SiO4 and AlO4 tetrahedrons (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 within the framework. Although the framework is sufficiently 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 (see ). 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 distributional 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 termed ordered; and they are distributed within the tetrahedrons of orthoclase in a manner usually characterized as only partly ordered. The disordered structure of sanidine 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 feldspars. The angle between the face that intersects the b axis 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 feldspars and ranges from about 86° to roughly 89°30′ for the triclinic feldspars; the deviations from 90° are not readily 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 irregularly 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 feldspars are commonly twinned. Some individual grains are twinned in two or more ways. Two common kinds of twinning—those designated Carlsbad twinning and albite twinning—are shown in . 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 classifying 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 feldspar grains can also be identified rather easily on the basis of macroscopic examination in the field, using properties such as those described in the remainder of this article.
Common properties of the group
As might be suspected on the basis of their similar chemical 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 general, 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 porcelaneous 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° distinguishes the feldspars from all other common rock-forming minerals except halite and the pyroxenes. The hardness (21/2) 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 mineral’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 plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars 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 polysynthetic 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 distinguish between alkali and plagioclase feldspars is their 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 characteristically 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 feldspars 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, anorthoclase 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 identified 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 plagioclases 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 accompanies albite or oligoclase; hornblende commonly occurs with andesine; and the pyroxenes, augite and/or hypersthene, 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 distributed in igneous rocks, which indicates that they have formed by crystallization from magma. Physical weathering 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 metamorphic 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. Some of the typical occurrences for the individual species are given in the table.
| Potassium feldspars* | |
|---|---|
| *Including perthites. In addition, anorthoclase occurs only in a few rather abnormal syenites (e.g., larvikite), and adularia—transparent, colourless to white, commonly opalescent potassium feldspar with a pseudo rhombohedral habit—occurs in some low-temperature hydrothermal veins. | |
| **Typical syenites consist of nearly 90 percent alkali feldspar. | |
| ***Typical anorthosites consist of about 90 percent plagioclase feldspar. | |
| sanidine | potassium-rich volcanic rocks and near-surface minor intrusions—e.g., rhyolites, trachytes, and high-temperature contact metamorphic rocks |
| orthoclase | potassium-rich dike rocks—e.g., rhyolite and trachyte porphyries; granites, granodiorites, and syenites**; moderate- to high-grade metamorphic gneisses and schists; and sandstones |
| microcline | granitic pegmatites, hydrothermal veins; granites, granodiorites, and syenites**; low- to moderate-grade metamorphic rocks; sandstones and conglomerates |
| Plagioclase feldspars | |
| albite | granites; granitic pegmatites; low-grade metamorphic gneisses and schists; sandstones |
| oligoclase | granodiorites and monzonites; sandstones; moderate-grade metamorphic rocks |
| andesine | diorites; andesites; moderate-grade metamorphic rocks, especially amphibolites |
| labradorite | gabbros and anorthosites***; diabases and basalts |
| bytownite | gabbros and anorthosites***; diabases and basalts |
| anorthite | gabbros; contact-metamorphosed impure limestones; and high-grade metamorphic rocks |
