In addition to being the most abundant and widespread minerals of the crust, or outermost part, of the Earth, feldspars are also major constituents of moon rocks

Feldspars are used in the manufacture of ceramics and ceramic glazes and as mild abrasives.

Feldspars are aluminosilicates, containing silicon and aluminum ions bound together by oxygen ions to form a three dimensional framework of atoms. Other ions, principally potassium, sodium, and calcium, occupy sites within the framework.

Extensive chemical variation, or solid solution, occurs between orthoclase and albite, with potassium and sodium substituting for each other. Feldspars in this range are called alkali feldspars.

Feldspars containing Ca++ as the cation charge balancing substitution of Al for Si are called Anorthite. Complete solid solution between albite and anorthite is also possible and unmixing of Na-rich from Ca-rich components results in fine scale intergrowths that produce irridescence (schiller)

Because these two ions have different charges, the proportions of aluminum to silicon in the aluminosilicate framework must also vary to maintain electrical charge neutrality.

Feldspars between albite (contains sodium (Na) and anorthite (contains calcium (Ca)) in composition are termed plagioclase.

There is little solid solution between alkali feldspars (contain potassium (K) +/- sodium (Na)) and anorthite ( contains Na and or Ca).

Although there is extensive mixing of Na and K or Na and Ca in feldspars when they are formed, as the minerals cool they would rather contain only K or Na or Ca (this is a simplification). This results in unmixing of feldspars when they are cooled, leading to play of color (schiller, labradorescence, or other terms).

THUS, a high temperature feldspar cools, albite separates and appears as small grains, bleb, or streaks within it. The resulting mixtures are termed perthites. The perthitic texture may be coarse and easily visible as slight variations in color in the crystal, or it may be microscopic and practically invisible. The coarseness of the texture depends mostly on the rate at which the feldspar was cooled the slower the cooling, the coarser the texture.

Feldspars are generally light colored minerals, white or buff to gray in color. One species, microcline, may also be light brick-red or even the green to blue green variety called amazonite.

Feldspars are slightly translucent and have a glassy, or vitreous, luster rather like that of glazed porcelain.

Because their atomic framework has planes of weakness, feldspars exhibit good cleavage, breaking readily into blocky pieces with smooth sides.

Because of their abundance, feldspars are used in the classification of IGNEOUS ROCKS. They are also abundant in METAMORPHIC ROCKS and in some sediments and SEDIMENTARY ROCKS.

The high-temperature potassium feldspars, sanidine and orthoclase, are monoclinic in their crystal symmetry; the others are triclinic, although they retain the general atomic pattern of the monoclinic species. Because triclinic feldspars have nearly monoclinic symmetry, they commonly occur in complex intergrowths of crystals called twins, which mimic the higher monoclinic symmetry. Plagioclase in particular exhibits a prominent twinning called albite twinning. Thin, plate like crystals, oriented so as to be mirror images of each other, are intergrown. On a cleavage surface, this intergrowth causes a finely striped pattern characteristic of plagioclase.

About a dozen different patterns of twinning have been recognized in the feldspars.

The three different but closely related species of alkali feldspar are sanidine, orthoclase, and microcline. The differences among these feldspars result from details of their atomic structures, principally the different ways in which aluminum is distributed, or ordered, in the aluminosilicate frameworks of each.

  • Sanidine is the form stable at the highest temperatures. It can accommodate sodium in all proportions, and a complete compositional series (with a slight structural change) runs from pure potassium sanidine to pure albite. Members of this series that are more than two thirds sodium are called anorthoclase and have triclinic symmetry.
  • At lower temperatures orthoclase is more stable than sanidine, and many igneous rocks contain orthoclase.
  • The compositional range of orthoclase extends only partway to albite, and orthoclase may occur with albite in rocks.

    In some igneous rocks and in most metamorphic rocks, microcline is the common potassium feldspar. Microcline can accommodate only a little sodium and, like orthoclase, may occur with albite. All the potassium feldspars can contain more sodium at high temperatures than at low.

    Alkali feldspars occur in many rocks. They are abundant in GRANITES, a family of intrusive igneous rocks composed chiefly of alkali feldspars and quartz. Granites constitute the cores of mountain ranges; they are formed by the melting of the Earth's crust as mountains are built. Sanidine is found chiefly in some lavas, where it is preserved by rapid chilling. When cooled slowly, sanidine changes to orthoclase. Crude crystals of microcline several feet wide (among the largest of any mineral) are commonly found in PEGMATITES, coarse-grained, granitelike rocks found in mountains. Pockets yield beautiful, sharp crystals, sometimes of the green variety amazonite. Pegmatites in the granite near Pikes Peak in Colorado are famous for such Alkali feldspars are also common in many metamorphic rocks that have crystallized at high temperatures. Micas and other minerals common in lower temperature rocks tend to break down into feldspar as the temperature increase during metamorphism.

    Alkali feldspars weather to clays, forming important deposits of china clay or kaolin. In arid regions, however, alkali feldspars accumulate in the sand and gravel formed by mechanical breakdown of rock. A SANDSTONE formed from this sand with more than 25 percent feldspar grains is called an ARKOSE.

    Plagioclase feldspars are those ranging from albite to anorthite in composition. Plagioclase feldspars are even more abundant than the alkali species and are found in many igneous and metamorphic rocks. Crystallization of plagioclase from a melt or molten rock magma has been studied in detail. As the melt cools, the feldspar that forms is richer in calcium than the melt itself. On further cooling, feldspar that is richer in sodium forms, and the feldspar already formed tends to react with the melt and become richer in sodium as well. If cooling takes place slowly, the feldspar will be homogeneous and have the same composition as the starting melt. Under conditions of relatively rapid cooling, however, the feldspar grains will not have fully reacted and will be zoned, richer in calcium in the centers and in sodium on the outsides.

    Igneous rocks that crystallize at high temperatures contain calcic plagioclase (rich in calcium), and rocks forming at lower temperatures contain sodic plagioclase (rich in sodium). A partly crystallized magma is separated into a solid rock containing calcic plagioclase and a still molten residue richer in sodium. If this residue is removed from the already solid rock, perhaps by a volcanic eruption, it forms a rock containing sodic plagioclase upon cooling. Thus a magma may be divided by igneous differentiation processes to form igneous rocks of different compositions. Plagioclases are among the most common minerals in igneous rocks; a few examples may show their variety.

    Anorthosite is a rock composed principally of calcic plagioclase, usually bytownite or labradorite. Anorthosite is uncommon on the Earth, but may constitute a substantial part of the mountainous or highland regions of the Moon.

    BASALT rocks contain plagioclase, magnesium, and iron-rich minerals, such as olivine or pyroxene. The sea floors and many oceanic islands, including Iceland and Hawaii, are composed of basalt. Basalt also makes up the lunar mare, or lowlands. ANDESITE, the volcanic rock of the Andes, the Cascades, and many other mountain ranges, contains substantial plagioclase. Plagioclase weathers more readily than alkali feldspars and is less common in sediments and sedimentary rocks. Albite, which may be considered either an alkali feldspar or a plagioclase, is found in some sedimentary rocks; in some sodium rich sediments it has actually grown as crystals rather than being detritus from the breakdown of other rocks.

    Plagioclase is widespread in metamorphic rocks. Albite is characteristic of metamorphic rocks formed below about 500 deg C; at higher temperatures more calcic species are characteristic. Pure anorthite is found in some highly metamorphosed marbles.