Felspar

Felspar (Ger. Feldspath, 'field-spar'), a general term in mineralogy for the most important rock-forming group of minerals. The feldspars are usually divided into two series—in one of which the minerals crystallise in monoclinic forms, and in the other in triclinic forms. They are all anhydrous silicates of alumina, containing either potash, soda, or lime alone, or two of those bases together. Thus they are often spoken of as potash-, soda-, lime-, soda-lime-feldspars, &c. They have a hardness between 6 and 7—that is to say, they can just be scratched with a good penknife. Their specific gravity is about 2.6. The most important feldspars are shown in the following table :

Orthoclase (Gr. orthos, 'straight,' klasis, 'a fracture') is so called because the two cleavage-planes of the mineral are at right angles to each other. As a rock-former, it occurs most frequently in the form of imperfect crystals or irregular crystalline aggregates. In porphyritic rocks, and especially in drusy cavities in such rocks as granite, it often appears in tolerably well-developed crystals. It is an essential constituent of granite, syenite, orthoclase-porphyry, and quartz-porphyry. The compact ground-mass of the two last-mentioned rocks is largely composed of micro-crypto-crystalline orthoclase. Orthoclase occurs also as an accessory ingredient in most plagioclase rocks, and is present in many of the crystalline schists, especially the gneissose rocks. A clear glassy variety of orthoclase called sanidine is a common constituent of many igneous rocks of Tertiary and Recent date, such as liparite, phonolite, trachyte, &c. Orthoclase, as a rule, is readily acted upon by the weather—the potash and some of the silica being removed in solution, while a fine-grained clay or Kaolin is left behind. Ordinary orthoclase is either gray, white, or flesh-coloured, and these tints of the feldspar generally determine the colour of the rock in which it occurs. Thus we have gray granites and red or pink granites.

Microcline has the same composition as orthoclase, and its cleavage-angles differ so very slightly from a right angle that it might well be looked upon as simply another form of orthoclase. It is frequently associated with the latter in plutonic and schistose rocks.

The other triclinic feldspars are grouped together as Plagioclase (Gr. plagios, 'oblique,' and klasis, 'a fracture;' in reference to the cleavage-planes, which are not at right angles to each other), and are among the most important rock-formers. According to Tschermak, they form a series of which anorthite and albite are the extremes, while the others are isomorphous mixtures of these two types in various proportions. The plagioclase feldspars often assume a tabular aspect when growth has taken place in the direction of the lateral axis. At other times the crystals, owing to extension in the direction of the principal axis, acquire a long prismatic form. As rock-formers they occur either as well-developed crystals, or as irregular crystalline granules, or sometimes as a crypto-crystalline matrix or ground-mass, through which other rock-forming minerals are disseminated. On fresh unweathered faces of an igneous rock the crystals frequently appear as clear glassy strips or rods, in which may often be detected by the eye or by a lens a fine parallel striation, formed by the union of many twins, sometimes thirty or more appearing in one and the same crystal. This structure is never seen in orthoclase. This repeated twin-lamellation has been developed artificially in untwinned plagioclase by subjecting the mineral to a high temperature, and the same molecular change has been superinduced by pressure. Plagioclase is an essential constituent of many igneous rocks, and it is likewise met with in many crystalline schists, in which it is the product of hypogene metamorphic action. Albite is a common constituent of some crystalline schistose rocks and certain crystalline limestones. As a secondary product it is occasionally met with also in igneous rocks. Oligoclase occurs both in the older and younger eruptive rocks—in hypogene rocks and in true volcanic rocks. Andesine is met with rarely in crystalline schists, but is a common constituent of some eruptive rocks, such as andesite. Labradorite is an essential constituent of such rocks as gabbro, basalt, &c. It occurs also in metamorphic rocks, sometimes in large crystalline masses. It often shows fine chatoyant reflections, receives a fine polish, and is occasionally employed in jewelry. Anorthite is not common as a rock-former. It occurs in some basalts and andesites, and sparingly in a few crystalline schistose and granitic rocks.

The feldspars which occur in rocks that contain much free silica (quartz) are generally orthoclase and microcline, but with these are frequently associated the highly silicated feldspars, albite and oligoclase. The chief habitat of the basic plagioclase feldspars are basic igneous rocks.

Common feldspar or orthoclase, under the name of Petunse or Pctuntze, is employed by the Chinese (along with some of the quartz associated with it) in the manufacture of porcelain. It is used with other materials as a flux, and alone to form an enamel or glassy covering, without which the porcelain would absorb moisture and grease. Kaolin (Chinese) is the name given to the fine clay which results from the decomposition of feldspar, as in the case of the highly weathered granites of Cornwall. Adularia is a transparent variety of orthoclase, often showing pearly opalescent reflection, and sometimes a play of colours. The finest specimens of this variety are cut as ornamental stones, and known as moonstones (some moonstones, however, appear to belong to albite and oligoclase) : another kind, found among rolled stones in Ceylon, and remarkable for the reflection of a pearly light, has occasionally been confounded with Cat's-eye (q.v.). Sunstone, or aventurine feldspar, is somewhat similar to the variety of quartz called Aventurine in the play of light which it exhibits—a property which seems to be due to disseminated crystals or plates of hematite or göthite. Aventurine feldspar is not always orthoclase, some of it belonging to albite and oligoclase. Amazon-stone is a bright verdigris-green orthoclase, fine specimens of which come from Pike's Peak, Colorado. Peristerite is a whitish adularia-like albite. Saccharite is a granular massive variety of andesine.