Monoclinic Section – Orthoclase, Potash Feldspar Composition

Composition. Potassium –aluminum silicate, KAISI3O8 = Silica 64.7, alumina 18.4, potash 16.9 Soda sometimes replaces a portion of the potash.

Crystallization. Monoclinic. Crystals are usually prismatic in habit and have as prominent forms, clinopinacoid, base, prism, with often smaller orthodomes. Frequently twinned; Carlsbad with clinopinacoid as twinning plane; Baveno with clinodome as twinning plane.

Structure. Usually crystallized or coarsely cleavable to granular; more rarely fine-grained, massive and cryptocrystalline.

Physical Properties. Two prominent cleavages (one parallel to base, perfect: the other parallel to clinopinacoid good), making an angle of 90° with each other. H. = 6-6.5.5 G. = 2.5-2.6. Luster vitreous. Colorless, white, gray, flesh-red, more rarely green. Streak white.

Varieties. Common feldspar is the usual opaque variety Adularia is white or colorless and translucent to transparent. Some Adularia shows an opalescent play of colors, and is called moonstone. Most of the moonstones, however, belong to the members of the plagioclase feldspar series. Sanidine, or glassy feldspar, is a variety occurring in glassy, often transparent, phenocrysts in eruptive rocks.

Tests. Difficultly fusible (5) Insoluble in acids. When mixed with powdered gypsum and heated on platinum wire gives the violet flame of potassium. Usually to be recognized by its color, hardness and cleavage. Distinguished from the other feldspars by its right-angle cleavage and the lack of striations on the best cleavage surface

Alteration. When acted upon by waters carrying g carbon dioxide in solution, orthoclase alters, forming a soluble carbonate of potassium and leaving as a residue either a mixture of kaolin (H₄A_l2Si2O₉) and quartz (SiO₂), or of muscovite (H₂K(AiSiO₄)₃) and quartz. Kaolin forms the chief constituent of clays and has been derived in this manner.

Occurrence. One of the most common of minerals. It is formed during the crystallization of igneous rocks; by pneumatolytic and hydrothermal agencies in pegmatite veins and in druses in the rocks. More rarely by crystallization from aqueous solutions sat low temperatures in veins. Widely distributed as a prominent rock constituent, occurring in all types of rocks; igneous, in granites, syenites, porphyries, etc., sedimentary in certain sandstones and conglomerates; metamorphic, in gneisses. Also in larger crystals and cleavable masses in pegmatite veins, associated chiefly with quartz, muscovite and albite. These veins are to be found where granite rocks abound. Larger veins of this character from which feldspar is quarried in considerable amounts occur in the New England and Middle Atlantic States, chiefly en Maine, Connecticut, NEW York, Pennsylvania and Maryland.

Name. The name orthoclase refers to the right-angle cleavage possessed by the terminal. Feldspar is derived from the German word field, field.

Use orthoclase is chiefly used in the manufacture of porcelain. It is ground very fine and mixed with kaolin, or clay, and quartz. When heated to high temperature the feldspar fuses and acts as a cement to bind the material together. Fused feldspar also furnishes the major part of the glaze on porcelain ware.

A rare barium feldspar, hyalophane (K2, Ba) Al₂Si₄O₁₂, belongs here.