Twinning: Common as simple, contact, or penetration twins according to the Carlsbad, Baveno, or Manebach laws. Sanidine occurs in volcanic and subvolcanic rocks. It is structurally similar to microcline but without the cross-hatched twinning. Orthoclase KAlSi3O8 °c 2001 Mineral Data Publishing, version 1.2 Crystal Data: Monoclinic. It is a type of potassium feldspar, also known as K-feldspar.The gem known as moonstone (see below) is largely composed of orthoclase. However, it has negative relief, is biaxial, and is often clouded by fine-grained alteration. Optical Properties : In thin section, orthoclase has low birefringence, moderate relief, and resemblance to quartz. For additional information, see the gemstone section on Orthoclase and Moonstone. Orthoclase and sanidine are both optically monoclinic K-feldspars. Orthoclase crystals and twins provide information on the formation of minerals and environmental factors. Orthoclase changes to kaolinite (Section 184.108.40.206) and sericite with the process of kaolinization and sericitization, respectively. In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. Sanidine ((K,Na)AlSi 3 O 8) is the monoclinic high-temperature alkaline K–Na feldspar, which usually contains about 30%, but sometimes up to 62% sodium (Ab) component. Orthoclase is industrially important in the manufacture of glass and ceramics. Adularia is a K-feldspar produced by low-temperature hydrothermal (veins) or authigenic processes. Orthoclase orthoclase is frequently known as potassium feldspar or k spar and is easily distinguished from the plagioclase feldspars in hand sample by its lack of striations. Has perfect cleavage on 001 and good cleavage on 010. Orthoclase, or orthoclase feldspar (endmember formula K Al Si 3 O 8), is an important tectosilicate mineral which forms igneous rock.The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Cleavable, granular, massive. Point Group: 2=m: Crystals commonly short prismatic along  or , tabular on , to 20 cm. Although some cross-hatched twinning may also occur in plagioclase, it is always very simple with only one or two cross twins per grain. It is dis-tinguished from sanidine by 2V and from micro-cline by its lack of plaid twinning. Orthoclase in thin section. Sanidine is more likely to form euhedral phenocrysts. This twinning is the most characteristic identifying feature of plagioclase, and makes its identification easy when present. You can find the explanation on the alkali feldspar page. Well shaped crystals and twins are highly sought after by mineral collectors, and the Moonstone variety is used as a gemstone. Orthoclase occurs in plutonic rocks.
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