For additional information, see the gemstone section on Orthoclase and Moonstone. Well shaped crystals and twins are highly sought after by mineral collectors, and the Moonstone variety is used as a gemstone. Sanidine is more likely to form euhedral phenocrysts. Orthoclase KAlSi3O8 °c 2001 Mineral Data Publishing, version 1.2 Crystal Data: Monoclinic. Orthoclase crystals and twins provide information on the formation of minerals and environmental factors. Orthoclase occurs in plutonic rocks. Orthoclase and sanidine are both optically monoclinic K-feldspars. Adularia is a K-feldspar produced by low-temperature hydrothermal (veins) or authigenic processes. This twinning is the most characteristic identifying feature of plagioclase, and makes its identification easy when present. Orthoclase changes to kaolinite (Section 5.2.1.2) and sericite with the process of kaolinization and sericitization, respectively. Point Group: 2=m: Crystals commonly short prismatic along [100] or [001], tabular on [010], to 20 cm. Orthoclase in thin section. Optical Properties : In thin section, orthoclase has low birefringence, moderate relief, and resemblance to quartz. Cleavable, granular, massive. 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. It is dis-tinguished from sanidine by 2V and from micro-cline by its lack of plaid twinning. Orthoclase is industrially important in the manufacture of glass and ceramics. 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. Twinning: Common as simple, contact, or penetration twins according to the Carlsbad, Baveno, or Manebach laws. However, it has negative relief, is biaxial, and is often clouded by fine-grained alteration. It is structurally similar to microcline but without the cross-hatched twinning. In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. 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. You can find the explanation on the alkali feldspar page. Sanidine occurs in volcanic and subvolcanic rocks. 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 a type of potassium feldspar, also known as K-feldspar.The gem known as moonstone (see below) is largely composed of orthoclase. Has perfect cleavage on 001 and good cleavage on 010.