alkali feldspar A group of silicate minerals that contain the alkali metal elements potassium and sodium. The normal feldspar minerals (including the calcium-bearing varieties) can be plotted on a chemical basis into a triangle which has KAlSi₃O₈ (potassium feldspar, sanidine, orthoclase (Or), or microcline), NaAlSi₃O₈ (sodium feldspar, albite, or Ab), and CaAl₂Si₂O₈ (calcium feldspar, anorthite, or An) at the three apices. The alkali feldspars are represented by the edge of the triangle joining KAlSi₃O₈ and NaAlSi₃O₈ and these minerals may also contain up to 10% by weight of the third phase (CaAl₂Si₂O₈). At high temperatures the alkali feldspars show complete solid solution between the potassium and sodium end-members, but as the temperature drops unmixing occurs and potassium feldspar and sodium feldspar separate out to produce a perthitic texture. Depending upon the final temperature, a range of perthites may result, from coarse (perthite), representing perthites formed during a large drop in temperature, to fine (microperthite), and finally to very fine (cryptoperthite), representing perthites invisible to the naked eye and often invisible under the microscope, but observed by X-ray diffraction (XRD) techniques. If the amount of potassium exceeds that of sodium, then potassium feldspar is the host and sodium feldspar occurs within the host mineral as blebs, irregular patches, etc. In the alkali feldspars, perthitic textures occur in the compositional range Or₈₅Ab₁₅ to Or₁₅Ab₈₅ (or Or₈₅ to Or₁₅). K-feldspar (KAlSi₃O₈) is the general name for the monoclinic, potassium-rich end-member: sp. gr. 2.6; hardness 6; white, sometimes with a reddish tint; vitreous lustre; crystalline, prismatic, with simple twins (see CRYSTAL TWINNING). It is an essential constituent of acid igneous rocks and arkoses and is used in the manufacture of glazes, porcelain, and pottery. Microcline has the same physical properties and composition as orthoclase, but is triclinic and is characterized by ‘cross-hatched’ twinning. It is greyish-white, but bright green in the variety known as ‘amazonstone’ (‘amazonite’). Anorthoclase is very similar to microcline, but the amount of sodium exceeds that of potassium. Crystal twinning is common particularly along the pericline and albite laws. Sanidine is the high-temperature variety of orthoclase and the inversion temperature is at 900°C. It occurs in quickly cooled lavas. Adularia is a variety of microcline, but with up to 10% sodium substituting for potassium. It may show an opalescent play of colours to give a variety known as ‘moonstone’. Albite (NaAlSi₃O₈) is the sodium-rich end-member of both the alkali feldspars and the plagioclase feldspars. The semi-precious moonstone, with its characteristic bluish sheen or schiller, is an example of a perthitic alkali feldspar.

orthoclase Essential mineral in acidic igneous rock and common in metamorphic rock. It is a potassium aluminium silicate, KAlSi₃O₈ with monoclinic system crystals. It is usually white but can be pink. Hardness 6–6.5; r.d. 2.5–2.6.

orthoclase See ALKALI FELDSPAR.