Perthite is either microcline or orthoclase with thin lamellae of exsolved albite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich). Alkali feldspar dominates, commonly represented by orthoclase and the exsolved lamellar albite, form perthite. Abelsonite occurs in association with albite, analcime, dolomite, mica, orthoclase, pyrite, and quartz. The main minerals are quartz, perthitic orthoclase, and plagioclase. The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). It contains also orthoclase, nepheline, biotite, aegirine, acmite. The metamorphic grade progresses from greenschist, biotite, garnet and finally orthoclase. Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite). The distinctive texture of rapakivi granite is due to oligoclase rims on orthoclase phenocrysts. It commonly occurs in association with orthoclase, sanidine, labradorite, olivine, leucite, amphiboles and other pyroxenes. It consists almost entirely of granodiorite with large crystals of orthoclase. Associated minerals include bixbyite, quartz, orthoclase, topaz, spessartine, pseudobrookite and hematite. The gem known as moonstone (see below) is largely composed of orthoclase. Its chemical formula is (KAlSiO), which is polymorphic to orthoclase.