Plagioclase feldspar is a group of common, widely-distributed feldspar minerals (the most abundant group of minerals on Earth), composed of varying proportions of the silicates of sodium and calcium with aluminum. They range from NaAlSi3O8 (albite) to CaAl2Si2O8, and are important ingredients in some igneous and metamorphic rocks. Off-white, or sometimes pink, green, or brown in color, they show an oblique cleavage, and have triclinic system crystals. Hardness 6–6.5; relative density 2.6. See also moonstone.