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Chiastolite is a variety of the mineral andalusite with the chemical composition Al2SiO5. It is noted for distinctive cross-shaped black inclusions of graphite. In areas around Georgetown, California, metamorphosed sediments contained andalusite and chiastolite in a graphite rich metasediment. The chiastolite crystals have been pseudomorphically altered by a mixture of muscovite, paragonite and margarite. The calcium rich margarite tends to form along the graphite rich crosses or bands within the chiastolite. Mineralogically the occurrence is important because all three white mica phases are present in an equilibrium assemblage.
There are several theories regarding the formation of the chiastolite cross, however the most widely accepted theory, proposed by Frondel in 1934, suggests that there is a selective attachment of impurities at the rapidly growing corners of andalusite crystals. As the concentration of these impurities (which consist primarily of graphite) increases, the growth of the crystal is slowed. This concentrated impurity deposit forms a re-entrant as it is absorbed by the growth of the andalusite porphyroblast. The cycle of growth-retardation-growth then repeats itself, creating a featherlike pattern of graphite along four radiating 'arms'.