Chiastolite

Three specimens of Chiastolite. Two are polished slabs (one quite large) polished on both sides. The third is polished on all sides. Chiastolite is a variety of the mineral andalusite with the chemical composition Al2SiO5. It is noted for distinctive cross-shaped black inclusions of graphite.There are several theories regarding the formation of the chiastolite cross, however the most widely accepted theory 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. The cycle of growth-retardation-growth then repeats itself, creating a featherlike pattern of graphite along four radiating 'arms'. .