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andalusite (chiastolite) A member of the Al2SiO5 (or Al2O3.SiO2) group of polymorphic minerals; the other members of this group are kyanite and sillimanite. The mineral mullite (3Al2O3.2SiO2) is related to this group of minerals; sp. gr. 3.2; hardness 6.5–7.5; orthorhombic; pink or red, sometimes grey-brown and green, transparent varieties used as gemstones; vitreous lustre; crystals prismatic with a pseudo-tetragonal form which has a characteristic cross-section, can also be massive; cleavage at right angles, distinct {110}; occurs principally in thermally metamorphosed rocks that show alteration to aggregates of white mica, in low-grade, regionally metamorphosed rocks, and also in granites and pegmatites together with corundum, tourmaline, topaz, and other minerals. The name is derived from Andalusia in Spain.