andalusite

{{Infobox mineral | boxbgcolor = #402815 | boxtextcolor = #fff | name = Andalusite | category = Nesosilicates | image = Andalusite - Malacheta, Minas Gerais, Brazil.jpg | imagesize = 260px | alt = | caption = Andalusite | formula = Al2SiO5 | molweight = |IMAsymbol=And| strunz = 9.AF.10 | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Pnnm | unit cell = a = 7.7980 Å, b = 7.9031 Å c = 5.5566 Å; Z = 4 | color = Pink, violet, yellow, green, white, gray; in thin section, colorless to pink or green | habit = As euhedral crystals or columnar aggregates having nearly square cross sections; fibrous compact to massive | twinning = On {101}, rare | cleavage = Good on {110}, poor on {100} | fracture = uneven to subconchoidal | mohs = 6.5–7.5 | luster = Vitreous | polish = | refractive = nα = 1.629 – 1.640 nβ = 1.633 – 1.644 nγ = 1.638 – 1.650 | opticalprop = double refractive, biaxial negative; chiastolite has anomalous aggregate reaction. | birefringence = δ = 0.009 – 0.010 | dispersion = r 2SiO5. This mineral was called andalousite by Delamétherie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the Spanish province of Guadalajara, not Andalusia.

Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.