Andalusite

Formula:  Al₂SiO₅

Origin of name:  After locality; Andalusia region, Spain

Crystal system:  Orthorhombic

Crystal class:  2/m 2/m 2/m

Twinning:  rare on {101}

Unit cell:  a = 7.79 Å, b = 7.90 Å, c = 5.56 Å

Colour: Pink, reddish brown, rose-red, whitish

Diaphaneity: Transparent to nearly opaque with inclusions

Luster:  Vitreous

Habit: Blocky, equant, prismatic, euhedral crystals 

Hardness:  6.5 to 7.5

Specific gravity:  3.13 to 3.16

Cleavage:  2; {110} distinct, {100} indistinct

Tenacity:

Streak:  Colourless to white

Synonyms/varieties:  Apyre, chiastolite (cross-like patterned var.), gosseletite (Fe,Mn-rich var.), viridine (green, Fe³⁺ Mn-rich var.), manqanandalusite

Comments:  Trimorphous with kyanite and sillimanite; series with kanonaite. Formed as a result of contact metamorphism of argillaceous sediments, also in regionally metamorphosed schists. Rare in granites and pegmatites, which however afford the largest crystals. Detrital in some sandstones.