Andalusite

Andalusite
Andalusite
General
Category	Nesosilicates
Formula (repeating unit)	Al2SiO5
IMA symbol	And
Strunz classification	9.AF.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnnm
Unit cell	a = 7.7980 Å, b = 7.9031 Å c = 5.5566 Å; Z = 4
Identification
Color	Pink, violet, yellow, green, white, gray; in thin section, colorless to pink or green
Crystal 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 scale hardness	6.5–7.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to nearly opaque with inclusions
Specific gravity	3.17 (± .04)
Optical properties	double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.[1]
Refractive index	nα = 1.629 – 1.640 nβ = 1.633 – 1.644 nγ = 1.638 – 1.650
Birefringence	δ = 0.009 – 0.010
Pleochroism	strongly trichroic
2V angle	71–86°
Dispersion	r < v strong
Ultraviolet fluorescence	non-fluorescent
References	[1][2]

Andalusite is an aluminium nesosilicate mineral with the chemical formula Al₂SiO₅. This mineral was called andalousite by Delamétehrie,^[3] 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.^[4]

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.^[5]

Kyanite

Andalusite

Sillimanite

Phase diagram of Al₂SiO₅
(aluminosilicates).^[6]