Sapphirine

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Sapphirine
Indigo-blue sapphirine crystal from Ft. Dauphin, Madagascar (size: 2.1 x 1.9 x 1.4 cm)
General
Category	Silicate mineral
Formula (repeating unit)	(Mg,Al)8(Al,Si)6O20
IMA symbol	Spr[1]
Strunz classification	9.DH.45
Crystal system	2M polytype: monoclinic 1A polytype: triclinic
Crystal class	2M polytype: prismatic (2/m) 1A polytype: pinacoidal (1)
Space group	1A polytype: P1 Other polytypes: 3A, 5A, 2M, 4M
Unit cell	a = 11.27 Å, b = 14.4 Å c = 9.93 Å; β = 125.5°; Z = 4
Identification
Colour	Light to dark blue or green, white, gray, pale red, yellow
Crystal habit	Anhedral, granular, tabular
Twinning	Uncommon on {001}
Cleavage	Poor to indistinct on {100}, {001}, and {010}
Fracture	Subconchoidal to uneven
Mohs scale hardness	7.5
Lustre	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.40–3.58
Optical properties	Biaxial (−)
Refractive index	nα = 1.701 – 1.729 nβ = 1.703 – 1.732 nγ = 1.705 – 1.734
Birefringence	δ = 0.004 – 0.005
Pleochroism	X = colorless, pale reddish, yellowish green, pale yellow; Y = sky-blue, lavender-blue, bluish green; Z = blue, sapphire-blue, dark blue
2V angle	Measured: 47 to 114°
References	[2][3][4]

Sapphirine is a rare mineral, a silicate of magnesium and aluminium, with the chemical formula (Mg,Al)₈(Al,Si)₆O₂₀ (with iron as a major impurity). Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed crystals are treasured and occasionally cut into gemstones. Sapphirine has also been synthesized for experimental purposes via a hydrothermal process.