Sapphirine

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
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

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.

Typical colours range from light to dark sapphire blue, bluish to brownish green, green, and bluish or greenish gray to black; less common colours include yellow, pale red, and pink to purplish pink. Sapphirine is relatively hard (7.5 on Mohs scale), usually transparent to translucent, with a vitreous lustre. Crystallising in the monoclinic system, sapphirine is typically anhedral or granular in habit, but may also be tabular or in aggregates: Twinning is uncommon. Fracture is subconchodial to uneven, and there is one direction of perfect cleavage. The specific gravity of sapphirine is 3.54–3.51, and its streak is white.

Sapphirine's refractive index (as measured by monochromatic sodium light, 589.3 nm) ranges from 1.701 to 1.718 with a birefringence of 0.006–0.007, biaxial negative. Refractive index values may correspond to colour: brownish green specimens will possess the highest values, purplish-pink specimens the lowest, and blue specimens will be intermediate between them. Pleochroism may be extreme, with trichroic colours ranging from: colourless, pale yellow or red; sky to lavender blue, or bluish-green; to dark blue. There is no reaction under ultraviolet light.