Sapphirine
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.
| 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 |
| 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 | |
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