16.59: Sapphirine
- Page ID
- 6487
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Figure \(\PageIndex{1}\): From Kolonne,Sri lanka
Sapphirine is a rare mineral, a silicate of magnesium and aluminum with the chemical formula (Mg,Al)8(Al,Si)6O20 (with iron as a major impurity). Named for its sapphire-like color, 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.
Chemical composition
(Mg,Al)8(Al,Si)6O20
Typical colors range from light to dark sapphire blue, bluish to brownish green, green, and bluish or greenish-gray to black; less common colors 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. Crystallizing 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.
Color
Dark blue, bluish to brownish green, green, gray.
Diaphaneity
Transparent to opaque.
Refractometer
1.701 to 1.718 with a Birefringence 0.006 to 0.007.
Specific gravity
The specific gravity of Sapphirine is 3.54–3.51.
Occurrence
Large crystals of fine clarity and color are known from very few locales: The Central Province (Hakurutale and Munwatte) of Sri Lanka has long been known as a source of facetable greenish blue to dark blue material, and crystals up to 30 mm or more in size have been found in Fianarantsoa (Betroka District) and Toliara Province (Androy and Anosy regions), southern Madagascar. Sapphirine's type locality is Fiskenaesset (Fiskenaes), Nuuk region, western Greenland, which is where the mineral was discovered in 1819.
Synthetics
Crystals have been produced with the hydrothermal method for experimental purposes.