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Category Borosilicates
(repeating unit)
Strunz classification 09.BJ.50
VIII/B.31-10 (8 ed)
Dana classification
Crystal system Orthorhombic
Unit cell a = 15.99 Å, b = 13.7 Å, c = 6.7 Å; Z=4
Color Colorless, white, grey, greenish, bluish, brown, black
Crystal habit Prismatic crystals, radiating, massive, fibrous
Crystal symmetry Orthorhombic dipyramidal
H-M symbol: (2/m 2/m 2/m)
Space group: C mcm
Cleavage Good on {110}
Mohs scale hardness 6 to 7
Luster Vitreous
Streak White
Diaphaneity Transparent to translucent and opaque
Specific gravity 3.29 - 3.35
Optical properties Biaxial (-)
Refractive index nα = 1.660 - 1.671 nβ = 1.673 - 1.683 nγ = 1.674 - 1.684
Birefringence δ = 0.014
Pleochroism X = colorless to green; Y = colorless, pale brownish yellow, pale yellowish green; Z = pale brownish green, green, light amber
2V angle Measured: 3° to 48°
References [1][2][3]

Kornerupine is a rare boro-silicate mineral with the formula (Mg,Fe2+)4(Al,Fe3+)6(SiO4,BO4)5(O,OH)2. It crystallizes in the orthorhombic - dipyramidal crystal system as brown, green, yellow to colorless slender tourmaline like prisms or in massive fibrous forms. It has a Mohs hardness of 7 and a specific gravity of 3.3 to 3.34. Its indices of refraction are nα=1.660 - 1.671, nβ=1.673 - 1.683 and nγ=1.674 - 1.684.

It occurs in boron-rich volcanic and sedimentary rocks which have undergone high grade metamorphism. It is also found in metamorphosed anorthosite complexes.[1]

Kornerupine is valued as a gemstone when it is found in translucent green to yellow shades. The emerald green varieties are especially sought after.

It was first described in 1884 for an occurrence in Fiskernaes in SW Greenland. It was named in honor of the Danish geologist, Andreas Nikolaus Kornerup (1857–1883).[2]