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Category Borosilicates
(repeating unit)
Strunz classification 9.BJ.50 (10 ed)
VIII/B.31-10 (8 ed)
Dana classification
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Cmcm
Unit cell a = 15.99, b = 13.7, c = 6.7 [Å]; Z = 4
Color Colorless, white, grey, greenish, bluish, brown, black
Crystal habit Prismatic crystals, radiating, massive, fibrous
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 forms a solid solution series with prismatine.[3]

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]