Kornerupine
General
Category	Borosilicates
Formula (repeating unit)	(Mg,Fe2+)4(Al,Fe3+)6(SiO4,BO4)5(O,OH)2
IMA symbol	Krn[1]
Strunz classification	9.BJ.50 (10 ed) VIII/B.31-10 (8 ed)
Dana classification	58.01.01.01
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
Identification
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	[2][3][4]

Kornerupine (also called Prismatine) is a rare boro-silicate mineral with the chemical 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.^[2]

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.^[4] Strongly pleochroic, it appears green or reddish brown when viewed from different directions. It has a vitreous luster.

It was first described in 1884 for an occurrence in Fiskernæs in southwest Greenland. It was named in honor of the Danish geologist, Andreas Nikolaus Kornerup Andreas Kornerup (1857–1881).^[3] Although kornerupine was named in 1884, it was not until 1912 that gem-quality material was found and it remains uncommon to this day.

Deposits are found in Burma (Myanmar), Canada (Quebec), Kenya, Madagascar, Sri Lanka, Tanzania, and South Africa.

References

• Mineral galleries