Kornerupine
Kornerupine (also called Prismatine) 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.
| Kornerupine | |
|---|---|
 | |
| General | |
| Category | Borosilicates |
| Formula (repeating unit) | (Mg,Fe2+)4(Al,Fe3+)6(SiO4,BO4)5(O,OH)2 |
| 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 | [1][2][3] |
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] 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 (1857–1883).[2] 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.