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Kornerupine

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Kornerupine
General
CategoryBorosilicates
Formula
(repeating unit)
(Mg,Fe2+)4(Al,Fe3+)6(SiO4,BO4)5(O,OH)2
IMA symbolKrn[1]
Strunz classification9.BJ.50 (10 ed)
VIII/B.31-10 (8 ed)
Dana classification58.01.01.01
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupCmcm
Unit cella = 15.99, b = 13.7, c = 6.7 [Å]; Z = 4
Identification
ColorColorless, white, grey, greenish, bluish, brown, black
Crystal habitPrismatic crystals, radiating, massive, fibrous
CleavageGood on {110}
Mohs scale hardness6 to 7
LusterVitreous
StreakWhite
DiaphaneityTransparent to translucent and opaque
Specific gravity3.29–3.35
Optical propertiesBiaxial (−)
Refractive indexnα = 1.660 – 1.671 nβ = 1.673 – 1.683 nγ = 1.674 – 1.684
Birefringenceδ = 0.014
PleochroismX = colorless to green; Y = colorless, pale brownish yellow, pale yellowish green; Z = pale brownish green, green, light amber
2V angleMeasured: 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 Kornerup [de] (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

[edit]
  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Mineral data publishing 2001 (PDF)
  3. ^ a b Webmineral data
  4. ^ a b Mindat with locality data