KORNERUPINE
Kornerupine – Mg3Al6(Si,Al,B)5O21(OH) – is an extremely rare sorosilicate that is sometimes used as a gemstone, particularly when it occurs in a transparent green form.
It is also referred to as “prismatine”—an incorrect designation, as prismatine is a distinct mineral with which kornerupine forms a solid solution. A very rare variety known as “star kornerupine” also exists.
Macroscopically, because it forms long prisms and displays colors ranging from green and blue to brown, it can be mistaken for tourmaline. It may contain Ca, Na, Ti, Mn, Li, and F.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Colorless, white, blue, green, yellow-brown, black.
Habit: Prismatic; it can form radial aggregates.
Cleavage: {110} good.
Tenacity: Brittle.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 6.5 – 7
Parting: No.
Streak: No information available.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 3.23 – 3.27
2. Geology and Deposits
Kornerupine is a rare mineral that forms in boron-rich sedimentary and volcanic rocks subjected to amphibolite- to granulite-facies metamorphism.
It also occurs in metamorphosed anorthosites.
3. Mineral Associations
It occurs with quartz, micas (biotite, muscovite, phlogopite), Al2SiO5 Group minerals (kyanite, andalusite, sillimanite), Fe-Ti oxides (magnetite, ilmenite, hematite, rutile), cordierite, scapolite, corundum, tourmaline, spinel, sapphirine, grandidierite, and dumortierite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.660 – 1.671 nβ: 1.673 – 1.683 nγ: 1.674 – 1.684
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: High.
Cleavage: Good {110} cleavage, generally clearly visible. Longitudinal sections show only one cleavage direction, while basal sections show two cleavage directions intersecting at ~90°. It may exhibit numerous fractures.
Habits: It exhibits a prismatic habit; the crystals can form radiating aggregates.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.014, corresponding to first-order colors ranging from gray and white to yellow and orange.
Extinction: Paralell.
Elongation sign: ES(-)
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(-), can simulate being U(-)
2V angle: 3 – 48º
Alterations: No information available.
May be confused with: various other high-relief minerals.
Prismatine is extremely similar; differentiation requires other analytical techniques.
Garnet is isotropic.
Sillimanite has positive elongation and more intense interference colors.
Olivine and beryl can be similar, but both are colorless in plane-polarized light, lack cleavage, and occur in different parageneses.
Corundum is very similar but lacks cleavage, showing only two partings that can mimic cleavage; however, corundum is colorless in plane-polarized light, whereas kornerupine exhibits pleochroism.
Orthopyroxenes (enstatite) exhibit ES(+) and pleochroism.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the analytical technique indicated for the identification of kornerupine.
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx