Kornerupine

kornerupine

grandidierite

sapphirine

sillimanite

Images

Formula: (Mg,Fe2+,Al,☐)10(Si,Al,B)5O21(OH,F)2
Sorosilicate (Si2O7 groups), boron-bearing mineral, forms a series with prismatine
Crystal system: Orthorhombic
Specific gravity: 3.29 to 3.35 measured, 3.288 calculated
Hardness: 6 to 7
Colour: Colourless, white, blue, green, yellow-brown, black
Common impurities: Ti,Mn,Ca,Li,Na,F
Environments

Metamorphic environments

Kornerupine occurs in boron-rich volcanic and sedimentary rocks subjected to amphibolite to granulite facies metamorphism, and in metamorphosed anorthosite complexes. Associated minerals include sapphirine, cordierite, spinel, corundum, tourmaline, grandidierite, dumortierite, kyanite, sillimanite, andalusite, biotite, phlogopite, magnetite, ilmenite, hematite and rutile (HOM),

Localities

At Larsemann Hills, Prydz Bay, Ingrid Christensen Coast, Princess Elizabeth Land, Eastern Antarctica, two occurrences of co-existing kornerupine and grandidierite have been found. Two distinct associations occur:
(1) At McCarthy Point, 1 to 10 mm thick late-tourmaline - kornerupine - grandidierite layers are hosted within quartzofeldspathic gneiss; the growth of these minerals predates, or is synchronous, with fold structures developed during a extensive granulite grade, normal shearing event.
(2) At Seal Cove, where coexisting kornerupine and grandidierite occur within coarse-grained, metamorphic segregations with magnesium-rich cores of cordierite - garnet - spinel - biotite - ilmenite and variably developed plagioclase halos. The segregations are hosted within biotite-bearing, plagio-feldspathic gneiss. The grandidierite- and kornerupine-bearing segregations largely postdate structures developed during a crustal thickening event that was coeval with peak metamorphism.
Average pressure calculations on assemblages that coexist with the borosilicates at Seal Cove, indicate the prevailing conditions were 5.2 to 5.5 kbar at ~750oC for formation of the grandidierite-kornerupine assemblage.
Textural relationships from these localities indicate the stability of co-existing kornerupine and grandidierite (MM 59.327-339).

At the type locality, Fiskenæsset old harbour, Qeqertarsuatsiaat area, Nuuk, Sermersooq, Greenland, new (in 1987) analyses of kornerupine in three specimens from the type locality indicate the presence of boron in all three. Textures and chemical compositions suggest that kornerupine crystallised in equilibrium in the following assemblages, all with anorthite and phlogopite:
(1) kornerupine-gedrite
(2) kornerupine-sapphirine
(3) kornerupine-hornblende
In the first assemblage, sapphirine and corundum are enclosed in radiating bundles of kornerupine; additionally sapphirine, corundum and/or gedrite occur with chlorite and "pinite" (cordierite?) as breakdown products of kornerupine.
Kornerupine may have formed by reactions such as: gedrite + sapphirine + corundum + B2O3 (in solution) + H2O → kornerupine + anorthite + Na-phlogopite under conditions of the granulite facies. Boron for kornerupine formation was most likely remobilised by hydrous fluids from meta-sedimentary rocks occurring along the upper contact of the Fiskemesset gabbro-anorthosite complex with amphibolite (MM 51.695-708).
Kornerupine from the Qeqertarsuatsiaat area - Image

At the Labwor Hills, Kotido, Northern Region, Uganda, a kornerupine-bearing rock sample is an eluvial cobble presumed to represent an iron-rich lens in granulites that contain sapphirine, garnet, sillimanite, orthopyroxene, spinel and cordierite. The cobble consists of a cellular aggregate of green spinel which contains abundant streaks and fine dust of magnetite. In parts of the cobble, interstices between the spinel aggregate are mostly filled with sillimanite in prisms up to nearly 1 mm across and several millimetres long. In other parts of the cobble the interstices are largely filled with kornerupine in prisms several millimetres across, having a crude parallel orientation. Sillimanite prisms mostly 0.05 to 0.2 mm across occur sparingly in kornerupine. Sapphirine in platelets and irregular grains up to 2 mm across is found throughout the cobble and is best developed along boundaries between spinel and kornerupine grains. Oxides besides spinel are magnetite and an ilmenite-hematite intergrowth. Biotite flakes 0.1 to 0.5 mm across are scattered in kornerupine or less commonly in oxide grains. Traces of K-feldspar(?) and zircon are also present. Seams of secondary aluminium hydroxides cut sapphirine and kornerupine (MM 48.550-552).

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