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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
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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