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Formula: MgAl3O2(BO3)(SiO4)
Nesosilicate (insular SiO4 groups), borosilicate
Crystal System: Orthorhombic
Specific gravity: 2.98 to 2.99 measured, 3.00 calculated
Hardness: 7½
Streak: White
Colour: Bluish green, greenish blue
Solubility: Insoluble in acids
Common impurities: Ti,Mn,Na,K
Environments
Pegmatites
Metamorphic environments
Grandidierite is a rare accessory mineral in aluminous boron-rich
rocks thermally and regionally metamorphosed to a high grade under low pressure; in
pegmatites,
aplite and gneiss,
and in xenoliths. Associated minerals include quartz,
K-feldspar, plagioclase,
biotite, garnet,
enstatite-ferrosilite,
spinel,
corundum, sillimanite,
andalusite, cordierite,
tourmaline, kornerupine,
sapphirine, serendibite
and sinhalite
(HOM, Mindat).
Stable, co-existing grandidierite-kornerupine assemblages
require high pressure and high temperature
(Dana).
Localities
At the Larsemann Hills, Prydz Bay, Ingrid Christensen Coast, Princess Elizabeth Land, Eastern Antarctica,
Two distinct associations occur:
(1) At McCarthy Point, 1 to 10 mm thick
tourmaline-kornerupine-grandidierite
layers are hosted within
quartz-feldspar
gneiss.
(2)
At Seal Cove, 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
plagioclase feldspar
gneiss.
Indications are that the prevailing conditions were 5.2 to 5.5 kbar at about 750oC for formation
of the grandidierite-kornerupine assemblage
(MM 59.327-339).
At Opinicon Lake, Frontenac County, Ontario, Canada grandidierite occurs with
kornerupine
(Dana).
At Horní Bory, Bory, Žďár nad Sázavou District, Vysočina Region, Czech Republic, a mineral assemblage of
grandidierite, ominelite,
boralsilite, werdingite,
dumortierite, tourmaline
and corundum, along with the matrix minerals
K-feldspar, quartz and
plagioclase, was found in a veinlet cutting leucocratic
granulite. Zoned crystals of
primary grandidierite to
ominelite enclosed in quartz
are locally overgrown by prismatic crystals of boralsilite and
iron-rich werdingite.
Boralsilite also occurs as separate cross-shaped plumose
aggregates with iron-rich
werdingite in quartz.
Grandidierite is commonly rimmed by a narrow zone of
secondary
tourmaline or is partially replaced by the assemblage
tourmaline + corundum
± hercynite.
Secondary
tourmaline of the
schorl-magnesio-fotite-foitite-olenite
solid solution occurs as a replacement product of grandidierite, rarely
boralsilite. Other accessory minerals in the veinlet include
monazite-(Ce), ilmenite,
rutile, ferberite,
srilankite, löllingite,
arsenopyrite and apatite.
The assemblage most probably originated from a H2O-poor system at temperature ~ 750°C and
pressure ~ 6–8 kbar
(AM 95.1533-1547).
At the Vohibola phlogopite deposit, Behara, Amboasary Sud District, Anosy, Madagascar, fine bright blue-green
and partially gemmy thumbnail-size crystals of grandidierite have been found; they show partial to
complete terminations and measure from 1.5 to a bit more than 2 cm in size
(Minrec 55.1.114).
At the type locality, Andrahomana, Andranobory, Anosy, Madagascar, grandidierite occurs as bluish green
masses up to several centimeters across, in quartz. A
pegmatite cross-cuts
migmatitic high-grade
granulite facies
metamosphosed mudstone
gneiss
(Mindat).
At Ihosy, Ihorombe, Madagascar, grandidierite occurs in
cordierite and garnet in
gneiss
(Dana).
At Almgjotheii, Flatestøl, Lund, Rogaland, Norway, iron-rich grandidierite is associated with
titanium-rich dumortierite
and tourmaline
(Dana).
At Bok se Puts Farm, Gordonia District, ZF Mgcawu, Northern Cape, South Africa, grandidierite occurs with
werdingite and kornerupine
in a sillimanite-hercynite
rich band in gneiss.
Werdingite occurs in association with grandidierite in
small lenticular pockets, a few centimeters across, within the
kornerupine layer. Contacts between
werdingite and kornerupine
are commonly marked by rims of grandidierite, and those between
werdingite and
oxide-hercynite clusters by broad lobes of intergroths of
grandidierite-hercynite
(AM 75.415-420).
At St. Joe Resources Company drillhole 1872, Russell Township, St. Lawrence county, New York, USA,
grandidierite has been found replacing serendibite
(Dana).
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