Formula: MgAl3O2(BO3)(SiO4)
Nesosilicate (insular SiO4 groups), borosilicate
Specific gravity: 2.98 to 2.99
Hardness: 7½
Streak: White
Colour: Bluish green, greenish blue
Solubility: Insoluble in acids
Common impurities: Ti,Mn,Na,K

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).


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 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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