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Formula: Mg2Al14Si4B4O37
Sorosilicate (Si2O7 groups), borosilicate
Crystal System: Triclinic
Specific gravity: 3.04 measured, 3.07 calculated
Hardness: 7
Streak: Buff-white
Colour: Brown-yellow, honey-yellow
Luminescence: No fluorescence under UV
Common impurities: Ti
Environments
Localities
At Horní Bory, Bory, Žďár nad Sázavou District, Vysočina Region, Czech Republic, a mineral assemblage involving
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 (light-coloured) 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-foitite - 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. Formation
of the borosilicate-bearing veinlet post-dates the development of foliation in the
host granulite. The assemblage most probably originated from a H2O-poor
system at a temperature about 750°C and pressure 6 to 8 kbar
(AM 95.10.1533–1547).
At Cap Andrahomana, Andranobory, Taolagnaro District, Anosy, Madagascar, five specimens of
grandidierite-bearing
pegmatite were studied; all five contain werdingite. The
pegmatite consists largely of
quartz and grey, perthitic
K-feldspar. Plagioclase is
subordinate to K-feldspar, and generally much finer grained.
Werdingite is found as inclusions in grandidierite or
garnet, as independent grains, and as grains mostly 0.2 to 1 mm across replacing
sillimanite. Werdingite is found in direct contact with
quartz, K-feldspar,
sillimanite, grandidierite,
hercynite and biotite.
Grandidierite, werdingite,
sillimanite and especially
andalusite are partially replaced by an aggregate of fine-grained
phyllosilicates distinct from muscovite
(CM 36.399-414).
At Almgjotheii, Flatestøl, Lund, Rogaland, Norway, a pegmatite
occurs whose host rocks are gneisses containing
garnet, cordierite,
spinel or sillimanite or both, and
traces of graphite; a particularly
graphite-rich layer crops out structurally below the
gneiss layers where the
pegmatite is exposed.
In hand specimens of the pegmatite,
grandidierite is visible as prisms up to 1.5 cm long and 0.5 cm across,
dark purple dumortierite occurs as prisms up to 1 cm in length and
sillimanite occurs as fibrous sprays. The dominant
K-feldspar is light grey in colour.
Werdingite has been definitively identified in only four of the 14 thin sections cut from one large specimen
collected in 1979; none was found in thin sections of 15 specimens collected since then.
Werdingite, commonly intergrown with boralsilite, forms bundles,
typically 2 to 3 mm long, of subparallel or somewhat radiating prisms, open clusters of such prisms, or intergrowths with
grandidierite. Prisms in the intergrowths with
boralsilite commonly also show a fine ladder-like texture that presumably
originated from exsolution. Other minerals in contact with werdingite are
plagioclase, K-feldspar,
quartz, blue and purple (titanium-poor)
dumortierite, and tourmaline.
Tourmaline has overgrown the intergrowths of werdingite with
boralsilite, and tourmaline
microveinlets have penetrated the intergrowths
(CM 36.401).
At the type locality, the Bok se Puts Farm, Kamiesberg Local Municipality, Namakwa District Municipality, Northern Cape,
South Africa, the host rocks form part of a
granulite facies terrane. Peak metamorphic conditions in
the granulite facies zone are estimated to be 800 to
850oC at 5 kbar pressure, and conditions at the sample site are assumed to have been close to this peak.
Werdingite forms millimeter-sized rounded grains and small clusters of grains in
kornerupine, and itself contains inclusions of
sillimanite, tiny rounded zircons
and slender, paralleloriented needles of a dark-coloured phase (rutile?).
Contacts between werdingite and kornerupine are commonly marked by
rims of grandidierite, and those between werdingite and
oxide-hercynite clusters by broad lobes of
grandidierite-hercynite
intergrowths. The intergrowths progressively replace werdingite and form embayments in adjacent
oxide-hercynite clusters.
Separated grains of werdingite are a translucent, honey-coloured brownish yellow with a vitreous lustre and a
conchoidal fracture
(AM 75.415-420.
Werdingite from the Bok se Puts Farm - Image
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