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Formula: Ca7(SiO4)3F2
Nesosilicate (insular SiO4 groups), chegemite subgroup,
humite group
Crystal System: Orthorhombic
Specific gravity: 2.91 calculated
Hardness: 5½ to 6
Streak: White
Colour: Colourless
Environments
Volcanic igneous environments
Metamorphic environments
Fluorchegemite is a relatively new mineral, approved in 2012. It formed in the
edgrewite-bearing zone of
endoskarn
(sanidinite facies) at the edge of an altered calciferous
xenolith within ignimbrite.
Localities
At the NW slope, Shadil-Khokh volcano, Kel’ volcanic area, Greater Caucasus Mountain Range, South Ossetia, Georgia,
fluorchegemite is associated with spurrite,
larnite, gehlenite,
merwinite, bredigite,
rondorfite and srebrodolskite
(HOM).
At the type locality, Xenolith no. 1, Lakargi Mountain, Upper Chegem volcanic caldera, Baksan Valley,
Kabardino-Balkaria, Russia, at the lower part of the volcanic sequence a
granodiorite-porphyry
stock has intruded into a 1.5 km thick series of
rhyolite-rhyodacite
ignimbrite and tuff. Higher in
the geological succession, a two-pyroxene
andesite lava overlies moraines of a previous glacial stage. Persilicic
ignimbrites of the Upper Chegem Caldera contain several small xenoliths
(up to 7 cm in size) belonging to different lithologies. In marginal zones of carbonate xenoliths
diopside and
grossular-andradite, and more
rarely wollastonite, have been identified, whereas the core of the
xenoliths is composed of recrystallized calcite
(marble). The large xenoliths (1–20 m in size) within
ignimbrites of the Upper Chegem Caldera composed of altered carbonate
silicate rocks were metamorphosed up to
sanidinite facies.
Minerals of the edgrewite -
hydroxyledgrewite series, associated with fluorchegemite, have
a very limited distribution; up to now (2015) they had only been detected in two small samples from xenolith no. 1.
Fluorchegemite was first discovered within kumtyubeite zones where
it forms small inclusions in large kumtyubeite grains. The
secondary minerals
bultfonteinite,
ettringite-thaumasite,
garnets of the
katoite-grossular series,
hydrocalumite, afwillite and
hillebrandite are widespread in the
kumtyubeite zones. The
primary minerals in these zones are
larnite, spurrite,
lakargiite, srebrodolskite,
perovskite, magnesioferrite,
wadalite and rondorfite.
Together with fluorchegemite, edgrewite and
eltyubyuite were discovered in the same samples.
Calcium-humite minerals are confined to whitish strongly altered zones.
Acicular fluorchegemite crystals up to 0.1 mm in length are irregularly distributed, associated with relatively large
edgrewite crystals. Calcium-humite
minerals are strongly replaced by late hydrosilicate minerals. Larger acicular fluorchegemite crystals, up to
0.2 mm in size, form aggregates replacing larnite in lens-shaped assemblages.
Wadalite-eltyubyuite,
rondorfite, lakargiite and
kerimasite are high-temperature accessory and minor minerals; the
secondary minerals include
bultfonteinite, killalaite,
hillebrandite, afwillite,
trabzonite and jennite
(CM 53.2.325-344).
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