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Formula: NaFe3+3Al6(Si6O18)(BO3)3O3F
Cyclosilicate (ring silicate), borosilicate,
tourmaline group
Crystal System: Trigonal
Specific gravity: 3.311 measured, 3.29 calculated
Hardness: 7
Streak: Light brown
Colour: Bronze-brown to dark brown
Luminescence: Not fluorescent under UV
Common impurities: Ti,Mn,Mg,Ca,K
Environments
For decades fluor-buergerite was thought to be a one-locality mineral. However, it has been reported from
alluvial deposits in the Ratnapura district in Sri Lanka, the Stanislaw quarry in Poland, and in Vlastejovice,
Bohemia, Czech Republic
(R&M 88.5.442-446).
Localities
The type locality, the fluor-buergerite occurence, Mexquitic de Carmona Municipality, San Luis Potosí, Mexico, was a
small tin deposit where cassiterite
was being mined from a rhyolite flow and adjacent
placer.
Fluor-buergerite occurs in the pneumatically altered rhyolite
flow. Many if not most fluor-buergerite specimens contain one or more individual 1 cm lustrous, dark brown to
black crystals attractively scattered over a flat surface of pale tan
rhyolite matrix. Less often, fluor-buergerite occurs in
divergent crystal sprays lying on the surface of rhyolite plates.
Some specimens of this sort are partially coated with lustrous, glassy botryoidal
hyalite opal. Other much rarer specimens contain larger essentially black
fluor-buergerite crystals up to 2 cm in size in dark reddish-brown
common opal, with no adhering
rhyolite. Rhyolite
plates with scattered fluor-buergerite crystals reach at least 30 cm across in rare instances. The
fluor-buergerite is often marked by a decided bronze schiller-like reflection from planes immediately beneath
the surface of the crystal and on cleavage planes.
Associated minerals, other than opal, are barely recognisable in the generally
very fine-grained rhyolite host. They include
biotite, a plagioclase feldspar,
and likely a K-feldspar. No
cassiterite was seen on any fluor-buergerite specimens
(R&M 88.5.442-446).
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