Fluor-buergerite

fluor-buergerite

opal

cassiterite

tourmaline

Images

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

Volcanic igneous 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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