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Formula: (Na,K)xV4+xV5+6-xO15 (0.5 < x < 0.9)
Vanadate
Crystal System: Monoclinic
Specific gravity: 3.5 measured, 3.55 calculated
Streak: Dark grey-black
Colour: Black, light brown on thin edges in transmitted light
Environments
Localities
The type locality, the Izalco Volcano, Sonsonate Department, El Salvador, is a
basaltic composite cone that has been intermittently active since its
birth in 1770.
Vanadium is a distinctive element in the minerals of the sublimate suite,
which includes such vanadium minerals as
shcherbinaite, stoiberite and
ziesite. Bannermanite is found in the fumaroles as a black incrustation
coating basaltic breccia
fragments. It occurs as subhedral to euhedral crystals up to approximately 250 microns in size, and is intimately
associated with shcherbinaite
(AM 68.634-642). Other associated minerals include fingerite,
chalcocyanite and chalcanthite
(HOM).
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