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Download hereGENERAL DESCRIPTION
Pentagonite is a calcium and vanadium silicate. This species is formed within vesicles, faults and fractures of basalts or tuffs.
Pentagonite is "polymorphous" to cavansite, that is to say that pentagonite and cavansite have both the same chemical composition but have two clearly distinct atomic structures. This explains why they have very close colours (due to the vanadyle ion, containing the vanadium atom) but different habit.
Pentagonite forms small spherical vitreous crystals, spherically arranged. The crystals seldom exceed millifounders in size. Pentagonite is found associated with cavansite, stilbite or with apophyllite.
Pentagonite has been found only in two localities: near the Owyhee Dam and the Chapman quarry in Oregon (USA) and in the Wagholi and Lonavala quarries, Poonah district (India).
The type of this species is kept at the National Museum of Natural History of Washington (USA). It was described in 1973 on a sample from the Owyhee Dam, located near the lake of the Owyhee State Park, county of Malheur, Oregon (USA).
HISTORY : Name inspired from the Greek word "πεντα" [penta] meaning "five", because of its tendancy to form aggregates of 5 crystals
Species first described in 1973 by Lloyd Staples, Howard Evans & James Lindsay
Type-locality : Lake Owyhee State Park, Malheur Co, Oregon, USA
CHEMICAL FORMULA : Ca (V4+ O) Si4 O10 4H2 O
CRYSTAL SYSTEM : Orthorhombic
COLOR : Blue, blue-green
DIAPHANIETY : Transparent
LUSTER : Vitreous
STREAK : Blueish white
MORPHOLOGIE : Prismatic crystals, as spherical roses
HARDNESS : 3,0-4,0
CHEMICAL CLASS: 2,33
DENSITY : VIII - Silicates
GROUP : Cavansite - pentagonite series
STRUNZ CLASS BEFORE 2001 : 8/H.36-20
STRUNZ CLASS AFTER 2001 : 9.EA.35
