Formula: CaV4+OSi4O10.4H2O
Phyllosilicate (sheet silicate), paramorph of
cavansite
Crystal System: Orthorhombite
Specific gravity: 2.33 calculated
Hardness: 3 to 4
Streak: Bluish white
Colour: Greenish blue
Environments
Pentagonite occurs as a fracture filling in tuff and
basalt, associated with cavansite,
heulandite, stilbite,
analcime, apophyllite and
calcite
(HOM, Mindat).
Localities
The type locality is the Owyhee Dam, Lake Owyhee State Park, Malheur county, Oregon, USA. Here
cavansite and pentagonite are found in a brown
tuff overlain by basalt in a roadcut near
Owyhee Dam. The roadcut exposes a face 17 feet high with a coating 2-3 mm thick of cavansite,
pentagonite, and associated minerals.
Pentagonite, found so far in the US only at Owyhee Dam, seems to have formed at a later stage than
cavansite. The principal associates of cavansite
and pentagonite are calcite, heulandite,
stilbite, analcime, and
apophyllite. The pentagonite is probably of two generations. The sequence of
deposition at Owyhee Dam appears to be: (1) amber-coloured calcite, (2)
cavansite, (3) pentagonite and heulandite,
(4) stilbite, (5) pentagonite, (6) calcite,
(7) analcime and (8) apophyllite. One specimen was
found that has both cavansite and pentagonite together. Here the
cavansite is perched in radiating groups on calcite,
over which large, clear, colourless crystals of heulandite have formed. Smaller, long prismatic
crystals of pentagonite fill the open space above the calcite and
cavansite, and penetrate the heulandite crystals
in all directions. This association of minerals suggests that this specimen during its formation passed through the temperature of
transition between cavansite and pentagonite, and that the latter is the
low-temperature form
(AM 58. 405-411).
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