Formula: CaV4+OSi4O10.4H2O
Phyllosilicate (sheet silicate), paramorph of cavansite
Specific gravity: 2.33 calculated
Hardness: 3 to 4
Streak: Bluish white
Colour: Greenish blue

Basaltic cavities

Pentagonite occurs as a fracture filling in tuff and basalt, associated with cavansite, heulandite, stilbite, analcime, apophyllite and calcite (HOM, Mindat).


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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