Formula: Ca(VO)Si4O10 · 4H2O

Species: Silicates – (Phyllosilicates)

Colour: Greenish blue

Lustre: Vitreous

Hardness: 3 – 4

Specific Gravity: 2.33 (Calculated)

Crystal System: Orthorhombic

Name: For the pseudosymmetrical five-fold habit of the typical pentagonite twins.

Type Locality: Owyhee Dam, Lake Owyhee State Park, Malheur Co., Oregon – USA

Dimorph of: Cavansite. Structurally related to plumbophyllite.
Cavansite might be a low-temperature form and pentagonite a high-temperature form.