Pentagonite

Pentagonite is a rare silicate mineral with formula Ca(VO)Si₄O₁₀·4(H₂O). It was named for the unusual twinning which produces an apparent five-fold symmetry.[1] It is a dimorph of cavansite.

Pentagonite
General
Category	Silicate mineral
Formula (repeating unit)	Ca(VO)Si4O10·4(H2O)
Strunz classification	9.EA.55
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	Ccm21
Unit cell	a = 10.386(4) Å, b = 14.046(7) Å, c = 8.975(2) Å; Z = 4
Identification
Color	Greenish blue
Crystal habit	Prismatic crystals, often as radiating clusters
Twinning	Multiple twins producing a pseudo pentagonal symmetry
Cleavage	Good on {010}
Tenacity	Brittle
Mohs scale hardness	3 - 4
Luster	Vitreous
Diaphaneity	Transparent
Specific gravity	2.33
Optical properties	Biaxial (-)
Refractive index	nα = 1.533 nβ = 1.544 nγ = 1.547
Birefringence	δ = 0.014
Pleochroism	Visible: X=Z= colorless Y= blue
2V angle	Measured: 50°
References	[1][2][3]

Pentagonite was first described in 1973 for an occurrence in Lake Owyhee State Park, Malheur County, Oregon.[2] It has also been reported from the Pune district of India. It occurs as fracture and cavity fillings in tuff and basalt. It occurs with cavansite, heulandite, stilbite, analcime, apophyllite and calcite.[1]