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Pentagonite from Wagholi, India
Category Silicate mineral
(repeating unit)
Strunz classification 9.EA.55
Crystal system Orthorhombic
Space group Orthorhombic pyramidal
H-M symbol: (mm2)
Space group: Ccm21
Unit cell a = 10.386(4) Å, b = 14.046(7) Å, c = 8.975(2) Å; Z=4
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 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 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]