Pentagonite

(repeating unit)Ca(VO)Si₄O₁₀·4(H₂O)IMA symbolPtg^[1]Strunz classification9.EA.55Crystal systemOrthorhombicCrystal classPyramidal (mm2)
(same H-M symbol)Space groupCcm2₁Unit cella = 10.386(4) Å,
b = 14.046(7) Å,
c = 8.975(2) Å; Z = 4IdentificationColorGreenish blueCrystal habitPrismatic crystals, often as radiating clustersTwinningMultiple twins producing a pseudo pentagonal symmetryCleavageGood on {010}TenacityBrittleMohs scale hardness3 - 4LusterVitreousDiaphaneityTransparentSpecific gravity2.33Optical propertiesBiaxial (-)Refractive indexn_α = 1.533 n_β = 1.544 n_γ = 1.547Birefringenceδ = 0.014PleochroismVisible: X=Z= colorless Y= blue2V angleMeasured: 50°References^[2][3][4]

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.^[2] It is a dimorph of cavansite.

Pentagonite was first described in 1973 for an occurrence in Lake Owyhee State Park, Malheur County, Oregon.^[3] 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.^[2]

References