Pentagonite

crystals of pentagonite, crystals of stilbite : Wagholi Quarry, Wagholi, Pune District (Poonah District), Maharashtra, India — Photo: Parent Géry · CC BY-SA 3.0

Pentagonite

A rare blue calcium-vanadyl phyllosilicate — its color comes from the vanadyl ion in the lattice.

Intro adapted from Wikipedia (CC BY-SA).

Pentagonite is an IMA-recognized mineral species in the silicate class, with the chemical formula CaV⁴⁺OSi₄O₁₀·4H₂O.

It crystallizes in the orthorhombic crystal system (space group Ccm2_1).

Key properties: Mohs hardness 3–4; specific gravity 2.33; white streak; Vitreous luster; refractive index 1.541; optically biaxial.

IMA symbol Ptg, IMA status: approved, named in 1971.

Found in coatings on a tuff.

It is found associated with Cavansite, heulandite, stilbite, analcime, apophyllite, calcite.

The name refers to the unusual pseudosymmetrical five-fold or pentagonal habit of the twinned crystals.