Chemistry:Pentagonite

Pentagonite
General
Category	Phyllosilicate minerals
Formula (repeating unit)	Ca(VO)Si4O10·4H2O
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
|re|er}}	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

Pentagonite is a rare phyllosilicate mineral with formula Ca(VO)Si₄O₁₀·4H₂O. Its characteristic blue color is due to the presence of the vanadyl (VO²⁺) cation in its crystal lattice. The oxidation state of vanadium in the vanadyl cation is +4; therefore, it is a divalent cation. It was named for the unusual twinning called a fiveling with an apparent five-fold symmetry. It is a dimorph of cavansite.

Pentagonite was first described in 1973 for an occurrence in Lake Owyhee State Park, Malheur County, Oregon. 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.