pentagonite

{{Infobox mineral | name = Pentagonite | category = Phyllosilicate minerals | image = Pentagonite-177796.jpg | alt = | caption = | formula = Ca(VO)Si4O10·4H2O | IMAsymbol = Ptg | strunz = 9.EA.55 | system = Orthorhombic | class = Pyramidal (mm2) (same H-M symbol) | symmetry = Ccm21 | unit cell = a = 10.386(4) Å, b = 14.046(7) Å, c = 8.975(2) Å; Z = 4 | color = Greenish blue | habit = Prismatic crystals, often as radiating clusters | twinning = Multiple twins producing a pseudo-pentagonal symmetry | cleavage = Good on {010} | tenacity = Brittle | mohs = 3 - 4 | luster = Vitreous | streak = | diaphaneity = Transparent | gravity = 2.33 | density = | polish = | opticalprop = Biaxial (-) | refractive = nα = 1.533 nβ = 1.544 nγ = 1.547 | birefringence = δ = 0.014 | 2V = Measured: 50° | pleochroism = Visible: X=Z= colorless Y= blue | 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 () 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.