| Pentagonite | |
| Category: | Silicate mineral |
| Formula: | Ca(VO)Si4O10·4(H2O) |
| Imasymbol: | Ptg[1] |
| 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 |
| Tenacity: | Brittle |
| Mohs: | 3 - 4 |
| Luster: | Vitreous |
| Diaphaneity: | Transparent |
| Gravity: | 2.33 |
| 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: | [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]