Cavansite vs. Pentagonite : risk of commercial trap

Pentagonite and cavansite are 2 hydrated calcium silicovanadates whose blue color is due to the presence of V⁴⁺. They are found associated especially in the vacuoles or cavities of the basalts of the traps of Deccan in India. The chemical formula of these two species is the same : Ca(V⁴⁺O) Si₄O₁₀ . 4H₂0 but their crystallographic structures are different, so called polymorphs. In our case it will be said that cavansite is dimorphic to pentagonite and vice versa. These two polymorphs crystallize in the same crystalline system (orthorhombic) however the differences in the arrangement of atoms in the crystal lattice result in distinct optical properties. Thus the cavansite is biaxial (+) while the pentagonite is biaxial (-).

It seems accepted that pentagonite forms rather long acicular crystals, sometimes more than 1 cm. However, there is no external evidence to differentiate a pentagonite from a cavansite, if it is not a characteristic five-branched pentagon-like cyclic twinning for the pentagonite (Owyhee Dam, Lake Owyhee State Park, Oregon, USA). Also without structural analysis, such as an X-ray diffraction for example (non-destructive), no one is able to prove the true nature of his sample. So there can be deception...

The figure to the right, modified from Howard T. Evans, Jr., shows the atomic structure of cyclic twinned pentagonite crystals seen in the basal section. The 5 red lines represent the 5 twinning planes.

In the mineral market, the term "pentagonite" is used very abusively. So you have to be careful. Many specimens are simply labeled "pentagonite" to justify higher prices...