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Pentagonite

Pentagonite is a rare hydrated calcium vanadium silicate with the formula Ca(VO)Si4O10·4H2O, prized for its vivid, pure sky-blue color derived from the vanadyl (VO2+) chromophore. Its name refers to the distinctive fiveling twins, star-shaped or bladed clusters that appear to show five-fold symmetry even though they actually form from six interpenetrating individuals. Crystallizing in the orthorhombic system, pentagonite is chemically identical to cavansite but structurally distinct, making the two minerals true dimorphs; some researchers propose that cavansite forms at lower temperature while pentagonite crystallizes at higher temperature within the same hydrothermal basaltic cavities. With a Mohs hardness of only 3 to 4 and a vitreous luster, specimens require careful handling, and genuine pentagonite is considerably scarcer on the market than its more common twin.

The type locality is Owyhee Dam in Lake Owyhee State Park, Malheur County, Oregon, USA, where it was first found in 1960 by Leslie and Mrs. Perrigo as a blue coating on basalt exposed in a road cut, and formally described in 1973 alongside cavansite. There it occurs as fracture and cavity fillings in tuff and basalt, associated with heulandite, stilbite, analcime, apophyllite and calcite. However, virtually all fine, gem-quality specimens seen in collections today come from the basalt quarries of the Deccan Traps near Pune, Maharashtra, India, especially the Wagholi Quarries and nearby Lonavala Quarry, where only a handful of the dozens of local quarries have ever produced this species. In these Indian basalt vugs, pentagonite forms sprays and rosettes of bladed crystals, often intergrown with or perched directly on cavansite, stilbite and heulandite, producing striking bicolor combination pieces highly sought by systematic and aesthetic collectors alike.

Because true pentagonite is frequently misidentified or mislabeled as cavansite due to their nearly identical appearance, distinguishing the two under strong blue-spectrum light, such as halogen lamps or direct sunlight, is essential: pentagonite reads as a very pure blue, while cavansite shows a subtler teal or greenish cast. Collectors value pentagonite for its genuine rarity, its striking color intensity, and its scientific interest as one of mineralogy's classic dimorphous pairs alongside cavansite, making well-formed twinned crystals from Wagholi or historic Oregon material especially desirable additions to a vanadium mineral or zeolite-association suite.

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