• Pentagonite is the dimorph of cavansite whose name indicates its composition as a calcium vanadium silicate: Ca(VO)Si4O10 4H2O. They were first described in 1973 and have exclusively been found in the Malheur county (Oregon, USA) and the Wagholi quarry (Pune, India). Pentagonite was named after the apparent five-fold symmetry of twins.
    The artistic picture shows the crystal structure of pentagonite with light blue Ca polyhedra, red square-pyramids of V(IV) and blue Si tetrahedra (O = small red spheres). Ca and V sites are arranged in zig-zag chains along the c-axis and these in turn are bridged on both sides through a silica network of six-fold rings. This contrasts the presence of four- and eight-fold silica rings in cavansite which also hosts dissociated water molecules (H3O+ / OH-) as opposed to H2O in pentagonite. Accordingly, Ishida et al. have concluded that pentagonite forms at higher temperatures under supercritical conditions (J. Miner. Petrol. Sci. 104 (2009) 241).
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