On the relative abundances of Cavansite and Pentagonite

Abstract

Cavansite is a visually stunning blue vanadosilicate mineral with limited occurrences worldwide, whereas Pentagonite is a closely related dimorph with similar physical and chemical properties, yet is extremely rare compared to Cavansite. The reasons behind Pentagonite's exceptional rarity remain largely unknown. In this study, (a) density functional theory (DFT) is utilized to investigate the electronic structures of Cavansite and Pentagonite at ground state and finite pressures; (b) a two-state Boltzmann probability model is then employed to construct a comprehensive phase diagram that reveals the abundance of each species across a wide range of pressure and temperature conditions; and (c) dehydration characteristics of these two minerals are explored. The present analysis reveals the key factors that contribute to the relative scarcity of Pentagonite, including differences in structural arrangement and electronic configurations between the two minerals. Specifically, it shows that (a) because of the peculiar arrangements of SiO4 polyhedra, Cavansite forms a compact structure (about 2.7% less in volume) resulting in lower energy; (b) at a temperature of about 650K only about 1% Pentagonite can form; (c) vanadium induces a highly localized state in both of these otherwise large-band-gap insulators resulting in an extremely weak magnetic phase that is unlikely to be observed at any reasonable finite temperature; and (d) water molecules are loosely bound inside the microporous crystals of Cavansite and Pentagonite, suggesting potential applications of these minerals in various technological fields.