Connoisseur’s Choice: Volborthite, Milpillas, Sonora, Mexico

Abstract

In April 2015 a single small pocket containing perhaps two dozen important specimens of volborthite instantly revolutionized the collecting world’s perception of this colorful but hitherto unprepossessing copper-vanadate-hydrate species. The new specimens featured 3.5-cm flowershaped rosettes of dark olive-green platy crystals, some with edges altering to light green vésigniéite, overgrowing and nestled among distinctive platy and bladed azurite crystals to 2 cm, with a final fillip of 0.25–0.5-cm green malachite balls, all on a blinding-white clay-coated matrix. Pieces ranged from 2-cm thumbnail-sized off-matrix rosettes to superb miniatures and large-cabinet plates 15 cm across with multiple azurite crystals set off by volborthite “flowers.” The find easily eclipsed the previous best for the species, also from Milpillas, that comprised an equally limited find of sharp, glassy, 1.0–2.5-cm single transparent dark olive-green platy crystals on a similar white matrix. Prior to the outstanding Milpillas finds, the largest reported volborthite crystals were radiating clusters of crystals to 1.5 cm from the Gold Quarry mine, Eureka County, Nevada (Anthony et al. 2000). The most prolific volborthite localities are in the Monument Valley area of Arizona where several occurrences have produced crusts of 1–3-mm scaly crystals and, less commonly, isolated platy crystals and rosettes in the 3–4-mm size range. Rare examples reach 1 cm. Volborthite, Cu3 V2 O7(OH)2·2H2O, crystallizes in the prismatic class of the monoclinic system (point group 2/m) where it commonly forms pseudohexagonal crystals. The crystal structure of volborthite was determined by Kashaev and Bakakin (1968) with refinement of the unit cell by Leonardsen and Petersen (1974). Volborthite occurs most commonly as fibrous or spongy crusts and coatings of tiny scales displaying triangular or hexagonal outlines. Less commonly it forms rosettelike aggregates, smooth spherical balls, and reticulated groups. Volborthite is translucent to opaque (rarely transparent), yellow-green to olive-green, with a waxy, vitreous, or pearly luster. The Mohs hardness is 3.5, and the specific gravity (measured) ranges from 3.5 to 3.8. Volborthite has one perfect cleavage (https://www.mindat. org/min-4200.html; accessed July 2022). Volborthite is isostructural with martyite and karpenkoite, the zinc and cobalt analogues, respectively. Volborthite closely resembles and is easily confused with tangeite, CaCu(VO4)(OH), which was formerly known as calciovolborthite (Basso, Palenzona, and Zefiro 1989; https://www. mindat.org/min-4200.html; accessed July 2022). The Milpillas mine in Sonora, Mexico, produces by far the finest examples of the species (https://www.mindat.org/ loc-158068.html; accessed July 2022; Jones and Megaw 2017, 2021), but there are 173 additional world occurrences of volborthite (https://www.mindat.org/min-4200.html; accessed Milpillas, Sonora, Mexico