Copper minerals from volcanic exhalations – a unique family of natural compounds: crystal-chemical review

The crystal-chemical characterization of oxysalts (sulfates, arsenates, vanadates, selenites, silicates, molybdates and borates), chlorides and oxides with species-defining Cu²⁺ formed in volcanic fumaroles (96 minerals representing 80 structure types; 81 species are endemic to fumarolic formation) is given. Copper minerals are known only from oxidizing-type fumaroles. The most diverse copper mineralization occurs at the Tolbachik volcano (Kamchatka, Russia). Copper minerals from fumarolic systems are subdivided into two genetic groups: Group I are minerals formed in the hot zones of fumaroles (>473 K, mainly 673–973 K) and Group II are minerals formed in the moderately hot zones of fumaroles (<473 K, mainly at 343–423 K). Group I includes 81 mineral species. Their most defining chemical feature is that all of them are hydrogen-free, and many of them contain the additional anion O²⁻. In comparison with minerals from other geological environments, in minerals of Group I the Cu²⁺ cation exhibits the strongest affinity for four- and fivefold coordinations and the strongest distortion of Cu²⁺-centred octahedra. Group II consists of 15 chlorides and sulfates including 13 H-bearing species. In these minerals the Cu²⁺ cation shows affinity for octahedral coordination, with OH⁻ and/or H₂O⁰ as ligands. In terms of crystal chemistry these minerals are closer to supergene minerals rather than to high-temperature fumarolic species. Temperature is the major factor governing the crystal chemistry of Cu²⁺ oxysalts and chlorides in low-pressure systems. The defining feature of fumarolic copper mineralization over this whole temperature range is the important role of alkali cations. The available data on complexes of Cu²⁺-centred polyhedra in the structures of natural oxysalts and halides are summarized and reviewed. Isomorphism in copper minerals from volcanic exhalations is discussed. The structures of high-temperature Cu oxysalts with additional O²⁻ anions (i.e. O atoms non-bonded to S⁶⁺, Mo⁶⁺, As⁵⁺, V⁵⁺, Se⁴⁺ or B³⁺) are also interpreted using an approach based on oxocentred tetrahedra.