Polymorphism and Isomorphic Substitutions in the $${\text{Cu}}_{3}^{{2 + }}$$(T 5+O4)2 Natural System with T = As, V, or P

Abstract—In the $${\text{Cu}}_{3}^{{2 + }}$$ (T5+O4)2 (T = As, V or P) natural system arsenates are trimorphous (lammerite, lammerite-β, and unnamed triclinic Cu3[(As,V)O4]2), as well as vanadates (mcbirneyite, pseudolyonsite, and borisenkoite), while phosphates are represented by a poorly studied lammerite-like Cu3[(P,As)O4]2 phase. In active oxidizing-type fumaroles related to the Tolbachik volcano in Kamchatka, Russia, all these minerals crystallized at temperature no lower than 250°C. The crystal structure of natural triclinic Cu3[(As,V)O4]2 was first studied on a single crystal from the Yadovitaya fumarole at Tolbachik; R = 4.89%. It is isostructural to stranskiite and mcbirneyite and crystallizes in the P-1 space group; the unit-cell parameters are: a = 5.0701(8), b = 5.4047(8), c = 6.3910(8) A, α = 70.091(13), β = 86.828(12), γ = 68.399(15)°, V = 152.61(4) A3, and Z = 1. This paper also contains new data on mcbirneyite (its novel As-enriched variety is characterized), lammerite, and lammerite-β. Original and previously published materials on natural members of the Cu3(T5+O4)2 system are summarized and isomorphism and other crystal chemical features of minerals belonging to this system are discussed in comparison with the data on related synthetic compounds.