Polymorphism and isomorphic substitutions in natural system Cu2+3(T5+O4)2 with T = As, V or P
Pekov I. V., Zubkova N. V., Yapaskurt V. O., Koshlyakova N. N., Turchkova A. G., Sidorov E. G., Pushcharovsky D. Yu.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 1. P. 108-130
In the natural system Cu2+3(T5+O4)2 (T = As, V, P), 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 poor-studied lammerite-like phase Cu3[(P,As)O4]2. In active oxidizing-type fumaroles related to the Tolbachik volcano (Kamchatka, Russia), all these minerals were crystallized at temperatures not lower than 250ºC. Crystal structure of the natural triclinic Cu3[(As,V)O4]2 was for the first time studied on a single crystal from the Yadovitaya fumarole (Tolbachik); R = 4.89%. This arsenate is isostructural to stranskiite and mcbirneyite, it crystallizes in the space group P-1; its unit-cell parameters are: a = 5.0701(8), b = 5.4047(8), c = 6.3910(8) Å, α = 70.091(13), β = 86.828(12), g = 68.399(15)°, V = 152.61(4) Å3 and Z = 1. The paper displays also new data on mcbirneyite (its novel As-enriched variety is characterized), lammerite and lammerite-β. Original and earlier published materials on natural representatives of the system Cu3(T5+O4)2 are summarized, isomorphism and other crystal chemical features of minerals belonging to this system are discussed in comparison with data on related synthetic compounds.
Keywords: copper arsenate, copper vanadate, polymorphism, lammerite, mcbirneyite, fumarole, Tolbachik volcano, Kamchatka