Andreybulakhite, Ni(C2O4) ⋅ 2H2O, the first natural nickel oxalate

Andreybulakhite, ideally Ni(C2O4) & sdot; 2H2O, is a new member of the humboldtine group, named in honour of Andrey Glebovich Bulakh of Saint Petersburg State University. The mineral was discovered at the Nyud-II (Nud-II) Cu-Ni sulfide deposit, Monchegorsk mafic-ultramafic pluton, Kola Peninsula, Russia. Andreybulakhite forms segregations of platy to prismatic crystals up to 2x1x1 mu m in size that are localized in the upper part of the fruiting bodies (apothecia) of Lecanora cf. polytropa lichen, whose colonies overgrow the oxidized surfaces of pyrrhotite-pentlandite-chalcopyrite ore. The mineral is monoclinic, with space group C2/c, a=11.8392(5) & Aring;, b=5.3312(2) & Aring;, c=9.8357(7) & Aring;, beta=126.723(5)degrees, V=497.59(3) & Aring;3 and Z=4. The Raman spectrum of andreybulakhite contains the following bands (cm-1): 1701 (C=O stretching vibrations and/or multiphonon processes); 1621 (H2O bending vibrations); 1454 and 924 (C-O and C-C stretching modes); 597 (Ni-O stretching, C-C-O and O-C-O bending vibrations); and 550, 307 and 226 (predominantly Ni-O stretching and deformation modes). The absorption bands of the infrared spectrum are (cm-1) 3389 (O-H stretching vibrations), 1640 (H2O bending vibrations), 1357 and 1315 (C-O stretching, C-C stretching), and 818 (Ni-O stretching, C-O and C-C stretching, C-C-O and O-C-O bending vibrations). The empirical formula calculated on the basis of (Ni + Cu + Mg + Co) =1 atom per formula unit is (Ni0.63Cu0.27Mg0.08Co0.02)Sigma 1.00(C2O4) & sdot; 2H2O. The absence of iron in the mineral is a result of oxidative Ni2+/Fe3+ fractionation during the secondary aqueous alteration of Ni- and Cu-rich sulfides. Andreybulakhite has synthetic Ni and Co counterparts; the latter implies the possibility of formation of its Co analogue in a related cobalt-rich environment.