Copper in Natural Oxide Spinels: The New Mineral Thermaerogenite CuAl₂O₄, Cuprospinel and Cu-Enriched Varieties of Other Spinel-Group Members from Fumaroles of the Tolbachik Volcano, Kamchatka, Russia

This paper is the first description of natural copper-rich oxide spinels. They were found in deposits of oxidizing-type fumaroles related to the Tolbachik volcano, Kamchatka, Russia. This mineralization is represented by nine species with the following maximum contents of CuO (wt.%, given in parentheses): a new mineral thermaerogenite, ideally CuAl2O4 (26.9), cuprospinel, ideally CuFe3+2O4 (28.6), gahnite (21.4), magnesioferrite (14.7), spinel (10.9), magnesiochromite (9.0), franklinite (7.9), chromite (5.9), and zincochromite (4.8). Cuprospinel, formerly known only as a phase of anthropogenic origin, turned out to be the Cu-richest natural spinel-type oxide [sample with the composition (Cu0.831Zn0.100Mg0.043Ni0.022)Σ0.996(Fe3+1.725Al0.219Mn3+0.048Ti0.008)Σ2.000O4 from Tolbachik]. Aluminum and Fe3+-dominant spinels (thermaerogenite, gahnite, spinel, cuprospinel, franklinite, and magnesioferrite) were deposited directly from hot gas as volcanic sublimates. The most probable temperature interval of their crystallization is 600⁻800 °C. They are associated with each other and with tenorite, hematite, orthoclase, fluorophlogopite, langbeinite, calciolangbeinite, aphthitalite, anhydrite, fluoborite, sylvite, halite, pseudobrookite, urusovite, johillerite, ericlaxmanite, tilasite, etc. Cu-bearing spinels are among the latest minerals of this assemblage: they occur in cavities and overgrow even alkaline sulfates. Cu-enriched varieties of chrome-spinels (magnesiochromite, chromite, and zincochromite) were likely formed in the course of the metasomatic replacement of a magmatic chrome-spinel in micro-xenoliths of ultrabasic rock under the influence of volcanic gases. The new mineral thermaerogenite, ideally CuAl2O4, was found in the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption. It forms octahedral crystals up to 0.02 mm typically combined in open-work clusters up to 1 mm across. Thermaerogenite is semitransparent to transparent, with a strong vitreous lustre. Its colour is brown, yellow-brown, red-brown, brown-yellow or brown-red. The mineral is brittle, with the conchoidal fracture, cleavage is none observed. D(calc.) is 4.87 g/cm3. The chemical composition of the holotype (wt.%, electron microprobe) is: CuO 25.01, ZnO 17.45, Al2O3 39.43, Cr2O3 0.27, Fe2O3 17.96, total 100.12 wt.%. The empirical formula calculated on the basis of 4 O apfu is: (Cu0.619Zn0.422)Σ1.041(Al1.523Fe3+0.443Cr0.007)Σ1.973O4. The mineral is cubic, Fd-3m, a = 8.093(9) Å, V = 530.1(10) Å3. Thermaerogenite forms a continuous isomorphous series with gahnite. The strongest lines of the powder X-ray diffraction pattern of thermaerogenite [d, Å (I, %) (hkl)] are: 2.873 (65) (220), 2.451 (100) (311), 2.033 (10) (400), 1.660 (16) (422), 1.565 (28) (511) and 1.438 (30) (440)

Crystal Chemistry and Refined Formula of Tounkite

New data on the crystal structure and isomorphism of extra-framework components in the cancrinite-group mineral tounkite have been obtained using chemical and single-crystal X-ray diffraction data, as well as infrared, Raman, ESR, UV–Vis–near-IR absorption and photoluminescence spectroscopy methods. The crystal structure of tounkite is based on the aluminosilicate framework formed by the САСАСВСВСАСВ stacking sequence with ordered Si and Al atoms The framework hosts Losod and liottite cages as well as columns of cancrinite cages. It is shown that tounkite is characterized by wide variations of the chemical composition. Its simplified crystal–chemical formula is (Na+3.89–5.18K+0.15–1.64Ca2+2.30–2.58(Al6Si6O24)(SO42−,S52−,S4) 2−x (Cl−, HS−)1+y·nH2O (x, y, n 2⦁− and S3⦁− radical anions may occur in some tounkite samples in minor amounts. These crystal–chemical features indicate that tounkite crystallizes under highly reducing conditions. All studied tounkite samples were polysynthetic twins. A large 10-layed cage formed at the border between twin components, connected by a rotation of 180° around the [001] axis, which may host the large S52− anion