Volume CLIV, Nº 4 (2025)

Metamorphic rocks of the Kichan structure of the Tiksheozero greenstone belt were formed after differentiated volcanic series and represented by gneisses, schists, and amphibolites. Conditions of metamorphism correspond to the high-pressure part of the epidote-amphibolite and amphibolite facies. The Gibbs energy minimization method has shown that mineral assemblages of metabasites (plagioclase + biotite + hornblende + garnet) occupy low- and medium-temperature regions in the P–T diagram (550–720 °C, 3–10 kbar). Metamorphic conditions for assemblages of kyanite-garnet-biotite and garnet + two-mica gneisses correspond to temperature 600–730 °C and pressure 6–8 kbar, which is consistent with petrographic observations. When using multiple mineral equilibria, the best intersections of reaction lines correspond to 640–650 °C, 7.8–8.0 kbar with decreasing to 620–630 °C, 6.2–6.5 kbar if Ti-containing phases and H₂O are taken into account. For metabasic mineral assemblages, the calculated water activity is close to 0.6, while for gneisses it reaches 0.9–1.0. According to predecessors, mineral assemblages and texture of the studied rocks were formed during the Proterozoic (1.80–1.76 Ga) stage of metamorphism of Archean volcanics synchronous with the late stages of the formation of the Svecofennian and Lapland-Kola orogens. Supracrustal series of volcanics were presumably formed in marginal-oceanic settings in the transition zone between the Karelian Craton and the Belomorian mobile belt.

For the first time on the Taimyr Peninsula, germanium mineralization, including its own mineral phases — minerals of the germanite group, namely germanocolusite, colusite and stibiocolusite, has been found and described. These minerals were discovered in calcite-sphalerite veins of the Koshka carbonatite massif, which belongs to the Middle-Late Triassic volcano-plutonic carbonate belt of the Taimyr Peninsula. In this members of the germanite group variations in the composition in the ratios of Ge, As, Sb and Sn are wide. Members of the colusite–germanocolusite isomorphous series are common whereas stibiocolusite is rare. These minerals form zonal crystals 5–50 µm in size in association with complex zonal sphalerite, chalcopyrite, pyrite, and less common galena. Colusite crystals exhibit both zoning with enrichment of the marginal parts by Sb and Sn, and oscillatory zoning, with rhytmic varying of the contents of many species-defining components. Germanocolusite contains up to 11 wt. % Ge (5.1 apfu Ge); colusite – up to 11.5 wt. % As (5.99 apfu As); stibiocolusite – up to 6.9 wt. % Sb (1.92 apfu Sb); the Sn content does not exceed 4.1 wt. % (1.19 apfu Sn). Reflectance spectra and unit-cell parameter of these minerals were also determined.

Perhamite — an hydrous calcium and aluminum silicate-phosphate with a Si : P = 3 : 4 ratio — was found in the oolitic iron ores of the Marsyaty deposit at the Northern Urals, Russia. The studied ores are divided into oxide (quartz-chamosite-goethite) and carbonate (kaolinite-quartz-siderite) ones. Perhamite occurs in carbonate ores in association with siderite. Perhamite is represented by spherulite aggregates of lamellar grains 20—30 μm across. In hand samples, perhamite has a yellow-brown color, in thin sections it is colorless, transparent, n ≈ 1.56, birefringence is very low. The identification of the mineral is confirmed by XRD. The chemical composition of perhamite is characterized by variations in the contents of Si, P, Al, Ca and Sr. In the case where the phosphorus content is minimal and silicon is maximal, the cationic part of the mineral formula has the form (Ca_2.2Sr_0.6Ba_0.1)_Σ=2.9(Al_7.3Fe_0.4)_Σ=7.7(Si_3.1P_3.9)_Σ=7.00 or, more simply, (Ca_2.0Sr_1.0)_Σ=3.0Al_7.7(Si_3.0P_4.0)_Σ=7.0. Minerals with higher phosphorus and lower silicon contents correspond to the perhamite–goyazite polysomatic series. The polysomatic series perhamite–iangreyite is also possible. The formation of perhamite occurred at the stage of early diagenesis of ferruginous deposits and is associated with the process of replacing initially sedimentogenic goethite oolites by siderite.

The new compound Na_15–2xCu_7+x(AsO₄)₈F₃C_l2 (x ~ 0.12) was obtained by chemical transport reactions. The crystal structure at 100 K (tetragonal, P4bm, a = 14.58917(16), c = 8.30072(14) Å, V = 1766.76(5) ų, Z = 2) was solved by direct methods and refined to R₁ = 0.025. The crystal structure is based upon copper-arsenate layers formed by combining lavendulan clusters based on Cu1 and Cu2 atoms through Cu3O4FCl octahedra. The crystal structure exhibits sharp axial asymmetry. The Na_15–2xCu_7+x(AsO₄)₈F₃C_l2 (x ~ 0.12) compound is closely related to axelite Na₁₄Cu₇AsO₄)₈F₂C_l2. Nonstoichiometry of composition is caused by isomorphism according to the scheme 2Na⁺ → Cu²⁺ + ☐.

This paper presents results of a study of the color of natural spinel from the Goron deposit. Mössbauer spectroscopy, optical absorption and luminescence spectroscopy were used to identify crystal chemical features of Goron spinel. The distribution of main chromophore centers in studied spinel samples was studied and the nature of color of spinel has been revealed. The main color centers are chromium, iron, and manganese ions which are located in tetrahedral and octahedral coordination polyhedra. Additionally, it was established that the main color is affected by exchange interactions of pairs of Fe³⁺_IV–Fe³⁺_VI ions that interact through a common oxygen ligand. As a result, a complex set of optically active centers was obtained which created a unique color (including the alexandrite effect) in spinel.

Samples of viscous fossil resins: rumanite, rumanite-like resins and succinite from the collections of the Mining Museum were investigated using Fourier-transform infrared spectroscopy (FTIR). In this paper, the characteristics of the IR spectra of fossil resins from the Carpathian region (Romania, Moldova), Sakhalin Island (Starodub locality, Russia), Kaliningrad region (Yantarny settlement, Russia) and Transcaucasian amber (Gorchu locality, Azerbaijan) are compared. It was established that resinites of the Starodub deposit belong to two types of viscous fossil resins. The more common type of resin has identical IR spectra with rumanite-like resins from Transcaucasia. Classical rumanite of the Carpathian region differ significantly from them in the IR spectra: the absence of absorption bands at 1325, 1180, 1095, 855, 814 cm^-1, a wide band in the range between 1030 and 975 cm^-1. The IR spectrum of classical rumanite is closest to the IR spectrum of succinite, from which it differs by the absence of a clearly expressed Baltic shoulder with a maximum at 1160 cm^-1 and absorption bands associated with exocyclic groups (3078, 1642 and 888 cm^-1). All studied types of resinites are characterized by the signs of viscous resins: the maxima for single and double C-O bonds in carboxyl groups are at 1160 and 1725 cm^-1. For rumanite-like resins, we have established a significant variability in the intensity of individual IR absorption bands between different samples from the same location and within the same sample.