Vol 61, No 7 (2019)

The Neoarchean subalkaline magmatism of the Keivy structure is expressed in the formation of the volcanoplutonic latite–monzonite–granite association (LMGA). The formation of LMGA magmas is assumed to occur due to melting of metasomatically altered mafic rocks during intrusion into the lower crust of basaltic melts initial for rocks of the dike complex and gabbro–labradorite massifs. The alkaline granites associated with LMGA have a close U–Pb age but a later formation time based on the geological data. With respect to LMGA, alkali granites have increased concentrations of SiO₂, alkalis (K₂O/Na₂O = 1.1–1.4), iron (F# = 84–98%), a high agpaitic index (K_agp = 0.86–1.2), and lower quantities of TiO₂, MgO, Fe_tot, and Al₂O₃, which probably resulted from the higher degree of differentiation of their initial melts compared to LMGA.

Three hundred thirty-six diamonds from deposits of the Rassolninskaya depression and 144 crystals from recent alluvial placers of the Krasnovishersky district were studied by IR absorption and photoluminescence spectroscopy. It is shown that crystals from the Rassolninskaya depression have a close-to-normal distribution for the nitrogen concentration. The average nitrogen content is 725 ppm, and no nitrogen-free crystals were detected. A sampling from recent alluvial placers contains 25% crystals with a nitrogen concentration smaller than 150 ppm; 3% of them are nitrogen-free. Among crystals from the Rassolninskaya depression, 12% are octahedral, 80% rhombododecahedral, and only one crystal has relicts of cubic faces. The collection from recent placers contains 3% cubic crystals, 10% individuals with relicts of cubic faces, 16% octahedroids, and 66% dodecahedra. Alluvial diamonds are often encountered with crescent-shaped cracks; however, they were observed only on a single crystal from the Rassolninskaya depression. It has been revealed that among alluvial placer diamonds, up to 95% of crystals contain nitrogen in the form of B1 defects. Thus, first, in morphological and structural-mineralogical features, diamonds from the Rassolninskaya depression differ from crystals of the nearest recent alluvial placers; second, they may belong to primary deposits based on the set of their characteristics.

A group of accessory ore minerals is revealed in rocks of the Galmoenan mafic-ultramafic massif (Koryak Upland, Kamchatka, Russia), represented by native metals (iron, copper, zinc, and silver), iron-nickel and cobalt-iron intermetallic compounds, sulfides (pentlandite, cobaltpentlandite, pyrrhotite, sphalerite, heazlewoodite, millerite, shandite, pyrite, chalcopyrite, and Cu₃S₂ phase), arsenides (maucherite, nicolite, orcelite, and modderite), and some other minerals. Their chemical compositions, crystal forms, and paragenetic associations are described. Formation of these minerals is attributed to serpentinization of rocks.

Magnetoplumbite-group minerals from various paragenetic assemblages, including a xenolith of altered garnet lherzolite from the Obnazhennaya kimberlite pipe in Yakutia, sulfide-free metasomatic Pb–Zn–Sb ore from the Pelagonian massif in the Republic of North Macedonia, and some other metasomatic occurrences have been studied with electron microprobe, as well as using IR and Raman spectroscopy. New data on isomorphic substitutions and crystal chemistry of the magnetoplumbite-group minerals have been obtained. Three potentially new mineral species belonging to this group have been identified: Al-dominant analogue of yimengite, Ba-dominant analogue of nežilovite, and Mn-dominant analogue of plumboferrite.

The pyrochlore-group minerals in ongonite and zwitter of the Verkhneurmiysky granite pluton in the Badzhal district, Russian Far East are reported. The composition, genesis, and crystallization sequence are characterized. Three rare species of pyrochlore, pyrochlore-I and pyrochlore-II in ongonite, and pyrochlore-III in zwitter are found for the first time in the Russian Far East. Pyrochlore-I is a possible new member of the pyrochlore group, bismuth-bearing “oxyferropyrochlore”; pyrochlore-II is an uranium-bearing and iron-bearing hydrokenopyrochlore; and pyrochlore-III is a lead-and iron-bearing hydrokenopyrochlore. All three pyrochlore species result from an alteration of early accessory minerals: niobium wolframite, samarskite, ishikawaite, wolframoixiolite, scheelite, and fergusonite. The Far East pyrochlore is characterized by extremely high Ta, W, and Fe contents. Compositional variations in pyrochlore are caused by substitution of Nb, Ta, W at the B site of the crystal structure and exchange of U, Pb, Bi, and Fe at the A site. The Ta, Bi, and Th concentrations decrease during transition from the late magmatic to greisen stage; the mineral forming significance of Nb, W, U, Y, Pb, and H₂O increases as followed: Ta, Nb, Bi, Fe, Th, As, P, Ca, Ti → Nb, Ta, W, U, Fe, HREE, Sc, Mn, Na, H₂O → Nb, Pb, W, Fe, Y, H₂O.

The rare beryllium silicate, meliphanite, from nepheline syenite pegmatite of the Sakharjok massif, Kola Peninsula has been studied. The chemical composition of the mineral has been refined using infrared and Raman spectroscopy; bulk chemical, thermal, and electron microprobe analyses; and singe crystal X-ray diffraction. The obtained data unambiguously support the presence of H₂O in the mineral and refine the crystal chemical formula of meliphanite from Sakharjok: Ca_4.00(Na_3.12Ca_0.88)_Σ4.00(Be_3.60Si_0.40)_Σ4.00Al_1.00(Si_6.74Be_0.26)_Σ7.00O_24.00[F_3.33(OH)_0.51O_0.16]_Σ4.00.

Esseneite from ultramafic xenoliths in dacite flow of the Ten’-01 paleovolcano at the Lena–Vilyui watershed, East Yakutia has been studied. The empirical formula of the mineral has been obtained using electron microprobe analysis: \({\text{C}}{{{\text{a}}}_{{0.99}}}{\text{Fe}}_{{0.52}}^{{3 + }}{\text{M}}{{{\text{g}}}_{{0.32}}}{\text{Fe}}_{{0.06}}^{{2 + }}{\text{T}}{{{\text{i}}}_{{0.05}}}{\text{Mn}}_{{0.01}}^{{2 + }}{\text{A}}{{{\text{l}}}_{{0.71}}}{\text{S}}{{{\text{i}}}_{{1.34}}}{{{\text{O}}}_{6}}\). Its crystal structure has been refined on the basis of the single crystal X-ray diffraction data, (R = 0.0152). The resulting crystal chemical formula is: \(^{{Ca}}{\text{C}}{{{\text{a}}}^{M}}{{\left( {{\text{Fe}}_{{0.48}}^{{3 + }}{\text{M}}{{{\text{g}}}_{{0.33}}}{\text{T}}{{{\text{i}}}_{{0.05}}}{\text{A}}{{{\text{l}}}_{{0.14}}}} \right)}^{T}}({\text{S}}{{{\text{i}}}_{{1.28}}}{\text{A}}{{{\text{l}}}_{{0.68}}}{\text{Fe}}_{{0.04}}^{{3 + }}){{{\text{O}}}_{6}}\). The monoclinic unit-cell parameters are: a = 9.7610(12), b = 9.8223(8), c = 5.3360(5) Å, β = 105.92(1), V = 441.89(8) ų, Z= 4, space group C2/c. The distribution of atoms over positions in the crystal structures of the Ca–Fe–Al clinopyroxenes and coordination polyhedron distortions, which reflect the formation conditions of the minerals, have been analyzed. The Yakutia esseneite has been formed in a highly oxidizing environment at 1200–950°C and approximate pressure of 2 kbar.