Том 58, № 8 (2016)

Phengite is a common metamorphic mineral stable in a wide pressure range. The dependence of pressure on silicon content established in the mid-20th century allowed us to propose a phengite-based geobarometer. Recently, the phengite geobarometer was calibrated by Caddik and Thompson (2008) but in the narrow pressure range. However, there attempts have been made to extend this range. We have analyzed the large number of published datasets on phengite composition. These data included both natural and experimental specimens of well defined P–T-conditions. For moderate temperatures (T < 750°C), two groups of phengite are identified. These groups are divided by silicon content value of 3.25 apfu. Different geobarometer equations were suggested for both groups. The precision of these geobarometers is ±0.34 GPa and ±0.56 GPa, respectively. There is no evidence of phengite used as a geobarometer at high temperatures (T > 750°C). The derived dependences were applied to study the conditions of gneiss and schist metamorphism of the Blyb metamorphic complex in the Northern Caucasus. This study shows that the peak pressure of gneiss and schist metamorphism is 2.0–2.2 ± 0.56 GPa. The latter agrees with previous data on the Blyb metamorphic complex.

Several types of both magmatic and metamorphic spinels have been found in Archean komatiites of the Sovdozero and Kostomuksha greenstone belts in the eastern part of the Fennoscandian Shield. Scanning electron microscopy and Raman spectroscopy revealed relics of cores of primary magmatic chrome-spinels with high Cr and Al contents. In the Sovdozero structure, the relics are better retained than those in the Kostomuksha structure, which is caused by a different degree of metamorphic transformation. The comparable 100 · Cr/(Al + Cr) values of spinel cores from Sovdozero and Kostomuksha reflect similar conditions of partitional melting in the mantle. These data agree with the fact that both komatiite complexes belong to the Al-undepleted petrogenic type. Wide variations in the Cr and Al contents in primary chrome-spinel cores together with a constant Mg/(Fe²⁺ + Mg) ratio correspond to low oxygen fugacity during magma crystallization. In general, the composition of these primary chrome-spinels is similar to that of accessory phases in peridotites from suprasubduction zones and agrees with hypothesis of komatiite complex formation in back-arc basins.

This study focuses on the morphological features, color cathodoluminescence, chemical composition, age, and source of zircons from the Ichet’yu occurrence. The isotopic U–Pb age of Paleo–Mezoproterozoic zircon grains varies within an interval of ~700 Ma from 2247 to 1478 Ma. The average roundness and well-preserved integrity of zircon grains allow us to suggest their proximal source. The available data show that the basement of the Middle Timan, composed of continental Paleo–Mezoproterozoic igneous rocks, is the most probable source of zircon in the Ichet’yu occurrence. These rocks are apparently a continuation of the Archean–Proterozoic Arkhangel’sk Mobile Belt.

A new mineral, tatarinovite, ideally Са₃Аl(SO₄)[В(ОН)₄](ОН)₆ · 12Н₂O, has been found in cavities of rhodingites at the Bazhenovskoe chrysotile asbestos deposit, Middle Urals, Russia. It occurs (1) colorless, with vitreous luster, bipyramidal crystals up to 1 mm across in cavities within massive diopside, in association with xonotlite, clinochlore, pectolite and calcite, and (2) as white granular aggregates up to 5 mm in size on grossular with pectolite, diopside, calcite, and xonotlite. The Mohs hardness is 3; perfect cleavage on (100) is observed. D_meas = 1.79(1), D_calc = 1.777 g/cm³. Tatarinovite is optically uniaxial (+), ω = 1.475(2), ε = 1.496(2). The IR spectrum contains characteristic bands of SO₄²⁻, CO₃²⁻, B(OH)₄⁻, B(OH)₃, Al(OH)₆^3-, Si(OH)₆^2-, OH^–, and H₂O. The chemical composition of tatarinovite (wt %; ICP-AES; H₂O was determined by the Alimarin method; CO₂ was determined by selective sorption on askarite) is as follows: 27.40 CaO, 4.06 B₂O₃, 6.34 A1₂O₃, 0.03 Fe₂O₃, 2.43 SiO₂, 8.48 SO₃, 4.2 CO₂, 46.1 H₂O, total is 99.04. The empirical formula (calculated on the basis of 3Ca apfu) is H_31.41Ca_3.00(Al_0.76Si_0.25)_Σ1.01 · (B_0.72S_0.65C_0.59)Σ_1.96O_24.55. Tatarinovite is hexagonal, space gr. P6₃, a = 11.1110(4) Å, c = 10.6294(6) Å, V = 1136.44(9) A³, Z = 2. Its crystal chemical formula is Са₃(Аl_0.70Si_0.30) · {[SO₄]_0.34[В(ОН)₄]_0.33[СO₃]0.₂₄}{[SO₄]_0.30[В(ОН)₄]_0.34[СО₃]_0.30[В(ОН)₃]_0.06}(ОН_5·73О_0.27) · 12Н₂O. The strongest reflections of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are 9.63 (100) (100), 5.556 (30) (110), 4.654 (14) (102), 3.841 (21) (112), 3.441 (12) (211), 2.746 (10) (302), 2.538 (12) (213). Tatarinovite was named in memory of the Russian geologist and petrologist Pavel Mikhailovich Tatarinov (1895–1976), a well-known specialist in chrysotile asbestos deposits. Type specimens have been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.

The morphology, optical properties, and composition of allanite-(Ce) and allanite-(Y) from raremetal tourmalinite of the Severnyi granitic pluton in the Chukchi Peninsula have been studied, as well as the composition and structure of host metasomatic rocks and the assemblage of rock-forming and accessory minerals. The hydrothermal origin of both allanite species and their stable combination have been established. Allanite-(Y) is partly replaced with allanite-(Ce), and metasomatic rims 2–10 μm wide enriched in LREE around allanite-(Y) have been identified. The degree of isomorphic Y+HREE substitution for LREE is estimated at 16%, on average; the maximum (Y+HREE)/LREE ratio does not exceed 0.25. It is assumed that the transition of allanite-(Y) into allanite-(Ce) might be caused by anincrease in acidity and decrease in aqueous fluid temperature at the late stage of the hydrothermal process.

The unique igneous rock (scapolite–diopside gabbro) from the Ilmeny Mountains in the southern Urals is described. Gabbro fills a segment of dike 1.3 m thick that cuts through calcite–dolomite carbonatite. Medium-grain pyroxenite with scapolite that occurs at selvages gradually passes to scapolite-bearing gabbro in the central part of the dike. Scapolite crystals display surfaces of concurrent growth, which are evidence of their magmatic origin. Scapolite (Me 63–70%) contains numerous pyrrhotite inclusions as platelets 0.001 mm thick oriented parallel to the cleavage plane {100}. The calculated pyrrhotite formula is consistent with its stoichiometry (Fe_1–xS). The morphology of the platelets (hexagonal sections) and their optical properties indicate a hexagonal symmetry of pyrrhotite. As follows from the insignificant difference between scapolite grains with and without pyrrhotite inclusions, scapolite and pyrrhotite should be regarded as products of synchronous magmatic melt crystallization.