Mineralogy and Petrology最新文献

The new mineral khurayyimite Ca₇Zn₄(Si₂O₇)₂(OH)₁₀·4H₂O occurs in colorless spherulitic aggregates in small cavities of altered spurrite marbles located in the northern part of the Siwaqa pyrometamorphic rock area, Central Jordan. It is a low-temperature, hydrothermal mineral and is formed at a temperature lower than 100 °C. Synchrotron single-crystal X-ray diffraction experiments have revealed that khurayyimite crystallizes in space group P2₁/c, with unit cell parameters a = 11.2171(8), b = 9.0897(5), c = 14.0451(10) Å, β = 113.297(8)º, V = 1315.28(17) ų and Z = 2. The crystal structure of khurayyimite exhibits tetrahedral chains of periodicity 6. The sequence of SiO₄ and ZnO₂(OH)₂-tetrahedra along the chain is Si–Si-Zn. The neighboring SiO₄-tetrahedra of the corrugated chains are bridged by additional ZnO₂(OH)₂-tetrahedra to form 3-connected dreier rings. The chains can be addressed as loop-branched sechser single chains {lB, 1¹_∞}[⁶Zn₄Si₄O₂₁]. The chains are linked by clusters of five CaO₆ and two CaO₇ polyhedra with additional OH groups and H₂O molecules in the coordination environment. Based on the connectedness and one-dimensional polymerisations of tetrahedra (TO₄)ⁿ⁻, chains of khurayyimite belong to the same group as vlasovite Na₂ZrSi₄O₁₁, since they can be described with geometrical repeat unit ^cT_r = ²T₄ ³T₄ and topological repeat unit ^cV_r = ²V₂ ³V₂.

As a part of the Kermanshah ophiolite in western Iran, the Cretaceous Nourabad-Dinavar ophiolitic complex is a remnant of the Neo-Tethys oceanic lithosphere and represents transitional mantle-crust and upper crust units in the Nourabad and Dinavar regions, respectively. All the units were affected by the two metamorphic regimes of static metamorphism and dynamic metamorphism. The whole-rock chemical data of the basic samples (i.e. gabbros, basalts, and dykes) show that they are related to the island-arc regime. The main reasons for this conclusion are as follows: their affinity with the calc-alkaline series, LREE enrichment, and subduction-related proxies such as the negative anomalies of Nb, Ta, Zr, and Hf and the positive anomaly of Th. On the other hand, the mineral chemistry analysis confirms that the studied ophiolitic complex is a MORB-type ophiolite emplaced in the supra-subduction zone. This is supported by mineralogical evidence including the compositional dependence of olivines (fo_90-91) on the spinel peridotite mantle facies, spinel minerals (Al-chromite and Mg/Cr-bearing hercynite), and Mg-rich orthopyroxenes (enstatite) in the harzburgites. The geochemical modeling implies that this complex evolved through the following successive magmatic steps: 1) the partial melting of a mixed NMORB-EMORB (50:50) source producing spinel harzburgite residues; 2) the fractional crystallization of the basic partial melts during their ascent to the surface and the formation of gabbro bodies; 3) the assimilation and fractional crystallization process as the NMORB components re-enter the chamber and produce basic pillow lavas, lava flows, and some fine-grained gabbro bodies (i.e. dykes). Accordingly, it can be interpreted that the emplacement history of the studied ophiolite succession has two stages: 1) an obduction stage in the Campanian; 2) an exhumation stage in the post-Cretaceous.

The new mineral nishanbaevite, ideally KAl₂O(AsO₄)(SO₄), was found in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with euchlorine, alumoklyuchevskite, langbeinite, urusovite, lammerite, lammerite-β, ericlaxmanite, kozyrevskite, and hematite. Nishanbaevite occurs as long-prismatic or lamellar crystals up to 0.03 mm typically combined in brush-like aggregates and crusts up to 1.5 mm across. It is transparent, colourless, with vitreous lustre. D_calc = 3.012 g cm^− 3. Nishanbaevite is optically biaxial (–), α = 1.552, β ≈ γ = 1.567. The chemical composition (average of seven analyses) is: Na₂O 3.79, K₂O 8.01, CaO 0.10, CuO 0.21, Al₂O₃ 30.08, Fe₂O₃ 0.50, SiO₂ 1.62, P₂O₅ 0.66, As₂O₅ 32.23, SO₃ 22.59, total 99.79 wt%. The empirical formula calculated based on 9 O apfu is: (K_0.57Na_0.41Ca_0.01)_Σ0.99(Al_1.99Fe³⁺_0.02Cu_0.01)_Σ2.02(As_0.95S_0.95Si_0.09P_0.03)_Σ2.02O₉. Nishanbaevite is orthorhombic, Pbcm, a = 15.487(3), b = 7.2582(16), c = 6.6014(17) Å, V = 742.1(3) ų and Z = 4. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 15.49(100)(100), 6.56(30)(110), 4.653(29)(111), 3.881(54)(400), 3.298(52)(002), 3.113(29)(121), and 3.038(51)(202, 411). The crystal structure, solved from single-crystal XRD data (R = 7.58%), is unique. It is based on the complex heteropolyhedral sheets formed by zig-zag chains of Al-centred polyhedra (alternating trigonal bipyramids AlO₅ and octahedra AlO₆ sharing edges) and isolated tetrahedra AsO₄ and SO₄. Adjacent chains of Al polyhedra are connected via AsO₄ tetrahedra to form a heteropolyhedral double-layer. Its topological peculiarity is considered and compared with those in structurally related compounds. The (K,Na) site is located in the interlayer space between SO₄ tetrahedra. The position of nishanbaevite among the arsenate-sulfates and their specific structural features are discussed. The mineral is named in honour of the Russian mineralogist Tursun Prnazorovich Nishanbaev (1955–2017).

The Neogene post-collisional volcanism in eastern Iran is represented by the Sikh Kuh and Bobak high-Na rocks including trachybasalt, trachyandesite, trachydacite, and dacite. We report whole rock geochemistry and Nd–Sr isotopic data which constrain the characteristics of the mantle source. The rocks are highly enriched in incompatible trace elements, suggesting a metasomatized subcontinental lithospheric mantle (SCLM) as the magma source. Felsic rocks record abundant petrographic evidence, major and trace element data, and isotopic (⁸⁷Sr/⁸⁶Sr(i) = 0.70727–0.70902) signatures indicative of fractional crystallization, and potentially, crustal assimilation. Such processes however, have not significantly affected the isotopic signatures (⁸⁷Sr/⁸⁶Sr(i) = 0.70417–0.70428) of the mafic members, suggesting that they are derived from a mantle source. The geochemical and isotopic data for the Sikh Kuh and Bobak volcanic rocks suggest that these Neogene magmas were derived from a small degree of partial melting (~ 2–10 vol%) of a spinel-bearing subcontinental lithospheric mantle source in a post-collisional setting. The generated more unfractionated mafic magmas erupted during an episode of extensional tectonics, presumably caused by extension that followed Eocene collision between the Lut and Afghan continental blocks. These melts interacted with continental crust during ascent, experiencing crystal fractionation, and crustal assimilation, to produce more evolved felsic volcanic rocks.

Structural characteristics of serpierite samples from Zvezdel, Bulgaria, and Lavrion, Greece, are reported. The thermal behaviour of serpierite from Lavrion is discussed. The chemical composition of the studied samples is analysed by energy-dispersive spectroscopy (EDS) and confirmed by single-crystal structure refinements. The obtained chemical formulas correspond well to that of serpierite with Cu:Zn ratio varying between 2.9 and 5.6. The sample from Zvezdel, with composition Ca[Cu_3.3Zn_0.7(OH)₆(SO₄)₂•3H₂O, crystallizes in the monoclinic crystal system, with space group I2 and unit-cell parameters a = 18.418(3), b = 6.220(1), c = 12.091(2) Å, β = 90.78(1)˚, whereas the one from Lavrion Ca[Cu_2.8Zn_1.2(OH)₆(SO₄)₂]•3H₂O, shows similar unit-cell parameters a = 18.394(1), b = 6.256(1), c = 12.097(1) Å, β = 90.92(1)˚, but higher I2/m space-group symmetry. Both studied crystals exhibit serpierite structure topology, but different stacking sequence of the octahedral layers. While in previously studied serpierite of Sabelli and Zanazzi (ActaCryst B24:1214-1221, 1968) there are two layers per unit cell, in currently studied samples there is only one. As a consequence, their unit-cell volumes are half than that of the first structurally characterized serpierite specimen with SG C2/c and unit-cell parameters a = 22.186(2), b = 6.250(2), c = 21.853(2) Å, β = 113.36(1)˚. Taking into account the structural peculiarities of the studied samples they are considered as serpierite polytypoids.

Minor ultramafic (dunite) and mafic (gabbroic) rock occurrences are exposed in South Andaman Island, Bay of Bengal. Dunite is in contact with serpentinite, while gabbroic rocks are in contact with the pyroxenite. Petrographic analysis using a petrographic microscope, major and trace element [including rare earth elements (REE)] analysis using an X-ray Fluorescence (XRF) spectrometer and the High Resolution Inductively Coupled Plasma Mass Spectrometer (HR-ICPMS), and mineral chemistry using an Electron Probe Micro-Analyzer (EPMA) were performed on selected ultramafic and mafic rocks. Petrographically, dunite is composed of olivine, clinopyroxene, and orthopyroxene, while olivine, clinopyroxene, orthopyroxene, and calcic plagioclase are present in olivine–gabbronorite. The bulk rock elemental relationship (Zr versus P₂O₅ and TiO₂ versus Zr/P₂O₅) indicate that the dunite and olivine–gabbronorite are tholeiitic in composition. The clinopyroxene with high Mg# [Mg²⁺/(Mg²⁺ + Fe²⁺)] and lower TiO₂ content is present in dunite, whereas the clinopyroxene with high Mg# and high TiO₂ content exists in olivine–gabbronorite. Cr₂O₃ versus Mg# in the clinopyroxene relationship and negative Nb, Ta, and Ti anomalies in these rocks imply high pressure arc related peridotite mantle source. Our results suggest that the dunite and gabbroic rocks were also intruded in the Andaman Ophiolitic suite of rocks during earlier subduction setting in Late Cretaceous time. Further, it is suggested that these ophiolites have been obducted on to the leading edge of the Eurasian continent during the Mid–Eocene to Late Oligocene event, prior to the current tectonically active Andaman–Java subduction, which was initiated in the Late–Miocene.

Floods in 1997 and 2010 exposed the Frýdek and Frýdlant formations of the Subsilesian Unit in the Ostravice River bed near Frýdek-Místek. In the sedimentary sequence of upper Campanian to Maastrichtian marls and paraconglomerates, clasts of strongly altered basic volcanic rock were found, accompanied by carbonate concretions and layers. Rare apatite, biotite, and a Cr-rich spinel subgroup mineral are the only relatively well-preserved primary minerals in the clasts. The matrix contains buddingtonite, albite, sanidine, kaolinite, illite-muscovite, a mineral of the smectite group, and possibly also a mixed structure mineral of the chlorite-smectite type. Laths of buddingtonite, identified by powder X-ray diffraction and wavelength-dispersive X-ray spectrometry, are not homogenous. Their compositions range from Bd₄₁ to Bd₅₉ molar component, with Kfs ranging between 26 and 35 mol%, Nafs between 5 and 27 mol%, and Ca-feldspar between 1 and 4 mol%. The matrix is irregularly dolomitized. Carbonates are also present in pseudomorphs after idiomorphic olivine and in fill of amygdaloidal cavities. These carbonates reveal complicated alteration rock history, having cores of magnesite passing into almost pure siderite outer parts. Calcite is always the youngest and most homogenous carbonate, probably connected with a different geological event. Accompanying carbonate concretions are composed of three dolomitic phases with quartz, calcite, and muscovite. We can conclude that buddingtonite originates in alteration of primary feldspar and/or volcanic glass during the catagenetic breakdown of kerogen in the sediment, surrounded by clayey sediments rich in decomposing organic matter. Volcanic clasts have similar texture and supposed pre-alteration phase composition as the rocks of teschenite association, namely monchiquites to picrites. However, the source of volcanic clast within the sediments remains unclear.

Intracrystalline exsolution textures in alkali feldspar are common in lithotypes from many alkaline complexes of the Eastern Ghats Granulite Belt (EGGB), India. However, the parentage of these textures and their compositional evolution is not well documented from this granulite belt. This study on the Koraput Alkaline Complex (KAC) in the EGGB documents the exsolution textures from several lithologies, establishes their igneous origin and finally links their compositional modifications with the evolutionary history of the complex. The studied exsolution textures belong to both perthite and mesoperthite. To estimate the temperature of formation of these textures, we used both two-feldspar thermometry, and one-feldspar thermometry following several models. In two-feldspar thermometry, compositions of exsolved alkali feldspar and the adjacent plagioclase feldspar pairs were used. In one-feldspar thermometry, the reintegrated compositions of exsolved alkali feldspars were used. The maximum temperature of formation of exsolution lamellae estimated from two-feldspar thermometry for mesoperthites in nepheline syenite is > 841 °C, and for perthites is > 759 °C, at 7 kbar pressure. Compositions of initially formed plagioclase feldspar lamellae and the host feldspar were more orthoclase rich and more albite rich respectively compared to the observed compositions. Using one-feldspar thermometry the calculated temperatures for alkali gabbro, syenite and alkali feldspar granite are > 870 °C, > 810 °C and > 730 °C, respectively. Compositions of alkali feldspars immediately before exsolution in these rocks were also estimated. Albite and orthoclase contents were nearly equal in mesoperthites; on the other hand, orthoclase content was higher than albite in perthites. Previous studies assigned their thermometric estimation with the minimum temperature of metamorphism that the KAC experienced, but the Ultra High Temperature (UHT) record obtained from the feldspar thermometry of the present study is difficult to correlate with these metamorphic events. Instead, these high temperatures may represent an igneous condition, which remained unaffected throughout the later metamorphic event as documented from nepheline syenite.