The uranyl tricarbonate mineral swartzite, CaMg[UO2(CO3)3]& centerdot;12H2O, was investigated using single-crystal X - ray diffraction on synthetic material between 295 and 100 K. Swartzite is composed of Mg(H2O)6 octahedra and {Ca(H2O)6[UO2(CO3)3]} complexes linked into a three-dimensional framework through an extensive hydrogen-bond network. At room temperature, the compound crystallizes in the monoclinic space group P21/m (a = 11.873(3), b = 14.638(3), c = 6.439(2) & Aring;, beta = 112.92(1)degrees, Z = 2, T = 295 K; non-standard unit cell). In this structure, the Mg(H2O)6 octahedron is centrosymmetric, whereas the {Ca(H2O)6[UO2(CO3)3]} complex exhibits mirror symmetry. Subtle disorder in two of the seven independent H2O groups gives rise to directionally disordered hydrogen bonds at room temperature. At approximately 238 K (- 35 degrees C), a reversible phase transition occurs, producing a low-temperature structure with space group P21/c (a = 11.850(3), b = 14.553(3), c = 12.837(2) & Aring;, beta = 113.39(1)degrees, Z = 4, T = 100 K). In this phase, the mirror plane is replaced by a c-glide plane, accompanied by doubling of the c axis. All atoms occupy general positions, and all H2O groups and hydrogen bonds become fully ordered. The phase transition is associated with small displacements and distortions of both the Mg(H2O)6 octahedra and the {Ca(H2O)6[UO2(CO3)3]} complexes. Atomic displacements between the 295 K and 100 K structures range from 0.02 to 0.28 & Aring; for non-hydrogen atoms. Final refinements converged at R 1 = 0.0164 for 2987 reflections at 295 K and R 1 = 0.0151 for 6627 reflections at 100 K. Additional diffraction measurements at 250 and 220 K constrain the phase transition process to this temperature range. Room-temperature neutron diffraction data complement the X-ray study and provide accurate hydrogen-bond geometries above the phase transition temperature.
A continuous igneous evolution has been recognized for the A-type rapakivi granites of the Mesoproterozoic Salmi Batholith in the Fennoscandian Shield. This evolution started with the creation of a 1546 Ma anorthosite-norite complex in the southern part of the batholith, which soon was succeeded by a 1540 − 1538 Ma coarse grained annite-bearing porphyric syenogranite (Type 1) in the central parts of the batholith. The evolution continued with the emplacement of 1538 − 1535 Ma medium-grained annite-bearing monzogranites, closer to the northwestern margin of the batholith (Types 2a and 2b) and culminated with the 1538 Ma siderophyllite-topaz-bearing Type 3 monzogranite. The latter can be subdivided into Type 3a (which is cut by fluorite veins and is capped with stockscheider pegmatites in contact with surrounding Archaean host rocks) and Type 3b (which is the most evolved phase and host to Sn-Zn-In-bearing greisen veins and considered to reflect the onset of magmatic–hydrothermal processes). This igneous evolution can be quantified by means of the Zr/Hf and Rb/Ba ratios as well as characteristic rare earth element (REE) contents, in particular Ce and Eu, as well as the presence/absence of topaz. Based on available U-Pb age data, it is shown that the time span for the evolution of the granitic phases took place within a few millions of years (from 1540 to 1535 Ma), which overlaps in time with the formation of the 1543 − 1539 Ma polymetallic (Cu-Zn-Pb-In-Sn-Fe) skarn ores of Pitkäranta.
Elemental concentrations in Cr-spinel of mantle peridotite are widely used to determine mantle melting conditions and tectonic setting. However, Cr-spinels with low Cr# [Cr/(Cr + Al)], indicative of mid-oceanic ridge (MOR) setting, are commonly observed in the residual peridotite associated with the subduction setting. The origin of this MOR-like Cr-spinel in subduction-related residual peridotite is enigmatic, creating ambiguity in distinguishing between tectonic environments. We present in-situ major-element concentrations of Cr-spinels along with whole-rock 143Nd/144Nd ratios in residual serpentinized peridotite from the Manipur ophiolite in the Indo-Burma Ranges, India. Cr-spinels are characterised by Al-Cr heterogeneity and exhibit reverse zonation. This reverse zonation likely results from the interaction of Cr-spinel with the slab-derived fluids or hydrous melt in the mantle wedge, which lowers the Cr/Al ratio, modifies Cr#, and complicates determination of the degree of mantle melting and tectonic setting. Furthermore, an earlier melt extraction from a metasomatized source and its subsequent fluid/hydrous melt impregnation with the residual mantle source in the forearc mantle wedge have been inferred. Based on Nd isotopic composition, trace elements, and Cr-spinel chemistry, we propose that Cr-spinels with low Cr# (< 0.6), which were previously suggested to be MOR-related in the Manipur ophiolite, initially originated with high Cr# in a subduction setting and subsequently were modified by late-stage fluid/hydrous melt metasomatism in the mantle wedge.
The Salmas region in northwestern Iran contains serpentinized peridotites that form part of a Mesozoic ophiolite mélange complex. Based on their mineral assemblages, these peridotites are classified as clinopyroxene-bearing harzburgites. Whole-rock compositions, with oxide contents of MgO 32.59–37.67 wt
Many studies have shown that geological processes, such as metamorphism and metasomatism, consist of multiple superimposed events developing over various timescales, with each stage overprinting the previous one and thus complicating reconstruction of the complete history. Here, we report the use of a natural calcium aluminum borosilicate mineral axinite-(Fe) [(Ca2Fe2+Al2BSi4O15(OH)], which occurs in skarn deposits, as a starting material in a series of experiments representing various metasomatic processes. The fluids used include NaF+H2O, NaCl+H2O, KCl+H2O, CaCl2+H2O, 2 M NaOH, 2 M KOH, Ca(OH)2+H2O, and Na2Si2O5+H2O, which are representative of common crustal fluids. The experiments were conducted at 600 °C; 200 MPa and at 800 °C; 500 MPa. The dominant experimental product was grandite garnet and, to a minor extent, wollastonite, titanite, and fluorite. Grandite garnet compositions vary significantly with respect to the fluid composition and the P-T conditions used, showing a wide range of grossular to andradite solid solutions. Under both P-T conditions, andradite-grossular zoning develops in experiments with metal-hydroxide–bearing fluids, whereas andradite-rich grandite garnet crystallizes in experiments utilizing Na2Si2O5+H2O. Despite varying physicochemical conditions, no differences are observed in the REE-chondrite normalized patterns from the product grandite garnet in each of the experiments. Wollastonite contains bustamite and ferrobustamite components whose composition is strongly controlled by the coexisting grandite garnet. These experiments provide important insights into understanding how first order skarn mineral assemblages may be altered during subsequent fluid-rock interaction under a variety of P-T conditions, highlighting how variations in the grandite garnet composition respond to variations in fluid composition.
