ALBERT

Description: Olenitic tourmaline with high amounts of tetrahedral B (up to 2.53 [4] B pfu) has been synthesized in a piston-cylinder press at 4.0 GPa, 700 °C, and a run duration of 9 days. Crystals are large enough (up to 30 x 150 μm) to allow for reliable and spatially resolved quantification of B by electron microprobe analysis (EMPA), single-crystal X-ray diffraction, and polarized single-crystal Raman spectroscopy. Tourmalines with radial acicular habit are zoned in [4] B-concentration [core: 2.53(25) [4] B pfu; rim: 1.43(15) [4] B pfu], whereas columnar crystals are chemically homogeneous [1.18(15) [4] B pfu]. An amount of 1.4(1) [4] B pfu was found in the columnar tourmaline by single-crystal structure refinement (SREF) ( R = 1.94%). The EMPA identify [T] Si –1 [V,W] O –1 [T] B 1 [V,W] (OH) 1 as the main and [X] –1 [T] Si –1 [X] Na 1 [T] B 1 as minor exchange vectors for [4] B-incorporation, which is supported by the SREF. Due to the restricted and well-defined variations in chemistry, Raman bands in the OH-stretching region (3000–3800 cm –1 ) are unambiguously assigned to a specific cation arrangement. We found the sum of the relative integrated intensity ( I rel ) of two low-frequency bands at 3284–3301 cm –1 (1) and 3367–3390 cm –1 (2) to positively correlate with the [4] B concentrations: [4] B [pfu] = 0.03(1) x [ I rel (1) + I rel (2)]. Hence, those bands correspond to configurations with mixed Si/B occupancy at the T site. Our semi-quantitative correlation also holds for well-characterized natural [4] B-bearing tourmaline from the Koralpe, Austria. This work shows the potential for Raman spectroscopy as a non-destructive method for the chemical classification of (precious) natural tourmaline, and as a tool to rapidly characterize chemical zonation of tourmalines in thin section.

Description: A spin-polarized electrical current leads to a variety of periodical magnetic structures in nanostripes. In the presence of the Ørsted field, which always assists an electrical current, the basic types of magnetic structures, i.e., a vortex-antivortex crystal and cross-tie domain walls, survive. The Ørsted field prevents saturation of the nanostripe and a longitudinal domain wall appears instead. Possible magnetization structures in stripes with different geometrical and material properties are studied numerically and analytically.

Description: The occurrence of a trioctahedral analog of illite, the dioctahedral interlayer-deficient K-mica, has long been debated. Due to the inherent difficulties of determining structure and chemical composition of the extremely fine-grained material, earlier descriptions based on separated material are equivocal. Here we describe low-temperature (diagenetic) formation of fluorophlogopite, which is interlayer-deficient and therefore analogous to illite, using high-resolution in situ methods (transmission electron microscopy, TEM, with preparation by focused ion beam milling, combined with wavelength-dispersive analysis by field-emission gun electron microprobe). The average composition is K 0.5 Mg 2.8 V 0.01 Fe 0.005 [Si 3.15 Al 0.85 O 10 (OH) 0.65 F 1.35 ], including minor amounts of NH 4 for charge compensation as determined by electron energy loss spectroscopy. The K-deficient Mg-mica occurs in layer packages of ~10 layers, and no indications for interlayering with other sheet silicate layers such as chlorite or vermiculite could be identified with TEM. X-ray powder diffraction patterns of separated material confirm the absence of smectite components. The mineral was identified in phosphorites from the lowermost Cambrian Tal Group, Mussoori Syncline, Lesser Himalayas, India. The rocks are alternating phosphatic mudstones and phosphatic dolostones, at times interbedded with phosphate-poor carbonate layers, which are rich in organic matter. Sedimentary fluorophlogopite occurs in both rock types and in two textural associations; one in vesicles filled with amorphic organic matter, the other as reaction rims around illite, which contains up to 5 wt% V 2 O 3 in its rims. Textural arguments favor an early diagenetic formation of both, V-bearing illite and fluorophlogopite, closely associated with organic matter and linked to dolomitization. The high-F content stabilizes phlogopite to low temperatures. Our findings confirm that the stability field of fluorophlogopite extends from magmatic to metamorphic and sedimentary conditions.

Description: The Volyn pegmatites from Volodarsk-Volynskyi in the Zhytomyr Oblast, NW Ukraine, are associated with granites genetically related to the Paleoproterozoic Korosten pluton. Their late-stage evolution is characterized by the formation of opal-cemented breccia. A polymineralic pseudomorph after beryl within the breccia includes bertrandite (±euclase) + F-muscovite (with tobelite component) + buddingtonite + organic matter (OM) + opal (+ traces of K-feldspar, albite, columbite, FeS 2 , barite, REE-minerals). Sector-zoned and platy to fibrous buddingtonite has variable (K+Na)- vs. NH 4 -contents (electron microprobe analyses) and some H 2 O or H 3 O + , as indicated by microscope infrared spectroscopy. We suggest that ammonium was produced by decay of OM, which is partly preserved in the pseudomorph. Energy-dispersive electron microprobe data of the OM show with increasing O–decreasing C-N-content due to degassing; the OM contains the high field strength elements Zr (≤7 at%), Y (≤3 at%), Sc (≤0.8 at%), REE (≤0.3 at%), Th (≤0.2 at%), and U (≤1.25 at%), which increase with increasing O-content. Transmission electron microscopy of the OM confirms the presence of N; Zr, Si, and O (with other HFSE) are concentrated in nanometer-sized areas and at the transition from OM to opal in nanometer-sized platy Zr-Si-O crystals. C-rich areas are amorphous but show poorly developed lattice fringes. OM is present in the pseudomorph also as brown pigmentation of opal and in pegmatitic beryl from Volyn as a component in late stage fluid inclusions, identified by C-H vibrational bands in infrared spectra. Stable isotope investigations of C and N of buddingtonite, black opal and kerite (fibrous OM known from the literature to occur in the Volyn pegmatites and interpreted as microfossils) indicate a biogenic origin of the OM. We propose that OM in the pseudomorph is condensed kerite, which achieved the high concentrations of high field strength elements via fluid-pegmatite interaction. Although no age determination of minerals in the pseudomorph is available, textural arguments and phase equilibria indicate its formation in a late stage of the pegmatite evolution, at P-T conditions below ~100 MPa/150 °C. We favor a conceptual model for the formation of the Volyn buddingtonite in analogy to Phanerozoic occurrences of buddingtonite, where over and around the shallow anorthosite-granite Korosten pluton hydrothermal convection cells introduced N-bearing hydrocarbons and its precursors into the cooling igneous rocks. Due to the elevated temperature, the OM disintegrated into degassing volatile and non-volatile residual components analogous to petroleum maturation. Organic N, released as NH 4 , was then incorporated into buddingtonite.

Description: We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H 2 O 〈 3 ppm) single rutile crystals. They were equilibrated with pure H 2 O in hydrothermal experiments at constant conditions of 600 °C, 400 MPa, and f O 2 near the Ni-NiO buffer, run time between ~25 min and 14 days. Slabs cut parallel to (110) of the reacted single crystals (1 to 2 mm 3 ) were analyzed for H + by FTIR. Hydration occurs almost spontaneously and the H 2 O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H 2 O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H 2 O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H 2 O, and rutile with 2000 ppm V does not incorporate H 2 O. During stepwise heating at atmospheric pressure of a reacted Nb-doped rutile, H + is quickly released between 450 and 550 °C. UV-VIS spectra of unreacted colorless and reacted deep blue pure rutile show the rutile-characteristic sharp absorption edge in the UV spectra. The reacted rutile has a broad absorption band at 6500 cm –1 wavenumber attributed to intervalence charge transfer transition Ti 3+ +Ti 4+ -〉 Ti 4+ +Ti 3+ . The reduction of Ti 4+ to Ti 3+ is charge balanced by the incorporation of H + . The Nb-doped rutile changed its color from light greenish-blue (untreated) to deep blue. In the untreated Nb rutile, the UV-VIS absorption band at 6500 cm –1 indicates that Nb 5+ is charge balanced by Ti 3+ . In the reacted Nb-rutile the absorption band is more intense, but compared with the pure rutile, H + incorporation is lower by the equivalent of Ti 3+ reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti 3+ /Ti 4+ IVCT band at ~6400 cm –1 with moderate intensity considering the high-H 2 O contents of ~900 ppm. The high-H + contents are best explained by the reduction of Cr 4+ to Cr 2+ . The UV-VIS spectra of the dark-blue to opaque V-doped rutile show a very strong absorption toward low energies, which is likely caused by reduction of Ti 4+ to Ti 3+ for charge balance of V 5+ . This forms a deep narrow window of transmittance in the range 25 000–20 000 cm –1 , which causes the dark-blue color. To explore the possible use of H-in-rutile as a geohygrometer, geothermobarometer, and oxybarometer, we measured the H + content in a natural rutile crystal from a retrograded eclogite with a zoned trace element (Fe, Nb, and Zr) content. The crystal reveals a slight correlation between the variable H 2 O (~200 to 900 ppm) and its trace element concentrations. The observations indicate that the preservation of H + contents in this natural rutile is a complicated interplay of diffusive reequlibration of fast H + , slower Fe and very slow other trace elements. An interpretation of the H 2 O contents of the natural crystal in terms of f O 2 or a H 2 O is not possible.

Description: The Ca-Sr fractionation between zoisite and, respectively, lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl 2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients K D (Sr-Ca) A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are K (Sr-Ca) zo A1-af = 3.38 to 41.08 for the A1 site and K (Sr-Ca) zo A2-af = 0.45 to 6.51 for the A2 site. Assuming Ca af = Sr af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca 2+(af) + Sr A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + Ca A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] and Ca 2+(af) + (M 2+ ) A1 Sr A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + (M 2+ ) A1 Ca A2 Al 3 [Si 3 O 11 (O/OH)] yields μ 0 = –29 kJ/mol and W Sr-Ca zo A1 = 5.5 kJ/mol for the A1 site and μ 0 = –1.1 kJ/mol and W Sr-Ca zo A2 = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P 2 1 /m form. Calculated lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K D (Sr-Ca) law Cmcm -af = 1.12 to 11.32 for orthorhombic and K D (Sr-Ca) law P 21 m -af = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca 2+(af) + SrAl 2 Si 2 O 7 (OH) 2 ·H 2 O = Sr 2+(af) + CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O assuming Ca af = Sr af and a symmetric mixing model yields similar values of μ 0 = –9 kJ/mol and W Sr-Ca law Cmcm = 10 kJ/mol for orthorhombic and μ 0 = –10 kJ/mol and W Sr-Ca law P 21 /m = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are D Sr zo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D Sr law Cmcm -af = 0.6 ± 0.2 for orthorhombic lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and lawsonite-bearing rocks. For low Sr-concentrations, x Sr zo directly reflects x Sr af and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.