ALBERT

Description: The study discusses the mineralogical, geochemical and thermometric properties of rock-forming blue quartz from eight worldwide occurrences. Compared to non-blue quartz, blue quartz contains significant amounts of submicron-sized (1 {micro}m-100 nm) and nanometre-sized (

Description: K-dominant tourmaline was synthesized in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -KCl-H 2 O at 700 °C and 4.0 GPa. The crystals were zoned and characterized by less-potassic cores (1.46 wt% K 2 O) and more-potassic rims (up to 3.44 wt% K 2 O). The K-dominant tourmaline rims are represented by the average structural formula (K 0.60(3) 0.36(3) )(Mg 2.60(7) Al 0.40(7) )(Al 5.98(3) Si 0.02(3) )Si 6 O 18 (BO 3 ) 3 (OH) 3.92(7) O 0.08(8) , which is analogous to the structural formula of dravite and is referred to here as "K-dravite"; the maximum analyzed K content (3.44 wt% K 2 O) represents occupancy of the X site by 0.71 K pfu. The addition of Na to the system in approximately equal molar proportions to K results in the crystallization of K-bearing, Na-rich dravitic tourmaline, dramatically reducing the K content to an average value of 0.47 wt% K 2 O, corresponding to 0.10 K pfu. This suggests that a K-dominated bulk composition is necessary for K-dominant tourmaline crystallization. Compositional zoning shows that solid solution exists between end-member compositions of "K-dravite" [KMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 (OH)] and dravite via the isovalent exchange X K( X Na) –1 , magnesio-foitite via the coupled substitution X K Y Mg( X Y Al) –1 , and "K-olenite" via the coupled substitution Y MgOH( Y AlO) –1 . Structural refinement of the powder X-ray diffraction data provides a unit-cell volume for the synthesized "K-dravite" of 1580.1(5) Å 3 , which is greater than that determined for K-bearing dravitic tourmaline synthesized at the same conditions [1574.9(4) Å 3 ]. We interpret this to reflect expansion of the crystal structure due to incorporation of the relatively large K + ion.

Description: Rutile, titanite, and zircon formed as relatively coarse-grained accessory minerals in several samples of high-grade calcite-dolomite marble with an early ultrahigh-pressure history. These minerals decomposed to a texturally complex set of secondary minerals during subsequent stages of retrograde metamorphism. The reactions involve several generations of geikielite-ilmenite as well as zirconolite [(Ca,Th,U)Zr(Ti,Fe,Nb,Ta) 2 O 7 ], kassite/cafetite [CaTi 2 O 4 (OH) 2 /CaTi 2 O 5 ·H 2 O], Ti-bearing humite group minerals, thorianite, and sometimes euxenite [(Ca,U,Th,REE)(Nb,Ta,Ti) 2 (O,OH) 6 ]. Stable coexistence of zircon and olivine is observed and stably coexisting titanite with olivine and/or humite-group minerals is reported here for the first time outside of carbonatites, kimberlites, or lamprophyres. Petrogenetic grids constructed for Ti- and Zr-bearing olivine/antigorite-saturated calcite-dolomite marbles show that geikielite is stable at highest pressures, followed by titanite and rutile, and that baddeleyite + diopside replaces zircon + calcite to higher pressures. The observed reaction textures are consistent with an earlier derived P-T path for the Kimi Complex. They corroborate a period of heating during decompression from 25 to 20 kbar and ca. 800 °C, where the assemblage olivine-diopside-spinel-rutile-zircon formed. This assemblage partially re-equilibrated during subsequent decompression and cooling, thus forming the observed reaction textures. Even though no memory of the UHP path is preserved in the accessory minerals, their reaction relationships turn out to be potentially very useful for geothermobarometry over a large range of metamorphic conditions.

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: Experiments were conducted to reproduce reaction rims of phlogopite ± diopside around olivine that have been observed within a wide range of potassic melts, including phonolite. Phlogopite is also a common secondary phase formed at the expense of olivine during metasomatic events involving K 2 O-and H 2 O-rich fluids or melts. Piston-cylinder experiments where olivine single crystals were reacted with synthetic phonolite melt at 10.7–14.7 kbar and 950–1000 °C recreate the mineralogy and textures documented in natural samples. Rim growth is parabolic with time, indicating a diffusion-controlled reaction. Fast diffusion in the melt and varying compositions across the phlogopite reaction rims suggest that diffusion through the rims, along grain boundaries is rate limiting. Reaction rates dramatically increase with temperature as well as the bulk water content of the sample charge. This is because of increasing amounts of atomically bound hydrous species along the grain boundaries that increase the rates of diffusion and thereby the rates of rim growth. Atomically bound hydrous species increase the rates of rim growth by lowering the activation energy for diffusion and by increasing the solubility of diffusing species in the grain boundary region. Transmission electron microscopy shows the presence of isolated pores and open grain boundaries. Most of these may have opened during quenching, but there is some evidence to suggest that a free fluid phase may have been locally present in experiments with high melt water contents (〉8 wt%). The measured rim growth rates at different conditions are used to estimate residence times of reacting olivine crystals in natural systems.

Description: We report the synthesis of h-magnetite, ideally h-Fe 3 O 4 with considerable amounts of substitutional cations (Cr, Mg, Al, Si) and quenchable to ambient conditions. Two types of experiments were performed at 18 GPa and 1800 °C in a multi-anvil press. In one, we used an oxide mixture with a majoritic stoichiometry Mg 1.8 Fe 1.2 (Al 1.4 Cr 0.2 Si 0.2 Mg 0.2 )Si 3 O 12 , with Si and Mg in excess as starting material (MA-367, MA-380). In the second type of experiment (MA-376), we started from an oxide mixture on the composition of the Fe-oxide phase obtained in MA-367. The Fe-oxide phases of both experiments were investigated by electron microprobe and transmission electron microscopy including electron diffraction tomography. Our investigations show that the Fe-oxide phases crystallize in the structure-type of h-magnetite. However, electron diffraction data show that keeping the cell setting from literature, this phase crystallizes in space group Amam and not in space group Bbmm as previously proposed. In the experiment MA-367, the Fe-oxide phase are mutually intergrown with majorite, the major phase of the run products. The formula for h-magnetite in this run was calculated as Fe1 (Fe 2+ 0.75 Mg 0.26 ) Fe2 (Fe 3+ 0.70 Cr 0.15 Al 0.11 Si 0.04 ) 2 O 4 . In the experiment on the bulk composition of the Fe-oxide, the main phase was h-magnetite with composition Fe1 (Fe 2+ 1.02 ) Fe2 (Fe 3+ 0.65 Cr 0.19 Al 0.13 Si 0.03 ) 2 O 4 and traces of nearly pure end-member wadsleyite and stishovite. Our results indicate that the substitution of 20 to 30% of Fe (0.7 to 0.9 atoms per formula unit) by smaller cations favored the preservation of the high-pressure form to ambient conditions. We prove that the h-magnetite-type oxide is also stable in chemical systems more complex than Fe-O. Based on our results, which were obtained at 18 GPa and 1800 °C in a system (MA-367) that is closely related to Fe-enriched oceanic lithospheric material, we suggest that a Fe 3 O 4 -rich phase may be present in environments connected to deeply subducted slabs and possibly associated with deep carbonatitic melting. Our observations show that Cr strongly partitions in the oxide phase such that the coexisting silicates are depleted in Cr compared to Fe 3 O 4 -free assemblages. This may significantly affect the chemical signature of melts produced in the deep mantle.

Description: Due to the similar ionic radius of K + and NH 4 + , K-silicates can incorporate a significant amount of NH 4 . As tourmaline is able to accommodate K in its crystal structure at high and ultrahigh pressure, we test if this also holds true for NH 4 . Piston-cylinder experiments in the system (NH 4 ) 2 O-MgO-SiO 2 -Al 2 O 3 -B 2 O 3 -H 2 O at 4.0 GPa, 700 °C, with B 2 O 3 and NH 4 OH in excess produce an assemblage of tourmaline, phengite, and coesite. The tourmaline crystals are up to 10 x 40 μm in size. EMP analyses indicate that the tourmalines contain 0.22 (±0.03) wt% (NH 4 ) 2 O and are solid solutions mainly along the magnesio-foitite and "NH 4 -dravite" join with the average structural formula X [(NH 4 ) 0.08(1) 0.92(1) ] Y [Mg 2.28(8) Al 0.72(8) ] Z [Al 5.93(6) Si 0.07(6) ] T [Si 6.00(5) O 18 ](BO 3 ) 3 (OH) 4 . NH 4 incorporation is confirmed by characteristic 〈N-H〉 stretching and bending modes in the IR-spectra of single crystals on synthetic tourmaline. Further evidence is the increased unit-cell parameters of the tourmaline [ a = 15.9214(9) Å, c = 7.1423(5) Å, V = 1567.9(2) Å 3 ] relative to pure magnesio-foitite. Incorporation of NH 4 in natural tourmaline was tested in a tourmaline-bearing mica schists from a high- P /low- T (〉1.2 GPa/550 °C) metasedimentary unit of the Erzgebirge, Germany, rich in NH 4 . The NH 4 -concentrations in the three main NH 4 -bearing phases are: biotite (~1400 ppm) 〉 phengite (~700 ppm) 〉 tourmaline (~500 ppm). This indicates that tourmaline can act as important carrier of nitrogen between the crust and the deep Earth, which has important implications for a better understanding of the large-scale light element cycle.

Description: In this study, deuteric coarsening of a lamellar cryptoperthite to a patch perthite has been experimentally induced for the first time. An homogeneous alkali feldspar, bulk composition Ab60Or40, was produced by molten salt ion-exchange with a gem quality orthoclase from Madagascar. This was then annealed isothermally for 32 days at 550 °C. This resulted in a coherent lamellar cryptoperthite (periodicity 28 nm) probably by spinodal decomposition and subsequent coarsening. The cryptoperthite was then reacted with 20 wt% H 2 O or 0.1 M HCl at 400 and 500 °C at 200, 400, and 1000 MPa for 96 h and for 192 h at 1500 MPa. At 1000 and 1500 MPa and 500 °C, the cryptoperthitic crystal fragments were partially replaced by a mosaic of albite and sanidine subgrains in the form of a patch perthite. Other than the Gibb’s free energy of reaction as derived from the chemical potentials of the reactant and product phases, the second principle mechanism responsible for driving this reaction is interface-coupled dissolution-reprecipitation driven by a reduction in the coherency strain energy stored within the initial cryptoperthite. The main indicator of this is a change in the microtexture due to the destruction of the coherent lamellar cryptoperthite and its replacement by an assemblage of incoherent subgrains. Detailed mass balancing, based on XRD and EPMA data, indicate that conversion of cryptoperthite to patch perthite is not isochemical. Experimental formation of patch perthite appears to take place in two steps. Between the original cryptoperthite and the patch perthite, TEM investigation identified a "transition zone," characterized by a significant coarsening of the preexisting lamellar microstructure, without any sign of a loss in lattice coherency. This zone is probably the result of hydrous species diffusing into the cryptoperthite causing the H 2 O-enhanced diffusion of Na and K and hence lamellar coarsening. In the second step, this coarsened microstructure is replaced by the patch perthite. The polycrystalline patch perthite is characterized by a very complex distribution of albite and sanidine sub-grains. Even the larger albite or sanidine patches are polycrystalline. This inevitably results in a three-dimensional network of grain boundaries characterized by high defect densities resulting from the structural misfit between the albite and sanidine sub-grains. An increase in average grain size with distance from the interface indicates that secondary coarsening of the initially, very fine-grained intergrowth occurred. This was driven by the reduction of interface and surface energy similar to the coarsening observed in multiphase ceramics. In addition, the newly formed patch perthite is highly porous despite the fact that the results obtained in this study show that the replacement of cryptoperthite by patch perthite is nearly isovolumetric (V ~ –0.6%). The observed reaction-induced porosity results from differences in relative solubilities between the cryptoperthite and the patch perthite. Porosity abundance has been estimated at approximately 10 to 11 vol% based on the amount of quartz formed during the experiments. Three-dimensional investigations via FIB serial sectioning indicate almost no interconnectivity between pores. A consequence of albite and sanidine sub-grain coarsening is the constant redistribution of the porosity and the sub-grain boundaries within the patch perthite mosaic over time. This suggests that a dynamically evolving porosity could provide an important transport mechanism for fluid transport through rocks even when the abundance and interconnectivity of the reaction-induced porosity is low.

Description: The Jaduguda uranium deposit in the Singhbhum shear zone is the most important and best-known uranium deposit in India. Uranium mineralization is hosted in hydrothermally altered, metamorphosed, and deformed chlorite and biotite schists. Uraninite is the most abundant and ubiquitous U ore mineral in the shear zone, including the deposit at Jaduguda. The deposit is known to have undergone multiple events of hydrothermal fluid flux. Integrating chemical texture, geochemistry, and in situ electron microprobe U-Th-Pb chemical dating, we aim at deciphering the geochemical and temporal evolution of uranium mineralization in the Jaduguda deposit. Geochemistry of uraninite and X-ray mapping of selected elements demonstrate that, though compositions of small uraninite grains (~10 μ m) were mostly or completely modified by subsequent hydrothermal alteration, the larger grains (~100 μ m) locally retain the footprints of the earlier events and capture the imprints of subsequent events. In general, all analyzed uraninite grains are poor in Th (〈2.5 wt % ThO 2 ) and most grains contain detectable rare earth elements (REEs; REE 2 O 3 : ~1.0–~12.5 wt %). Y and heavy rare earth elements (HREEs) are ubiquitous and most important in terms of concentrations. Based on the concentrations of U, Pb, and REEs, the Jaduguda uraninite can be classified into three compositional types: group-1a with low U, high Pb, and low REEs; group-1b with low U and high Pb, but high REEs; and group-2 with high U, low Pb, and moderate REEs. These three compositional types represent three discrete hydrothermal events. Chemical textures of uraninite indicate that the earliest recognized hydrothermal event is manifested in the formation of nearly pure uraninite (group-1a), which is depleted in REEs and other minor elements. The second hydrothermal event is characterized by Y-HREE (+ Ca + Fe) metasomatism that altered the compositions of existing uraninite, shifting its composition toward that of group-1b. The third event further modified the compositions of the previous uraninite, mainly by removing part of the REEs, resulting in uraninite of group-2 composition. Using U-Th-Pb chemical ages (with a mean error of ~25 m.y.) and taking into account the effect of cation exchange on chemical age during alteration, we propose that the uranium mineralization (represented by group-1a uraninite) in the Jaduguda deposit is Paleoproterozoic in age and took place sometime circa 1.80 to 1.90 Ga. The timing of HREE metasomatism is not well constrained. However, published data and this study indicate a ~1.66 Ga age of HREE metasomatism. Based on a strong cluster of ages of group-2 uraninite, we place the latest hydrothermal event, which affected the Singhbhum shear zone in general and the Jaduguda deposit in particular, at ~1.0 Ga.