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 American Mineralogist   [SJR: 1.185]   [H-I: 104]   [14 followers]  Follow         Hybrid journal (It can contain Open Access articles)    ISSN (Print) 0003-004X    Published by GeoScienceWorld  [16 journals]
• Looking for "missing" nitrogen in the deep Earth
• Authors: Zedgenizov, D. A; Litasov, K. D.
Pages: 1769 - 1770
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6218
Issue No: Vol. 102, No. 9 (2017)

• Crystal structure of richetite revisited: Crystallographic evidence for
the presence of pentavalent uranium
• Authors: Plašil; J.
Pages: 1771 - 1775
Abstract: Revision of crystal structure of the rare U-oxide mineral richetite provided crystallographic evidence for the presence of pentavalent U. The structure of richetite, space group P, a = 12.0919(2), b = 16.3364(4), c = 20.2881(4) Å, α = 68.800(2), β = 78.679(2), g = 76.118(2)°, with V = 3600.65(14) Å3 and Z = 1, was solved by charge-flipping algorithm and refined to an agreement index (R) of 5.6% for 9955 unique reflections collected using microfocus X-ray source. The refined structure, in line with the previous structure determination, contains U-O-OH sheets of the α-U3O8 type (protasite topology) and an interstitial complex comprising Pb2+, Fe2+, Mg2+ cations and molecular H2O. However, the polyhedral geometry, the bond-valence sum incident at one U site within the sheet (U17) together with charge-balance requirements, indicate that U17 site is occupied by U5+. The U177 (: O, OH) polyhedra is distorted, with two shorter U–O bond-lengths (~2.01 Å), four longer U–O bond-lengths (~2.2 Å) and one, very long U–O bond (2.9 Å). The color of richetite also supports the presence of U5+ in the structure The current results show that α-U3O8 type of sheet can incorporate U5+. Richetite is the third mineral containing pentavalent uranium that occurs in nature.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6092
Issue No: Vol. 102, No. 9 (2017)

• Mobilization and agglomeration of uraninite nanoparticles: A
nano-mineralogical study of samples from the Matoush Uranium ore deposit
• Authors: Schindler, M; Lussier, A. J, Bellrose, J, Rouvimov, S, Burns, P. C, Kyser, T. K.
Pages: 1776 - 1787
Abstract: The occurrence of uraninite nanoparticles in the alteration zones of uranium ore deposits suggests potential mobilization of U(IV) under reducing conditions, which is important for understanding the mobility of uranium in contaminated sites and potential repositories for nuclear waste. This study investigates the occurrence of uraninite nanoparticles in the outer alteration zone of the Matoush uranium ore deposit, Quebec, Canada. Selected samples with finely disseminated uraninite from the outer alteration zone of the deposit are examined by X-ray fluorescence spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy on specimens prepared using the focused ion beam milling technique. Uraninite nanoparticles occur as single particles, in clusters, and in larger aggregates in close association with the Cr-rich phases chromceladonite (Cr-rich mica), ideally KMgCr3+(Si4O10)(OH)2, eskolaite, ideally Cr23+O3, bracewellite, Cr3+OOH, and an amorphous Cr-rich oxide matrix as well as with fluorapatite and galena. Nanoparticles on the surface and in the outer rim of single uraninite crystals indicate the growth of larger uraninite crystals via crystallization through particle attachment and Oswald ripening. The flow texture of the uraninite nanoparticles in the amorphous Cr-rich oxide matrix, their aggregation on the surface of nanocrystals of bracewellite, the absence of products of a redox reaction involving U(VI) and Fe(II), and the occurrence of amorphous Fe-depleted alteration layers between uraninite and eskolaite, and uraninite and Cr-rich mica indicate that the uraninite nanoparticles have been mobilized under reducing conditions (leaching of Fe2+ from the alteration layer) at low T (amorphous character of the alteration layer) after the main mineralization event from the center of the mineralization to the outer parts of the Matoush dike complex. These results indicate that fluids can mobilize U(IV) under reducing conditions in the form of uraninite nanoparticles albeit over limited distances. The potential mobilization of these nanoparticles may also explain the occurrence of proximal mineralized zones in U-ore deposits that lack common products resulting from the reduction of U(VI) by Fe2+ (e.g., hematite and other Fe3+-phases).
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-5984
Issue No: Vol. 102, No. 9 (2017)

coal mining in the Lower Silesian basin (Czech Republic)
• Authors: Čurda, M; Goliaš, V, Klementova, M, Strnad, L, Matěj, Z, Škoda, R.
Pages: 1788 - 1795
Abstract: The isotopic composition of lead (207Pb/206Pb, 208Pb/206Pb, and 210Pb) in a recently formed galena from burning heaps after coal mining in Radvanice, Markoušovice, and Rybníček, the Lower Silesian basin, Czech Republic, was studied in detail. 210Pb activity in galena varied from 135 ± 9 to 714 ± 22 Bq/g and calculated integral doses ranged from 2.21 x 1011 to 6.11 x 1011 α/g. The radioactivity of the galena causes micro-deformations in its crystal structure as indicated by the Williamson-Hall graphs, showing that the level of micro-strain depends on the length of time that galena samples were exposed to the radiation. However, the crystal structure of galena is affected very inhomogenously; according to TEM investigations there are domains of fully crystalline, polycrystalline, and fully metamict galena within one crystal. Inductively coupled plasma-mass spectrometry (ICP-MS) was used to determine the isotopic composition of the studied galena. The stable isotope ratios of Pb varied for 207Pb/206Pb from 0.8402 to 0.8435 and for 208Pb/206Pb from 2.0663 to 2.0836. The average ratios 207Pb/206Pb = 0.8312 and 208Pb/206Pb = 2.0421 were obtained for coal from the same localities. These isotope ratios show that there is no isotopic fractionation taking place during the coal burning and subsequent galena crystallization from hot gases.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6036
Issue No: Vol. 102, No. 9 (2017)

• Element mobility during regional metamorphism in crustal and subduction
zone environments with a focus on the rare earth elements (REE)
• Authors: Ague; J. J.
Pages: 1796 - 1821
Abstract: This paper explores bulk-rock geochemical data for a wide array of metamorphosed mafic, quartzo-feldspathic, pelitic, and metacarbonate rocks using a quantitative mass-balance approach to assess fluid-driven element mobility—particularly of the rare earth elements (REE)—in regional metamorphic and some high-pressure subduction zone environments (40 examples; over 240 individual analyses). Most examples are from focused fluid flow settings, such as veins and lithologic contacts, where fluxes are large and metasomatic signals are thus strong. A variety of REE behaviors are observed, including little or no REE mobility (roughly a third of the data set); light REE (LREE), mid-REE (MREE), and/or heavy (HREE) mobility; europium "anomalies"; overall REE losses; and local REE redistribution. The REE are typically fractionated by mass transfer, with the exception of several examples that underwent fairly uniform overall losses of REE. The fractionation reflects strong mineralogical controls on REE uptake/loss by a comparatively small number of phases. Examples include: HREE mass changes associated with garnet, xenotime, and sphene; LREE and MREE changes associated with apatite, monazite, and allanite; and Eu changes associated with plagioclase and lawsonite. As mineralogy is a dominant control, the nature of the metasomatism is not strongly correlated with metamorphic grade, other than obvious mineralogical differences between settings (e.g., plagioclase in Barrovian metamorphism, lawsonite in subduction zones). Extensive mobilization of non-REE major and trace elements can happen without significant open-system transport of the REE. If REE mobility occurs, it is always accompanied by mobilization of other non-REE. When mobile, neighboring REE (e.g., Sm and Nd) typically have strongly correlated mass changes indicating that both were mobilized to about the same degree. Although individual examples of metasomatism can show correlations between patterns of mass transfer for the REE and the non-REE, little such correlation is evident across the entire data set, with the exception of P. Once again, this highlights the importance of individual minerals in controlling REE systematics. Broad correlations of REE and P mobility suggest REE transport by P complexes, or REE and P transport together by some other complexing agent. Mass changes for REE and Y are more strongly coupled, reflecting the geochemical similarity of these elements and perhaps indicating a role for Y complexing as well.The REE are the most mobile of the high field strength elements (HFSE). On a percentage mass basis, the amount of HFSE mobility decreases roughly in the order: REE> U> Nb> Ti> Th ~ Zr. Th mobility is rare but when present is positively correlated with U and REE mobility. The mobility of the more refractory HFSE is low in aqueous fluids, but is a larger concern in more extreme environments such as magmatic/magmatic-hydrothermal systems, charnockite metamorphism, and supercritical fluids in high-pressure/ultrahigh-pressure settings. The mobility of certain HFSE (e.g., Nb) can be large enough to affect rock plotting positions on petrotectonic discrimination diagrams.Potassium and related (Rb, Ba, Cs) large-ion lithophile elements (LILE) are typically lost in association with mica breakdown. As the data set focuses on high fluid flow environments, LILE changes will likely be smaller for rocks that undergo less fluid-rock interaction. Gains of K and related LILE are coupled to volatile gain and Na loss, consistent with fluid flow down temperature gradients, or infiltration from K-rich sources (e.g., pegmatites). In Barrovian settings, two other mass transfer trends are evident in addition to K mass transfer. First, silica loss is coupled to volatile loss, illustrating the dependence of silica mass transfer on devolatilization. Most of the silica loss reflects local mass transfer into adjacent vein fluid flow conduits were additional silica is precipitated; indeed, wallrock inclusions in veins can be highly silicified. Second, Na and Ca mass gains and losses are well correlated in the Barrovian examples, reflecting control of plagioclase growth or destruction. Strontium and Pb behaviors also appear to be largely related to plagioclase behavior (in subduction settings, this role can be played by phases such as lawsonite or epidote group minerals).Carbon dioxide mass transfer by conventional devolatilization as well as near-stoichiometric CaCO3 dissolution or precipitation is represented in the data set. Determining the relative roles of these processes as functions of metamorphic grade, intensity of fluid-rock interaction, and tectonic setting will be an important challenge for future research.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6130
Issue No: Vol. 102, No. 9 (2017)

• Subsolidus hydrogen partitioning between nominally anhydrous minerals in
garnet-bearing peridotite
• Authors: Demouchy, S; Shcheka, S, Denis, C. M. M, Thoraval, C.
Pages: 1822 - 1831
Abstract: Hydrogen distribution between nominally anhydrous minerals (NAMs) of a garnet-lherzolite under subsolidus conditions has been investigated. Separated NAMs from a garnet-peridotite from Patagonia (Chile) are annealed together (olivine, orthopyroxene, clinopyroxene, and garnet) using a piston-cylinder at 3 GPa and 1100 °C using talc-pyrex cell assembly for 10, 25, and 100 h. The talc-pyrex assembly provides enough hydrogen in the system to re-equilibrate the hydrogen concentrations at high pressure. The three coexisting nominally anhydrous minerals (NAMs, i.e., olivine, orthopyroxene, and clinopyroxene) were successfully analyzed using FTIR. The resulting hydrogen concentrations exceed significantly the initial hydrogen concentration by a factor of 13 for olivine and a factor of 3 for both pyroxenes. Once mineral-specific infrared calibrations are applied, the average concentrations in NAMs are 115 ± 12 ppm wt H2O for olivine, 635 ± 75 ppm wt H2O for orthopyroxene, and 1214 ± 137 ppm wt H2O for clinopyroxene, garnet grains are dry. Since local equilibrium seems achieved over time (for 100 h), the calculated concentration ratios are interpreted as mineral-to-mineral hydrogen partition coefficients (i.e., Nernst’s law) for a garnet-peridotite assemblage. It yields, based on mineral-specific infrared calibrations, DOpx/Ol = 5 ± 1, DCpx/Ol = 10 ± 2, and DCpx/Opx = 1.9 ± 0.4. While DCpx/Opx is in agreement (within error) with previous results from experimental studies and concentration ratios observed in mantle-derived peridotites, the DPx/Ol from this study are significantly lower than the values reported from mantle-derived xenoliths and also at odd with several previous experimental studies where melt and/or hydrous minerals co-exists with NAMs. The results confirm the sensitivity of hydrogen incorporation in olivine regarding the amount of water-derived species (H) in the system and/or the amount of water in the coexisting silicate melt. The results are in agreement with an important but incomplete dehydration of mantle-derived olivine occurring at depth, during transport by the host magma or during slow lava flow cooling at the surface. The rapid concentration modification in mantle pyroxenes also points out that pyroxenes might not be a hydrogen recorder as reliable as previously thought.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6089
Issue No: Vol. 102, No. 9 (2017)

• OH defects in quartz as monitor for igneous, metamorphic, and sedimentary
processes
• Authors: Stalder, R; Potrafke, A, Billstrom, K, Skogby, H, Meinhold, G, Gogele, C, Berberich, T.
Pages: 1832 - 1842
Abstract: Oriented sections of more than 500 quartz grains from sediments, igneous, and metamorphic rocks from different localities in Sweden, Austria, Germany, and South Africa were analyzed by FTIR spectroscopy, and their OH defect content was determined with respect to the speciation and total defect water content. Systematic variations of defect speciation and statistical evaluation of total defect contents were used to evaluate the potential of FTIR spectroscopy on quartz as a thermometer in quartzite, as a tool for differentiation trends in granitic systems, and for provenance analysis of sedimentary rocks. In addition to the analyses of natural crystals, high-pressure annealing experiments at lower crustal conditions (1–3 kbar and 650–750 °C) were performed to document the effect of high-grade metamorphism on the defect chemistry. Results indicate that (1) quartz grains from unmetamorphosed granite bodies reveal interesting differentiation trends; (2) sediments and sedimentary rocks are valuable archives to preserve the pre-sedimentary OH defect chemistry, where individual signatures are preserved and can be traced back to potential source rocks; (3) OH defects are retained up to 300 °C over geological time scales; (4) long-term low-grade metamorphic overprint leads to a continuous annealing to lower defect water contents, where Al-specific OH defects survive best; and (5) middle to high-grade annealing drives toward a homogeneous defect partitioning from grain to grain, where the degree of attainment of equilibrium depends on temperature and duration of the thermal event.In summary, OH defects in quartz crystals monitor parts of their geological history, and the systematic investigation and statistical treatment of a large amount of grains can be applied as an analytical tool to study sedimentary, metamorphic, and igneous processes.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6107
Issue No: Vol. 102, No. 9 (2017)

• Quantitative electron backscatter diffraction (EBSD) data analyses using
the dictionary indexing (DI) approach: Overcoming indexing difficulties on
geological materials
• Authors: Marquardt, K; De Graef, M, Singh, S, Marquardt, H, Rosenthal, A, Koizuimi, S.
Pages: 1843 - 1855
Abstract: Electron backscatter diffraction (EBSD) data yield plentiful information on microstructure and texture of natural as well as experimentally produced mineral and rock samples. For instance, the characterization of microstructures and textures by EBSD allows for the evaluation of phase equilibria. Furthermore, determination of the preferred orientations of crystals using EBSD yields constraints on deformation mechanisms and history of the minerals/rocks. The latter affects bulk-rock properites, for example, through the relation between lattice preferred orientation and electrical conductivity and seismic anisotropy. EBSD is also applied to advance our understanding of various phenomena such as seismic wave attenuation in the Earth deep interior and its relation to the presence of interfacial small degrees of melt fractions, or free fluid phases.In standard EBSD software solutions, the original EBSD patterns are rarely saved and indexing routines result in many artifacts, such as pseudo-symmetry or unindexed pixels at interfaces that may be misinterpreted as amorphous material, such as a melt.Here we report the first application of the dictionary indexing (DI) approach proposed by Chen et al. (2015), an alternative indexing routine, which we extended to be applicable to multiphase geologic materials. The DI method is independent of the EBSD system, and thus of the used detector/software. The DI routine generates simulated EBSD patterns for all possible crystal orientations, taking the sample composition and experimental setups into account. The resulting pattern database is called a dictionary. The experimental electron backscattering pattern (EBSP) images are indexed by comparing them to the dictionary using a dot-product algorithm. We evaluate the new DI method in comparison to standard routines and highlight advantages and disadvantages.To test and compare the DI’s reliability and performance, we apply the routine to two scientifically challenging samples: (1) A nominally anhydrous ("dry") residual eclogite composed of garnet (cubic), clinopyroxene (monoclinic) and an amorphous melt, where the different degrees of hardness of the phases cause surface topology; and (2) a pure forsterite (olivine) polycrystalline sample produced by vacuum sintering. The acquired EBSD patterns are of low quality for the latter as a result of fast data acquisition to reduce the on-line machine time.We conclude that the new DI method is highly precise and surpasses the performance of previously available methods, while being computer time and computer memory consuming. We find that the DI method is free of pseudo-symmetry-related problems. Interpolation of data becomes obsolete and high reproducibility is obtained, which minimizes the operator impact on the final data set. The latter is often caused by applying several cleaning steps on EBSD maps with low indexing fraction. Finally, much higher scientific integrity is ensured by image collections as described above, which requires that all patterns are saved. This in turn allows later re-analyses if required. The DI routine will help to achieve more reliable information on interface properties of geological samples, including amorphous materials, and thus in the long run help to improve the accuracy of large-scale Earth mantle process models.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6062
Issue No: Vol. 102, No. 9 (2017)

• Trace element inventory of meteoritic Ca-phosphates
• Authors: Ward, D; Bischoff, A, Roszjar, J, Berndt, J, Whitehouse, M. J.
Pages: 1856 - 1880
Abstract: Most extraterrestrial samples feature the two accessory Ca-phosphates (apatite-group minerals and merrillite), which are important carrier phases of the rare earth elements (REE). The trace-element concentrations (REE, Sc, Ti, V, Cr, Mn, Co, As, Rb, Sr, Y, Zr, Nb, Ba, Hf, Ta, Pb, Th, and U) of selected grains were analyzed by LA-ICP-MS and/or SIMS (REE only). This systematic investigation includes 99 apatite and 149 merrillite analyses from meteorites deriving from various asteroidal bodies including 1 carbonaceous chondrite, 8 ordinary chondrites, 3 acapulcoites, 1 winonaite, 2 eucrites, 5 shergottites, 1 ureilitic trachyandesite, 2 mesosiderites, and 1 silicate-bearing IAB iron meteorite.Although Ca-phosphates predominantly form in metamorphic and/or metasomatic reactions, some are of igneous origin. As late-stage phases that often incorporate the vast majority of their host’s bulk REE budget, the investigated Ca-phosphates have REE enrichments of up to two orders of magnitude compared to the host rock’s bulk concentrations. Within a single sample, each phosphate species displays a uniform REE-pattern, and variations are mainly restricted to their enrichment, therefore indicating similar formation conditions. Exceptions are brecciated samples, i.e., the Adzhi-Bogdo (LL3-6) ordinary chondrite. Despite this uniformity within single samples, distinct meteorite groups do not necessarily have unique REE-patterns. Four basic shapes dominate the REE patterns of meteoritic Ca-phosphates: (1) flat patterns, smoothly decreasing from La-Lu with prominent negative Eu anomalies (acapulcoites, eucrites, apatite from the winonaite and the ureilitic trachyandesite, merrillite from ordinary chondrites); (2) unfractionated patterns, with only minor or no anomalies (mesosiderites, enriched shergottites, IAB-iron meteorite); (3) LREE-enriched patterns, with either positive or slightly negative Eu anomalies (chondritic apatite); and (4) strongly LREE-depleted patterns, with negative Eu anomalies (depleted shergottites). The patterns do not correlate with the grade of metamorphism (petrologic type), specific adjacent mineral assemblages or with Ca-phosphate grain size. Neither the proportions of different REE, nor particular REE patterns themselves are universally correlated to a specific formation mechanism yet Eu (i.e., magnitude of the Eu anomaly) is a sensitive indicator to evaluate the timing of plagioclase and phosphate crystallization. Based on our data, U and Th abundances in apatite increase (almost linearly) with the grade of metamorphism, as well as with the differentiation of their host rock.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6056
Issue No: Vol. 102, No. 9 (2017)

• Insights into solar nebula formation of pyrrhotite from nanoscale
disequilibrium phases produced by H2S sulfidation of Fe metal
• Authors: Gainsforth, Z; Lauretta, D. S, Tamura, N, Westphal, A. J, Jilly-Rehak, C. E, Butterworth, A. L.
Pages: 1881 - 1893
Abstract: Lauretta (2005) produced sulfide in the laboratory by exposing canonical nebular metal analogs to H2S gas under temperatures and pressures relevant to the formation of the Solar System. The resulting reactions produced a suite of sulfides and nanophase materials not visible at the microprobe scale, but which we have now analyzed by TEM for comparison with interplanetary dust samples and comet Wild 2 samples returned by the Stardust mission. We find the unexpected result that disequilibrium formation favors pyrrhotite over troilite and also produces minority schreibersite, daubréelite, barringerite, taenite, oldhamite, and perryite at the metal-sulfide interface. TEM identification of nanophases and analysis of pyrrhotite superlattice reflections illuminate the formation pathway of disequilibrium sulfide. We discuss the conditions under which such disequilibrium can occur, and implications for formation of sulfide found in extraterrestrial materials.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-5848
Issue No: Vol. 102, No. 9 (2017)

• Unraveling the presence of multiple plagioclase populations and
identification of representative two-dimensional sections using a
statistical and numerical approach
• Authors: Cheng, L; Costa, F, Carniel, R.
Pages: 1894 - 1905
Abstract: Many plagioclase phenocrysts from volcanic and plutonic rocks display quite complex chemical and textural zoning patterns. Understanding the zoning patterns and variety of crystal populations holds clues to the processes and timescales that lead to the formation of the igneous rocks. However, in addition to a "true" natural complexity of the crystal population, the large variety of plagioclase types can be partly artifacts of the use of two-dimensional (2D) petrographic thin sections and random cuts of three-dimensional (3D) plagioclase crystals. Thus, the identification of the true number of plagioclase populations, and the decision of which are "representative" crystal sections to be used for detailed trace element and isotope analysis is not obvious and tends to be subjective.Here we approach this problem with a series of numerical simulations and statistical analyses of a variety of plagioclase crystals zoned in 3D. We analyze the effect of increasing complexity of zoning based on 2D chemical maps (e.g., backscattered electron images, BSE). We first analyze the random sections of single crystals, and then study the effect of mixing of different crystal populations in the samples. By quantifying the similarity of the compositional histogram of about a hundred 2D plagioclase sections it is possible to identify the so-called reference and ideal sections that are representative of the real 3D crystal populations. These section types allow filtering out the random-cut effects and explain more than 90% of the plagioclase compositional data of a given sample. Our method allows the identification of the main crystal populations and representative crystals that can then be used for a more robust interpretation of magmatic processes and timescales.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-5929CCBYNCND
Issue No: Vol. 102, No. 9 (2017)

• Refractive indices of minerals and synthetic compounds
• Authors: Shannon, R. C; Lafuente, B, Shannon, R. D, Downs, R. T, Fischer, R. X.
Pages: 1906 - 1914
Abstract: This is a comprehensive compilation of refractive indices of 1933 minerals and 1019 synthetic compounds including exact chemical compositions and references taken from 30 compilations and many mineral and synthetic oxide descriptions. It represents a subset of about 4000 entries used by Shannon and Fischer (2016) to determine the polarizabilities of 270 cations and anions after removing 425 minerals and compounds containing the lone-pair ions (Tl+, Sn2+, Pb2+, As3+, Sb3+, Bi3+, S4+, Se4+, Te4+, Cl5+, Br5+, I5+) and uranyl ions, U6+. The table lists the empirical composition of the mineral or synthetic compound, the ideal composition of the mineral, the mineral name or synthetic compound, the Dana classes and subclasses extended to include beryllates, aluminates, gallates, germanates, niobates, tantalates, molybdates, tungstates, etc., descriptive notes, e.g., structure polytypes and other information that helps define a particular mineral sample, and the locality of a mineral when known. Finally, we list nx, ny, nz, (all determined at 589.3 nm), , deviation of observed and calculated mean refractive indices, molar volume Vm, corresponding to the volume of one formula unit, anion molar volume Van, calculated from Vm divided by the number of anions (O2–, F–, Cl–, OH–) and H2O in the formula unit, the total polarizability , and finally the reference to the refractive indices for all 2946 entries. The total polarizability of a mineral, , is a useful property that reflects its composition, crystal structure, and chemistry and was calculated using the Anderson-Eggleton relationship ${{\upalpha }}_{\hbox{ AE }}=\frac{\left({n}_{\hbox{ D }}^{2}-1\right){V}_{\hbox{ m }}}{4{\uppi }+\left(\frac{4{\uppi }}{3}-c\right)\left({n}_{\hbox{ D }}^{2}-1\right)}$where c = 2.26 is the electron overlap factor. The empirical polarizabilities and therefore, the combination of refractive indices, compositions, and molar volumes of the minerals and synthetic oxides in the table were verified by a comparison of observed and calculated total polarizabilities, derived from individual polarizabilities of cations and anions. The deviation between observed and calculated refractive indices is
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6144
Issue No: Vol. 102, No. 9 (2017)

• Can we use pyroxene weathering textures to interpret aqueous alteration
conditions' Yes and No
• Authors: Phillips-Lander, C. M; Legett, C, Elwood Madden, A. S, Elwood Madden, M. E.
Pages: 1915 - 1921
Abstract: Pyroxene minerals are a significant component of Shergottite-Nakhlite-Chassignite (SNC) meteorites (e.g., Velbel 2012) and detected across large areas of Mars’ surface (e.g., Mustard et al. 2005). These minerals are associated with chloride, sulfate, and perchlorate salts that may represent briny waters present in Mars’ history. Previous textural analyses by Velbel and Losiak (2010) comparing pyroxenes and amphiboles from various natural weathering environments showed no correlation between apparent apical angles (describing the morphology of denticular weathering textures) and mineralogy or aqueous alteration history in relatively dilute solutions. However, high-salinity brines preferentially dissolve surface species, potentially leading to different textures dependent on the brine chemistry. In this study, we performed controlled pyroxene dissolution experiments in the laboratory on a well-characterized diopside to determine if aqueous alteration in different high-salinity brines, representative of potential weathering fluids on Mars, produce unique textural signatures.Following two months of dissolution in batch reactors, we observed denticles on etch pit margins and pyroxene chip boundaries in all of the solutions investigated: ultrapure water (18 M cm–1; aH2O = 1); low-salinity solutions containing 0.35 M NaCl (aH2O = 0.99), 0.35 M Na2SO4 (aH2O = 0.98), and 2 M NaClO4 (aH2O = 0.9); and near-saturated brines containing 1.7 M Na2SO4 (aH2O = 0.95), 3 M NaCl (aH2O = 0.75), and 4.5 M CaCl2 (aH2O = 0.35). No systematic change in denticle length or apical angle was observed between any of the solutions investigated, even when altered in brines with significantly different salinity, activity of water, and anion composition. Based on these and previous results from natural systems, apical angle measurements are not a useful proxy for determining the extent or nature of aqueous alteration. However, since denticles form relatively slowly during weathering at circum-neutral pH, denticle length may be a useful proxy for chemical weathering duration. All of the experimental solutions produced median denticle lengths ≤1 μm, likely due to the brief weathering experiments. However, perchlorate brines produced a significantly wider range of denticle lengths than those observed in all the other experimental solutions tested. Since perchlorate is likely a common constituent in martian soils (Glotch et al. 2016), denticle length measurements should be used cautiously as proxies for extent of aqueous alteration on Mars, particularly in samples that also contain perchlorate.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6155
Issue No: Vol. 102, No. 9 (2017)

• Phase relations and formation of K-bearing Al-10 A phase in the MORB+H2O
system: Implications for H2O- and K-cycles in subduction zones
• Authors: Tao, R; Zhang, L, Liu, X, Bader, T, Fei, Y.
Pages: 1922 - 1933
Abstract: The potassium (K) and water (H2O) cycles in subduction zones are predominately controlled by the stability of K- and H2O-bearing minerals, such as K-mica, lawsonite, and dense hydrous magnesium silicates (DHMS). K-micas (muscovite or phlogopite) are the principal H2O and K hosts in subduction zones and Earth’s upper mantle and play a significant role in the deep H2O and K cycles. The Mg-10 Å phase, normally appearing in hydrated peridotite in high-pressure experiments, has been considered as an important water-carrier in subducted hydrated peridotite. In this study, we found a K-bearing Al-10 Å phase in the MORB+H2O system (hydrated basalt) at high pressures according to X-ray diffraction and stoichiometry. We experimentally constrained its stability field at high pressure. By considering newly and previously documented compositions of the 10 Å phase and micas, we confirmed a continuous solid solution or mixed layering between the 10 Å phase and K-mica at the interlayer site, suggesting that the K cycle and the H2O cycle in subduction zones are coupled. From the discussion of the effect of fH2O on stability of the Al-10 Å phase, we conclude that a cold subduction zone can host and carry more bulk H2O and K into Earth’s deep mantle than a hot one. This work expands the stability regions of the 10 Å phase from the ultramafic system (Mg-10 Å phase) to the mafic system (Al-10 Å phase), and emphasizes the significance of the 10 Å phase for the deep H2O and K cycle in subduction zone.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6025
Issue No: Vol. 102, No. 9 (2017)

• Effect of alkalis on the reaction of clinopyroxene with Mg-carbonate at 6
GPa: Implications for partial melting of carbonated lherzolite
• Authors: Shatskiy, A; Podborodnikov, I. V, Arefiev, A. V, Litasov, K. D, Chanyshev, A. D, Sharygin, I. S, Karmanov, N. S, Ohtani, E.
Pages: 1934 - 1946
Abstract: The reaction between clinopyroxene and Mg-carbonate is supposed to define the solidus of carbonated lherzolite at pressures exceeding 5 GPa. To investigate the effect of alkalis on this reaction, subsolidus and melting phase relations in the following systems have been examined at 6 GPa: CaMgSi2O6+2MgCO3 (Di+2Mgs); CaMgSi2O6+NaAlSi2O6+2MgCO3 (Di+Jd+2Mgs); CaMgSi2O6+Na2Mg(CO3)2 (Di+Na2Mg); and CaMgSi2O6+K2Mg(CO3)2 (Di+K2Mg). The Di+2Mgs system begins to melt at 1400 °C via the approximate reaction CaMgSi2O6 (clinopyroxene) + 2MgCO3 (magnesite) = CaMg(CO3)2 (liquid) + Mg2Si2O6 (orthopyroxene), which leads to an essentially carbonate liquid (L) with composition Ca0.56Mg0.44CO3 + 3.5 mol% SiO2. The initial melting of the Di+Jd+2Mgs system occurs at 1350 °C via the reaction 2CaMgSi2O6 (clinopyroxene) + 2NaAlSi2O6 (clinopyroxene) + 8MgCO3 (magnesite) = Mg3Al2Si3O12 (garnet) + 5MgSiO3 (clinopyroxene) + 2CaMg(CO3)2 (liquid) + Na2CO3 (liquid) + 3CO2 (liquid and/or fluid), which yields the carbonate liquid with approximate composition of 10Na2CO3·90Ca0.5Mg0.5CO3 + 2 mol% SiO2. The systems Di+Na2Mg and Di+K2Mg start to melt at 1100 and 1050 °C, respectively, via the reaction CaMgSi2O6 (clinopyroxene) + 2(Na or K)2Mg(CO3)2 (solid) = Mg2Si2O6 (orthopyroxene) + (Na or K)4CaMg(CO3)4 (liquid). The resulting melts have the alkali-rich carbonate compositions Na2Ca0.4Mg0.6(CO3)2 + 0.4 mol% SiO2 and 43 K2CO3·57Ca0.4Mg0.6CO3 + 0.6 mol% SiO2. These melts do not undergo significant changes as temperature rises to 1400 °C, retaining their calcium number and a high Na2O, K2O, and low SiO2. We suggest that the clinopyroxene–Mg-carbonate reaction controlling the solidus of carbonated lherzolite is very sensitive to the carbonate composition and shifts from 1400 to 1050 °C at 6 GPa, which yields K-rich carbonate melt if the subsolidus assemblage contains the K2Mg(CO3)2 compound. Such a decrease in solidus temperature has been previously observed in the K-rich carbonated lherzolite system. Although a presence of eitelite, Na2Mg(CO3)2, has a similar effect, this mineral cannot be considered as a potential host of Na in carbonated lherzolite, because the whole Na added into the system dissolves as jadeite component in clinopyroxene if bulk Al/Na ≥ 1. The presence of jadeite component in clinopyroxene has little impact on the temperature of the solidus reaction decreasing it to 1350 °C at 6 GPa.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6048
Issue No: Vol. 102, No. 9 (2017)

• Synthesis and crystal structure of LiNbO3-type Mg3Al2Si3O12: A possible
indicator of shock conditions of meteorites
• Authors: Ishii, T; Sinmyo, R, Komabayashi, T, Ballaran, T. B, Kawazoe, T, Miyajima, N, Hirose, K, Katsura, T.
Pages: 1947 - 1952
Abstract: LiNbO3-type Mg2.98(2)Al1.99(2)Si3.02(2)O12 (py-LN) was synthesized by recovering a run product from 2000 K and 45 GPa to ambient conditions using a large volume press. Rietveld structural refinements were carried out using the one-dimensional synchrotron XRD pattern collected at ambient conditions. The unit-cell lattice parameters were determined to be a = 4.8194(3) Å, c = 12.6885(8) Å, V = 255.23(3) Å3, with Z = 6 (hexagonal, R3c). The average A-O and B-O distances of the AO6 and BO6 octahedra have values similar to those that can be obtained from the sum of the ionic radii of the averaged A- and B-site cations and oxygen (2.073 and 1.833 Å, respectively). The present compound has the B-site cations at the octahedral site largely shifted along the c axis compared with other LiNbO3-type phases formed by back-transition from perovskite (Pv)-structure, and as a result, the coordination number of this site is better described as 3+3. It appears therefore that the B-site cation in the octahedral position cannot be completely preserved during the back-transition because of the small size of Si and Al, which occupy usually a tetrahedral site at ambient conditions. The formation of py-LN can be explained by the tilting of BO6 octahedra of the perovskite structure having the pyrope composition and formed at high P-T conditions. The tilting is driven by the decrease in ionic radius ratio between the A-site cation and oxygen during decompression. This also explains why there is no back-transition from the Pv-structure to the ilmenite-structure during decompression, since this is a reconstructive phase transition whose activation energy cannot be overcome at room temperature. Py-LN may be formed in shocked meteorites by the back-transformation after the garnet-bridgmanite transition, and will indicate shock conditions around 45 GPa and 2000 K.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6027
Issue No: Vol. 102, No. 9 (2017)

• Single crystal synthesis of {delta}-(Al,Fe)OOH
• Authors: Kawazoe, T; Ohira, I, Ishii, T, Ballaran, T. B, McCammon, C, Suzuki, A, Ohtani, E.
Pages: 1953 - 1956
Abstract: Single crystals of -AlOOH, -(Al0.953,Fe0.047)OOH, and -(Al0.878,Fe0.122)OOH with dimensions up to ~0.4–0.6 mm were synthesized by the high-pressure hydrothermal method. Synthesis experiments were performed at 21 GPa and 1470 K for 4 h using a Kawai-type multi-anvil apparatus. The crystals of -AlOOH, -(Al0.953,Fe0.047)OOH, and -(Al0.878,Fe0.122)OOH were colorless, yellowish green, and brown, respectively. Mössbauer spectra showed 95–100% Fe3+/Fe at the octahedral site in -(Al,Fe)OOH. Chemical compositions of -(Al0.953,Fe0.047)OOH and -(Al0.878,Fe0.122)OOH are homogeneous with Fe/(Al+Fe) of 0.0469(8) and 0.122(3), respectively. Unit-cell parameters of -AlOOH are consistent with those of previous studies, and they increase with Fe/(Al+Fe). These results confirm that -AlOOH can form a solid solution with -FeOOH. The crystals contained a small number of fluid inclusions. The syntheses of large single crystals of -(Al,Fe)OOH will facilitate investigation of their phase stability, physical properties including elasticity and elastic anisotropy, behavior of hydrogen bonding, and spin state of Fe, which will improve models of the water and oxygen cycles in the deep Earth.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6153
Issue No: Vol. 102, No. 9 (2017)

• EosFit-Pinc: A simple GUI for host-inclusion elastic thermobarometry
• Authors: Angel, R. J; Mazzucchelli, M. L, Alvaro, M, Nestola, F.
Pages: 1957 - 1960
Abstract: Elastic geothermobarometry is a method of determining metamorphic conditions from the excess pressures exhibited by mineral inclusions trapped inside host minerals. An exact solution to the problem of combining non-linear Equations of State (EoS) with the elastic relaxation problem for elastically isotropic spherical host-inclusion systems without any approximations of linear elasticity is presented. The solution is encoded into a Windows GUI program EosFit-Pinc. The program performs host-inclusion calculations for spherical inclusions in elastically isotropic systems with full P-V-T EoS for both phases, with a wide variety of EoS types. The EoS values of any minerals can be loaded into the program for calculations. EosFit-Pinc calculates the isomeke of possible entrapment conditions from the pressure of an inclusion measured when the host is at any external pressure and temperature (including room conditions), and it can calculate final inclusion pressures from known entrapment conditions. It also calculates isomekes and isochors of the two phases.
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-6190
Issue No: Vol. 102, No. 9 (2017)

• New Mineral Names,
• Authors: Belakovskiy, D. I; Camara, F, Gagne, O. C, Uvarova, Y.
Pages: 1961 - 1968
PubDate: 2017-09-05T06:04:14-07:00
DOI: 10.2138/am-2017-NMN102920
Issue No: Vol. 102, No. 9 (2017)

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