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American Mineralogist
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Institución detectada | Período | Navegá | Descargá | Solicitá |
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No detectada | desde feb. 1998 / hasta dic. 2023 | GeoScienceWorld |
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Tipo de recurso:
revistas
ISSN impreso
0003-004X
ISSN electrónico
1945-3027
País de edición
Estados Unidos
Fecha de publicación
1916-
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doi: 10.2138/am-2021-7929
A first-principles study of water in wadsleyite and ringwoodite: Implication for the 520 km discontinuity
Wenzhong Wang; Zhongqing Wu
<jats:title>Abstract</jats:title> <jats:p>The seismic discontinuity around 520 km is believed to be caused by the phase transition from wadsleyite to ringwoodite, the dominant minerals in the mantle transition zone (MTZ). Both wadsleyite and ringwoodite can contain more than 1.0 wt% water at MTZ’s conditions, but it is not well known how water affects the wadsleyite-ringwoodite transition. Here we investigated water partitioning between wadsleyite and ringwoodite and the water effect on this phase boundary using first-principles calculations. Our results show that the presence of water will shift the phase boundary to higher pressures, and the width of the two-phase coexistence domain in the Mg2SiO4-H2O system is insignificant at mid-MTZ conditions. For the (Mg0.9Fe0.1)2SiO4 system, the incorporation of 1.0 wt% water can narrow the effective width of two-phase coexistence by two-thirds. Together with elastic data, we find that velocity and impedance contrasts are only mildly changed by the water partitioning. We suggest that compared to the anhydrous condition, the presence of 1.0 wt% water will increase velocity gradients across the wadsleyite-ringwoodite transition by threefold, enhancing the detectability of the 520 km discontinuity.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1361-1368
doi: 10.2138/am-2020-7483
Inclusions in calcite phantom crystals suggest role of clay minerals in dolomite formation
Stefan Farsang; Péter Pekker; Giulio I. Lampronti; Zsombor Molnár; Rastislav Milovský; Mihály Pósfai; Daniel Ozdín; Timothy D. Raub; Simon A.T. Redfern
<jats:title>Abstract</jats:title> <jats:p>Micro- and nano-inclusions embedded in calcite phantom crystals from Gemerská Ves, Slovak Republic, have been characterized by a combination of Raman spectroscopy, scanning and transmission electron microscopy, X-ray powder diffraction, and C and O isotope analysis. Whereas the outer, colorless part of the phantom crystal is relatively homogeneous and cavity and inclusion-free, the inner terracotta-colored part contains abundant cavities, dolomite, hematite, goethite, titanite, phyllosilicates (mainly kaolinite and illite), and apatite inclusions and nanostructures that have formed on the walls of cavities. The nanostructures comprise hematite and goethite particles sandwiched between either two phyllosilicate crystals or a phyllosilicate and a carbonate (calcite or dolomite) crystal. Our observations suggest that all inclusions in the terracotta calcite originate from the terra rossa (a common soil type in karstic areas) and limestone outcropping adjacent to the calcite crystals. While the micrometer-sized phyllosilicate and hematite particles were likely transported from the terra rossa and attached to the surface of growing calcite, the presence of phyllosilicates that are only a few atomic layers thick and of euhedral hematite, goethite, and dolomite crystals suggests that these particles precipitated along with the phantom calcite in situ, from an aqueous solution carrying terra rossa-derived and limestone-derived solutes. The compositional differences between the terra rossa (e.g., smectite as the only major Mg-rich phase) and terracotta calcite inclusions (e.g., dolomite as the only major Mg-rich phase and the presence of only Mg-free clays) hint that a smectite-illite conversion provides the Mg necessary for the precipitation of dolomite and possibly the Fe associated with the iron oxyhydroxide nanostructures. Phyllosilicate nucleation on calcite and dolomite nucleation on phyllosilicates, as inferred from nanoscale mineralogical associations, suggest that carbonates and phyllosilicates may mutually enhance nucleation and growth. This enhancement may result in the formation of large-scale clay-carbonate successions in aqueous settings, including the enigmatic, pink-colored cap dolostones succeeding late Neoproterozoic “Snowball Earth” deposits. The distribution of inclusions in the terracotta calcite and the preferred nucleation of hematite and goethite on phyllosilicate, rather than on carbonate surfaces, indicates that phyllosilicates have a potential to not only disrupt crystal growth and trigger the formation of cavities in the structure of the calcite host, but also to provide surfaces for the precipitation of different phases in the cavities and to uniformly distribute otherwise incompatible materials in a calcite host crystal. This calls for further exploration of the potential application of phyllosilicates in composite structure development.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1369-1377
doi: 10.2138/am-2022-8086
Crystal-chemical reinvestigation of probertite, CaNa[B5O7(OH)4]·3H2O, a mineral commodity of boron
G. Diego Gatta; Enrico Cannaò; Valentina Gagliardi; Oscar Fabelo
<jats:title>Abstract</jats:title> <jats:p>The crystal chemistry of probertite, a mineral commodity of B (B2O3 ~50 wt%) with ideal formula CaNa[B5O7(OH)4]·3H2O from the Kramer Deposit (Kern County, California, type locality), was investigated by a multi-methodological approach [i.e., single-crystal X-ray (at 293 K) and neutron (at 20 K) diffraction, EPMA-WDS, LA-ICP-MS, and LA-MC-ICP-MS]. As recently determined for other hydrous borates, the real chemical formula of probertite from the Kramer Deposit is virtually ideal, i.e., the fractions of other elements are insignificant. Therefore, excluding B, probertite does not act as a geochemical trap of other industrially relevant elements (e.g., Li, Be, or REE). Our experimental results confirm that the structure of probertite is built up by the so-called pentaborate polyanion [B5O7(OH)4]3− (topology: 5(2Δ + 3T)], which consists of oxygen-sharing B-tetrahedra and B-triangular units. The five (geometrical) components of the polyanion are BO3, BO2OH, BO4, BO3OH, and BO2(OH)2 groups. The pentaborate building units are connected to form chains running along [100]. Clusters of distorted Ca-polyhedra [CaO5(OH)3(OH2), CN = 9] and Na-polyhedra [NaO(OH)2(OH2)3, CN = 6] are mutually connected by edge-sharing and, in turn, connected to the pentaborate chains by edge-sharing (with the Ca-polyhedron) and corner-sharing (with the Na-polyhedron). The hydrogen-bonding scheme of the probertite structure is complex and pervasive, with 10 independent H sites (belonging to hydroxyl groups or H2O molecules) and 11 of the 14 oxygen sites being involved in H-bonds as donor or acceptors. Hence, the H-bonding network likely plays an important role in the stability of probertite. In addition, the potential utilizations of probertite are discussed.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1378-1384
doi: 10.2138/am-2022-8119
Crystal structure determination of orthorhombic variscite2O and its derivative AlPO4 structure at high temperature
Matteo Ardit; Brian L. Phillips; David L. Bish
<jats:title>Abstract</jats:title> <jats:p>Variscite [Al(PO4)·2H2O] is an uncommon secondary phosphate mineral but is important in a variety of environmental and technological applications. It exists in at least one monoclinic (metavariscite) and two orthorhombic polymorphs (“Lucin-type” and “Messbach-type”), but the fine-grained nature of the “Messbach-type” variscite has hampered the determination of its crystal structure. The crystal structure of the latter from Tooele County, Utah, was solved and refined using laboratory powder X-ray diffraction (XRD) data, charge-flipping, and the Rietveld method. Both variscite modifications belong to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms. Topological analysis reveals that the two structures are polytypes. Based on our results and our structural interpretations, we refer to “Lucin-type” variscite as variscite1O and the “Messbach-type” as variscite2O, to be consistent with modern polytype terminology. The similarity of these two structures suggests that 1O-2O interstratifications may exist in nature, which is consistent with observed broadening of diffraction peaks of the Tooele material. 31P and 27Al MAS/NMR measurements are consistent with the XRD-determined crystal structure, and they show distinct signals for each of the two independent P and Al positions in variscite2O.</jats:p> <jats:p>High-temperature XRD, thermal analyses, and NMR measurements were applied to study the nature of the transformation of variscite2O to a derivative AlPO4 structure above 473 K. Charge-flipping analysis showed that the crystal structure of the new anhydrous AlPO4 phase (AlPO4-var2O in analogy to its parent structure) can be described as a 3D framework of alternating AlO4 and PO4 tetrahedra linked by bridging O atoms. Thermogravimetric analyses revealed almost complete dehydration above ~450 K, and NMR results were consistent with tetrahedral Al and P atoms.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1385-1395
doi: 10.2138/am-2021-7888
Transformation of Fe-bearing minerals from Dongsheng sandstone-type uranium deposit, Ordos Basin, north-central China: Implications for ore genesis
Liang Yue; Yangquan Jiao; Mostafa Fayek; Liqun Wu; Hui Rong; Huili Xie
<jats:title>Abstract</jats:title> <jats:p>Iron-bearing mineral assemblages and their distribution patterns directly reflect the redox environment in sediments, which plays a decisive role in the migration and precipitation of U. The Dongsheng sandstone-type U deposit hosted in fluvial and/or deltaic sandstones of the lower member of the Middle Jurassic Zhiluo Formation in the northeastern Ordos Basin has experienced multiple fluid events that impacted the redox conditions. Highly enriched in barren gray sandstones, pre-ore U (Umean = 12.05 ppm) associated with Fe-Ti oxides, clay minerals, and organic matter is likely one of the key sources of U for the mineralization. Different contents of Fe-bearing minerals, including biotite, Fe-Ti oxides, pyrite, hematite, goethite, and chlorite that were formed or altered under different redox conditions, resulted in sandstone units with distinct colors. The red sandstone is hematite-rich, indicating a highly oxidizing environment. The green sandstone is chlorite-rich and formed because of reducing hydrocarbon-rich fluids that overprinted the hematite-rich sandstone. The barren and mineralized gray sandstones consist of pyrite (with a higher content in mineralized sandstones), Fe-Ti oxides, and carbonaceous debris, which are indicators of a reducing environment. Based on the paragenetic relationship and sulfur isotopic compositions of ore-stage pyrite, bacterial sulfate reduction was responsible for the formation of framboidal pyrite (δ34S = –31.2 to –3.8‰), and the sulfur of this pyrite mainly came from the oxidation of pre-ore pyrite (δ34S = –19.1 to +20.3‰). Euhedral and cement pyrite overprinting framboids were produced via Ostwald ripening with δ34S values ranging from –56.9 to –34.3‰, lower than any values of framboidal pyrite. Therefore, these mineralogical and geochemical characteristics of the Dongsheng deposit suggest U mineralization involves both biogenic and abiogenic redox processes.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1396-1409
doi: 10.2138/am-2021-7940
Vaterite in a decrepitated diamond-bearing inclusion in zircon from a stromatic migmatite in the Chinese Sulu ultrahigh-pressure metamorphic belt
Chenhui Fei; Jingbo Liu
<jats:title>Abstract</jats:title> <jats:p>Vaterite was identified in a decrepitated carbonaceous material (CM) bearing inclusion in zircon from a stromatic migmatite in the Chinese Sulu ultrahigh-pressure (UHP) metamorphic terrane. It is associated with nanometer to micrometer anhedral diamonds, aragonite, calcite, amorphous C-Si-O, and amorphous Zr-Si-O materials. The inclusion developed offshoots and abundant indigenous holes. The C-Si-O material is carbon-rich and porous and shows diagnostic Raman bands of highly disordered CM, whereas the Zr-Si-O material is spherulitic or banded with little or no carbon. The observations from focused ion beam–scanning electron microscope (FIB-SEM) and transmission electron microscope (TEM) verify that both diamond and highly disordered CM are of indigenous origin. The formation pathway of vaterite means that an amorphous calcium carbonate (ACC) phase occurred as the precursor of vaterite. The highly disordered CM contains the subsidiary bands at 1150 and 1250 cm−1 on the low-frequency side of the D1 band, suggesting that there exist aliphatic hydrocarbon chains. Thus, the highly disordered CM was derived from carbonization of some kind of organic species in the fluid inclusion. Decrepitation of inclusion resulted in an extremely high supersaturation state of the fluid that induced the precipitation of amorphous materials and released residual fluid out of the inclusion, which became dry and preserved vaterite and amorphous materials.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1410-1424
doi: 10.2138/am-2022-7970
Oxygen diffusion in garnet: Experimental calibration and implications for timescales of metamorphic processes and retention of primary O isotopic signatures
Maria Rosa Scicchitano; Michael C. Jollands; Ian S. Williams; Jörg Hermann; Daniela Rubatto; Noriko T. Kita; William O. Nachlas; John W. Valley; Stéphane Escrig; Anders Meibom
<jats:title>Abstract</jats:title> <jats:p>Knowledge of oxygen diffusion in garnet is crucial for a correct interpretation of oxygen isotope signatures in natural samples. A series of experiments was undertaken to determine the diffusivity of oxygen in garnet, which remains poorly constrained. The first suite included high-pressure (HP), nominally dry experiments performed in piston-cylinder apparatus at: (1) T = 1050–1600 °C and P = 1.5 GPa and (2) T = 1500 °C and P = 2.5 GPa using yttrium aluminum garnet (YAG; Y3Al5O12) cubes. Second, HP H2O-saturated experiments were conducted at T = 900 °C and P = 1.0–1.5 GPa, wherein YAG crystals were packed into a YAG + Corundum powder, along with 18O-enriched H2O. Third, 1 atm experiments with YAG cubes were performed in a gas-mixing furnace at T = 1500–1600 °C under Ar flux. Finally, an experiment at T = 900 °C and P = 1.0 GPa was done using a pyrope cube embedded into pyrope powder and 18O-enriched H2O. Experiments using grossular were not successful.</jats:p> <jats:p>Profiles of 18O/(18O+16O) in the experimental charges were analyzed with three different secondary ion mass spectrometers (SIMS): sensitive high-resolution ion microprobe (SHRIMP II and SI), CAMECA IMS-1280, and NanoSIMS. Considering only the measured length of 18O diffusion profiles, similar results were obtained for YAG and pyrope annealed at 900 °C, suggesting limited effects of chemical composition on oxygen diffusivity. However, in both garnet types, several profiles deviate from the error function geometry, suggesting that the behavior of O in garnet cannot be fully described as simple concentration-independent diffusion, certainly in YAG and likely in natural pyrope as well. The experimental results are better described by invoking O diffusion via two distinct pathways with an inter-site reaction allowing O to move between these pathways. Modeling this process yields two diffusion coefficients (D values) for O, one of which is approximately two orders of magnitude higher than the other. Taken together, Arrhenius relationships are:logDm2s-1=-7.2(±1.3)+(-321(±32)kJmol-12.303RT)</jats:p> <jats:p>for the slow pathway, andlogDm2s-1=-5.4(±0.7)+(-321(±20)kJmol-12.303RT)</jats:p> <jats:p>for the fast pathway. We interpret the two pathways as representing diffusion following vacancy and inter-stitial mechanisms, respectively. Regardless, our new data suggest that the slow mechanism is prevalent in garnet with natural compositions, and thus is likely to control the retentivity of oxygen isotopic signatures in natural samples.</jats:p> <jats:p>The diffusivity of oxygen is similar to Fe-Mn diffusivity in garnet at 1000–1100 °C and Ca diffusivity at 850 °C. However, the activation energy for O diffusion is larger, leading to lower diffusivities at P-T conditions characterizing crustal metamorphism. Therefore, original O isotopic signatures can be retained in garnets showing major element zoning partially re-equilibrated by diffusion, with the uncertainty caveat of extrapolating the experimental data to lower temperature conditions.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1425-1441
doi: 10.2138/am-2021-7682
Oxidation state of iron and Fe-Mg partitioning between olivine and basaltic martian melts
Andrew K. Matzen; Alan Woodland; John R. Beckett; Bernard J. Wood
<jats:title>Abstract</jats:title> <jats:p>We performed a series of experiments at 1 atm pressure and temperatures of 1300–1500 °C to determine the effect of oxygen fugacity on the oxidation state of Fe in a synthetic martian basalt. Ferricferrous ratios were determined on the quenched glasses using Mössbauer spectroscopy. Following the conventional doublet assignments in the spectrum, we obtain a Fe3+/ΣFe value of 0.19 at 1450 °C and an oxygen fugacity corresponding to the QFM buffer. If we apply the Berry et al. (2018) assignments the calculated Fe3+/ΣFe drops to 0.09, and the slope of log(XFeO1.5melt/XFeOmelt) vs. log(fO2) changes from 0.18 to 0.26.</jats:p> <jats:p>Combining oxidation state data together with results of one additional olivine-bearing experiment to determine the appropriate value(s) for the olivine (Ol)-liquid (liq) exchange coefficient, KD,Fe2+-Mg = (FeO/MgO)Ol/(FeO/MgO)liq (by weight), suggests a KD,Fe2+-Mg of 0.388 ± 0.006 (uncertainty is one median absolute deviation) using the traditional interpretation of Mössbauer spectroscopy and a value of 0.345 ± 0.005 following the Mössbauer spectra approach of Berry et al. (2018).</jats:p> <jats:p>We used our value of KD,Fe2+-Mg to test whether any of the olivine-bearing shergottites represent liquids. For each meteorite, we assumed a liquid composition equal to that of the bulk and then compared that liquid to the most Mg-rich olivine reported. Applying a KD,Fe2+-Mg of ~0.36 leads to the possibility that bulk Yamato 980459, NWA 5789, NWA 2990, Tissint, and EETA 79001 (lithology A) represent liquids.</jats:p>
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. 1442-1452
doi: 10.2138/am-2022-8502
Low-temperature crystallography and vibrational properties of rozenite (FeSO4·4H2O), a candidate mineral component of the polyhydrated
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. No disponible
doi: 10.2138/am-2022-8584
Magnetic Contributions to Corundum-Eskolaite and Corundum-Hematite Phase Equilibria: a DFT Cluster Expansion Study
Palabras clave: Geochemistry and Petrology; Geophysics.
Pp. No disponible