Editor’s Notes
Total Results: 1697
Volume 104 : September 2019 Issue
Nixonite, Na2Ti6O13, a new mineral from a metasomatized mantle garnet pyroxenite from the western Rae Craton, Darby kimberlite field, Canada
https://doi.org/10.2138/am-2019-7023
Anzolini et al. (page 1336) report the discovery and full description of a new mineral, nixonite, ideally Na2Ti6O13 (IMA 2018-133). Nixonite was found as a part of a complex reaction rim around a rutile grain within a heavily-metasomatized pyroxenite xenolith from the Darby kimberlite field, beneath the west-central Rae Craton, Canada. Nixonite is the first occurrence in nature of Na2Ti6O13, which was previously known only as a synthetic material used in batteries, and is the Na-rich analogue of jeppeite, K2Ti6O13, which is commonly observed as a groundmass mineral in lamproites. This finding represents not only the first natural occurrence of Na2Ti6O13, but also the unique coexisting assemblage of the minerals rutile, priderite, perovskite, freudenbergite, ilmenite, and nixonite. We suggest that this complex Na-K-Ti rich metasomatic mineral assemblage may have been produced by an unusual metasomatic melt that percolated through the lithospheric mantle beneath the Darby field. https://doi.org/10.2138/am-2019-7023
Goldschmidtite, (K,REE,Sr)(Nb,Cr)O3: A new perovskite supergroup mineral found in diamond from Koffiefontein, South Africa
https://doi.org/10.2138/am-2019-6937
Meyer et al. (page 1345) report the discovery of the new mineral Goldschmidtite. New minerals are not common after years of mineral research, and new perovskite-structured minerals are even more rare. This new mineral, coming from a diamond, highlights a case of extreme metasomatic conditions in the lithosphere. Remarkably, this mineral is a well-known synthetic in ceramic science. It is also, justifiably, named after one of, if not the, founders of geochemistry, whose scientific impact is well known and far reaching. https://doi.org/10.2138/am-2019-6937
Edscottite, Fe5C2, a new iron carbide mineral from the Ni-rich Wedderburn IAB iron meteorite
https://doi.org/10.2138/am-2019-7102
Chi Ma and Alan Rubin (page 1351) report the discovery of the new mineral edscottite that occurs with low-Ni iron (kamacite), taenite, nickelphosphide (Ni-dominant schreibersite), and minor cohenite in the Wedder-burn iron meteorite, a Ni-rich member of the group IAB complex. The end-member formula is Fe5C2. The new mineral is named in honor of Edward (Ed) R.D. Scott, a pioneering cosmochemist at the University of Hawai’i at Manoa, for his seminal contributions to research on meteorites. Since the Carbon Mineral Challenge was launched in 2015, edscottite is the first carbide mineral approved by the IMA-CNMNC. https://doi.org/10.2138/am-2019-7102
The stability of Fe5O6 and Fe4O5 at high pressure and temperature
https://doi.org/10.2138/am-2019-7097
Hikosaka et al. (page 1356) present the results of their study of the stability of Fe5O6 and Fe4O5 at high pressures. While several new iron oxides were discovered above 10 GPa in the last decade, their stabilities at high pressure and temperature (P-T ) are not understood yet. Here we examined phase relations in both Fe5O6 and Fe4O5 and found that both decompose into the FeO + Fe3O4 assemblage above ~40 GPa. It indicates that the intermediate compounds between FeO and Fe3O4 (i.e., Fe4O5, Fe5O6, Fe7O9, and Fe9O11) are formed only in the deep upper mantle to the shallow lower mantle. https://doi.org/10.2138/am-2019-7097
NEW MINERAL NAMES
https://doi.org/10.2138/am-2019-NMN104914
Belakovskiy et al. report on the new mineral names (page 1360). The paper contains entries for nine new minerals, including argentotetrahedrite-(Fe), bytízite, calamaite, fluorlamprophyllite, honzaite, katerinopoulosite, meitnerite, melcherite, and rozhdestvenskayaite-(Zn). http://dx.doi.org/10.2138/am-2019-NMN104914
Volume 104 : August 2019 Issue
Metamorphism and the evolution of subduction on Earth
https://doi.org/10.2138/am-2019-6956
This issue of American Mineralogist starts with the MSA Presidential Address given by Brown (Past-President) and Johnson (page 1065): Metamorphism and the evolution of subduction on Earth. Whether Earth always had plate tectonics or, if not, when and how a globally-linked network of narrow plate boundaries emerged are matters of debate. Here the authors use a dataset of the pressure (P), temperature (T), and age of metamorphic rocks to evaluate secular change associated with subduction and collisional orogenesis at convergent plate boundaries. The widespread appearance of two types of metamorphism with different thermobaric ratios (T/P) at the beginning of the Neoarchean is interpreted to be evidence of the stabilization of subduction during the emergence of plate tectonics.
Experimental investigation of FeCO3 (siderite) stability in Earth’s lower mantle using XANES spectroscopy
https://doi.org/10.2138/am-2019-6428
Cerantola et al. (page 1083) studied the stability of siderite (FeCO3) in the Earth's lower mantle. The experimental results using Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy are supported by first-principles calculations and match well with recently reported observations on FeCO3 at extreme conditions. At conditions of the mid-lower mantle, ~50 GPa and ~2200 K, FeCO3 melts and partially decomposes to high-pressure Fe3O4. Diamond and oxygen are also inferred products of the reaction. Moreover, the incongruent melting of FeCO3 could be a key mechanism that partially preserves FeCO3 from decomposition, potentially supporting carbon influx into the deep Earth via carbonate subduction.
Comparison of fluid processes in coexisting wolframite and quartz from a giant vein-type tungsten deposit, South China: Insights from detailed petrography and LA-ICP-MS analysis of fluid inclusions
https://doi.org/10.2138/am-2019-6958
Pan et al. (page 1092) conducted a comparative study of fluid inclusions in coexisting wolframite and quartz crystals from a giant vein-type tungsten deposit, South China. A combined in situ analytical procedure, including cathodoluminescence (CL) imaging, infrared microthermometry, Raman microspectroscopy, and fluid inclusion LA-ICP-MS analysis, was used to reconstruct the detailed fluid evolution history. Based on elaborate petrography on the crystal scale, fluid inclusions in wolframite and coexisting quartz are found to exhibit distinct chemical signatures, despite sharing similar overall ranges of salinity and homogenization temperature. The revealed fluid evolution history provides insight into the fluid source and the wolframite deposition mechanism in vein-type tungsten deposits.
Monazite and xenotime solubility in hydrous, boron-bearing rhyolitic melt
https://doi.org/10.2138/am-2019-6931
Rusiecka and Baker (page 1117) obtained new data on monazite and xenotime solubility in boron-bearing rhyolitic melts at 1000-1400 °C and 800 MPa in a piston cylinder apparatus, as well as the diffusivity of the components of these two minerals (LREE, P, and Y). This study provides first-ever data on the solubility of xenotime-(Y) in rhyolitic melt, as well as new data on monazite solubility in the boron-bearing rhyolitic melt. The authors also discussed implications of the results on the understanding of natural, silicic (granitic/rhyolitic), and magmatic systems.
Solving the iron quantification problem in low-kV EPMA: An essential step toward improved analytical spatial resolution in electron probe microanalysis—Olivines
https://doi.org/10.2138/am-2019-6865
Moy et al. (page 1131) report a new low-accelerating voltage electron probe microanalysis (EPMA) method to quantify Fe using the combined Fe L X-ray lines. This method takes advantage of the reduction of the electron beam offered by Schottky field emission source in conjunction with the reduction of the electron interaction volume permitted by decreasing the accelerating voltage from traditional 15-20 kV to 7 kV, to accurately quantify submicrometer-sized features. The method has been successfully applied to olivines as a test bench. Compared with large errors using the traditional EPMA quantification method using the Fe L X-ray line, the new method gives much more accurate results, with average relative deviations of 3.6% from actual compositions.
Transition metals in komatiitic olivine: Proxies for mantle composition, redox conditions, and sulfide mineralization potential
https://doi.org/10.2138/am-2019-6914
Locmelis et al. (page 1143) present the results of a comprehensive study on the concentrations of first-row transition elements, Ga and Ge in olivine from komatiites measured via laser ablation ICP-MS. The data show that (1) elevated Ga/Sc ratios in olivine reflect garnet retention in the komatiite source, (2) high Ge contents in olivine may be indicative of melting under hydrous conditions, (3) V/Sc and Mn/Fe ratios in olivine can potentially be used to constrain local oxygen fugacity in the komatiite magma, and (4) Cu-abundances reflect the sulfide saturation state of a komatiite magma during olivine crystallization.
Formation of saponite by hydrothermal alteration of metal oxides: Implication for the rarity of hydrotalcite
https://doi.org/10.2138/am-2019-7043
Tao et al. (page 1156) report a mechanism of hydrotalcite (Ht) conversion into saponite after formed by the hydrothermal alteration of metal oxides. The conversion was through a hydration-dissolution-precipitation pathway. It was favored under the conditions of relatively low Mg/Al ratios with high Al and Si contents, and greatly accelerated by the Al3+—Si4+ substitution in silicate oligomers to generate the extra negative charge in tetrahedral sheets. During the process, CO2 was initially incorporated into the interlayer of Ht as CO32-, but was expelled by the formation of saponite, due to the layer charge reversal from positive in Ht to negative in saponite. These findings provide an explanation for the rare occurrence of hydrotalcite deposits on the Earth's surface.
Near end-member shenzhuangite, NiFeS2, found in Muong Nong-type tektites from Laos
https://doi.org/10.2138/am-2019-6930
Křížová et al. (page 1165) report the occurrence of shenzhuangite close to its ideal end-member composition (NiFeS2), Ni1.007Fe0.998Cu0.016Co0.058S1.922, in Australasian Muong Nong-type tektites from Laos. This was the first discovery of shenzhuangite in terrestrial materials; originally it was found in meteorite Suizhou. This shenzhuangite was identified by electron probe microanalysis and electron back scatter diffraction. The authors also presented a Raman spectrum with a tentative assignment of spectral bands based on the analogy with synthetic chalcopyrite-structured phases.
Pressure-induced velocity softening in natural orthopyroxene at mantle temperature
https://doi.org/10.2138/am-2019-6935
Wang et al. (page 1173) conducted acoustic velocity measurements of a natural orthopyroxene, (Mg1.77Fe0.22Ca0.01)Si2O6, up to 13.5 GPa and 873 K. It is known that the end-member orthoenstatite, Mg2Si2O6, undergoes velocity softening at high pressure and room temperature, due to its transition to the metastable, high-pressure clinoenstatite. This study reports a similar, high-pressure velocity softening in Fe-bearing orthopyroxene at temperatures up to at least 673K, providing an upper bound on the P-T conditions where such softening may occur in subduction zones. The direct measurement of velocity jump across the orthopyroxene to high-pressure clinopyroxene transition further proves that it could be a possible contributor to the seismic X-discontinuity.
The role of mineral nanoparticles at a fluid-magnetite interface: Implications for trace-element uptake in hydrothermal systems
https://doi.org/10.2138/am-2019-6996
Yin et al. (page 1180) present a potential mechanism for the uptake of trace element at a fluid-mineral interface in hydrothermal systems. More specifically, they determine the role of mineral nanoparticles at a fluid-magnetite interface using high-resolution transmission electron microscopy. The results show that the Al concentration in magnetite measured on a micron-scale is caused by three different effects: Al solid solution, Al-rich nanometer-sized lamellae and zinc spinel nanoparticles in the host magnetite, and the authors propose a genetic relationship among the three phases. The fluid-mineral interface in this mechanism has been repeatedly utilized during crystal growth, providing an efficient way for the uptake of trace element from a related undersaturated bulk fluid.
Melting temperature depression due to the electronic spin transition of iron
https://doi.org/10.2138/am-2019-6948
Deng and Lee (page 1189) investigated the effects of the electronic spin transition of iron on the melting temperature of Earth's two most abundant minerals, ferropericlase, and bridgmanite, using both Lindemann's Law and thermodynamic analyses. Lindemann's Law predicts a negligible melting temperature depression for bridgmanite but a substantial depression for ferropericlase across the spin transition of iron, consistent with extant experimental results. Thermodynamically, the melting depression likely derives from a more negative Margules parameter for a liquid mixture of high- and low-spin end-members as compared to that of a solid mixture. This melting depression across the spin transition of iron may be the process responsible for the formation of a deep molten layer during the crystallization of a magma ocean in the past, and a reduced viscosity layer at present.
Epidote spherulites and radial euhedral epidote aggregates in a greenschist facies metavolcanic breccia hosting an UHP eclogite in Dabieshan (China): Implication for dynamic metamorphism
https://doi.org/10.2138/am-2019-6980
Chen et al. (page 1197) investigated epidote spherulites and radial euhedral epidote aggregates in a greenschist facies metavolcanic breccia hosting an ultrahigh-pressure eclogite in Dabieshan, China, and discussed the implications for dynamic metamorphism. Because these non-equilibrium textures would recrystallize into equilibrium ones if the P-T -H2O conditions were maintained sufficiently long, they likely formed in response to P-T and fluid pulses, possibly related to seismicity.
A new (Mg0.5Fe3+0.5)(Si0.5Al3+0.5)O3 LiNbO3-type phase synthesized at lower mantle conditions
https://doi.org/10.2138/am-2019-7070
Liu et al. (page 1213) synthesized a new (Mg0.5Fe3+0.5)(Si0.5Al3+0.5)O3 LiNbO3-type phase (space group R3c) at lower-mantle conditions. Fe3+ and Al3+ cations substitute into A (Mg2+) and B (Si4+) sites, respectively, through a charge-coupled substitution mechanism. This phase is probably recovered from bridgmanite at lower-mantle conditions by a diffusionless transition because of the displacement of A cations and distortion of BO6 octahedra on releasing pressure. Bridgmanite can thus contain the FeAlO3 component (50 mol%) beyond the previously reported solubility limit (37 mol%). The present study shows that the Earth's most abundant elements form a new FeAlO3-dominated LiNbO3-type compound from bridgmanite at lower mantle conditions. It provides new insight into the complicated crystal chemistry of LiNbO3-type phase/bridgmanite and constrains the P-T conditions for shocked meteorites.
Volume 104 : July 2019 Issue
Fate of cobalt and nickel in mackinawite during diagenetic pyrite formation
https://doi.org/10.2138/am-2019-6834
Swanner et al. (page 917) in their article “Fate of cobalt and nickel in mackinawite during diagenetic pyrite formation” report their investigations of Ni and Co incorporation into freshly-precipitated mackinawite, and after experimental diagenesis to pyrite at 65 °C. Fe and S K-edge micro-X-ray absorption near edge spectroscopy identified the oxidation state and mineralogy within experimentally synthesized and diagenetically transformed minerals. Results indicate that Co and Ni may inhibit the transformation of mackinawite to pyrite or slow it down. Cobalt concentrations in the solid diminished by 30% during pyrite transformation, indicating that pyrite Co may be a conservative tracer of seawater or porewater Co concentrations. Ni concentrations increased several-fold after pyrite formation, suggesting that pyrite may have scavenged Ni from the dissolution of primary mackinawite grains. Nickel in pyrites thus may not be a reliable proxy for seawater or pore water metal concentrations.
Influence of aluminum on the elasticity of majorite-pyrope garnets
https://doi.org/10.2138/am-2019-6771
Liu et al.’s research on page 929 in the “Influence of aluminum on the elasticity of majorite-pyrope garnets” used an ultrahigh pressure convection technique to fabricate a series of gem-quality majorite-pyrope garnets and measured the velocity of these garnets by means of ultrasonic interferometry measurements. They found that both velocity and elastic moduli increase linearly with increasing Al along the majorite-pyrope system. The Al component plays a dominant role for the variation of elasticity (velocity and modulus) for majorite-pyrope garnets, while the phase transition due to cation ordering or disordering cannot significant affect these elastic properties. Therefore, seismic velocity modeling of a garnet-bearing mantle transition zone is more associated with garnet’s composition rather than the phase transitions due to cation order or disorder in garnet.
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