Editor’s Notes
Total Results: 1697
Volume 106 : August 2021 Issue
Effect of magnesium on monohydrocalcite formation and unit-cell parameters
https://doi.org/10.2138/am-2021-7673
Vereshchagin et al. report the phase diagrams for monohydrocalcite (MHC), calcite, aragonite, and dypingite precipitation, as a function of pH and Mg/Ca ratio in solution. They demonstrate that elevated magnesium content and low temperature are favorable for MHC formation. On the basis of changes in the unit cell parameters, the possibility of the significant incorporation of magnesium into the crystal structure of MHC has been unequivocally proven. The increase in Mg in MHC is accompanied by an increase in water content, which leads to multidirectional changes in a and c cell parameters.
Formation pathway of norsethite dominated by solution chemistry under ambient conditions
https://doi.org/10.2138/am-2021-7348
"""Dolomite problem"" is a long-standing puzzle in geology. Zhang et al. investigated low-temperature formation of norsethite &91;BaMg(CO3)2&93; as a dolomite-analog under different solution chemistry conditions. The results reveal that the initial Mg/Ba ratio in solution is a crucial factor controlling the formation pathway of norsethite. At low Mg/Ba ratios, a multistep pathway occurs, i.e., a precursor witherite first forms, followed by norsethite precipitation and transformation from witherite to norsethite. In contrast, at Mg/Ba ratio > 20, norsethite can be directly precipitated from aqueous solution. This is the first report on direct precipitation of dolomite analogs under ambient conditions, and the findings provide new insights into the formation pathways of dolomite analogs and thus low-temperature dolomite."
A model for the kinetics of high-temperature reactions between polydisperse volcanic ash and SO2 gas
https://doi.org/10.2138/am-2021-7691
Scientists have collected volcanic ash from eruptions and noticed that tiny salt crystals exist on their surfaces. Wadsworth et al. use experiments and mathematics to work out how quickly those salt crystals grow. Because the salts contain sulfur, they propose that some of the sulfur that would have ended up in the atmosphere actually ends up on ash surfaces. They further propose that these equations could tell us just how much sulfur is removed from the atmosphere in this process.
Redox control and measurement in low-temperature (<450 °C) hydrothermal experiments
https://doi.org/10.2138/am-2021-7687
Fang and Chou propose a new oxygen buffer technique for hydrothermal experiments carried out at temperatures below 450 °C and demonstrate its success in redox control and monitoring. Their work extends the low-temperature limit of previously well-developed redox control techniques to, at least, 200 °C, and it has promising prospects of application in low-temperature hydrothermal experiments.
Heat capacity and thermodynamic functions of partially dehydrated sodium and zinc zeolite A (LTA)
https://doi.org/10.2138/am-2021-7726
Dickson et al. offer a thermodynamic perspective into cation exchange, water sorption, and sorbate-induced structural transformations in zeolites. Heat capacities and thermodynamic functions are reported for industrially important zeolite A and for the ion-exchanged version zinc zeolite A. In addition, zeolitic water in sodium zeolite A is compared with that in other zeolites. Inflections common to all the zeolitic water heat capacities suggest that the hydration-influenced framework flexibility that has been found in zeolite A may be present in other zeolites as well.
P-V-T measurements of Fe3C to 117 GPa and 2100 K: Implications for stability of Fe3C phase at core conditions
https://doi.org/10.2138/am-2021-7581
McGuire et al. measured the unit-cell volume of an iron carbide, Fe3C, by X-ray diffraction at simultaneous extreme pressure and temperature conditions relevant to the deep interior of the Earth. These measurements are used to construct a thermodynamic model of Fe3C for the Earth's core. They find that existing measurements support two possible scenarios for the Earth's core in the Fe-C system: (1) Fe3C is the stable end-member at inner core conditions but cannot explain the inner core density; and (2) Fe7C3 is stable and may explain the inner core density at relatively low temperatures.
NEW MINERAL NAMES
https://doi.org/10.2138/am-2021-NMN106818
New Mineral Names
ERRATUM
https://doi.org/10.2138/am-2021-E106817
Erratum
BOOK REVIEW
https://doi.org/10.2138/am-2021-B106819
Book Review: The Pinch Collection at the Canadian Museum of Nature. (2020)44378
Volume 106 : July 2021 Issue
Tourmaline composition and boron isotope signature as a tracer of magmatic-hydrothermal processes
https://doi.org/10.2138/am-2021-7495
Qiu et al. in their paper make three key points. They found that tourmalines crystallized from magmatic-hydrothermal fluids with different oxygen fugacities and salinities; that degassing during fluid evolution leads to significant B isotope fractionation and increases oxygen fugacity; and that tourmaline composition and the boron isotope signature can be tracers of magmatic and hydrothermal processes, as well as used to distinguish mineralized from barren host-granites.
Deformation and strength of mantle relevant garnets: Implications for the subduction of basaltic-rich crust
https://doi.org/10.2138/am-2021-7587
Garnet is an abundant mineral in the upper mantle, and there have been no previous measurements of both its room-temperature strength and deformation mechanism. Vennari et al. deformed three compositions of garnet between two diamonds and probed its deformation with X-rays perpendicular to the compression axis. These measurements allowed them to determine garnet's resistance to plastic deformation and show that its strength is similar to other mantle phases. Vennari et al. also provide a modeling analysis to explore the deformation mechanism on an atomistic scale when axial stress is applied.
Ultra-reduced phases in ophiolites cannot come from Earth’s mantle
https://doi.org/10.2138/am-2021-7612
It is claimed that ophiolites carry highly reduced minerals and that the source regions of ophiolites in the Earth's mantle are located in the transition zone or lower mantle. By showing experimentally that highly reduced phases such as moissanite (SiC) are unstable in a FeO-bearing mantle, Ballhaus et al. refute this hypothesis. They caution that accessory diamond and ultra-reduced phases may not provide sufficient evidence to question existing models of ophiolite genesis.
Olivine from aillikites in the Tarim large igneous province as a window into mantle metasomatism and multi-stage magma evolution
https://doi.org/10.2138/am-2021-7521
Wang et al. report the first aillikite in China. Phlogopite- and carbonate-rich metasomatic veins are involved in the source of the aillikite magma. Phenocrystic olivines in the aillikite are cognate cumulates formed by multi-stage crystallization at temperatures of 906-1136 degrees C.
Precise determination of the effect of temperature on the density of solid and liquid iron, nickel, and tin
https://doi.org/10.2138/am-2021-7509
Kamiya et al. precisely measured the density of solid Sn, Ni, Fe, and liquid Fe using a high-temperature furnace. The density of solid Sn shows a drastic decrease near melting temperature (Tm). By contrast, the densities of solid Ni and Fe decrease linearly with increasing temperature up to Tm without any drastic decrease near Tm. The thermal expansion of liquid Fe was also determined to depend on temperature. The results are important for understanding the elastic behavior of solid and liquid metals at high temperatures.
Timescales of crystal mush mobilization in the Bárðarbunga- Veiðivötn volcanic system based on olivine diffusion chronometry
https://doi.org/10.2138/am-2021-7670
Caracciolo et al. applied diffusion chronometry to olivine crystals erupted at different ages from a single volcanic system. They carried out a chemical characterization of olivine crystals coupled with a detailed Fe-Mg, Mn, and Ni diffusion study from temporally diverse (sub-glacial to historical time) eruptive units to retrieve timescales of crystal mush disaggregation in the volcanic system in Iceland. One important implication of their study is that magma replenishment and mush disaggregation events, possibly triggered by rifting-assisted processes, do not result in imminent eruption. In addition, only a few data exist on timescales of magmatic processes in mid-Ocean Ridge settings, and this is the first detailed study of the temporal variation of crystal-mush-to-eruption timescales in any volcanic system in any magmatic setting.
Chemical reactions in the Fe2SiO4-D2 system with a variable deuterium content at 7.5 GPa
https://doi.org/10.2138/am-2021-7697
The reactions examined in the paper by Efimchenko et al. are expected to occur under natural conditions in the interiors of the icy satellites and lead to the removal of fayalite from rocks and minerals due to its dissolution in hydrogen fluid or replacement by iron, ferrosilite, or silica depending on the molar ratio H2/Fa.
High-pressure syntheses and crystal structure analyses of a new low-density CaFe2O4-related and CaTi2O4-type MgAl2O4 phases
https://doi.org/10.2138/am-2021-7619
The high-pressure polymorphs of MgAl2O4 are considered important and abundant components that are stable under lower-mantle conditions in alumina-rich rocks, such as basalt, upper continental crust, and sediment. In this study, Ishii et al. synthesized single crystals of CaTi2O4 (CT)-type, CaFe2O4 CF-type, and a novel MgAl2O4 phases at 27 GPa and 2500 °C by conventional multi-anvil technique. They also synthesized CT-type MgAl2O4 at 45 GPa and 1727 °C using an advanced multi-anvil technique that they developed. Their single-crystal X-ray diffraction measurements showed that the novel phase has a completely new structure with a smaller density than the ambient pressure phase of spinel, despite its high-pressure synthesis, indicating that this phase is a back-transformed phase from a high-pressure phase during the recovery. This finding solves the crystal structure of a previously unknown phase. CT-type MgAl2O4 has been considered to be stable at more than 40 GPa. Ishii et al.’s finding that CT-type phase is synthesized even at 27 GPa indicates that the CT phase in natural-rock compositions may be stable in wide lower mantle conditions, especially upwelling hot plume conditions. They also suggest that the novel phase can be found in shocked meteorites and can be used as a shock indicator.
Phase diagram and thermal expansion of orthopyroxene-, clinopyroxene-, and ilmenite-structured MgGeO3
https://doi.org/10.2138/am-2021-7685CCBY
Despite the ubiquity of studies of low-pressure analogs for Earth-forming phases, these systems are often incompletely characterized. Frequently, parts of the system remain unmeasured, but an analogue is only as good as its characterization. Here, Hunt et al., in their open access paper, complete the characterization of the lowest pressure phases of MgGeO3 and reconcile the disagreement between previous measurements. The reconciliation points to important differences between MgSiO3 and its low-pressure analog MgGeO3.
Mass transfer associated with chloritization in the hydrothermal alteration process of granitic pluton
https://doi.org/10.2138/am-2020-7353
This study by Yuguchi et al. focuses on the petrography and mineral chemistry of hornblende chloritization, K-feldspar chloritization, and fracture-filling chlorite, in order to better understand mass transfer among the reactants, products, and hydrothermal fluids during chloritization. A combination of this study and Yuguchi et al. (2015) dealing with biotite chloritization reveals the characteristics of mass transfer in the overall chloritization process.
Nonlinear effects of hydration on high-pressure sound velocities of rhyolitic glasses
https://doi.org/10.2138/am-2021-7597
Gu et al. measured sound velocities in hydrous rhyolitic glasses containing up to ~6 wt% H2Ot at pressures to 3 GPa. The results show that hydration has a non-linear effect on sound velocities at high pressure, in contrast to the linear effects of hydration previously observed at ambient pressure. These results imply that the different water species, OH- and H2Om, play unique roles in altering sound velocities in rhyolitic glasses. The non-linear effects of hydration found here provide some insight into the behavior of hydrous silicate melts in felsic magma chambers at depth.
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