Editor’s Notes
Total Results: 1697
Volume 99 : January 2014 Issue
Shedding light on bone material
https://doi.org/10.2138/am.2014.4725
On page 1 of this issue, Hughes provides an overview of new work on the mineralogy of bone material, by Pasteris et al., which appears on page 16 of this issue. As noted by Hughes, this study shows that channels within the apatite structure are filled with water molecules, which help stabilize the channel structure even when as much as 80% of hydroxyl sites are depopulated and that in bone mineral, molecular water does not occur accidentally. Instead, water is an essential structural constituent in bone apatite, and thus bone comprises a definable variety of this mineral.
Weathering of cobalt arsenides: Natural assemblages and calculated stability relations among secondary Ca-Mg-Co arsenates and carbonates
https://doi.org/10.2138/am.2014.4540
On page 44 of this issue, Markl et al. investigate the conditions under which Ca-Mg-Co arsenates and carbonates are stable. Their work uncovers the pH conditions and Ca2+, Mg2+ activities that control the precipitation of various Co- and As-bearing phases, some of which can be used to immobilize Co and As near ore deposits. This work also explains the precipitation of various rare minerals near ore bodies, some of which may buffer Co and As concentrations in accompanying fluids.
Thermal equation of state and spin transition of magnesiosiderite at high pressure and temperature
https://doi.org/10.2138/am.2014.4553
One possible means of C storage in the deep mantle may involve magnesiosiderite (Mg0.35Fe0.65CO3), provided that such a phase is a stable part of a deep mantle assemblage. On page 84 of this issue, Liu et al. show that the low-spin form of magnesiosiderite is significantly more dense compared to end-member magnesite and high-spin magnesiosiderite. These results imply that at >50 GPa, low-spin ferromagnesite (which contains up to 20 mol% Fe) could be a stable form of carbonate at higher pressures and thus provide a significant storehouse for C in the deep mantle.
The sound velocity measurements of Fe3S
https://doi.org/10.2138/am.2014.4463
Seismic studies have long shown that Earths core cannot consist only of crystalline Fe, as seismic velocities for both the inner and outer core are too low; hence, the core must contain one or more light alloying elements, and sulfur is a leading candidate for such. On page 98 of this issue, Kamada et al. present new data that should help to advance our ability to for S in the inner core. They present an orientation-averaged value for Vp of Fe3S (a likely stoichiometry of Fe sulfide) at 24 < P < 85 GPa and 300 K, i.e. above the magnetic transition, and they calculate the dependence of Vp on density at pressures appropriate to the inner core using Birchs Law. They show that at the density of the inner core, both Fe3S and hcp-Fe yield a Vp that is too high relative to observed values, which means that either Brichs Law can be more temperature sensitive than now recognized, or that the inner core contains alloying elements/compounds that little effect the thermal sensitivity of Birchs Law.
A comparative analysis of the mechanical behavior of carbon dioxide and methane hydrate-bearing sediments
https://doi.org/10.2138/am.2014.4620
Methane hydrates, which are dispersed amongst marine sediments, may provide an important source of energy in the coming decades. One proposed means of methane production involves CO2-CH4 exchange, as CO2 is injected into methane reservoirs. If feasible, this technology may not only spur energy production, but provide a means to simultaneously sequester CO2 in marine sediments. On page 178 of this issue, Hyodo et al present triaxial deformation experiments which indicate that sediment-bearing CO2-hydrates should be mechanically stable following CO2-CH4 exchange and CH4 extraction, which adds promise to this form of methane mining.
Crystal structures and stabilities of cristobalite-helium phases at high pressures
https://doi.org/10.2138/am.2014.4637
"On page 184 of this issue, Matsui et al. show that helium can be far from the inert gas it is assumed. Helium has previously been shown to stiffen silica glass, but Matsui et al. show that it may enter the interstices of the cristobalite framework to form stoichiometric SiO2-He structures in which it occupies well-defined crystallographic sites. The SiO2-He phases have molar volumes more than 20% larger than their SiO2 counterparts, and the presence of helium may modify both the positions of phase boundaries and symmetries of the structures. Helium may thus not always be the ""ideal gas"" for high-pressure experiments."
Compositional zoning in dolomite from lawsonite-bearing eclogite (SW Tianshan, China): Evidence for prograde metamorphism during subduction of oceanic crust
https://doi.org/10.2138/am.2014.4507
On page 206 of this issue, Li et al. investigate zoning profiles within eclogite-hosted dolomite, from the Tianshan area of northwest China. Thermodynamic modeling shows that Fe-Mg zoning patterns within dolomite record increases in T during prograde, subduction-driven metamorphism. These zoning profiles also show that Fe-rich magnesite, which occurs as inclusions in matrix dolomite and dolomite inclusions in garnet, grows at high-pressure metamorphic conditions, and thus that Fe-bearing magnesite is not an unambiguous indicator of ultra-high pressure metamorphism.
Volume 98 : November - December 2013 Issue
Developing vanadium valence state oxybarometers (spinel-melt, olivine-melt, spinel-olivine) and V/(Cr+Al) partitioning (spinel-melt) for martian olivine-phyric basalts
https://doi.org/10.2138/am.2013.4622
On page 2193 of this issue, Papike et al. develop oxybarometers based on vanadium partitioning between four different pairs of phases: V spinel/melt, V/(Cr+Al) spinel/melt, olivine/melt, and spinel/olivine. Except for the spinel/olivine oxybarometer, the models apply over a wide range of fO2 conditions (IW 1 to QFM). And while the spinel/olivine oxybarometer may be restricted in fugacity range (IW 1 to IW + 1), it may still be useful for the characterization of some martian, lunar, and 4 Vesta basalts. For martian meteorite Y98, the oxybarometers indicate a quite reduced fO2 for the martian mantle, at least compared to terrestrial basalts, with a window of IW + 0.8 to IW + 1.6.
Lessons from a lost technology: The secrets of Roman concrete
https://doi.org/10.2138/am.2013.4643
On page 1917 of this issue, Elsen et al. provide perspective on the recent paper by Jackson et al. (2013), which reveals the mineralogic and geologic aptitude of the Romans during the Axial age. Their study uncovered the Roman secrets for formulating some of the most long-lasting concrete yet discovered. Elsen et al. emphasize that our ability to unlock the secrets of ancient concrete formulas is dependent upon interdisciplinary analytical approaches utilized by the Jackson et al. (2013) group.
North American microtektites are more oxidized than tektites
https://doi.org/10.2138/am.2013.4505
On page 1930 of this issue, Giuli et al. (2013) confirm earlier results from tektites and microtektites, that microtektites recovered from North America are significantly more oxidized than those recovered from near equatorial or southern hemisphere latitudes. Their current work shows that the observed range in oxidations states is unrelated to bulk composition but, at least in the case of the North American microtektites, is related to flight distance from the source crater. The reasons for the contrasts are still uncertain, but may be related to the thermal conditions of microtektite and tektite formation, and/or interaction of tektites with water-rich vapor plumes that are generated upon impact.
Clay mineral evolution
https://doi.org/10.2138/am.2013.4425
On page 2007 of this issue, Hazen et al. expand on their seminal 2008 paper on the evolution of minerals, in this case by examining the evolution of the quantity and diversity of clay minerals through time. The timescale of clay genesis spans the age of the solar system, ranging from clay minerals formed by aqueous alteration and shock processes in chondrites, to biogenic clay growth on Earth during the Phanerozoic. Like no other mineral group, clays reflect the interconnection between geosphere, hydrosphere, and biosphere, which, as noted by Hazen, is likely to be vastly better understood, both on Earth and on other planets that have had a hydrosphere (Mars) when geochemists begin to analyze these minerals in earnest.
Thermodynamic basis for evolution of apatite in calcified tissues
https://doi.org/10.2138/am.2013.4537
On page 2037 of this issue, Rollin-Martinet et al. show and quantify on an experimental basis that, beyond biologically mediated processes, the existence of a thermodynamic driving force dictates the inexorable evolution of apatite biominerals as observed during bone and enamel maturation. These results probably explain, on a physical basis, the need for bone remodeling in vertebrates for retaining bone mineral role in homeostasis. Biological processes initially precipitate metastable but highly bioactive nonstoichiometric apatites; these crystals are then inexorably driven towards a more stable state (lower Gibbs free energy, largely through decreases in enthalpy) closer to stoichiometry but less bioactive. This work leads to avenues of further research on nanocrystalline apatites in view of more effective bone replacement, and even has implications regarding the origin of life on Earth (as life evolves from carbonate-based shells to more versatile phosphate-based skeletons).
Acoustic velocity measurements for stishovite across the post-stishovite phase transition under deviatoric stress: Implications for the seismic features of subducting slabs in the mid-mantle
https://doi.org/10.2138/am.2013.4145
On page 2053 of this issue Asahara et al. examine Brillouin scattering measurements for SiO2 at room temperature, but at high pressure and high differential stress. They find that the stishovite/post-stishovite phase transition occurs at lower pressures at high differential stress, and that the post-stishovite phase is accompanied by a smaller than expected discontinuity in seismic wave speeds. Their results may explain a lack of seismic features in subducted slabs at depths of >1500 km, and hint at the possibility that the stishovite/post-stishovite phase transition may occur in subducted slabs at depths shallower than would be predicted under hydrostatic conditions.
Size distributions of nanoparticles from magnetotactic bacteria as signatures of biologically controlled mineralization
https://doi.org/10.2138/am.2013.4429
On page 2105 of this issue Jandacka et al. examine the size distributions of biogenic and inorganic magnetite. Laboratory-cultured bacterial magnetite crystals are observed to evolve to an extreme valued distribution, while uncultured natural samples exhibit a mixture of extreme value and log-normal distributions. Analysis of several populations of magnetite nanoparticles from martian meteorite ALH 84001 show that half of these follow an extreme value distribution, providing very tentative support for a biogenic origin for magnetite growth for such samples.
Medium-range order in disordered K-feldspars by multinuclear NMR
https://doi.org/10.2138/am.2013.4448
"On page 2115 of this issue Snchez-Muoz et al. for K-feldspars, which improve our understanding of order/disorder relationships among K-feldspars. This study shows that there exists medium-range Si-Al-K or molecular-like order (MRO), involving not only the tetrahedral sites but also the alkali cation position, which cannot be captured by standard diffraction methods. This MRO allows K-feldspars to be divided into two groups, based on number and location of Al atoms in the four-membered rings of tetrahedral sites: microcline and orthoclase, on the one hand, and valencianite and sanidine on the other. This differentiation reveals important structural contrasts that are otherwise hidden from average structure solutions, and suggest to consider ""valencianite"" as an additional mineral species in the K-feldspars group."
Melting and subsolidus phase relations in the system Na2CO3-MgCO3±H2O at 6 GPa and the stability of Na2Mg(CO3)2 in the upper mantle
https://doi.org/10.2138/am.2013.4418
On page 2172 of this issue, Shatskiy et al. examine phase relations in the system Na2CO3-MgCO3 at 6 GPa and 900-1400 degrees C, and under dry and hydrous conditions. Their results show that eitolite (Na2Mg(CO3)2), while stable at high pressures, would likely melt for reasonable P-T ascent paths in kimberlites, where eitolite occurs as inclusions in spinel or olivine. But under dry conditions, eitolite partial melting in the presence of magnesite may control the solidus of carbonated peridotite in a subducting slab.
Volume 98 : October 2013 Issue
The crystal structure and vibrational spectroscopy of jarosite and alunite minerals
https://doi.org/10.2138/am.2013.4486
On page 1633 of this issue, Spratt et al. review the alunite supergroup of minerals (which includes jarosite) that are of current interest due to their discovery on the surface of Mars. Their study carries implications for the presence of acidic, sulfur-rich water on the Martian surface. This review focuses on the structure and vibrational spectroscopic properties of alunite and jarosite, and may allow more rapid and certain identification of such minerals by vibrational spectroscopic methods.
Analysis of H2O in silicate glass using attenuated total reflectance (ATR) micro-FTIR spectroscopy
https://doi.org/10.2138/am.2013.4466
On page 1660 of this issue Lowenstern and Pitcher calibrate a new method for the determination of water contents for natural and experimental silicate liquids (glasses). The authors use an alternative FTIR-based technique called Attenuated Total Reflectance (ATR). This method provides accuracy that is comparable to transmission FTIR, and can be used to analyze spot sizes as small as 5 micrometers. Then, on page 1788 of this issue, Morizet et al. provide a new model for estimating CO2 contents in glasses by a micro-Raman approach. The ATR model of Lowenstern and Pitcher can be applied to glasses containing from 0 to 6 wt% H2O, while the Morizet model can be used for silicate glasses containing up to 16% CO2.
Unlocking the secrets of Al-tobermorite in Roman seawater concrete
https://doi.org/10.2138/am.2013.4484
On page 1669 of this issue, Jackson et al. unlock the secrets of Roman harbor concretes, in their analysis of Al-tobermorite from a 2000-year-old concrete breakwater. The ancient Roman concrete was formulated from a mixture of lime, pumiceous volcanic ash and zeolite-bearing tuff (the volcanic materials are from Flegrean Fields). These components together reacted with seawater to form a binder that consists of poorly crystalline calcium-aluminum-silicate-hydrate (C-A-S-H). In contrast to modern Portland cement, which produces calcium-silicate-hydrate (C-S-H) binder, the Al-rich C-A-S-H binder is more chemically stable. Jackson et al. examine the chemical and temperature conditions under which the very stable Al-tobermorite crystals formed, and the kinds of starting materials required for its use in modern concrete synthesis.
The determination of hydrogen positions in superhydrous phase B
https://doi.org/10.2138/am.2013.4475
On page 1688 of this issue Trots et al. present a most convincing data set based on a neutron powder diffraction experiment to pinpoint the controversial hydrogen/deuterium positions in the structure of superhydrous phase B. Superhydrous phase B is one of the many nominally hydrous dense magnesium silicates that may act as carriers of H from the surface into the deep Earth. Their work shows that hydrogen is stored in channels formed by edge-sharing octahedra, rather than along the edges of MgO6 octahedra. The exact structure including precise hydrogen positions of hydrous phases is important in order to properly assess the stability and physical properties of hydrogen carrying phases stable in the deep Earth.
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