Editor’s Notes
Total Results: 1697
Volume 100 : February - March 2015 Issue
The catalytic effect of bound extracellular polymeric substances excreted by anaerobic microorganisms on Ca-Mg carbonate precipitation: Implications for the “dolomite problem”
https://doi.org/10.2138/am-2015-4999
On page 483 of this issue, Zhang et al. investigate a new means by which disordered dolomite may be precipitated. Their experiments indicate that dolomite may precipitate directly from Ca-Mg-bearing solutions through the catalytic effect of extracellular polymeric substances (EPS), formed in association with anaerobic, fermenting bacteria. They posit that hydrogen bonds form between H and OH in EPS layers and O in CO32– at carbonate surfaces. The adsorbed EPS weakens bonding in the hydration shell about surface Mg cations, leaving Mg available to bond with carbonate. Their work provides the first demonstration that anaerobic bacteria may play a role in the direct precipitation of dolomite.
Volume 100 : January 2015 Issue
The American Mineralogist at 100 years, and a mineralogy renaissance
https://doi.org/10.2138/am-2015-Ed10012
On pages 1-2 of this issue, we present an editorial on the future of mineralogy and the American Mineralogist: Journal of Earth and Planetary Materials
At the blurry edge of mineralogy
https://doi.org/10.2138/am-2015-5147
Defining Minerals & Mineralogy
Mineral evolution and Earth history
https://doi.org/10.2138/am-2015-5101
On page 4 of this issue, Dwight Bradley provides an overview of a series of publications by Robert Hazen and co-workers, including Hazen et al. (2011, 2012, 2013b) and Grew and Hazen (2014). Bradley suggests that an important step in mineral-evolution studies will involve the isotopic history of long-extant mineral species, such as zircon and calcite, whose isotopic ratios possibly track supercontinent cycles/subduction flux rates, but from different perspectives: continental growth and subduction flux on the one hand (zircon), and continental weathering and plate generation flux on the other (calcite).
Franciscan geologic history constrained by tectonic/olistostromal high-grade metamafic blocks in the iconic California Mesozoic-Cenozoic accretionary complex
https://doi.org/10.2138/am-2015-4850
On page 6 of this issue Gary Ernst presents the inaugural paper in our Invited Centennial Article Series, which celebrates American Mineralogist's 100th volume. In this paper, Ernst presents a model for the origin of high-pressure (HP) meta-basaltic blocks in the Franciscan Complex. The HP blocks contain rinds of slightly younger actinolite that appear to record equilibration with harzburgite. Ernst suggests that such rinds record the means by which such blocks are transported towards the surface, i.e., by being carried upwards in subduction-channel muddy mlanges, or in buoyant serpentinized peridotite diapirs. Ernst also finds that some portions of such rock packages are exposed at the surface and eroded, so as to be later deposited back into the trench as olistostromes.
The rapid expansion of environmental mineralogy in unconventional ways: Beyond the accepted definition of a mineral, the latest technology, and using nature as our guide
https://doi.org/10.2138/am-2015-4749
On page 3 of this issue Peter Heaney provides some context of Caraballo et al. (page 14), whose Outlook paper examines the troublesome issues of classification of nano-materials. These materials may fall outside the boundaries of generally accepted mineral-defining characteristics, and yet be fundamental to naturally occurring and environmentally important systems. Heaneys overview provides food for thought regarding the importance of classification generally and in mineralogy especially, the latter topic of which is coincidentally covered from an historical standpoint in our Editorial.
The structure of water-saturated carbonate melts
https://doi.org/10.2138/am-2015-4856
On page 35 of this issue, Foustoukos and Mysen report on the structure of melts in the systems MgO-H2O-CO2 and CaO-H2O-CO2. They hypothesize that carbonates subducted as sediment and serpentinized peridotite may not reach much greater than sub-arc depths. Instead, carbonates may completely partially melt prior to the completion of slab-dehydration. In such a case, subducted carbonates would not contribute C to the deep mantle and would instead contribute a CO2 flux into the crust and arc volcanoes, even in relatively cold subduction conditions.
In-situ measurements of fluorine and chlorine speciation and partitioning between melts and aqueous fluids in the Na2O-Al2O3-SiO2-H2O system
https://doi.org/10.2138/am-2015-4859
On page 47 of this issue, Dalou et al. examine the structure of F- and Cl-bearing melts in simple alkali-silicate systems (Na2O-Al2O3-SiO2-H2O). Their work shows that F and Cl in silicate melts may be sequestered in the form of SiF and NaCl complexes, and the solubility of such complexes may increase with decreasing pressure. This result can explain the relative late degassing of F and Cl from volcanic systems, relative to degassing of CO2 and H2O.
The use of cation-cation and anion-anion bonds to augment the bond-valence model
https://doi.org/10.2138/am-2015-4810
On page 148 of this issue Wander et al. improve the Bond Valence Model (BVM) for the prediction of local chemical structure. The BVM defines the valence of a bond as a function of bond length and two additional empirical parameters, and then uses Paulings second rule to solve for atomic valence. Current BVMs consider only cation-anion interactions, but Wander et al. show that the model can be improved by including anion-anion interactions (oxygen-oxygen in their case); they suggest that cation-cation interactions, while not useful in the system H-Al-Si-O investigated by them, might be useful for other systems. The hope is that a more accurate BVM may prove a useful tool to augment molecular dynamics simulations.
An in situ high-pressure NMR study of sodium coordination environment compressibility in albite glass
https://doi.org/10.2138/am-2015-5081
On page 326 of this issue, Gaudio et al. examine the compressibility of albite (NaAlSi3O8)-composition glasses using a novel in situ high-pressure solid-state NMR technique. Pressure-induced changes to the 23Na NMR spectra indicate that with increasing pressures, decreases in volume are accommodated almost entirely by shortening of Na-O bonds, rather than changes in coordination number for any of Na, Al, or Si. The volume of the Na-O coordination spheres decreases by nearly 7% between 1 atm and 2 GPa. As Gaudio et al. note, such understanding has important implications for models of viscosity, elasticity and trace element partitioning in silicate melts. For example, the shortening of Na-O bond with increasing pressure may increase the effective field strength of Na, which would then stabilize high-coordinated Al, resulting in a reduction in viscosity of the melt with increasing pressure.
Volume 99 : November - December 2014 Issue
Merrillite and apatite as recorders of planetary magmatic processes
https://doi.org/10.2138/am-2014-5075
This issue features two Highlights and Breakthroughs articles. On page 2161 of this issue, Brad Jolliff highlights the work of McCubbin et al. (page 1347 of the July issue). Their careful analysis of phosphate minerals in the Shergotty (martian) meteorite shows that the occurrence of anhydrous merrillite (vs. hydrous whitlockite) is not necessarily an indicator of anhydrous crystallization conditions. Instead, the competing phase assemblages of apatite + merrillite vs. apatite + whitlockite may reflect the role of temperature in controlling H activity in these phosphates.
Comparing clays from Mars and Earth: Implications for martian habitability
https://doi.org/10.2138/am-2014-5089
On page 2163 of this issue John Bridges provides a review of Treiman et al. (2014; page 2234 of this issue), who examine a terrestrial analog of martian smectities observed using the ChemMin XRD instrument aboard the Mars Science Laboratory. Their spectroscopic work shows that their analog mineral, Griffithite, contains significant Fe3+, which they hypothesize to be secondary, forming during diagenesis. They find that very near IR reflectance (VNIR) spectra vary with Fe3+ content, indicating that VNIR may be used to remotely assess oxidation states on planetary surfaces.
Data-driven abductive discovery in mineralogy
https://doi.org/10.2138/am-2014-4895
On page 2165 of this issue, Bob Hazen considers abductive reasoning in the Earth Sciences, and the newly emerging role of large databases. Abduction is a logical form posited by Charles Pierce, which effectively means guessing. For example, if you come home to find your favorite biking shorts in shreds, your dog has a guilty look, and bits of Lycra are scattered about his bedding, you may abduce that your dog ate your biking shorts. Or course, they may have been shredded by a strange dog that hopped the fence and entered through the doggy door, or by an ex-cycling partner who stole into your house with a pair of scissors. But your black lab is a more likely suspect. Hazen argues that newly-developed large data sets in the Earth and Biological sciences can lead to a new era of data-driven, abduction-oriented discovery. The new data sets are expected to be a rich source of useful working hypotheses in the coming decades. But Hazen warns that to yield their full potential, such databases require concerted efforts on the parts of publishers, authors and data compilers, to ensure that data are properly systematized and archived, and that such archived data are of high quality.
Experimental study of phlogopite reaction rim formation on olivine in phonolite melts: Kinetics, reaction rates, and residence times
https://doi.org/10.2138/am-2014-4821
"On page 2211 of this issue, Grant et al. experimentally investigate the growth of phlogopite reaction rims on olivine, for phonolitic whole rock compositions. Their work shows that the growth rates of phlogopite reaction rims are limited by diffusion along phlogopite-phlogopite grain boundaries of the growing rim, and that increased water contents greatly increased rim growth rates. This is due to the lowering of diffusion rates by the presence of ""atomically bound"" fluid at grain boundaries. Their results also appear to indicate presence of pore cavities, which might not have formed during de-pressurization; rather, pores with negative phlogopite geometries may indicate the presence of ""free"" fluid at grain boundaries that formed at peak experimental (and bulk fluid-undersaturated) P-T conditions. If this latter observation is valid, then disequilibrium fluids may have an important effect on our interpretation of various diffusion-mediated processes in magmatic systems"
The role of water in generation of group II kimberlite magmas: Constraints from multiple saturation experiments
https://doi.org/10.2138/am-2014-4893
On page 2292 of this issue Sokol et al. present results from partial melting studies that may shed light on the origin of Group II (phlogopite-bearing) kimberlites. Their experiments are performed on a synthetic kimberlite, representing a mean of published Group II compositions. If such published compositions approximate a magmatic composition, then the experiments by Sokol et al. delimit several intensive parameters by which such liquids were generated. They obtain liquids saturated with olivine + orthopyroxene + garnet, which are compositionally similar to minerals retrieved from peridotite inclusions. None of their liquids were saturated with clinopyroxene at any conditions examined. Their data allow that kimberlite-like liquids can be produced by partial melting of a carbonated garnet harzburgite source at 6.3 to 7.5 GPa and 1500 oC, with water contents restricted to <5 wt%. The water may have come from reaction of resident phlogopite with K-Ca-rich carbonated melts derived from a deeper mantle source.
A new interpretation of decomposition products of serpentine under shock compression
https://doi.org/10.2138/am-2014-5021
On page 2374 of this issue, Zhang et al. examine the pressure-density relationships (through the Hugoniot equation) for various decompositional equilibria for antigorite. They find that the high pressure phase H, MgSiO4H2, may be a stable decomposition product of antigorite in the pressure range 40-70 GPa. If valid, these results imply that antigorite might not dehydrate until it reaches lower mantle depths, in which case phase H may be an important water delivery system from the near surface into the lower mantle. The authors also suggest that antigorite and possibly phase H (if it is quenchable to low pressure) may survive shock impact pressure and so provide a means of hydrating planetary interiors during planetary accretion at near-solar distances.
Steinhardtite, a new body-centered-cubic allotropic form of aluminum from the Khatyrka CV3 carbonaceous chondrite
https://doi.org/10.2138/am-2014-5108
On page 2433 of this issue, Bindi et al. report on a new mineral named steinhardtite, which represents the first natural occurrence of a body centered cubic (bcc) polymorph of an Al-bearing compound, from the Khatyrka meteorite. The structure exhibits solid solution with Ni and Fe with a formula of Al0.38Ni0.32Fe0.30. A key result of this work is that the solution of Fe and Ni into the structure reduces the pressure at which Al undergoes an hcp to bcc transition, the latter of which should be stable in Earth's core. These results thus indicate that within the Earths core, Al might be a plausible candidate as a light alloying element.
Volume 99 : October 2014 Issue
Pauling’s rules, in a world of non-spherical atoms
https://doi.org/10.2138/am-2014-5048
"This issue features three Highlights and Breakthroughs articles. On page 1817 of this issue, Bob Downs provides a review of Gibbs et al. (2014) re-analysis of Pauling's rules, and his use of a saddle point in the electron density between two bonded atoms so as to mark a non-spherical radii. This new view of the ""shape"" of an atom provides a challenge to our use of certain Pauling's Rules, where, for example, a spherical atom is assumed so as to determine coordination number from radius ratios. The model presented by Gibbs et al. also provides a new perspective on the meaning of bond strength."
Theoretical and applied implications of the structural order of irradiated vermiculite
https://doi.org/10.2138/am-2014-5041
Then on page 1818 of this issue, Marcos discusses new work by Kaur et al. (2014; page 2018 of this issue), whose studies of gamma-irradiated vermiculite show that this phase has high radiation shielding properties and chemical stability despite radiation bombardment; these properties open the door for the development of new types of opto-electronic devices, and a possible new radiation dosimeter.
An examination of the Ti-in-quartz thermobarometer in rocks that contain dynamically recrystallized quartz: Re-equilibration of [Ti] during recrystallization
https://doi.org/10.2138/am-2014-5050
Finally, on page 1820 of this issue, John Hughes reviews work by Ashley et al. (2014; page 2025 of this issue), who use single-crystal XRD and atomistic simulations to test two modes by which Ti may be redistributed in recrystallized quartz. They conclude that strain-driven redistribution may be important, with localized thermodynamic equilibration with the intergranular medium buffering Ti solubility in quartz. Ti-in-quartz thermometry may thus be compromised when quartz is dynamically recrystallized.
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