Editor’s Notes
Total Results: 1697
Volume 100 : July 2015 Issue
The mobility of Nb in rutile-saturated NaCl- and NaF-bearing aqueous fluids from 1–6.5 GPa and 300–800°C
https://doi.org/10.2138/am-2015-5031
On page 1600 of this issue, Tanis et al. investigate the solubility of Nb in rutile-saturated fluids. Their work shows that Nb solubility in fluids increases not only with increasing T (as the rutile effectively dissolves) but also with increasing chloride concentrations, although rutile always contains more Nb than coexisting fluids. Their results are important in that they may explain HFSE enrichments in subduction-related gabbros. One hypothesis is that HFSE enrichments may derive by dehydration of serpentinites. This idea is bolstered by this work given that serpentinite-derived fluids may be chloride-rich. Natural fluid compositions might also be just right so as to allow HFSE to be mobilized by the dehydration of blueschist in the formation of eclogite. HFSE are thus perhaps much more mobile than previously thought.
Volume 100 : May - June 2015 Issue
Experimental confirmation of high-temperature silicate liquid immiscibility in multicomponent ferrobasaltic systems
https://doi.org/10.2138/am-2015-5285
On page 1304 of this issue, Hou and Veksler show the results of experiments that test whether Fe-rich silicate liquids may be immiscible at geologic temperatures (1150-1200 C). Their work verifies some previous work, which shows that silicate liquid immiscibility may be viable in some geologically relevant high-Fe systems, although implied consolute points derived from these experiments tend to be higher than in some previous studies. In any case, this work provides greater impetus to the growing view that silicate liquid immiscibility may be an important mechanism of magma differentiation for certain natural compositions.
Discovery of stishovite in Apollo 15299 sample
https://doi.org/10.2138/am-2015-5290
On page 1308 of this issue, Kaneko et al. report the first example of a high-pressure silica polymorph from a lunar sample collected as part of the Apollo missions. They suggest that the stishovite possibly formed by the Imbrium impact or subsequent local impact event(s) in the Procellarum KREEP Terrane (PKT) of the near side of the Moon. The authors suggest that re-examination of Apollo samples may allow us to identify and date specific impact event on the Moon, and thus it may be valuable to revisit lunar samples from a high-pressure mineralogy perspective.
Reaction pathways toward the formation of dolomite
https://doi.org/10.2138/am-2015-5269
On page 1017 of this issue Carlos M. Pina provides an overview of Rodriguez-Blanco et al. (page 1172), which elucidates the transformation kinetics for the growth of dolomite. Dolomite is a common mineral, but uncommonly resistant to our uncovering the nature of its formation. While perhaps not completely solved, the classic dolomite problem is at least now better understood. Rodriguez-Blanco et al. show that the process begins with aqueous precipitation of a Mg-Ca amorphous phase, which transforms to proto-dolomite at 25-200 C through a spherulitic growth mechanism. Proto-dolomite then transforms to dolomite through an Ostwald ripening-like process, at temperatures >140 C.
The many facets of apatite
https://doi.org/10.2138/am-2015-5193
On page 1033 of this issue, John Hughes provides a paean to the mineral apatite, which may have the reader convinced that apatite is the most important naturally crystalline substance on Earth. Most readers will be quite familiar with the use of apatite for various age dating studies. But its relevance to the future of agriculture is rather compelling, as apatite is now a major source of P for fertilizers, and investors are curious about the possibility of peak P (like the one in oil that may be plural). And then of course, there are applications to the sequestration of radioactive nuclides and its importance for understanding bio-skeletal materials and its action as a reservoir for Ca and P in biogenic systems. So, is there a more important mineral? Well, maybe pyroxene.
First-principles prediction of pressure-enhanced defect segregation and migration at MgO grain boundaries
https://doi.org/10.2138/am-2015-5143
On page 1053 of this issue, Karki et al. use density functional theory (DFT) to examine grain boundaries in MgO, whose polymorphs are important in the lower mantle. Their results show that native defects and impurities (Ca, Al) segregate to grain boundaries, and that the segregation increases as pressure increases. The enthalpies of migration are also estimated to be lower for impurities residing at grain boundaries, which implies that material transport should be increasingly be dominated by anisotropic grain boundary processes, compared to bulk diffusion as depth increases in the lower mantle.
Modeling siderophile elements during core formation and accretion, and the role of the deep mantle and volatiles
https://doi.org/10.2138/am-2015-5052
On page 1098 of this issue, Righter provides a review of aspects of metal/silicate partitioning and the implications of such for understanding core formation and residual silicate mantle compositions (see also Introductory Remarks by Rushmer and Watson on page 1093). Righters review shows that distribution coefficients for moderately siderophile elements vary smoothly with P and T when a thermodynamic approach is used for modeling, which leaves out the apparent discontinuities observed in different approaches. When such models are applied to Mn, Cr, V, Nb, or Ta, show that rather different courses of history are obtained, depending upon whether the modern shallow mantle concentrations of such elements are affected by metal/silicate partitioning alone, or are also affected by deep primitive layers or contrasts in lower and upper mantle mineral/liquid partitioning. If these elements are controlled solely by metal/silicate partitioning, a hotter or more reduced early earth is required. Finally, metal/silicate partitioning of S, C, H, and N may ultimately control the content of these elements in the mantle, and the nature of the light element in the core -- there is much additional work required to fully understand these multicomponent systems and their effects on siderophile element partitioning.
Very large differences in intramolecular D-H partitioning in hydrated silicate melts synthesized at upper mantle pressures and temperatures
https://doi.org/10.2138/am-2015-4940
On page 1182 of this issue, Wang et al. examine deuterium (D)/hydrogen (H) ratios in synthetic silicate glasses through NMR. They discover that inter-molecular contrasts in D/H ratios within the glasses vary to a far greater extent than can be explained by entropy contrasts in the molecular species. Instead, it appears that D prefers sites of low partial molar volume. Wang et al. refrain from predicting melt/fluid partitioning of D/H during early Earth magma ocean conditions, but the qualitative implications are clear: as P increases, Ds preferences for lower molar volume sites means that in a water-saturated magma ocean, D should be preferably held within a silicate melt, while H is preferentially partitioned into a fluid phase. D/H ratios upon mantle dehydration are thus expected to be much lower for the early Earth, compared to today, where cooler upper mantle conditions translates to lower pressures of melting and possibly volatile saturation.
Volume 100 : April 2015 Issue
Clays are messy—also on Mars
https://doi.org/10.2138/am-2015-5229
On page 669 of this issue, Javier Cuadros reviews a new open access paper by Bristow et al. (page 824) on martian clay minerals. As Cuadros explains, compared to remote infrared data, new XRD measurements of clay minerals and Fe-oxides from the Curiosity mission may complicate our picture of water retention and water-driven reactions on the martian surface. The new data indicate low water/rock ratios, but push to later dates the time interval for water-rock interaction on the martian surface. Bristow et al.'s study also indicates that the weathering processes that created martian clays occurred over periods of thousands to hundreds of thousands of years, placing a minimum time interval on which life could evolve. Bristow et al. also point to H2 production in redox reactions may provide at least a potential energy source for biologic activity.
Tweed, Twins, and Holes: A link between mineralogy and materials science
https://doi.org/10.2138/am-2015-5231
On page 671 of this issue Wilfried Schranz provides an overview of our invited Centennial article in the February-March issue by Ekhard Salje (page 342). Salje shows that physicists and mineralogists are pursuing non-intersecting lines of research in regard to microstructural mineral features, such as twins, holes and tweeds (the latter being the precursors to twin structures). Physicists are making use of such defects to produce new electronic devices, but without the benefit of understanding of the mineralogical context in which such defects occur. In the mean time, however, breakthroughs with regard to super-conductivity and ferro-elastic behavior has led to new physics-derived insights that can aid mineralogists in understanding transport and deformation properties of natural materials.
Bursting the bubble of melt inclusions
https://doi.org/10.2138/am-2015-5254
On page 672 of this issue Jacob Lowenstern provides a review of Moore et al. (page 806), who examine the effect of vapor bubbles on estimates of volatile concentrations in melt inclusions (MI). Bubbles are common in MI and are thought to occur as MI are cooled and decompressed along an isochore (with minerals walls serving as a fixed-volume container). Moore et al. show that bubbles contain a significant fraction of low-solubility volatiles, such as CO2, comprising anywhere from 40-90% of the total CO2 budget for MI. For CO2-rich systems, the volatile budgets of bubbles are ignored at the risk of confusing pre-eruptive degassing conditions with post-entrapment CO2 melt/vapor fractionation.
Mineralogy, materials science, energy, and environment: A 2015 perspective
https://doi.org/10.2138/am-2015-5130
On page 674 of this issue, Alexandra Navrotsky provides a remarkably fitting overview of how Mineralogy segues into various disciplines of interest to society. For example, she compares materials science advances in vapor deposition and the precipitation of phases from the early solar system. As another example, she notes how mineralogists have developed high-P equations of state that are useful to materials scientists (to understanding bonding), but that advances in understanding perovskite defect structures in the materials sciences may also provide new analog structures for studying water sequestration in deep planetary interiors. As she concludes, partnerships between mineralogists, materials scientists, and industry are needed to facilitate this natural research synergy, and should enable advances on the most important problems, and place our society on an environmentally sustainable footing.
Toward theoretical mineralogy: A bond-topological approach
https://doi.org/10.2138/am-2015-5114
"On page 696 of this issue, Frank Hawthorne presents a fascinating approach to determining why some atomic arrangements are stable while others are not. An intriguing aspect of this work is the application of bond topology to structural units in minerals. For example, oxysalt structures can be divided into a strongly bonded structural units, which act as Lewis bases, and weakly bonded interstitial units, which act as Lewis acids, and the corresponding basicities and acidities can be calculated. Stable structures are those where Lewis acidities and basicities match. As an example, Hawthorne shows how this theory predicts stable stoichiometries for MgN(PO4)(OH)m; only when N=2 and m (less than or equal to) 2 is the valence-matching principle satisfied, and these are the only stoichiometries observed in minerals. For oxysalts in general, Hawthorne also shows why stoichiometries with octahedrally coordinated to tetrahedrally coordinated cations in the range 0.25 to 4.0 are stable, whereas stoichiometries outside this range do not occur. This theoretical approach to Mineralogy has the potential to explain atomic arrangements and chemical compositions of terrestrial planetary objects, and with further development, may provide an understanding of the relation between atomic arrangements and their thermodynamic properties. On page 897 of this issue, Kohn et al. model Zr budgets and zircon growth along common P-T paths under a range of P-T conditions. Kohn et al. find that zircon growth and modal abundances can best be predicted by considering decreasing Zr solubility in co-existing, low-Zr phases. Kohn et al. find that most zircon saturation and growth is expected to occur upon exhumation and cooling, rather than under ultrahigh-pressure (UHP) metamorphic conditions. Identifying zircons that record prograde metamorphism thus faces serious challenges. A key finding, then, is that zircon age dates, especially bulk mineral TIMS ages, are unlikely to record the times of peak metamorphism, but should rather systematically underestimate the ages of peak UHP metamorphism.
Atom probe tomography of isoferroplatinum
https://doi.org/10.2138/am-2015-4998
On page 852 of this issue, Steve Parman presents an application of atom probe tomography (APT) to a natural sample of isoferroplatinum. As noted by Parman, APT is more commonly used in the materials sciences, but as demonstrated by this work, can also be applied to more compositionally complex natural crystals. The APT provides an atom-by-atom mapping providing both compositional, isotopic and structural information. It is not clear that APT can yet be applied to analyze covalently bonded silicates, but in the mean time, oxides, metals, and sulfides provide potentially useful targets, and if the silicate problem is solved, APT may serve to revolutionize the way minerals are analyzed and understood.
Volume 100 : February - March 2015 Issue
Anhydrite: An important sulfur binder limiting the climatic impact of subaerial volcanic eruptions
https://doi.org/10.2138/am-2015-5169
On page 341 of this issue, Nowak provides an overview of experimental work by Huang and Keppler (page 257 of the January 2015 issue) who examine sulfur solubility in model rhyolite melts. They find that under reducing conditions, total CaO content has no effect on the partitioning of S between fluid and co-existing melts and S partitions strongly into a co-existing fluid phase. In contrast, under oxidizing conditions (NNO +0.5 or higher), where anhydrite is stable, increasing CaO contents cause a net decrease in S that is partitioned into a coexisting fluid, largely because with increased CaO, a greater degree of S is locked up in crystalline phase (anhydrite in this case). This result, and earlier work on the kinetics of anhydrite decomposition, imply that oxidizing conditions may inhibit the release of S into the atmosphere on the short time scales of volcanic eruptions.
The chemical behavior of fluids released during deep subduction based on fluid inclusions
https://doi.org/10.2138/am-2015-4933
On page 352 of this issue, Frezzotti and Ferrando review fluid inclusion compositions from high pressure (HP) and ultra-high pressure (UHP) lithologies. They suggest that the solutes in HP fluid inclusions are dominated by chloride salts, alkalis, and Si and Al, similar to species that are found in crustal fluids. At UHP conditions, however, solutes are dominated by a wide range of aluminosilicate components as well as sulfate and carbonate species, which appear to record the increasing solubility of mineral components into fluid phases. Of particular interest are the high amounts of carbonate at sub-arc pressures, which indicates that the transport of C in subduction zones may be as much affected by the transport of aqueous fluids as by other phases.
Prevalence of growth twins among anhedral plagioclase microlites
https://doi.org/10.2138/am-2015-4809
On page 385 of this issue, Brugger and Hammer explain variations in plagioclase microlite morphologies, and how such variations might be related to decompression rates and the formation of twin-plane boundaries. Their experiments show that twin planes in plagioclase likely form at the early stages of crystal growth as a result of defects in the crystal structure. Such defects, and twinning itself, is much more prevalent at high effective undercooling. Of key importance is that these high-energy twin surfaces can inhibit new growth at or across twin boundaries. These findings suggest that melt inclusions, swallowtails, and hopper structures might not result from diffusion-controlled growth, but rather due to preferential crystal growth on lower-energy surfaces away from twin planes.
Intrinsic conditions of magma genesis at the Lunar Crater Volcanic Field (Nevada) and implications for internal plumbing and magma ascent
https://doi.org/10.2138/am-2015-4812
On page 396 of this issue, Cortes et al. examine the mantle source components for two phases of both closely and distantly spaced volcanoes in the Lunar Crater Volcanic Field of central Nevada. They examine four volcanoes that formed within two time windows; in both cases, the magmas appear to be generated at very similar depths and to record very similar transport and cooling histories. In the earlier window, however, both volcanoes are just hundreds of meters apart and yet tap a slightly more heterogeneous source compared to the two later volcanoes, which yield lava flows with much more homogenous compositions—despite being separated by several kilometers, and tens of thousands of years for their eruptions.
Alkali influence on the water speciation and the environment of protons in silicate glasses revealed by 1H MAS NMR spectroscopy
https://doi.org/10.2138/am-2015-5004
On page 466 of this issue, Le Losq et al. show that the structural volume of H depends upon the radius of alkali cations dissolved in a hydrous silicate melt. The total amount of water dissolved in their experimental glasses has no apparent effect on H volume. However, H volume is smaller when the melts dissolve alkalis with larger ionic radii. This probably participates in determining larger partial molar volume and higher solubility of water in Na- compared to K-bearing silicate melts. These results imply that both water solubility and water-controlled magma buoyancy are intrinsically affected by magma composition.
Carbonate mineralization in percolated olivine aggregates: Linking effects of crystallographic orientation and fluid flow
https://doi.org/10.2138/am-2015-4913
On page 474 of this issue, Peuble et al. examine the role of crystal-preferred orientations (CPO) with regard to olivine dissolution and carbonate precipitation. In natural peridotites affected by asthenospheric flow, olivines are oriented so that (010) planes are mostly horizontal (b-axis vertical), and (100) planes are mostly vertical (a-axis horizontal). In this experimental study, the authors find that upon injection of CO2-bearing fluids into such an anisotropic regime, olivine dissolves in the (010) direction, and carbonate precipitation is controlled by fluid flow along fractures in the 10 direction; with precipitation of carbonate, the latter event effectively blocks further fluid transport, limiting the re-emission of CO2. If their experimental trials can be scaled upward, such anisotropy may provide a leak-resistant means for storing CO2.
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