Editor’s Notes
Total Results: 1697
Volume 102 : September 2017 Issue
EosFit-Pinc: A simple GUI for host-inclusion elastic thermobarometry
https://doi.org/10.2138/am-2017-6190
On page 1957 of this issue, Angel et al. present new software that can be used to more accurately determine the P-T paths of mineral-in-mineral inclusions. Their new programs do this by taking into account the differing equations of state (EoS) for inclusions and hosts, and by allowing the user to input customized EoS. Their approach uses an “isomeke”, which is a curve in P-T space that provides the locus of point where the fractional volume change of the host, induced by some external change of pressure, yields a comparable fractional volume change in an inclusion. Among the intriguing results is that quartz included in garnet may experience pressures along a prograde metamorphic path that are 30-40% lower than a host garnet; in contrast, pressures experienced by rutile in the same garnet may differ by <5%, due to the similar EoS for these materials.
Volume 102 : August 2017 Issue
How many boron minerals occur in Earth’s upper crust?
https://doi.org/10.2138/am-2017-5897
On page 1573 of this issue Grew et al. use B-bearing minerals to examine how the Large Numbers of Rare Events (LNRE) model has fared in predicting the total number of as-yet-to-be-discovered minerals. The LNRE model was used to predict that perhaps as few as 1600 new minerals were to be discovered (Hazen et al. 2015). But the LNRE approach assumes a fixed rule set, and so does not account for new technological methods that might be applied to discover new minerals (let alone shifts in the definition of a mineral); potentially leading to under-estimates of natural mineral diversity. Thus the LNRE model yields a predicted total of ca. 500 B mineral species (ca. 200 as yet to be discovered). The authors also note that as few as 19% of all B minerals were known as synthetic analogs prior to their discovery. However, what appears to be lacking in the LNRE approach is intent. For example, does the low fraction of pre-synthesized B minerals represent limits on our ability to create new compounds or a lack of compelling reasons to take on an apparently mindless and unproductive task? The highs and lulls of new mineral descriptions may be related to intent as well, but these issues do not blunt the authors’ fundamental contention that surprises are ahead.
Network analysis of mineralogical systems
https://doi.org/10.2138/am-2017-6104CCBYNCND
On page 1588 of this issue, Morrison et al. introduce Network Analysis as a means to explore possible genetic relationships between minerals. The use of Network Analysis will already be familiar to Am Min readers, mostly from their wide use in sociology;like mapping social media networks or predict voting behavior. Applied to mineralogy, the idea is that Network Analysis may reveal new associations that are characteristic of a time, place, or conditions of mineral formation. One might immediately respond that we have a governing theory--thermodynamics--that obviates the need for a statistical approach. The promise of Network Analysis, though, is to examine very large data sets to predict mineral occurrence in rare minerals that nonetheless might characterize an environment, an era, or a tectonic setting. These authors use Cu-bearing minerals as a striking example of how network analysis can be employed to examine hundreds of species at thousands of localities, and in the process reveal how Cu-mineral diversity has increased in both magnitude and kind since Archean.
Geochemistry of the Cretaceous Kaskanak Batholith and genesis of the Pebble porphyry Cu-Au-Mo deposit, Southwest Alaska
https://doi.org/10.2138/am-2017-6053
On page 1597 of this issue Olson et al. examine the conditions of Cu, Au, and Mo mineralization in the Pebble Porphyry deposits of southwest Alaska, by examining the precursor and host granite plutons and dikes. The authors find that the very high degree of enrichment in Cu and Au are related to the differentiation of mafic, hydrous, calc-alkaline, and alkalic magmas under strongly oxidizing conditions, the latter of which inhibit the saturation of sulfide melts, and so promote the retention of Cu and Au in increasingly sulfate rich magmas. The authors also hypothesis that certain trace element enrichments may have occurred in a so-called “MASH” zone where primitive magmas may have interacted with sulfide-rich cumulates in a lower crust setting, although the depths of differentiation and emplacement were not estimated.
Nitrides and carbonitrides from the lowermost mantle and their importance in the search for Earth’s “lost” nitrogen
https://doi.org/10.2138/am-2017-6101
On page 1667 of this issue Kaminsky and Wirth report on a new set of nitride minerals (Fe2N, Fe3N, and Fe9(N,C)4) found as inclusions in what appear to be a lower mantle-derived diamond. The authors infer that the nitride minerals derive from the core-mantle boundary, having formed within the core itself, apparently based on the association of Fe7C3 in other inclusions in the same diamond and the presumed usefulness of high P variants of Fe7C3 and Fe7N3 to explain some seismic anomalies of the solid core. However, these same inclusions carry carbonatite-association minerals. Thus, it is not clear whether we must accept all the components that occur in such inclusions as being necessarily derived from the core/mantle boundary or as indices of a more shallow provenance for all of the included phases, in what may be a highly reduced domain in the upper mantle; which would appear to be required in any case to generate those diamonds presumed to form in the uppermost mantle.
Single-crystal equations of state of magnesiowüstite at high pressures
https://doi.org/10.2138/am-2017-5966
On page 1709 of this issue, Finklestein et al. conduct high P experiments on a magnesiouwustite with 78 mol% Fe, to determine its elastic properties. These experiments are motivated by the hypothesis that Ultra Low Velocity Zones (ULVZs) at the base of Earth’s mantle might reflect the partitioning of Fe into oxides phases relative to ambient bridgmanite (or putative post-peroviskite phases). Their work confirms, at least at low temperatures, that the bulk modulus (K) of mw decreases with increased iron content, and that the first pressure derivative (K’) is unchanged with composition. Two pressing questions emerge from this work: first, do these elastic property relationships hold at elevated temperature; second, could ULVZs be explained solely by the way Fe partitions between mw and high pressure silicate phases or is an absolute enrichment in bulk Fe contents needed to obtain observed ULVZ anomalies?
Mn-Fe systematics in major planetary body reservoirs in the solar system and the positioning of the Angrite Parent Body: A crystal-chemical perspective
https://doi.org/10.2138/am-2017-6112
On page 1759 of this issue, Papike et al. compare the compositions of olivine and pyroxene from Earth, Moon, Mars, and Angrite meteorites. They conclude that Fe/Mn ratios of these minerals are similar enough between Angrites and Earth to require that the Angrite parent body was initially coagulated in some proximity to Earth. The idea is based on the observation that the Fe contents of the silicate portions of the terrestrial planets decreases with increasing distance from the sun. This result is in distinction with Sossi et al. (2016: EPSL) who suggest that Fe/Mn ratios are not only higher than for Earth within the error of their bulk measurements, but that Fe/Mn ratios are meaningfully and positively correlated with Fe isotope ratios, although these authors come to the same conclusion, i.e., that inter-planetary contrasts in Fe/Mn are controlled not just by variation in core mass and Fe segregation into a metallic phase, but also by the loss of the more volatile Mn.
Volume 102 : July 2017 Issue
A new high JIF for American Mineralogist (by all early indications), why you shouldn’t care, and a note on values
https://doi.org/10.2138/am-2017-ed102713
On page 1369 of this issue, we show our failed prediction of a much higher JIF (it went up, but only by 0.1 from last year), relating JIFs to college rankings, and discuss how these and other rankings reflect values. The failed prediction lends support to our proposal that journals adopt a variant of the JIF (introduced in our editorial) that is more easily reproduced. Average citation rates have in fact increased significantly at Am Min, and as we show, some Am Min papers can acquire vastly more citations than comparably themed papers in Science or Nature. But purveyors of citation statistics have no incentive to transparency, and our discrepant calculations should oblige Editors and journal critics alike to consider independent estimates and indices. (And hats off to JGR editors for joining the bandwagon.)
Sapphire, a not so simple gemstone
https://doi.org/10.2138/am-2017-6105
On page 1373 of this issue, Lin Sutherland provides a review of Palke and Breeding’s new study of needle-like rutile in sapphire from several localities, such inclusions being the cause of “silk” and “star” varieties of the gemstone. Palke and Breeding find that rutile (and possibly other Fe-Ti oxides) is not necessarily formed by exsolution. In this interpretation, rutile-bearing sapphires are not indicative of initially high temperatures of crystallization; they also indicate that the trace elements contained within the inclusions may help to characterize both geologic and geographic origin. Additionally, this new work lends support to the idea that blue-colored sapphires may acquire their optical characteristics from Ti-Fe charge transfer within the host sapphire.
Radon emanation coefficients of several minerals: How they vary with physical and mineralogical properties
https://doi.org/10.2138/am-2017-6017
On page 1375 (temporary link) of this issue Krupp et al. measure Rn loss rates for a variety of minerals from various geologic settings and show how such loss rates vary with a number of interesting factors, including grain size, mineral density, temperature, U and Th concentrations, and mineral melting points. They find that Rn emission rates are greatest for minerals with low density and low melting points, and they show that when fission tracks are annealed, Rn loss rates are reduced. The authors thus suggest that Rn may readily diffuse along nuclear tracks. The study does not apply multivariate methods so as to untangle these effects, but this work nonetheless shows that mineralogy, environment, and geologic history can each have a significant effect on Rn concentrations in the atmosphere and groundwaters, and may explain discordant dates in some U-Pb systems.
Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete
https://doi.org/10.2138/am-2017-5993CCBY
On page 1435 of this issue, Jackson et al. present yet another American Mineralogist publication that has garnered very widespread attention in popular news media. In this work, the authors examine phillipsite and Al-tobermorite: mineral cements in certain Roman marine concrete. The ancient concrete is formed by the interaction of seawater with “Pozzolan”, a silica-rich material (often derived from volcanic ash) that upon reaction with seawater produces a highly durable concrete, perhaps more durable than limestone-based Portland derivatives. This new work shows that the growth of zeolite (phillipsite) and Al-tobermorite during seawater-volcanic ash reactions may be key to providing at least some of the structural resiliency of these concrete structures formed in the harbors of ancient Rome. Their work should also aid the identification of natural pozzolans that should optimize the growth of these phases, and so yield durable structures.
Dehydration studies of natrolites: Role of monovalent extra-framework cations and degree of hydration
https://doi.org/10.2138/am-2017-5902
On page 1462 of this issue, Lee et al. present the results of dehydration experiments involving natrolite, containing extra-framework cations (EFC) of various size and charge. They find that dehydration temperatures are inversely proportional to EFC size, which might not be unexpected given a possible Coulomb’s Law effect on bonding strength. The authors also find that during dehydration, EFCs migrate from the middle to the edges of the open channels that characterize zeolite structures, which then causes a change in channel orientations relative to crystallographic axes. The authors suggest that EFC type has a greater influence over the stability of zeolite structures than degree of dehydration, with stability increasing with decreasing EFC radius.
Mineralogical and compositional features of rock fulgurites: A record of lightning effects on granite
https://doi.org/10.2138/am-2017-5971
On page 1470 of this issue, Elmi et al. present a study of fulgurites from a lightning strike of granite exposed at Mt. Mottarone, Baveno (Italy). Fulgurites have long been a curiosity--and perhaps little more--but in this study, the authors delve into the thermal conditions and compositional changes that occur upon lightning-induced partial melting of granite. Their work identifies a mineralogy and energetic conditions that indicate temperatures of ca. 1700 °C, if the system approached equilibrium. They also find that voids in the fulgurite structure result from the degassing of burned organic matter. Beyond the curio aspect, this work tells us that fulgurites may trap ambient gases and reveals much about how materials are affected by lightning strikes, which may then lead to better mitigation of thermal shocks associated with such.
Experimental evidence for the survival of augite to transition zone depths, and implications for subduction zone dynamics
https://doi.org/10.2138/am-2017-5959
On page 1516 of this issue, Xu et al., to better understand the conditions under which subducted slabs may stall in the upper mantle, conduct experiments to measure the physical properties of natural augite to 27 GPa and 700 K. Existing studies of pyroxenes focus on idealized end-member compositions, or involve measurements at high P or high T, but not both, forcing modelers to rely on mixing relationships and extrapolate equations of state. This work shows that natural augite-composition clinopyroxenes, if metastably preserved in a sinking plate, do indeed have a sufficiently low density so as to inhibit subduction beyond transition zone depths. Their modeling efforts indicate that natural augite has sufficiently high Fe so as to cause slabs to sink above transition zone depths, but that at the base of the transition zone, if temperatures remain cool, metastable augite may cause slabs to become neutrally buoyant, at least relative to PREM densities.
17O NMR evidence of free ionic clusters Mn+...CO32– in silicate glasses: Precursors for carbonate-silicate liquids immiscibility
https://doi.org/10.2138/am-2017-6133
On page 1561 of this issue, Morizet et al. examine the structural characteristics of carbonate-rich, low-Si melts, analogous to kimberlite and melilitite bulk compositions. Their NMR study reveals that carbonate dissolves in such de-polymerized melts so as to form Free Ionic Clusters (FIC), instead of network-forming complexes. These FIC are composed of carbonate units bonded to network-modifying cations that donate most or all of their charge to the carbonate complex, as opposed to neighboring silicate structural units (analogous to what may be deduced of mineral structures using Pauling’s Rules). The FIC are thus nearly completely disconnected from ambient silicate melt constituents, and thus may form the nuclei upon which immiscible carbonate melts exsolve from a silicate solution.
Volume 102 : June 2017 Issue
Uranium-bearing opals: Products of U-mobilization, diffusion, and transformation processes
https://doi.org/10.2138/am-2017-5700
On page 1154 of this issue, Schindler et al., examine the crystal structural controls on the incorporation of U into opal. These authors build on prior work that shows that U may be structurally bound by amorphous Si for millions of years. Their new study shows that U is most likely to be retained along fibers or grain boundaries of opal as various domains are precipitated or transformed by various dissolution-re-precipitation reactions. The U complexes are apparently especially mobile along the boundaries separating domains of opal-CT, a variety that contains microcrystalline cristobalite and tridymite (compared to opal-A, an aggregate of non-crystalline silica). The U so captured may be released as opal transforms to microcrystalline quartz.
High-resolution geochemistry of volcanic ash highlights complex magma dynamics during the Eyjafjallajökull 2010 eruption
https://doi.org/10.2138/am-2017-5860
On page 1173 of this issue, Laeger et al. present new analyses of pumice and minerals contained in tephra deposits erupted on May 18 and 22, from the Eyjafjallajökull 2010 eruption in Iceland. Their major and trace element data allow these authors to identify two distinct episodes of mixing, between a mafic recharge basalt magma, and two magma compositions (a trachyandesite and a rhyolite) that geophysical evidence indicates were intruded and stored as sills starting in 1994. Perhaps most interesting is that their merger of mineralogical and geophysical data appears to show that the arrival in 2010 of what is termed “BAS 1” (a basalt composition) signaled what Colin Wilson would call a “tipping point”, sending this Icelandic magma plumbing system from a state where the crust can absorb all those magmas delivered from the mantle, into a state of instability, where new batches of mantle-derived magma, in mixed form, are erupted.
The spin state of Fe3+ in lower mantle bridgmanite
https://doi.org/10.2138/am-2017-5917
On page 1263 of this issue Sinmyo et al. use Mössbauer spectroscopy to measure the valence and spin state of Fe in bridgmanite, at elevated P and ambient T. Prior work has indicated that Fe3+ in bridgmanite may transition from a high spin to an intermediate or low spin state as pressure increases, which may in turn be measurable by a resulting change in magnetic character. The authors find that the spin transition in bridgmanite occurs at 900 km (35 GPa) in a pyrolite mantle, and 1200 km (50 GPa) in MORB-like domains, with the transition occurring at greater depths still at higher temperature. And the transition will, of course, be quite sensitive to fO2. Their results suggest that the spin crossover will not yield detectable contrasts in elastic properties, but might be observed in geomagnetic surveys. This work implies an urgency to measure the depth of a spin crossover in Earth’s mantle by geophysical means, which may then be as helpful for understanding the T and fO2 conditions of the deep mantle, as seismic velocities were in determining the existence of bridgmanite itself.
NanoSIMS study of seismically deformed zircon: Evidence of Y, Yb, Ce, and P redistribution and resetting of radiogenic Pb
https://doi.org/10.2138/am-2017-5975
On page 1311 of this issue, Kovaleva and Klötzli examine zircons in mylonites from the Ivrea-Verbano zone of northern Italy to determine to how zircon trace element and isotope compositions may vary within lattice defects that are generated by seismic activity. They find that post-growth deformation bands can both gain and lose certain trace elements (e.g., Hf, Ti, and P) relative to undeformed parts of the same grain and that these same deformation bands appear to undergo systematic Pb loss. The greatest Pb loss appears to be associated with planar deformation bands that occur at the highest angle relative to an unstrained host lattice. Such Pb loss should lead to the possibility of using coupled structural and isotopic studies of zircon to age date mylonite formation events other any other deformation process that affects zircon microstructures.
Magma mush chemistry at subduction zones, revealed by new melt major element inversion from calcic amphiboles
https://doi.org/10.2138/am-2017-5928
On page 1353 of this issue, Zhang et al. provide new “chemometric” equations that may be used to reconstruct a silicate liquid using an amphibole composition. Amphiboles are a common phase in arc-related igneous rocks, and because of their compositional complexity, they would appear to offer a powerful tool for determining silicate liquid compositions, which at most arc volcanoes are often highly obscured due to the very common and efficient mixing of diverse magma types. This study revisits an important earlier attempt by Ridolfi and Renzulli (2012) to use an amphibole composition to infer the composition of the silicate liquid in crystallized from. This new work focuses on basanitic to rhyolitic melt compositions, and is accompanied by an interesting test: their new models can successfully predict matric glass compositions in selected ignimbrite samples when using coexisting amphibole rim compositions as input. Their application to natural systems also shows that the variety of liquid compositions, at least at one volcano, may be much greater than one might infer from the bulk composition erupted products alone.
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