American Mineralogist: Journal of Earth and Planetary Science:
All our papers are special, but each month, the American Mineralogist editors pick a few to be "Noted Papers". We hope this information is enjoyable and useful.
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Editors Selections, July 2017
A New High Journal Impact Factor (Not really) and How Rankings Reflect Values
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.)
Highlights and Breakthroughs
Sapphire – Blue with Inclusions
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.
Invited Centennial Articles
Explaining Radon Loss in Minerals
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.
Mineralogy of Durable Concrete from Ancient Rome
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.
Zeolite Dehydration and Extra-Framework Cations
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.
What Happens When Lightning Hits Rock?
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.
Pyroxenes Stop Subducted Slabs Cold
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.
Carbonate-Silicate Liquid Immiscibility Explained
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.