Editor’s Notes
Total Results: 1697
Volume 102 : May 2017 Issue
Defining minerals in the age of humans
https://doi.org/10.2138/am-2017-6045
On page 925 of this issue, Peter Heaney provides a provocative overview of the immensely popular article by Hazen et al. (2017; this volume) regarding minerals that have an anthropogenic parentage. As Heaney notes, textbooks have largely given up on the notion that minerals form only by inorganic processes; he then asks whether we are “prepared to take the next step”: to accept synthetic crystals as minerals (as already proposed by Zalasiewicz et al. 2014). Hazen et al. do not require such in their definition of an Anthropocene epoch they attempt to play by IMA rules. Some of the most intriguing arguments appear to begin with how hypothetical visitors to, or evolved species on, a late Earth might classify crystalline structures. What is technology at present is archeology to a future age. What is uniquely anthropogenic now, may be a primitive step along a biogenic continuum to some future species. As our language evolves over centuries or even decades, it is sure to vary across species and millennia.
Bottled samples of Earth’s lower mantle
https://doi.org/10.2138/am-2017-6047
On page 927 of this issue, William Bassett reviews a new work by Kaminsky and Lin (2017; this volume) who speculate on the composition of Earth’s lower mantle, based on the compositions of minerals included in diamonds and their synthetic analogs as produced in high pressure experiments. In the new work, Kaminsky and Lin (2017) examine the co-existing compositions of ferropericlase and bridgmanite (the latter of which is quenched to an orthopyroxene structure, but is presumed to preserve the composition of the high pressure, proto-mineral phase). They find that in some inclusions, the total amounts of Fe in both phases are high relative to what might be expected had they equilibrated from a pyrolite bulk mantle composition. And yet another set of inclusions contain lower-than-expected Ni. These observations lead Kaminsky and Lin to infer that the lowermost mantle may contain an Fe-rich zone that might even be saturated in a metallic Fe phase that may retain Ni. This suggestion carries with it the implication that mantle plumes from the lowermost mantle should yield volcanic rocks that are enriched in Fe, but depleted in Ni and other siderophile elements.
Two ways of looking at chemical bonding
https://doi.org/10.2138/am-2017-6037
On page 928 of this issue I. David Brown discusses a new paper by Bickmore et al. (2017; this volume), which attempts to connect bond energy to electrostatic characteristics, in this case electronegativity. The work is inspired by the observation that bond energies vary greatly across bond types. Perhaps the most intriguing implication is one left unsaid by the authors: they provide a possible fifth explanation for the rarity of many minerals, as enumerated by Hazen and Ausubel (2016; Am Min). Those authors suggested that rarity is typical for the more than 5000 minerals yet named, due to restricted P-T-X stability, planetary constraints, ephemeral stable conditions and collection bias. Bickmore et al. (2017) provide, if not an additional cause for rarity, then at least a possible rationale for ephemeral P-T-X stability, by postulating that peroxides are rare but persuflides are common. Peroxides are built upon O-O bonds, which have intrinsically less energy per bond valence than more ionic M-O bonds.
Diamonds from the lower mantle?
https://doi.org/10.2138/am-2017-6061
On page 929 of this issue, Andrew Thomson reflects on some new and important observations by Uenver-Thiele et al. (2017) regarding the putative depths of mantle-derived diamonds. Some diamonds are thought to derive from depths of at least >800 km, on the basis of their containing magnesiowüsite (Mg,FeO), a phase that is not expected to be stable under subsolidus conditions in a pyrolite bulk composition (but is stable to atmospheric pressure in other bulk compositions). Even greater depths are suggested where some magnesioferrite phases occur as exsolution lamellae within Mg,FeO—but such inferences have not verified by experiments at ambient pressure. Uenver-Thiele et al. (2017) conduct the needed experiments and ironically find that such exsolution relationships are likely to occur only at P<10 GPa. Their new work does not preclude entrapment of inclusions at greater depths, but indicates that the exsolution textures in question represent an upper mantle phenomenon.
A review and update of mantle thermobarometry for primitive arc magmas
https://doi.org/10.2138/am-2017-5783
On page 931 of this issue, Christy Till reviews the conditions under which arc magmas are generated in the mantle wedge. She finds that while published P-T estimates of arc magma genesis vary greatly, the largest source of variation in these estimates is differences in their calculation methodology. Once calculated with a consistent approach, wet primitive arc magmas reveal origins via water-saturated partial melting at 20-35 kbar, with subsequent equilibration (prior to transport to the crust) over relatively narrow P and T intervals of 8-19 kbar and 1075-1300 °C. Anhydrous arc magmas reveal similar depths of mantle equilibration en route to the surface but at higher temperatures (1290-1450 °C), facilitated by channelized rather than porous flow. Till finds that these processes are sufficient to explain a rather wide range of arc magmas, from boninites to high Mg andesite to hydrous calc-alkaline basalt, leaving a lherzolite residue for the latter and a harzburgite residue for the former two. More importantly, though, Till is able to show how these P-T conditions can be mapped into different and distinct parts of the mantle wedge, which means that various volcanic composition suites may be used to describe the evolution of an ancient or evolving arc system.
Sperrylite saturation in magmatic sulfide melts: Implications for formation of PGE-bearing arsenides and sulfarsenides
https://doi.org/10.2138/am-2017-5631
On page 975 of this issue, Schmädicke and Gose measure water contents in both garnet and omphacite in coesite-bearing eclogite. A key inference from their work is that molecular water in these phases is secondary, related to fluids that come into contact with the system following peak metamorphism. (The opposing possibility is that molecular water represents structural water that is exsolved upon decompression). In addition, molecular water may even secondarily enhance the content of structural water. In this case, only structural water of samples devoid of molecular water is useful for estimating water budgets in a downgoing basaltic slab, and these authors thus infer that such contents are quite low, on the order of 300-650 ppm, with most pre- and syn-metamorphic water being released to hanging wall rocks (e.g., the mantle wedge or exhuming high-pressure slices).
Dry annealing of metamict zircon: A differential scanning calorimetry study
https://doi.org/10.2138/am-2017-5901
On page 1066 of this issue Pidgeon et al. examine the recrystallization of radiation-damaged zircon. The recrystallization process first involves the formation of tetragonal zirconia, which then reacts with silica to recreate zircon. Their experiments indicate, however, that such recrystallization is highly unlikely in natural systems, and Pb and He loss would thus be permanent. They further note that recrystallization would be no more likely in synthetic zircons that are used to sequester radioactive waste, in which case radionuclides would be not be protected against reactions with fluids.
Crystal structure of abelsonite, the only known crystalline geoporphyrin
https://doi.org/10.2138/am-2017-5927
On page 1129 of this issue, Hummer et al. provide the structure of one of the very few known organic minerals, in this case, a naturally crystalline geoporphyrin (porphyrins being a type of organic compound) called abselonite. This and related compounds are believed to be due to the breakdown under diagenesis of chlorophyll a, and the transformation to abselonite involves a highly specific change of cations from Mg to Ni(II).
Volume 102 : April 2017 Issue
Mineral precipitation and dissolution in the kidney
https://doi.org/10.2138/am-2017-5778
On page 701 of this issue, Hill et al. present a new computer model, based on current thermodynamic measurements, that can be used to predict the solubility of Ca phosphates in body fluids. As noted by the authors, the human body must in places remain supersaturated in hydroxyapatite for healthy bones and teeth but that oversaturation in the kidneys can lead to the development of unwanted stones. This new model attempts to predict mineral saturation under some of the various complex fluids of the human body, and so better inform the conditions that lead to kidney stone development, and thus better inform preventative measures.
Magmatic graphite inclusions in Mn-Fe-rich fluorapatite of perphosphorus granites (the Belvís pluton, Variscan Iberian Belt)
https://doi.org/10.2138/am-2017-5895
On page 728 of this issue, Perez-Soba et al. present a case for what they argue is the first occurrence of graphite (in a granite host) that is of magmatic origin. Their study focuses peraluminous granites of the Variscan Belt, and three different types of magmatic apatite contained in these. Because peraluminous compositions have a high solubility for apatite, these minerals tend to record late stage processes. They find that some fluorapatite grains of apparent magmatic origin contain needle-like inclusions of graphite. They interpret the graphite inclusions as forming at a very late stage of crystallization, where the fluorapatite is in equilibrium with a fluid phase and two immiscible (perphosphorous and peraluminous composition) magmatic liquids and they suggest that a C-rich system may induce liquid immiscibility. These intriguing results point to the usefulness of apatite as a means to investigate the P-T conditions of very late-stage magmatic processes….
Barometric constraints based on apatite inclusions in garnet
https://doi.org/10.2138/am-2017-5898
On page 743 of this issue, Ashley et al. calibrate an equation of state for complex apatite solid solutions. They find that their equation of state may provide a powerful tool to obtain P estimates from apatite included in garnet. They find that garnet is an ideal host in that this isotropic mineral also has a very high bulk modulus, and so more ably preserves the stress state of included minerals. Their tests further indicate that the pressures recorded by apatite are not highly sensitive to T. Apatite inclusions in garnet may this provide an accurate barometer in whatever systems they may be found, and may be especially useful when P-sensitive exchange equilibria are absent.
A comparison of olivine-melt thermometers based on DMg and DNi: The effects of melt composition, temperature, and pressure with applications to MORBs and hydrous arc basalts
https://doi.org/10.2138/am-2017-5879
On page 750 of this issue, Pu et al. calibrate a new Ol-liquid thermometer, based on the partitioning of Ni between Ol and co-existing silicate melt. The thermometer is calibrated with some several limitations: it uses experimental data collected at 1 bar only, of liquids saturated only with Olivine, containing ≤0.1 wt% NiO. But the resulting experimental calibration data set still spans a wide range of T (1170-1650 °C) and liquid compositions (37-66 wt% SiO2), and the resulting model ably predicts T from experiments performed at 10 kbar, and so may have an advantage over Mg-exchange based thermometers in requiring no knowledge of P (or rather to allow P to be estimated, at least at P<10 kbar, by determining the P at which the Ni and Mg exchange thermometers yield identical temperatures). Finally, their study also hints at the possibility that their Ni exchange thermometer may also be only slightly dependent on magmatic water (H2O, more appropriate than water) contents, thus offering the promise of a highly versatile thermometer that requires minimal input parameters for application.
Water transfer during magma mixing events: Insights into crystal mush rejuvenation and melt extraction processes
https://doi.org/10.2138/am-2017-5793
On page 766 of this issue, Pistone et al. conduct rather clever experiments to examine the role of recharge magmas in aiding the expulsion of melt from overlying felsic mush systems. Their experiments, which place aphyric, hydrous andesite below a dacite mush, show a significant transfer of water from the andesite to the dacite, and in increase in the SiO2 contents of dacitic mush glasses as the addition of water apparently drives the dissolution of quartz in the mush. The authors suggest that mush rejuvenation occurs in two stages, the first dominated by heat transfer and the second by water transfer, from the mafic recharge magma to the felsic mush. Both these processes lead to an increase in melt fraction in the dacitic mush, and a decrease in melt density and viscosity, which are expected to aid felsic melt extraction.
Fluvial transport of impact evidence from cratonic interior to passive margin: Vredefort-derived shocked zircon on the Atlantic coast of South Africa
https://doi.org/10.2138/am-2017-5857CCBYNCND
On page 813 of this issue Montalvo et al. survey some 11,000 zircon grains recovered from sedimentary deposits that may have the Verdefort impact site as a possible provenance. From that survey, the authors recovered three zircon grains that show tell-tale signs of shock-induced formation ({112} twins), with age dates of ca. 3 Ga that are consistent with the bedrock ages of the impact site. This work reveals the possibility for determining impact locations (and apparently maximum ages for impact) by examining the detrital zircon record.
Iron partitioning in natural lower-mantle minerals: Toward a chemically heterogeneous lower mantle
https://doi.org/10.2138/am-2017-5949
On page 824 of this issue, Kaminsky and Lin compare and find discrepancies between ferropericlase (fPer) and nominal bridgmanite (Bridg) (transformed to enstatite, so not true Bridg) compositions that occur as inclusions in diamonds, and as obtained from experiments. Some experiments are performed on non-Al bearing bulk compositions, and this work implicitly shows why experiments performed on natural compositions are so crucial. In any case, the authors find that some natural fPer have both high Fe and low Ni, from which the authors infer equilibration with a Fe-Ni alloy, a phase that presumably only inhabits the deepest mantle. They also find that certain nominal Bridg grains exhibit high Al, which is also inferred to be a lowermost mantle signature, on the expectation that Al-in-Bridg increases with increased P. The highest Al-in-Bridg crystals do not coexist with the lowest Ni-in-fPer grains, so the precise interpretation is unclear, but these compositions may still indicate important heterogeneity in the lower mantle. And while pyrolite is a highly flexible term (intended only to indicate rough proportions of basalt and peridotite), this study may still lead to the identification of distinct mantle domains—which if real, should connect to ocean island basalt compositions, or at least those exhibiting high mantle potential temperatures.
Fluorwavellite, Al3(PO4)2(OH)2F·5H2O, the fluorine analog of wavellite
https://doi.org/10.2138/am-2017-5948
On page 909 of this issue, Kampf et al. describe the new mineral, fluorwavellite, and its solid solution behavior relative to the non-fluorinated wavellite. In surveying wavellite from various localities, the authors find complete solid solution between the two end-members, but no connection between F content and mode of occurrence (e.g., F contents are not different for minerals obtained from pegmatites, ore deposits, or hydrothermally altered systems). The authors thus infer that F contents are controlled by the activities of Al, P, and F, and perhaps also by pH, since wavellite-fluorwavellite are highly soluble only under the uncommon conditions of pH>9. At lower pH, the authors suggest that wavellite-fluorwavellite is an important near-surface sink for F.
Volume 102 : March 2017 Issue
Nickel variability in Hawaiian olivine: Evaluating the relative contributions from mantle and crustal processes
https://doi.org/10.2138/am-2017-5763
On page 507 of this issue Lynn et all examine NiO contents of high forsterite (Fo88) olivine crystals from various Hawaiian lavas (from Koolau, Kilauea, Loihi, Mauna Kea, and Mauna Loa). They find that over a 2.5X variation at individual volcanoes, Ni-in-Ol contents have no relationship to either host whole rock compositions, and that inter-island whole rock Ni contents strongly overlap and are not correlated to Ni contents in Ol. These authors show that variations in Ni-in-Ol at high Fo can be explained by very minor variations in the Ni contents of parental magmas (0.09-0.11 wt% NiO), and that observed ranges in Ni-in-Ol can be explained by a combination of fractional crystallization and magma mixing, and that to the extent that Ol may record mantle mineralogy, large, oriented crystals are needed to correct for intra-crystalline diffusion.
Co-variability of S6+, S4+, and S2– in apatite as a function of oxidation state: Implications for a new oxybarometer
https://doi.org/10.2138/am-2017-5907
On page 548 of this issue, Konecke et al. examine the oxidation states of S in natural apatite and experimentally equilibrated apatite + liquid pairs. Their work reveals perhaps the first report of a mineral that can incorporate three oxidation states of S (S2-, S4+, and S6+), which apply to experimental systems at oxygen fugacities ranging from FMQ to FMQ+3. These results show the potential for calibrating S6+/S4+ and or S4+/S2- ratios as oxybarometers for apatite saturated systems, although such a function is not calibrated, it probably can be from the data presented. The authors also note that the partitioning of various oxidation states of S may, in part, explain the non-Henrian behavior S portioning in apatite.
On the mineralogy of the “Anthropocene Epoch”
https://doi.org/10.2138/am-2017-5875
Most Am Min readers will know that some 400 news organizations have already covered the work of Hazen et al. (2017), which appears on page 595 of this issue. Hazen et al. show that 200 new minerals that occur exclusively or primarily because of human activities, may leave a decipherable stratigraphic signature, visible to future mineralogists. Some durable crystalline materials might not be classified as minerals today, as they are intentional products of engineering or social activities (Portland cement, semiconductors, various abrasives, etc.) rather than products of nature. Other substances at the boundary of natural and human activities occur because of human influences on near-surface conditions. Future mineralogists may be unable to decipher whether such substances were intentionally manufactured. Nevertheless, they are so ubiquitously distributed across Earth's surface that they will leave a robust mineral-like signature of what may be termed the Anthropocene.
Phase relations of MgFe2O4 at conditions of the deep upper mantle and transition zone
https://doi.org/10.2138/am-2017-5871
On page 632 of this issue Uenver-Thiele et al. present new experimental studies of the stability field and breakdown reactions of magnesioferrite (MgFe2O4). They find that above 8 GPa and 1000 degrees C it breaks down into Fe2O3 and MgO, and at yet higher temperatures, the reaction involves an unquenchable phase with an approximate stoichiometry of Mg5Fe2O8 or Mg4Fe2O7. Their results may shed light on magnesioferrite inclusions in diamond (often included in (Mg,Fe)O). Either such inclusions formed within the upper mantle, or if a lower mantle genesis is inferred, say from other phases, then the magnesioferrite must have formed from some other precursor phase.
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