Editor’s Notes
Total Results: 1697
Volume 108 : January 2023 Issue
Repeat, fast, and high-resolution mapping of fine-scale trace element distribution in pyrite and marcasite by LA-Q-ICP-MS with the Aerosol Rapid Introduction System (ARIS)
https://doi.org/10.2138/am-2022-8168
Minor and trace element compositions of sulfides provide critical insights into a variety of geological processes. Reynolds et al. detail a new method for mapping minor and trace element distribution in pyrite and marcasite by LA-Q-ICP-MS using the Aerosol Rapid Introduction System (ARIS) and novel operating conditions. This approach allows for increased mapping speed, decreased limits of quantification, and decreased cost of consumables. As a result, extraordinarily high-resolution maps can be created without an excessive compromise of speed, cost, or limits of quantification.
Continuous Be mineralization from two-mica granite to pegmatite: Critical element enrichment processes in a Himalayan leucogranite pluton
https://doi.org/10.2138/am-2022-8353
Beryllium is a critical metal typically concentrated in highly fractionated granitic rocks. Liu et al. study beryl mineralization in a typical leucogranite pluton at Pusila in the central Himalaya, a striking example of continuous Be mineralization. (1) Beryl is the most common beryllium mineral and is ubiquitous at Pusila. The beryl compositions trend to Na-Li-Cs enrichment and Fe-Mg depletion and show continuous decreasing Na/Li and Mg/Fe, and increasing Cs/Na ratios, which are positively correlated with increasing whole-rock Rb/Sr ratios. Mineral chemistry characteristics highlight beryl as a monitor of magmatic evolution processes. (2) The enrichments of Be and other critical elements are associated with strongly fractionated granites or their equivalents. Despite the low to moderate fractionation, the biotite- and two-mica granites contain unusually high Be contents, indicative of an initial magma relatively enriched in Be. The gneisses of GHS, considered as the protolith, also show high Be abundances, which could be the source reservoir of Be. Therefore, a high degree of fractionation is likely not the only factor for beryl saturation. A source relatively enriched in Be could effectively reduce the dependence on the degree of fractionation to concentrate Be sufficiently to reach saturation. (3) This study reveals the differing behaviors of incompatible elements (e.g., Be, Cs) in melt. Once the system is beryl saturated, beryllium is no longer incompatible, and the Be concentrations would be controlled by multiple processes such as fractional crystallization and the buffering action of beryl crystallization. This combination of processes could be applicable to other trace and incompatible elements. (4) The term “delayed crystallization,” describing the process of beryl crystallization at the late stage of granite solidification in two-mica granite and muscovite granites, is the crucial part of the continuous crystallization model. This continuous crystallization model reveals the evolutionary mechanisms of continuous Be mineralization with a progressive increase in whole rock Be contents and concurrently could provide new insights into the enrichment processes of other critical metal elements, like Cs, Li, Zr, etc., which are commonly rare and incompatible in granitic melts.
An evolutionary system of mineralogy, Part VI: Earth’s earliest Hadean crust (>4370 Ma)
https://doi.org/10.2138/am-2022-8329
Part VI of the evolutionary system of mineralogy, by Morrison et al., imagines Earth’s earliest mineralogy from a time before the oldest known mineral grain, more than 4.37 billion years old. Based on the evidence of minerals from rocky asteroids, planets, and moons, amplified by experimental petrology, geochemistry, and models of planetary evolution, they postulate 262 mineral species that formed within a few tens of millions of years of the Moon’s violent formation.
Oxidation or cation re-arrangement? Distinct behavior of riebeckite at high temperature
https://doi.org/10.2138/am-2022-8073
Fe-rich amphiboles are important rock-forming silicates, and their composition, in particular their Fe3+/Fetot ratio, may provide insight into the redox state of the geological system where they grew. Della Ventura et al. studied the oxidation process at high temperature in riebeckite by combining Mossbauer spectroscopy and powder XRD and showed that riebeckite follows two distinct paths depending on the external environment. Under oxidizing conditions, riebeckite is stable in the hydrous form up to relatively low temperatures (400-450 °C), and then it undergoes a rapid transformation into an oxo-amphibole which is stable up to ~900 °C. Under vacuum conditions, no Fe oxidation is observed; riebeckite undergoes a significant cationic re-arrangement and is stable in the hydrous form up to much higher temperatures (750-800 °C). These findings imply that characterization of the oxidation state of iron in riebeckite does not necessarily provide the redox and thermal conditions of formation.
Fe3+/FeT ratios of amphiboles determined by high spatial resolution single-crystal synchrotron Mössbauer spectroscopy
https://doi.org/10.2138/am-2022-8115
This work by Ratschbacher et al. presents the first documentation of amphibole Fe3+/FeT ratios determined by single-crystal synchrotron Mossbauer spectroscopy (SMS). SMS provides a high-spatial resolution and precision to avoid analyzing inclusions and alteration features and thus allows detecting intra-grain compositional variations in Fe3+/FeT ratios, which then can be interpreted in the context of changing crystallization condition and/or post-crystallization oxidation. SMS does not require reference spectra, therefore eliminating that source of uncertainty in the final results.
How clay delamination supports aseismic slip
https://doi.org/10.2138/am-2022-8195
Delamination between clay layers due to interlayer water lubrication was suspected of supporting aseismic slip but lacked a mechanistic insight. The variable hydration state of a clay mineral adds to the complexity. Here Zhou et al. use atomic-scale simulations to investigate the role of clay minerals in aseismic slip. The reported shear behaviors of clay minerals at different temperatures and pressures provide constraints on fault mechanics. Increased pore fluid pressure leads to more intercalated water, resulting in lower frictional strength and enhanced velocity-strengthening behavior. This work suggests that knowledge of the hydration state of a clay mineral is a necessity when studying fault mechanics.
The influence of Al2O3 on the structural properties of MgSiO3 akimotoite
https://doi.org/10.2138/am-2022-8257
Akimotoite, a MgSiO3 polymorph present in Earth’s lower transition zone, will contain some Al, raising the question of how this will affect its crystal structure and properties. Siersch et al. investigate MgSiO3 akimotoite - Al2O3 corundum solid solutions by single-crystal X-ray diffraction in order to examine their crystal chemistry. Results show a strong non-linear behavior of the a- and c-axes as a function of Al content, which arises from fundamentally different accommodation mechanisms in the akimotoite and corundum structures and suggests that an immiscibility gap may be present around the 50:50 compositions in this solid solution. Furthermore, samples belonging to the akimotoite - corundum solid solutions were investigated at high pressure in order to determine the different compression mechanisms associated with the cation substitution. Al2O3-bearing akimotoite becomes more compressible at least up to 20 mol% Al2O3, which is likely driven by an increase in compressibility as the Al cation is incorporated into the SiO6 octahedron. This observation is in strong contrast to the stiffer corundum end-member with respect to that of the akimotoite end-member. These findings have important implications for mineral physics models of elastic properties, which have in the past assumed linear mixing behavior between the MgSiO3 akimotoite and Al2O3 corundum end-members in order to calculate sound wave velocities for Al-bearing akimotoite at high pressure and temperature.
Atomistic insight into the ferroelastic post-stishovite transition by high-pressure single-crystal X-ray diffraction
https://doi.org/10.2138/am-2022-8458
Zhang et al. studied atomistic information and distortion modes across the ferroelastic transition from stishovite to post-stishovite. Using the synchrotron single-crystal X-ray diffraction technique, they determined crystal structures of the stishovite and post-stishovite phases in a diamond-anvil cell up to 75.3 GPa. Across the transition at ~51.4 GPa, the oxygen coordinate splits, the apical and equatorial Si-O bond lengths cross over, and a rotational lattice distortion occurs. This structural information has been correlated with shear modulus softening and Landau parameters, including spontaneous strains and order parameters reported in the literature. Importantly, they found that the symmetry-breaking spontaneous strain and the order parameter are proportional to the SiO6 rotation angle due to the appearance of the rotational mode. Together with atomistic insights of other types of ferroelastic transitions (such as the proper-type in albite and the improper-type in CaSiO3 perovskite), Zhang et al. found that the ferroelastic transition always occurs through changes of structural angle and an occurrence of a distortion mode. This finding could further shed light on the abnormal seismic properties and geodynamics in the regions where the ferroelastic transition occurs.
Epidote as a conveyor of water into the Earth’s deep mantle in subduction zones: Insights from coupled high-pressure and high-temperature experiments
https://doi.org/10.2138/am-2022-8252
Li et al. found that epidote begins to decompose at 1223 K and fully breaks down at 1373 K at atmospheric pressure. Furthermore, there is no phase transition in epidote at temperatures and pressures up to 1172 K and 14.0 GPa. These results suggest water can be conveyed downward into the mantle transition zone through epidote in cold, subducting mafic oceanic crust.
Potential link between antigorite dehydration and shallow intermediate-depth earthquakes in hot subduction zones
https://doi.org/10.2138/am-2022-8271
The dehydration of antigorite has been associated with intermediate-depth earthquakes in subduction zones. However, the reaction of antigorite → forsterite + talc has not yet been considered to explain some of these earthquakes because this reaction was not thought to occur in subducting mantle. In the hot subducting mantle beneath Cascadia and southwestern Japan, however, the reaction fits shallow intermediate-depth earthquakes. To explore whether the reaction can trigger these earthquakes, Shao et al. conducted a series of isothermal experiments on the kinetics of antigorite dehydration. Avrami modeling of the experimental data combined with microstructural observations of run products suggests that the reaction is controlled by a heterogeneous nucleation and growth mechanism. The fast fluid production rate of the reaction may lead to high fluid pressure in the subducting mantle but does not necessarily cause embrittlement. Superplasticity or velocity weakening of fine-grained forsterite and velocity weakening of antigorite by water and/or talc may be responsible for earthquake nucleation and propagation in a heterogeneous system, which can be either antigorite and its dehydration products within a serpentinized fault zone or the mixture of antigorite and surrounding peridotite in hot subduction zones (<2 GPa).
Stability of Fe5O6 and its relation to other Fe-Mg-oxides at high pressures and temperatures
https://doi.org/10.2138/am-2022-8370
Woodland et al. report new results on the stability of high-pressure Fe and Mg-Fe oxide phases. Such information is important for understanding the mineralogy of the Earth’s mantle, particularly in certain domains that appear to be iron-rich. The existence of such domains is documented by certain mineral inclusions found in diamonds and, by inference, may also be involved in the process of diamond formation. From these results, they conclude that either Fe4O5 or Fe5O6 could occur in such Fe-rich environments. However, the observed limited solubility of Mg in Fe5O6 implies that it will not be present as an accessory mineral in a peridotite bulk composition. Rather a Fe4O5 phase would be more likely to be stable in the “average” mantle composition.
From schwertmannite to natrojarosite: Long-term stability and kinetic approach
https://doi.org/10.2138/am-2022-8288
Jimenez et al. find that schwertmannite cannot be considered an efficient phase for the immobilization of contaminating metals in the short term, whereas natrojarosite remains stable under conditions of high acidity for long reaction times and can be considered more reliable for the retention of heavy metals. Moreover, an exhaustive spectroscopic study confirms the mineralogical results and suggests that the crystal structure of jarosite-like minerals may offer interesting geochemical information about the aqueous solutions in which they were formed.
Trace element and isotopic (S, Pb) constraints on the formation of the giant Chalukou porphyry Mo deposit, NE China
https://doi.org/10.2138/am-2022-8142
Porphyry-type Mo deposits have supplied most of Mo to the world. However, the source of the Mo and the controls on its enrichment in such deposits are still a matter of great debate. Zhao et al. present in situ trace element and isotopic data for a giant porphyry Mo deposit and use these data to address this issue. The trace element chemistry of pyrite and S and Pb isotopic compositions have made it possible to interpret the evolution of hydrothermal fluids during the formation of the Chalukou porphyry Mo deposit. The in situ LA-ICP-MS results lead to the conclusion that the absolute content of metal in the ore fluid was not the key factor controlling metallic mineral deposition and that molybdenite saturation was controlled by the system temperature. Another important conclusion of the study is that magmas unusually enriched in Mo due to crustal contamination, and crystallized as the syn-ore intrusions, were the source of Mo for the Chalukou deposit. This shows, contrary to previous studies, that magma fertility (enrichment in Mo) may play an essential role in generating large porphyry Mo deposits. Finally, the study highlights the importance of in situ compositional analyses (including S and Pb isotopes and trace elements) of ore sulfides as tools for constraining the origin and chemical evolution of ore fluids.
Textural and chemical evolution of magnetite from the Paleozoic Shuanglong Fe-Cu deposit: Implications for tracing ore-forming fluids
https://doi.org/10.2138/am-2022-8400
Zhang et al. identified three types of magnetite (MA, MB, and MC) in the Shuanglong Fe-Cu deposit, modified by the mushketovitization, as well as the dissolution and reprecipitation. Scheelite and wolframite inclusions in MA-type magnetite formed by the W expulsion during mushketovitization. Textural and chemical information reveals that the synchronous oscillatory zoning in magnetite and coexisting chlorite was caused by the variation of fluid temperature.
Jingwenite-(Y) from the Yushui Cu deposit, South China: The first occurrence of a V-HREE-bearing silicate mineral
https://doi.org/10.2138/am-2022-8373
Liu et al. report the occurrence of jingwenite-(Y), the first V-HREE-bearing silicate mineral discovered in nature, and its structure represents a new structural type of nesosilicate and can be classified as a new mineral group.
Wenjiite, Ti10(Si,P,□)7, and kangjinlaite, Ti11(Si,P)10, new minerals in the ternary Ti-P-Si system from the Luobusa ophiolite, Tibet, China
https://doi.org/10.2138/am-2022-8226
Xiong et al. demonstrate that wenjiite and kangjinlaite are more than just another pair of new minerals. Wenjiite has implications for understanding polymorphism in intermetallic compounds. It is isostructural with over 175 compounds, including two other minerals, but this structure type is only one of four principal types reported for the 5:3 stoichiometry. Kangjinlaite has not been previously reported either in nature or as a product of synthesis. Discovering and characterizing kangjinlaite and wenjiite was only possible thanks to the recently developed 3-dimensional electron diffraction technique. The new technique enables the collection of in situ structural data suitable for ab-initio solution and refinement of crystal structures from single grains of less than 1 micrometer in size.
Evaluating the physicochemical conditions for gold occurrences in pyrite
https://doi.org/10.2138/am-2022-8207
Gold extraction from other minerals is often of great interest to the mining industry. The incorporation mechanism of gold in the host mineral is a chief factor affecting the extraction of gold. For a long time, various incorporation mechanisms have been recognized through spectroscopy, but with little theoretical consideration. In this study, He et al. consider such incorporation mechanisms from an energetic perspective; they propose a theoretical framework investigating the occurrence of elements in mineral structures and employing DFT calculations to examine the occurrence of gold in pyrite.
Volume 107 : December 2022 Issue
Oxidation of arcs and mantle wedges: It’s not all about iron and water
https://doi.org/10.2138/am-2022-8802
Callum Hetherington wrote a commentary on Song et al. (2022) Oxidation of arcs and mantle wedges by reduction of manganese in pelagic sediments during seafloor subduction. American Mineralogist, 107, 1850-1857.
Paragenesis of Li minerals in the Nanyangshan rare- metal pegmatite, Northern China: Toward a generalized sequence of Li crystallization in Li-Cs-Ta-type granitic pegmatites
https://doi.org/10.2138/am-2022-8285
Yang et al. describe a generalized crystallization sequence of lithium minerals in LCT-type pegmatites. As the typical and the largest one of hundreds of LCT pegmatite dikes in eastern Qinling orogenic district, North China, the Nanyangshan pegmatite is strongly Li-mineralized with a clear series of lithium minerals, including spodumene, montebrasite, lithiophilite, elbaite, lepidolite, and possible former petalite. They discuss the potential factors (mostly volatile activity) controlling the stability of lithium minerals under conditions of pegmatite crystallization. They collect Li-mineral data of 58 Li-Cs-Ta pegmatites worldwide, which are used to infer a general consensus of successive evolution of Li minerals with crystallization of LCT pegmatites.
The new mineral tomiolloite, Al12(Te4+O3)5[(SO3)0.5(SO4)0.5] (OH)24: A unique microporous tellurite structure
https://doi.org/10.2138/am-2022-8368
Missen et al. describe a new mineral, tomiolloite, which has a unique structure and chemical composition. It was found in the oxidation zone of a tellurium-gold mine near Moctezuma, Sonora, Mexico. Tomiolloite has a microporous structure containing channels. The chemical composition of tomiolloite includes aluminium cations and four anions: the tellurite anion, the sulfate and sulfite anions (unusual to find in the same structure), and hydroxide anions.
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