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American Mineralogist, Volume 106, pp 1596-1605; https://doi.org/10.2138/am-2021-7784
Low-temperature omphacite has peculiar microstructures called “antiphase domains (APDs),” which can be formed via phase transition from disordered C2/c to ordered P2/n structure during cooling. Hence morphological analyses of the APDs of undeformed omphacite have a potential to unravel the temperature-time (T-t) histories of the eclogite. We investigated five omphacite inclusions in a euhedral garnet porphyroblast obtained from low-temperature eclogite in Syros. The garnet (~6 mm in size) exhibits a distinct prograde chemical zoning and contains abundant mineral inclusions. Transmission electron microscope (TEM) observations of the focused ion beam (FIB) foils confirmed a heterogeneous distribution of equiaxed APDs (10–280 nm in diameter) and columnar APDs. Size distributions of the equiaxed APDs are characterized by kurtosis values of –0.45–3.91, which are larger than those in the matrix omphacite. The columnar APDs are subdivided into two types: dislocation-related (Type I) and inclusion–host interfacial (Type II). The presence of Type I APDs may suggest the inclusions were deformed prior to the host garnet growth. In contrast, Type II APDs, which are characterized by a bundle of stripe-like APDs (~40 nm in width) aligned perpendicular to the host garnet, imply the simultaneous growth of omphacite and garnet in a non-deformation state. The presence of these two contrasting APDs of omphacite inclusions in the single prograde-zoned garnet prevents a simple application of geospeedometry based on APD sizes. Nevertheless, our observations demonstrate that APDs are keys to understanding thermodynamic equilibrium states and the mineral growth kinetics during eclogitization.
American Mineralogist, Volume 106, pp 1690-1703; https://doi.org/10.2138/am-2021-7779
Secondary hydrothermal reworking of REEs has been widely documented in carbonatites/alkaline rocks, but its potential role in the REE mineralization associated with these rocks is currently poorly understood. This study conducted a combined textural and in situ chemical investigation on the REE mineralization in the ~430 Ma Miaoya carbonatite-syenite complex, central China. Our study shows that the REE mineralization, dated at ~220 Ma, is characterized by a close association of REE minerals (monazite and/or bastnäsite) with pervasive carbonatization overprinting the carbonatites and syenites. In these carbonatites and syenites, both the apatite and calcite, which are the dominant magmatic REE-bearing minerals, exhibit complicated internal textures that are generally composed of BSE-bright and BSE-dark domains. Under BSE imaging, the former domains are homogeneous and free of pores or mineral inclusions, whereas the latter have a high porosity and inclusions of monazite and/or bastnäsite. In situ chemical analyses show that the BSE-dark domains of the apatite and calcite have light REE concentrations and (La/Yb)N values much lower than the BSE-bright areas. These features are similar to those observed in metasomatized apatite from mineral-fluid reaction experiments, thus indicating that the BSE-dark domains formed from primary precursors (i.e., represented by the BSE-bright domains) through a fluid-aided, dissolution-reprecipitation process during which the primary light REEs are hydrothermally remobilized. New, in situ Sr-Nd isotopic results of apatite and various REE minerals, in combination with mass balance calculations, strongly suggest that the remobilized REEs are responsible for the subsequent hydrothermal REE mineralization in the Miaoya complex. Investigations of fluid inclusions show that the fluids responsible for the REE mobilization and mineralization are CO2-rich, with medium temperatures (227–340 °C) and low salinities (1.42–8.82 wt‰). Such a feature, in combination with C-O isotopic data, indicates that the causative fluids are likely co-genetic with fluids from coeval orogenic Au-Ag deposits (220–200 Ma) in the same tectonic unit. Our new findings provide strong evidence that the late hydrothermal upgrading of early cumulated REEs under certain conditions could also be an important tool for REE mineralization in carbonatites, particularly for those present in convergent belts where faults (facilitating fluid migration) and hydrothermal fluids are extensively developed.
American Mineralogist, Volume 106, pp 1668-1678; https://doi.org/10.2138/am-2021-7686
The intrinsic anharmonicity plays an important role in the thermodynamic properties of minerals at the high-temperature conditions of the mantle. To investigate the effect of iron on the thermodynamic properties of olivine, the most abundant mineral in the upper mantle, we collected in situ high-temperature and high-pressure Raman spectra of natural Fo89Fa11 and synthetic Fo58Fa42 samples. Fo58Fa42 dissociates to enstatite + quartz + Fe2O3(+Fe) at 893 K. All the Raman-active modes systematically shift to lower frequencies at elevated temperatures, whereas to higher frequencies with increasing pressure. The Ag mode at ~960 cm–1 is more sensitive to the variations of temperature and pressure than other internal modes. The crystal-field splitting of the vibrational energy states becomes slightly weakened at high temperatures but strengthened at elevated pressures. We calculated the isobaric (γiP) and isothermal (γiT) mode Grüneisen parameters for these olivine samples. The intrinsic anharmonic parameters (ai) are negative for both the lattice and internal vibrations, and our calculations indicate that the intrinsic anharmonicity makes positive contributions to the thermodynamic properties of olivine at high temperatures, such as the internal energy (U), heat capacities (CV and CP), and entropy (S). Iron incorporation further increases the magnitudes of these anharmonic contributions. In addition, the Fe effect on the intrinsic anharmonicity may also apply to other thermodynamic properties in olivine, such as equations of state and equilibrium isotopic fractionations, which are important in constraining physical and chemical properties of the upper mantle.
American Mineralogist, Volume 106, pp 1622-1639; https://doi.org/10.2138/am-2021-7711
Natural graphite, a polygenic mineral, is a product of regional, contact, impact metamorphism, and magmatic or fluid deposition. In fluid-deposited graphite, aqueous C-O-H systems play a special role in determining the characteristics of hydrothermal products by shifting the chemical equilibrium. From this viewpoint, the recently discovered carboniferous mineralization in the Baikal hydrothermalites has attracted increasing interest with regard to graphite crystallization under the influence of low-pressure low-temperature (LPLT) carboniferous H2O-rich fluids. Herein, we studied graphite mineralization in the geyserites and travertines of the Baikal geyserite paleovalley (Eastern Siberia, Russia) by applying a multitude of mineralogical studies. Optical, scanning, transmission electron, and atomic force microscopy, energy-dispersive spectroscopy, Raman spectroscopy, and carbon isotopic composition analyses of graphite, carbonate carbon, and oxygen in both the hydrothermalites and host rocks were conducted. The obtained results revealed several peculiar features regarding the graphite in geyserites and travertines. We found that Baikal graphite, earlier predicted to be a product of hydrothermalites, generally occurs as a relict graphite of the host metamorphic rocks with partial in situ redeposition. The newly formed LPLT fluid-deposited graphite is characterized by micrometer- and submicrometer-sized idiomorphic crystallites overgrown on the relict metamorphic graphite seeds and between calcite sinter zones during the last stage of travertine formation. The results present additional valuable data for understanding the mechanism, range of the formation conditions, and typomorphism of fluid-deposited graphite with probable crystallization from carbon solution in the C-O-H system at LPLT conditions.
American Mineralogist; https://doi.org/10.2138/am-2022-8039
American Mineralogist; https://doi.org/10.2138/am-2021-7951
American Mineralogist; https://doi.org/10.2138/am-2022-8214ccby
American Mineralogist; https://doi.org/10.2138/am-2022-8213
American Mineralogist; https://doi.org/10.2138/am-2021-7752
American Mineralogist; https://doi.org/10.2138/am-2022-7989
American Mineralogist; https://doi.org/10.2138/am-2022-8057
American Mineralogist; https://doi.org/10.2138/am-2022-8174
American Mineralogist; https://doi.org/10.2138/am-2022-8141
American Mineralogist; https://doi.org/10.2138/am-2021-7802
American Mineralogist, Volume 106, pp 1580-1585; https://doi.org/10.2138/am-2021-7719
The crystal structure of dixenite, ideally Cu+Fe3+Mn142+(As5+O4)(As3+O3)5(SiO4)2(OH)6, from Långban, Sweden, was refined to an R1-index of 1.58%, and the structure proposed by Araki and Moore (1981) was confirmed and details elucidated. The structure, crystallizing in space group R3 with a = 8.2204(3) and c = 37.485(3) Å, consists of layers of (Mn2+,Fe3+)(O,OH)6 octahedra linked by (As5+O4) and (SiO4) tetrahedra, (As3+O3) trigonal pyramids, and (CU+As43+) tetrahedra. There are five distinct layers in the repeat unit of the cell, four of which are very similar to the layers in mcgovernite. An unusual aspect of one of the trimers of octahedra is that there is a triangular-prismatic hole through the center of the cluster. The (CU+As43+) tetrahedra are parts of larger clusters: [Cu+(As3+O3)4] in which four (As3+O3) groups link to a central Cu+ that occupies the positions normally taken by the stereoactive lone-pairs of electrons that generally characterize As3+ in triangular-pyramidal coordination by O. Thus, the stereoactive lone-pair behavior that is characteristic of (As3+O3) trigonal pyramids is suppressed by the coordination of Cu+ by four As3+ ions.
American Mineralogist; https://doi.org/10.2138/am-2022-8034
American Mineralogist; https://doi.org/10.2138/am-2021-7665
American Mineralogist; https://doi.org/10.2138/am-2022-8281
American Mineralogist, Volume 106, pp 1654-1667; https://doi.org/10.2138/am-2021-7674
Native Au-Ag alloys (electrum) are the predominant precious metal host in Au-bearing volcanogenic massive sulfide (VMS) deposits. The chemical composition and distribution of electrum records crystal growth and post-crystallization processes. In this study, we present detailed textural and compositional data of electrum from the Ming (Canada) and Boliden (Sweden) Au-bearing VMS deposits. Electron probe micro-analyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of electrum enable characterization of chemical zoning in heterogeneous electrum grains. Electrum from Ming exhibits Ag-rich cores, in gradational contact with an outer Au-rich transition zone also enriched in S, Fe, Cu, Zn, and Pb, which is in sharp contact with Ag-rich rims. The textural observations, coupled with in situ LA-ICP-MS data, highlight that the electrum zoning arises from a complex interaction between fluid facilitated solid-state diffusion (SSD) within the grain and coupled dissolution and reprecipitation (CDR) reactions at the grain interface, in response to changing fluid composition and extrinsic parameters, such as temperature, pH, and redox state at Ming. Electrum from Boliden, in contrast, shows an Au-rich core in contact with a gradually increasing Ag-rich rim enriched in Se, Bi, Sb, Te, Sn, S, and Zn, which indicates the formation by fluid facilitated SSD reactions. The different local re-equilibrium caused by SSD from two deposits are attributed to different transport ligands and effects of physicochemical parameters of fluids (e.g., pH and fO2), resulting in different compositional zoning patterns within the electrum. The long-lived metamorphic events that affected the occurrence and compositions of electrum at both VMS deposits, probably provided the elevated temperature and deformation to allow pervasive fluids to remobilize trace metals in electrum, which resulted in the complex chemical zoning in electrum. This study provides insights from in situ, textural and chemical analyses to understand the formation of complex chemical zoning in electrum in metamorphosed VMS deposits.
American Mineralogist, Volume 106, pp 1679-1689; https://doi.org/10.2138/am-2021-7786
Mixing of cogenetic magmas represents an important process in granite petrogenesis but is difficult to identify and is consequently often overlooked due to the absence of obvious isotopic distinctions between the mixed melts. We have conducted in situ elemental and O isotope analyses on apatite from Cretaceous Zhangzhou calc-alkaline granite in southeast China. We integrated these data with micro-analyses on other minerals (plagioclase, zircon, and titanite) as well as whole-rock geochemistry to decipher the mixing history of this granitic complex. The apatite occurs as an early crystallizing phase forming inclusions in biotite, plagioclase, and titanite, and is characterized by core-rim zonation textures with a dark core and bright rims in backscattered images. The core domains have remarkably higher SO3 and Li concentrations but much lower SiO2, REE, and Y concentrations than the rim domains. However, both the cores and rims show geochemical compositions similar to that from typical I-type granite and also have mantle-like O isotope compositions (the core has δ18O = 5.3–6.8‰ and the rim has δ18O = 5.2–6.4‰, respectively), indicating crystallization from granitic melts derived from newly accreted crust. The combined major and trace element and O isotope compositions of apatite and whole-rock geochemistry suggest that compositional evolution of the Zhangzhou granite involved mixing between two cogenetic magma batches, with variable degrees of subsequent differentiation. Batch I magma was a low-SiO2 and high-SO3 melt, whereas Batch II magma was a high-SiO2 and low-SO3 melt that experienced devolatilization. The high-S content in apatite cores further suggests the parental magma of the Zhangzhou granite likely originated from a sulfur-rich source comprising mainly newly accreted arc crust in response to subduction of the paleo-Pacific Ocean. The geochemical records of these magmatic processes are rarely observed in coeval zircon, titanite, and plagioclase. Our study, therefore, demonstrates that apatite geochemistry is potentially a more suitable monitor of complex magmatic evolution, including devolatilization and mixing of isotopically indistinguishable magmas.
American Mineralogist; https://doi.org/10.2138/am-2022-8100
American Mineralogist; https://doi.org/10.2138/am-2021-7919
American Mineralogist; https://doi.org/10.2138/am-2022-7996
American Mineralogist, Volume 106, pp 1537-1543; https://doi.org/10.2138/am-2021-nmn106921
In this issue This New Mineral Names has entries for 11 new species, including bohuslavite, fanfaniite, ferrierite-NH4, feynmanite, hjalmarite, kenngottite, potassic-richterite, rockbridgeite-group minerals (ferrirockbridgeite and ferrorockbridgeite), rudabányaite, and strontioperloffite.
American Mineralogist; https://doi.org/10.2138/am-2022-8105
American Mineralogist, Volume 106, pp 1420-1437; https://doi.org/10.2138/am-2021-7680
Xenocrysts and xenoliths in Upper Cretaceous pyroclastics on Mount Carmel (northern Israel) represent a series of similar magma-fluid systems at different stages of their evolution, recording a continuous decrease in oxygen fugacity (fO2) as crystallization proceeded. Corundum coexisting with Fe-Mg-Cr-Al spinels, other Fe-Mg-Al-Na oxides, and Fe-Ni alloys in apparent cumulates crystallized at fO2 values near the iron-wüstite (IW) buffer (fO2 = IW±1) and is zoned from high-Cr cores to lower-Cr rims, consistent with fractional crystallization trends. The reconstructed parental melts of the cumulates are Al-Cr-Fe-Mg oxides with ca. 2 wt% SiO2. Corundum in other possible cumulates that contain Cr-Fe (Fe 45 wt%) alloys has low-Cr cores and still lower-Cr rims. Corundum coexisting with Cr0 (fO2 = IW-5) in some possible cumulates has low-Cr cores, but high-Cr rims (to >30% Cr2O3). These changes in zoning patterns reflect the strong decrease in the melting point of Cr2O3, relative to Al2O3, with decreasing fO2. The electron energy loss spectroscopy (EELS) analyses show that all Cr in corundum that coexists with Cr0 is present as Cr3+. This suggests that late in the evolution of these reduced melts, Cr2+ has disproportionated via the reaction 3Cr2+(melt) → 2Cr3+(Crn) + Cr0. The most Cr-rich corundum crystallized together with β-alumina phases including NaAl11O17 (diaoyudaoite) and KAl11O17 (kahlenbergite) and β″-alumina phases; residual melts crystallized a range of (K,Mg)2(Al,Cr)10O17 phases with the kahlenbergite structure. The parental melts of these assemblages appear to have been Al-Cr-K-Na-Mg oxides, which may be related to the Al-Cr-Fe-Mg oxide melts mentioned above, through fractional crystallization or liquid immiscibility. These samples are less reduced (fO2 from IW to IW-5) than the assemblages of the trapped silicate melts in the more abundant xenoliths of corundum aggregates (fO2 = IW-6 to IW-10). They could be considered to represent an earlier stage in the fO2 evolution of an “ideal” Mt. Carmel magmatic system, in which mafic or syenitic magmas were fluxed by mantle-derived CH4+H2 fluids. This is a newly recognized step in the evolution of the Mt. Carmel assemblages and helps to understand element partitioning under highly reducing conditions.
American Mineralogist, Volume 106, pp 1503-1519; https://doi.org/10.2138/am-2021-7499
The genesis of the Dabaoshan stratabound base metal deposit has remained in dispute since its discovery. Scheelite is commonly present in both the Cu-S orebody and adjacent porphyry-style Mo-W mineralization and can provide insights into the hydrothermal history. In the stratabound Cu-S orebodies, there are three stages of mineralization: early-stage, Cu-(W), and late-stage. Two types of scheelite (here referred to as SchA and SchB) are identified in the Cu-(W) mineralization stage. SchA is anhedral and disseminated in massive sulfide ores. It coexists with chalcopyrite and replaces the preexisting arsenic-bearing pyrite. SchA exhibits chaotic cathodoluminescent (CL) textures and contains abundant mineral inclusions, including pyrite, chalcopyrite, arsenopyrite, uraninite, and minor REE-bearing minerals. Chemically, SchA displays middle REE (MREE)-enriched patterns with negative Eu anomalies. SchB occurs in veins crosscutting the stratabound orebodies and shows patchy textures in CL images. Based on CL texture, SchB is subdivided into SchB1 and SchB2. SchB1 is CL-dark and occasionally shows oscillatory zoning, whereas SchB2 is CL-bright and relatively homogeneous. Chemically, SchB1 has a high-U content (mean = 552 ppm) and REE patterns varying from MREE-enriched to MREE-depleted. In contrast, SchB2 is depleted in U (mean = 2.5 ppm) and has MREE-enriched patterns. Compared with SchB, SchA is significantly enriched in Ba. Scheelite in the stratabound orebodies has similar Y/Ho ratios and trace-element characteristics as Sch3 in the Dabaoshan porphyry system. In situ U-Pb dating of hydrothermal apatite, collected from the Sch3-bearing veins in the footwall of stratabound orebodies, yielded a mineralization age of 160.8 ± 1.1 Ma. Zircon from the Dabaoshan granite porphyry yielded a U-Pb age of 161.8 ± 1.0 Ma. These two ages are consistent within uncertainty, suggesting that the ore-forming fluid responsible for tungsten mineralization in the stratabound orebodies was derived from the porphyry system. When fluid emanating from the deep porphyry system encountered the overlying Lower Qiziqiao Formation and stratabound orebodies, replacement reactions resulted in dramatic variations in physiochemical conditions (e.g., decrease in fO2, increase in Ca/Fe, As, Ba). During this process, U6+ was reduced to U4+, and As and Ba were leached out of the preexisting pyrite and host rock. Fluid-rock interactions triggered a rapid discharge of fluids, forming SchA with chaotic CL textures and abundant inclusions, but uniform REE patterns. SchB (characterized by patches with different chemical characteristics) may have been formed from repeated injection of ascending fluids into fractures crosscutting the preexisting massive sulfide ores. We propose that Jurassic porphyry Mo-W mineralization contributed to tungsten mineralization in the stratabound orebodies. Considering that Cu and W mineralization are genetically related, not only in the footwall but also in the stratabound orebodies, we infer that Cu in the stratabound orebodies was locally sourced from the Jurassic porphyry mineralization.
American Mineralogist, Volume 106, pp 1470-1479; https://doi.org/10.2138/am-2021-7554
The shock stages of 14 L6 ordinary chondrites are estimated using the random X-ray diffraction patterns of polished thin section samples and the in-plane rotation method. The mean lattice strains and grain size factors for olivine and orthopyroxene are determined from the analyses based on the Williamson–Hall plots, which depict the tangent Bragg angle and integral breadth β. The lattice strain in olivine, eOl, is distributed from ~0.05% to ~0.25%, while that in orthopyroxene, eOpx, is distributed from ~0.1 to ~0.4%, where we selected the isolated peaks of olivine and orthopyroxene. The olivine peaks have Miller indices of (130), (211), (222), and (322), while the orthopyroxene peaks have Miller indices of (610), (511), (421), (631), and (12.1.2). The intercept for integral breadth β0O1 and β0Opx for the Williamson–Hall plots correlates with the grain size of the constituent minerals. The grain size is proportional to the inverse of β0 since the β intercept increases with the shock stage. Introducing a new parameter, –ε/log β0 for olivine (0.04–0.16) and orthopyroxene (0.07–0.32) reveals a clear relationship between them: –εOpx/log β0Opx = –0.01+ 2.0 (–εOl/log β0O1) (R > 0.9). In addition, the isolated peak of plagioclase (201) systematically changes as the shock stage increases, suggesting the progress of amorphization (maskelynitization). Another parameter, (I/FWHM)Pl(201) reveals additional relationships: –εOl/log β0O1 = 0.14(±0.01) − 5.2 × 10−5 (±5.7 × 10−6) × (I/FWHM)Pl(201), and –εOpx/log β0Opx = 0.25(±0.04) − 8.9 × 10−5 (±2.6 × 10−5) × 10–5 × (I/FWHM)Pl(201). These three parameters systematically change with the shock stage, suggesting that they are suitable shock barometers. The present method is useful to evaluate the shock stage of L6 chondrites and potentially enables quantitative shock stage classification for stony meteorites.
American Mineralogist, Volume 106, pp 1453-1469; https://doi.org/10.2138/am-2021-7622
The metamorphic rocks from the Torres del Paine contact aureole (Patagonia, Chile) show field, petrographic, and geochemical evidence for small amounts of igneous fluid infiltration due to the solidification of the granite complex. Hydrogen isotope ratios (D/H) in the contact aureole first decrease while approaching the intrusion and subsequently increase toward the granite contact. Initial decrease with metamorphic grade is due to preferential loss of the 2H isotopes due to Rayleigh fractionation during prograde dehydration reactions. The infiltration of high-δD fluids from the intrusion increases δD within the last 150 m. In contrast, (18O/16O) ratios show no systematic changes, indicating that neither oxygen loss by Rayleigh fractionation nor oxygen exchange by fluid infiltration was significant enough to dominate original variations seen in the oxygen isotope ratio of the protolith. Calculated volume of fluid using the position of the hydrogen isotope exchange front gives a relatively low time-integrated fluid flux of about 4 m3/m2 at the contact for the igneous fluid. These small amounts of fluid flux are in agreement with whole rock oxygen isotope data that are not affected in the contact aureole. Chlorine content of metamorphic biotite crystals, in contrast to oxygen isotopes, supports infiltration of igneous fluids. Indeed, relatively high-Cl concentrations in biotite were measured in some samples close to the intrusion (up to 0.2 wt%), while chlorine concentrations in biotite are constant everywhere else in the entire contact aureole, having low concentrations (0.01–0.06 wt%). The absence of a well-marked Rayleigh fractionation trend in Cl concentrations with increasing metamorphism is surprising since chlorine strongly fractionates into the fluid. This is best explained by slow diffusive exchange of chlorine in biotite in the cooler outer aureole. Hence recrystallization of biotite would be required to modify its Cl composition. Biotite grains from samples close to the intrusion with high-Cl content also have lower Ti content (0.4 pfu) than biotite (0.5 pfu) from other samples containing biotite with lower Cl content located at the same distance from the contact. Since Ti content in biotite is a function of temperature, this is a good indication that magmatic fluid infiltration started post peak, early during cooling of the metamorphic rocks. Episodes of fluid flow appear to have been nearly continuous during cooling as evidenced by numerous retrogression textures, such as secondary muscovite (above 470 °C) or chlorite + muscovite intergrowth after cordierite or biotite (slightly below 470 °C). This might be related to crystallization of subsequent batches of granites or the onset of minor fluid convection during cooling of the aureole. Nevertheless, only minor secondary muscovite has been found, and fresh cordierite is present throughout the aureole confirming small amounts of fluid infiltration. The time-integrated fluid flux computed from the hydrogen isotope exchange front is two orders of magnitude lower than values computed for metacarbonate in many other contact aureoles, suggesting low permeabilities of pelitic rocks. In conclusion, Cl contents and hydrogen isotope compositions of hydrous minerals provide a sensitive tool to identify small fluid-rock interaction events, much more sensitive than oxygen isotope compositions of the whole rock or minerals.
American Mineralogist, Volume 106, pp 1480-1487; https://doi.org/10.2138/am-2021-7455
A new indentation-based method was developed that will impact and facilitate the elastic property measurements of rocks and minerals, especially those possessing unusual deformation behavior, including brittle materials and those with complex architectures. The novel feature employed is a metallic film that uniformly transfers the load from the indenter tip to the sample. The film also absorbs the damage caused by the penetrating indenter, shielding the material from highly localized deformation that can impact its response to loading. Many geologically relevant materials have resisted traditional indentation testing because they are either brittle in nature or possess highly anisotropic architectures, such as layered or lamellar structures. In both cases, the highly localized deformation from direct indentation significantly affects the indenter unloading stiffness, from which the elastic properties are determined. The indirect indentation method developed here has demonstrated accurate determination of the elastic properties of many common geological materials as well as materials that have resisted elastic characterization such as galena and talc.
American Mineralogist, Volume 106, pp 1520-1529; https://doi.org/10.2138/am-2021-7766
Schreibersite, (Fe,Ni)3P, the most abundant cosmic phosphide, is a principal carrier of phosphorus in the natural Fe-Ni-P system and a likely precursor for prebiotic organophosphorus compounds at the early stages of Earth’s evolution. The crystal structure of the mineral contains three metal sites allowing for unrestricted substitution of Fe for Ni. The distribution of these elements across the structure could serve as a tracer of crystallization conditions of schreibersite and its parent celestial bodies. However, discrimination between Fe (Z = 26) and Ni (Z = 28) based on the conventional X-ray structural analysis was for a long time hampered due to the proximity of their atomic scattering factors. We herein show that this problem has been overcome with the implementation of area detectors in the practice of X-ray diffraction. We report on previously unknown site-specific substitution trends in schreibersite structure. The composition of the studied mineral encompasses a Ni content ranging between 0.03 and 1.54 Ni atoms per formula unit (apfu): the entire Fe-dominant side of the join Fe3P-Ni3P. Of 23 schreibersite crystals studied, 22 comprise magmatic and non-magmatic iron meteorites and main group pallasites. The near end-member mineral (0.03 Ni apfu) comes from the pyrometamorphic rocks of the Hatrurim Basin, Negev desert, Israel. It was found that Fe/Ni substitution in schreibersite follows the same trends in all studied meteorites. The dependencies are nonlinear and can be described by second-order polynomials. However, the substitution over the M2 and M3 sites within the most common range of compositions (0.6 < Ni <1.5 apfu) is well approximated by a linear regression: Ni(M2) = 0.84 × Ni(M3) – 0.30 apfu (standard error 0.04 Ni apfu). The analysis of the obtained results shows a strong divergence between the variation of unit-cell parameters of natural schreibersite and those of synthetic (Fe,Ni)3P. This indicates that Fe/Ni substitution trends in the mineral and its synthetic surrogates are different. A plausible explanation might be related to the differences in the system equilibration time of meteoritic schreibersite (millions of years) and synthetic (Fe,Ni)3P (~100 days). However, regardless of the reason for the observed difference, synthetic (Fe,Ni)3P cannot be considered a structural analog of natural schreibersite, and this has to be taken into account when using synthetic (Fe,Ni)3P as an imitator of schreibersite in reconstructions of natural processes.
American Mineralogist, Volume 106, pp 1388-1419; https://doi.org/10.2138/am-2021-7760
Part V of the evolutionary system of mineralogy explores phases produced by aqueous alteration, metasomatism, and/or thermal metamorphism—relicts of ancient processes that affected virtually all asteroids and that are preserved in the secondary mineralogy of meteorites. We catalog 166 historical natural kinds of minerals that formed by alteration in the parent bodies of chondritic and non-chondritic meteorites within the first 20 Ma of the solar system. Secondary processes saw a dramatic increase in the chemical and structural diversity of minerals. These phases incorporate 41 different mineral-forming elements, including the earliest known appearances of species with essential Co, Ge, As, Nb, Ag, Sn, Te, Au, Hg, Pb, and Bi. Among the varied secondary meteorite minerals are the earliest known examples of halides, arsenides, tellurides, sulfates, carbonates, hydroxides, and a wide range of phyllosilicates.
American Mineralogist, Volume 106, pp 1488-1502; https://doi.org/10.2138/am-2021-7678
The Helukou deposit, with proven reserves of 33 752 t WO3, is one of the newly exploited medium-scale tungsten (W) deposits in the Guposhan ore field, Nanling Range of South China. Skarn-type and less abundant altered granite-type tungsten orebodies were identified in this deposit. The ore mineralization in this district was a product of two-stage magmatism, as shown by LA-ICP-MS U-Pb dating of zircons and Re-Os dating of molybdenite. The former yielded U-Pb ages of 184.0 ± 3.6 Ma (MSWD = 0.15) and 163.8 ± 1.5 Ma (MSWD = 0.41) for fine-grained biotite granite and muscovite granite, respectively, as well as a U-Pb age of 181.5 ± 2.1 Ma (MSWD = 0.75) for zircon grains from altered granite-type tungsten ore. The latter yielded molybdenite Re-Os ages of 183.5 ± 2.8 Ma (without MSWD owing to a limited number of samples) and 163.4 ± 2.8 Ma (MSWD = 0.71) for altered granite-type and skarn-type tungsten deposits, respectively. Thus, two separate tungsten mineralization events occurred during the Early Jurassic and Middle Jurassic. Trace-element compositions suggest that scheelite I was controlled by the coupled substitution reactions of 2Ca2+ = Na+ + REE3+ and Ca2+ + W6+ = Nb5+ + REE3+, whereas scheelite II was controlled by the coupled reactions of 2Ca2+ = Na+ + REE3+ and 3Ca2+ = ☐Ca + 2REE3+ (where ☐ is a site vacancy). High Mo and low Ce contents suggest that both scheelite I and scheelite II were precipitated from oxidizing magmatic-hydrothermal fluids. Based on the mineral assemblage of the altered granite-type ores and geochemical characteristics of scheelite I [i.e., negative Eu anomalies (0.02–0.05; mean = 0.03 and STD = 0.01), and high 87Sr/86Sr ratios (0.70939–0.71932; mean = 0.71345 and STD = 0.00245)], we infer that fluid-rock interaction played an important role in modifying Early Jurassic ore-forming fluids. Scheelite II exhibits a geo-chemical composition [i.e., 87Sr/86Sr ratios (0.70277–0.71471; mean = 0.70940 and STD = 0.00190), Eu anomalies (0.14–0.55; mean = 0.26 and STD = 0.09), and Y/Ho ratios (16.1–33.7; mean = 27.9 and STD = 2.91)] similar to that of the Middle Jurassic Guposhan granites, suggesting inheritance of these features from granite-related magmatic-hydrothermal fluids. These results provide new insights into the two-stage magmatic and metallogenic history of the Nanling Range during the Jurassic Period.
American Mineralogist, Volume 106, pp 1530-1533; https://doi.org/10.2138/am-2021-7928
A continuously increasing number of research groups are adopting elastic geobarometry for retrieving pressures and temperatures of entrapment of inclusions into a host from both natural and experimental samples. However, a few misconceptions of some of the general concepts underlying elastic geobarometry are still widespread. One is the difference between various approaches to retrieve the residual pressures and residual strains from Raman measurements of inclusions. In this paper, the estimation of uncertainties and the validity of some general assumptions behind these methods are discussed in detail, and we provide general guidelines on how to deal with inclusion strain, measurements, inclusion pressure, and their uncertainties.
American Mineralogist, Volume 106, pp 1534-1535; https://doi.org/10.2138/am-2021-7865
Cuadros et al. (2019) used a wide range of data from dioctahedral and trioctahedral Fe3+-bearing, 2:1 phyllosilicates to propose a model describing how tetrahedral occupancy by Fe3+ takes place in both dioctahedral and trioctahedral 2:1 phyllosilicates. The partition coefficient approach (Decarreau and Petit 2014) focusing on the distribution of Al3+ and Fe3+ between octahedral and tetrahedral sites of dioctahedral smectites has been disregarded in the study of Cuadros et al. (2019). This approach was applied here on the set of data from Cuadros et al. (2019). The partition coefficient value linked to the distribution of Al3+ and Fe3+ between octahedral and tetrahedral sites determined from natural and synthetic dioctahedral smectites applies well to trioctahedral phyllosilicates too. Data from synthetic iron-rich 2:1 smectites also fit well with both Cuadros et al. (2019) and Decarreau and Petit (2014) models.
American Mineralogist; https://doi.org/10.2138/am-2022-7958
American Mineralogist; https://doi.org/10.2138/am-2022-8167
American Mineralogist; https://doi.org/10.2138/am-2021-7682
American Mineralogist; https://doi.org/10.2138/am-2022-7959
American Mineralogist; https://doi.org/10.2138/am-2022-7969
American Mineralogist, Volume 106, pp 1544-1544; https://doi.org/10.2138/am-2021-b106922
American Mineralogist; https://doi.org/10.2138/am-2021-7924
American Mineralogist; https://doi.org/10.2138/am-2022-8112
American Mineralogist; https://doi.org/10.2138/am-2021-7937
American Mineralogist; https://doi.org/10.2138/am-2022-8027
American Mineralogist; https://doi.org/10.2138/am-2021-7874
American Mineralogist; https://doi.org/10.2138/am-2022-7971
American Mineralogist; https://doi.org/10.2138/am-2022-8058
American Mineralogist; https://doi.org/10.2138/am-2022-8121
American Mineralogist; https://doi.org/10.2138/am-2022-7861