Editor’s Notes
Total Results: 1697
Volume 107 : December 2022 Issue
Authigenic anatase nanoparticles as a proxy for sedimentary environment and porewater pH
https://doi.org/10.2138/am-2022-8330
Titanium has long been considered to be immobile during weathering and diagenetic processes, and it is widely used for normalization of elemental concentrations in weathering profiles. However, Hong et al. demonstrate that authigenic titania is commonly formed in fine-grained siliciclastics of a wide variety of facies through weathering of Ti-bearing silicates, authigenic euhedral anatase is present ubiquitously as nanoparticles, and its morphology varies in a predictable manner over a range of depositional environments. The crystal habit of authigenic anatase nanoparticles is controlled primarily by porewater pH during early diagenetic alteration of Ti-bearing silicates. Their findings suggest that authigenic anatase could be a sensitive proxy for the sedimentary environment and sedimentary porewater chemistry and will likely prove useful in depositional facies analysis.
Color effects of Cu nanoparticles in Cu-bearing plagioclase feldspars
https://doi.org/10.2138/am-2022-8325
This study by Jin et al. bridges the gap between colloidal optics in material science and the color effects in minerals and gemstones. The different colors and pleochroism in Cu-bearing feldspars are explained quantitatively for the first time. The result also has implications for understanding the crystal chemistry of feldspars, as well as the igneous processes that create them, by providing a way to quantify the diffusion of Cu in feldspars through optical spectroscopy. It also inspires designing and engineering novel optical materials.
Expanding the speciation of terrestrial molybdenum: Discovery of polekhovskyite, MoNiP2, and insights into the sources of Mo-phosphides in the Dead Sea Transform area
https://doi.org/10.2138/am-2022-8261
Britvin et al. describe a new mineral, polekhovskyite, MoNiP2, the first terrestrial Mo phosphide and a phosphorus-rich homologue of meteoritic monipite, MoNiP. Polekhovskyite represents a novel terrestrial Mo speciation. Having 44 wt% Mo in its composition, the mineral is a striking example of selective Mo enrichment. The origin of Mo in phosphides of the Southern Levant is likely related to the processes of the Dead Sea Rift formation.
Sound speed and refractive index of amorphous CaSiO3 upon pressure cycling to 40 GPa
https://doi.org/10.2138/am-2022-8081
Geballe et al. investigate amorphous silicate, CaSiO3, that is created through several different pressure-temperature routes. The samples exhibit highly reproducible properties at room temperature and at pressures up to 40 GPa, especially in comparison to other silicates. This suggests that the amorphous solid may mimic the liquid over the pressure range investigated. The basis of this assessment is three complementary data sets: sound-speed measurements, refractive index measurements, and previously published NMR and Raman spectroscopy.
Calorimetric study of skutterudite (CoAs2.92) and heazlewoodite (Ni3S2)
https://doi.org/10.2138/am-2022-8337
Thermodynamic properties of chalcogenide minerals are important for modeling and understanding ore deposits. In particular, nickel and cobalt arsenides, sulfarsenides, and sulfides occur in many hydrothermal ore deposits but their thermodynamic properties of these phases are not well known. Majzlan et al. determined a full set of thermodynamic properties for heazlewoodite and skutterudite using high-temperature oxide-melt solution calorimetry and low-temperature relaxation calorimetry.
Melting phase equilibrium relations in the MgSiO3-SiO2 system under high pressures
https://doi.org/10.2138/am-2022-8004
Moriguti et al. determined the melting relations in the system MgSiO3-SiO2 at 13.5 GPa and up to 2900 °C using the Kawai-type of multi-anvil apparatus. At such extreme temperatures, generated pressure calibration was also carried out. The mass-balance calculations on E-chondrites model indicate that Si content in the core would be between 2.7 to 8.6 wt%, which is within the range of 2 to 9 wt% Si in the core as predicted by metal-silicate element partitioning. Thus, the E-chondrite model could still have high potential to explain the Bulk Earth composition if the Si depletion in the core has worked well through Earth's history.
Effects of hydrostaticity and Mn-substitution on dolomite stability at high pressure
https://doi.org/10.2138/am-2022-8248
Wang et al. collected high-pressure Raman spectra of natural Mg-dolomite CaMg(CO3)2 and Mn-dolomite kutnohorite Ca1.11Mn0.89(CO3)2 samples up to 56 GPa at room temperature in a diamond anvil cell using helium or neon as a pressure-transmitting medium (PTM). Phase transitions in CaMg(CO3)2 were observed at 36.1(25) GPa in helium and 35.2(10) GPa in neon PTM for dolomite-II to -III, respectively. Moreover, the onset pressure of Mn-dolomite Ca1.11Mn0.89(CO3)2-III occurs at 23−25 GPa, about 10 GPa lower than that of Mg-dolomite-III. These results reveal a significant effect of hydrostaticity and cationic radius on the kinetics of the pressure-induced structure transformations in the dolomite group. The results provide new insights into deep carbon carriers within the Earth's mantle.
Crystallization of bastnäsite and burbankite from carbonatite melt in the system La(CO3)F-CaCO3-Na2CO3 at 100 MPa Anna M. Nikolenko, Konstantin M. Stepanov, Vladimir Roddatis, and
https://doi.org/10.2138/am-2022-8064
Nikolenko et al. present the first experimental data on the crystallization of bastnaesite and two other REE carbonates, burbankite and lukechangite, from carbonatitic melt in the model system La(CO3)F-CaCO3-Na2CO3 at 100 MPa and temperatures between 625 and 850 ºC. Liquidus phases in the run products are calcite, nyerereite, Na carbonate, bastnaesite and burbankite solid solution and lukechangite. Addition of 10 wt% Ca3(PO4)2 to a ternary mixture resulted in massive crystallization of La-bearing apatite and monazite, and complete disappearance of bastnäsite and burbankite. Thus, primary magmatic crystallization of REE carbonates and fluorocarbonates from natural carbonatitic melts is unlikely (with a possible exception of the uniquely REE-rich and P2O5-poor carbonatites at Mountain Pass).
Crystal shapes, triglyphs, and twins in minerals: The case of pyrite
https://doi.org/10.2138/am-2022-8280
Arrouvel analyzed pyrite samples using XRD, SEM and EDS coupled with atomistic simulations to study their crystal growth, twinning and anisotropy. The findings include: (1) {120} and {210} pyritohedral pyrite can be distinguished by the orientation of the striations on their surfaces. (2) The striations are linked to kinetic growth with a higher rate. (3) The directions of the striations are compatible to the sulfur network orientated along the 6 <001> directions. (4) The so-called negative striated {120} pyrite crystals are rare specimens that can be encountered in geothermal areas. (5) Striations help identify [uvw]° rotations in merohedral twinning. (6) Simulations confirm that mirror (001) twin and [001]90° iron-cross twinning are energetically the most favorable grain boundaries.
Nanostructure reveals REE mineral crystallization mechanisms in granites from a heavy REE deposit, South China
https://doi.org/10.2138/am-2022-8309
Shi et al. report unusual nanostructure of Ce-poor bastnasite-(La,Nd,Y) from the South China granites, whose weathering crusts form HREE deposits. In-situ SRXD and high-resolution TEM analyses show the REE-mineral grew as disordered nanocrystals, and coaligned, nearly coaligned nanoparticle aggregations. They provide a novel nonclassical crystallization by particle attachment, and further support that the HREE-rich mineral precipitation in the parental granites underwent sudden, rapid crystallization in a highly oxidized environment. This work has practical implications for mineral exploration, as it provides criteria for the identification of similar HREE-rich deposits elsewhere.
Paratobermorite, Ca4(Al0.5Si0.5)2Si4O16(OH)·2H2O·(Ca·3H2O), a new tobermorite-supergroup mineral with a novel topological type of the microporous crystal structure
https://doi.org/10.2138/am-2022-8284
Pekov et al. discovered a new mineral, paratobermorite, ideally Ca4(Al0.5Si0.5)2Si4O16(OH)˙2H2O˙Ca˙3H2O), a member of the tobermorite supergroup, at the Bazhenovskoe chrysotile asbestos deposit, Central Urals, Russia. Paratobermorite significantly differs in the topology of crystal structure [mutual arrangement of (Si,Al)O4 tetrahedra and Ca polyhedra] from tobermorite and other tobermorite-supergroup minerals. Due to the original structure type and the presence of a significant amount of Al which substitutes Si, paratobermorite can be considered a novel microporous material and a perspective cation-exchanger.
Morphological and chemical characterization of secondary carbonates in the Toki granite, central Japan, and the evolution of fluid chemistry
https://doi.org/10.2138/am-2022-8229
Yuguchi et al., in serial works (Yuguchi et al., 2015, 2019, 2021, and this study), provide new insights into the mass transfer due to hydrothermal alterations and groundwater-rock interactions. This study focuses on the petrography and mineral chemistry of calcites in the Toki granite, central Japan. It explores the sequential variations in mass transfer during the sub-solidus stages in granite, which were overlooked or not fully appreciated in earlier studies.
Characteristics and formation of corundum within syenite in the Yushishan rare metal deposits in the northeastern Tibetan Plateau
https://doi.org/10.2138/am-2022-8223
Liu et al. present newly discovered in-situ corundum in syenite and discuss the characteristics, origin, and geological processes of corundum. Abundant mineral inclusions were found and categorized into primary and secondary based on morphological and reacting characteristics. They indicate two geological processes. Trace-element characteristics and oxygen isotopes are also consistent with corundum of magmatic origin. The implication is that corundum crystallized in melts with the involvement of Al-rich and Si-poor crustal material.
Hydrogen solubility in FeSi alloy phases at high pressures and temperatures
https://doi.org/10.2138/am-2022-8295
Light elements alloying with metallic Fe can greatly affect its properties and play a key role in dynamics of planetary cores. H and Si are two candidate light elements in cores. However, the H storage in the Fe-Si system under relevant pressure and temperature is still unclear. Here Fu et al. found dramatic decrease of H content in FeSi alloys when Si is present. Their experiments indicate that H remains in the structure of FeSi alloys when recovered to 1 bar. The unusual property of FeSiHx alloys will help understand important geochemical processes involving hydrogen in future studies.
First evidence of dmisteinbergite (CaAl2Si2O8 polymorph) in high-grade metamorphic rocks
https://doi.org/10.2138/am-2022-8505
Wannhoff et al. present the first finding and characterization of dmisteinbergite inside crystallized anatectic melt inclusions (MI) in garnet from three locations on two continents. These MI were originally entrapped inside a garnet growing or recrystallizing in presence of crustal melts during orogenesis continental collision under high T and highly variable P. The work on dmisteinbergite in anatectic MI, combined with previous results reported in literature, helps (a) to identify more precisely which factors and P-T conditions control the formation of this particular polymorph and (b) to better constrain the crystallization behavior of silicate melts in small pores. Overall, these new findings contribute to shed light on high T minerals which increasingly appear to be a common occurrence in the metamorphic evolution of the continental crust.
NEW MINERAL NAMES
https://doi.org/10.2138/am-2022-NMN1071218
New Mineral Names
Volume 107 : November 2022 Issue
The Zn, S, and Cl isotope compositions of mare basalts: Implications for the effects of eruption style and pressure on volatile element stable isotope fractionation on the Moon
https://doi.org/10.2138/am-2022-8290
The stable isotope compositions of volatile elements are sensitive tracers for the loss of volatiles throughout planetary evolution. Gargano et al. measure three distinct volatile element isotope systems of Zn, S, and Cl in a suite of lunar mare basalts to better understand the causal mechanisms for the Moon's depletion of volatile elements and wide-ranges in volatile element stable isotope compositions. Their data show that the Moon is enriched in the heavy isotopes of Zn, S, and Cl when compared to the Earth — likely resulting from vaporization associated with the Moon-forming Giant Impact. Additionally, they show that the Zn, S, and Cl isotope compositions of lunar low and high-titanium mare basalts are similar despite sampling reservoirs that crystallized early and late, respectively, from the Lunar Magma Ocean. As such, they conclude that the Giant Impact likely dominated the fractionation of these volatile-element stable isotope systems in the lunar interior, with minimal contribution from later mare volcanism.
An ab-initio study on the thermodynamics of disulfide, sulfide, and bisulfide incorporation into apatite and the development of a more comprehensive temperature, pressure, pH, and composition-dependent model for ionic substitution in minerals
https://doi.org/10.2138/am-2022-8250
Recent studies of natural and experimental apatite demonstrate that the oxidation state of S (S6+, S4+, S1– and S2–) in apatite varies systematically as a function of oxygen fugacity (fO2). To establish a theoretical understanding of S in apatite, Kim et al. adopted quantum-mechanical methods to explore the thermodynamics and geometry of incorporation of disulfide (S22–), sulfide (S2–), and bisulfide (HS–) as possible forms of reduced S species in apatite.
Experimental partitioning of fluorine and barium in lamproites
https://doi.org/10.2138/am-2022-8289
The halogens (F, Cl, Br, and I) are elements essential to life and are cycled by exchanges between the Earth's interior and the biosphere, hydrosphere, and atmosphere. Whilst some attention has been paid to the recycling of these elements on the surface of our planet, little is understood about their long-term storage within the Earth and how the exchange between the deep Earth and surface takes place over geological time. Lamproites, ultrapotassic melts, have the highest known F and Ba content of any primary mantle melts, but little is known about the source mineralogy that generates these unusual melts. Ezad and Foley experimentally determined the partitioning of F and Ba between phlogopite mica, a primary halogen-bearing mineral in the mantle, and lamproite melts. Using their partition coefficients, they investigate several mantle lithologies to determine which upon melting can produce lamproitic melts.
Nb and Ta intracrustal differentiation during granulite-facies metamorphism: Evidence from geochemical data of natural rocks and thermodynamic modeling
https://doi.org/10.2138/am-2022-8260
Both continental crust and depleted mantle are characterized by subchondritic Nb/Ta, leading to a mass imbalance compared to the bulk Earth. Even though several potential high Nb/Ta reservoirs in Earth's core and mantle have been proposed, little attention has been given to those in the crust. Huang et al. present bulk-rock and rutile geochemical data for samples of lower crustal pelitic granulite from the North China Craton, which exhibit systematic variations in Nb and Ta contents. High-temperature (HT) and ultra-high temperature (UHT) granulite residues exhibit Nb/Ta ratios that are close to chondritic and subchondritic, respectively; whereas leucosomes from UHT granulites mostly have higher Nb/Ta. These variations are best explained via competition for Nb and Ta between biotite and rutile during metamorphism, although initial bulk-rock Nb/Ta values also have an effect. When significant amounts of biotite remain in the residue, biotite dehydration melting generates a high-Nb/Ta residue and low-Nb/Ta melt because biotite preferentially incorporates Nb over Ta. However, geochemical modeling suggests that once biotite is depleted, the Nb/Ta ratio of the system is instead controlled by rutile growth, which promotes formation of a lower Nb/Ta residue and higher Nb/Ta melt, even though the amount of these melts might be lower. Huang et al. propose that in-situ to in-source leucosome and leucocractic veins in UHT terranes may maintain a high Nb/Ta geochemical signature, whereas residual crustal-derived A-type granite that experienced significant fractionation of Nb- or Ta-bearing minerals or contamination from other low-Nb/Ta sources could not maintain high Nb/Ta ratio, even though these ratios would still be higher than S-type granite.
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