Editor’s Notes
Total Results: 1697
Volume 104 : May 2019 Issue
A new occurrence of yimengite-hawthorneite and crichtonite-group minerals in an orthopyroxenite from kimberlite: Implications for mantle metasomatism
https://doi.org/10.2138/am-2019-6741
Rezvukhin et al. (p. 761) recognized LILE-enriched chromium titanates of the magnetoplumbite (AM12O19) and crichtonite (ABC18T2O38) groups as abundant inclusions in orthopyroxene grains in a mantle-derived xenolith from the Udachnaya-East kimberlite pipe, Daldyn field, Siberian craton. The studied xenolith consists of three parts: an orthopyroxenite, a garnet clinopyroxenite, and a garnet-orthopyroxene intermediate domain between the two. Within the host enstatite (Mg# 92.6) in the orthopyroxenitic part of the sample, titanate inclusions are associated with Cr-spinel, diopside, rutile, Mg-Cr-ilmenite, and pentlandite. Crichtonite-group minerals also occur as lamellae inclusions in pyrope grains of the intermediate domain adjacent to the orthopyroxenite, as well as in interstitial-to-enstatite oxide intergrowths together with Cr-spinel, rutile, and ilmenite. Yimengite-hawthorneite inclusions in enstatite contain (wt%) 3.72-8.04 BaO, 2.05-3.43 K2O, and 0.06-0.48 CaO. Their composition is transitional between yimengite and hawthorneite end-members with most grains exhibiting K-dominant chemistry. A distinct feature of the studied yimengite-hawthorneite minerals is a high content of Al2O3 (5.74-7.69 wt%). Crichtonite-group minerals vary in composition depending on the occurrence in the xenolith: inclusions in enstatite are moderate-to-high in TiO2 (62.9-67.1 wt%), moderately Cr-rich (12.6-14.0 wt% Cr2O3), Ba- or K-specific in the A site, and contain low ZrO2 (0.05-1.72 wt%), whereas inclusions in pyrope are moderate in TiO2 (61.7-63.3 wt% TiO2), relatively low in Cr (8.98-9.62 wt% Cr2O3), K-dominant in the A site, and are Zr-enriched (4.64-4.71 wt% ZrO2). Crichtonite-group minerals in polymineralic oxide intergrowths show highly diverse compositions even within individual aggregates, where they are chemically dominated by Ba, Ca, and Sr. P-T estimates indicate the orthopyroxenite equilibrated at ~800 °C and 35 kbar. Preferentially oriented lamellae of enstatite-hosted Cr-spinel and diopside, as well as pyrope, diopside, and Cr-spinel grains developed around enstatite crystals, are interpreted to have been exsolved from the high-T, Ca-Al-Cr-enriched orthopyroxene precursor. The observed textural relationships between inclusions in enstatite and exotic titanate compositions imply that the studied orthopyroxenite has undergone metasomatic processing by a mobile percolating agent; this highly evolved melt/fluid was enriched in Ba, K, HFSE, and other incompatible elements. The prominent textural and chemical inhomogeneity of the interstitial oxide intergrowths is either a consequence of the metasomatic oxide crystallization shortly prior the kimberlite magma eruption, or arose from the intensive modification of pre-existing oxide clusters by the kimberlite melt during the Udachnaya emplacement. Our new data provide implications for the metasomatic treatment of orthopyroxenites in the subcontinental lithospheric mantle from the view of exotic titanate occurrences.
Discovery of asimowite, the Fe-analog of wadsleyite, in shock-melted silicate droplets of the Suizhou L6 and the Quebrada Chimborazo 001 CB3.0 chondrites
https://doi.org/10.2138/am-2019-6960
Bindi et al. (p. 775) report the first natural occurrence and single-crystal X-ray diffraction study of the Fe-analogue of wadsleyite [a = 5.7485(4), b = 11.5761(9), c = 8.3630(7) Å;, V = 556.52(7) Å3; space group Imma], spinelloid-structured Fe2SiO4, a missing phase among the predicted high-pressure polymorphs of ferroan olivine, with the composition (Fe2+1.10Mg0.80Cr3+0.04Mn2+0.02Ca0.02Al0.02Na0.01)Σ2.01(Si0.97Al0.03)Σ1.00O4. The new mineral was approved by the International Mineralogical Association (No. 2018-102) and named asimowite in honor of Paul D. Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at the California Institute of Technology. It was discovered in rare shock-melted silicate droplets embedded in Fe,Ni-metal in both the Suizhou L6 chondrite and the Quebrada Chimborazo (QC) 001 CB3.0 chondrite. Asimowite is rare, but the shock-melted silicate droplets are very frequent in both meteorites and most of them contain Fe-rich wadsleyite (Fa30-45). Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)2SiO4 at high temperature (> 1800 K) and pressure are clearly needed. This may also have a significant impact on the temperature and chemical estimates of the mantle's transition zone in Earth.
New Mineral Names
https://doi.org/10.2138/am-2019-NMN104510P
Belakovskiy et al. (p. 779) present names for 11 new minerals, including ammoniovoltaite, belousovite, chlorellestadite, clino-suenoite, marcobaldiite, markeyite, martinandresite, parisite-(La), plumbopharmacosiderite, somersetite, and ziminaite.
Volume 104 : April 2019 Issue
Deep carbon cycle through five reactions
https://doi.org/10.2138/am-2019-6833
This issue of American Mineralogist starts with an article to introduce the special collection “Earth in Five Reactions: A Deep Carbon Perspective” by Li et al. (page 465). This special collection features review papers on the role of carbon-related reactions in Earth’s dynamics and evolution, and includes original studies on carbon-bearing phases and the impact of chemical and polymorphic reactions on Earth’s deep carbon cycle.
Earth in five reactions: Grappling with meaning and value in science
https://doi.org/10.2138/am-2019-6745
Hazen (page 468) then describes the five reactions that influence the Earth’s history, which were identified at the Earth in Five Reactions Workshop held at the Carnegie Institution for Science, Washington, D.C., March 22–23, 2018. The workshop posed two challenges: (1) the formulation of a conceptual definition of “reaction” and (2) the identification and ranking of the “most important reactions” in the context of planetary evolution.
In-situ iron isotope analyses reveal igneous and magmatic-hydrothermal growth of magnetite at the Los Colorados Kiruna-type iron oxide-apatite deposit, Chile
https://doi.org/10.2138/am-2019-6623
Knipping et al. (page 471) determined grain-to-grain and intra-grain Fe isotope variations in magnetite grains from the Los Colorados Kiruna-type iron oxide-apatite (IOA) deposit, Chile, using in situ femtosecond laser ablation MC-ICP-MS. The results reveal an igneous and magmatic-hydrothermal growth mechanism of magnetite, which is consistent with the formation model of this IOA deposit proposed based on trace element zonation in its magnetite grains. This is a contribution to the special collection “From Magmas to Ore Deposits”.
Carbon and nitrogen isotopes and mineral inclusions in diamonds from chromitites of the Mirdita ophiolite (Albania) demonstrate recycling of oceanic crust into the mantle
https://doi.org/10.2138/am-2019-6751
Wu et al. (page 485) report the discovery of ophiolite-hosted diamonds in the podiform chromitites of the Skenderbeu massif of the Mirdita ophiolite in the western part of Neo-Tethys. Carbon and nitrogen isotopes and mineral inclusions in diamonds demonstrate recycling of oceanic crust into the mantle. This discovery not only provides new evidence of diamonds in these settings but also sheds light on deep cycling of subducted oceanic crust and mantle composition.
Geochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite and the role of halogens
https://doi.org/10.2138/am-2019-6668
Pe-Piper et al. (page 501) investigated the geochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite using SEM, Raman spectroscopy, LA-ICP-MS and in situ U-Pb analysis. In particular, they determined the role of halogens in the mobility of Ti and associated Zr, U and REE, which led to hydrothermal alteration of magmatic rutile in syenite. The complexities of rutile chemistry in this hydrothermal setting could be reproduced in deeper subduction settings as a result of variations in halogen content of fluids released by prograde metamorphism.
The incorporation of chlorine into calcium amphibole
https://doi.org/10.2138/am-2019-6768
Jenkins (page 514) synthesized calcium amphiboles from ferro-pargasite and hastingsite bulk compositions at 600–950 °C, 0.1–0.45 GPa and logfH2 of 1.4 to 2.4 for durations of 111–672 h, and determined how variations in formation conditions (temperature, pressure, hydrogen fugacity), bulk composition (Na and K ratio), and choice of starting material salts affect the Cl contents of synthesized calcium amphiboles. The results imply that the crystal-chemical controls for Cl incorporation in calcium amphiboles are dominated by substitution of Fe2+ for Mg, TAl for Si, and K for Na into the crystallographic A site with a linear dependence at the rate of 0.45 Cl per FeAlK index above a minimum value of about 0.34.
Crystal size distribution of amphibole grown from hydrous basaltic melt at 0.6–2.6 GPa and 860–970 °C
https://doi.org/10.2138/am-2019-6759
Zhang et al. (page 525) performed three series of amphibole crystallization experiments from hydrous basaltic melt at 0.6–2.6 GPa and 860–970 °C and measured the amphibole crystal size distribution (CSD). The results show that the amphibole growth rate increases with increasing temperature in the isobaric series and with increasing pressure at constant temperature. By contrast, the growth rate is negatively correlated with crystallization time at constant temperature and pressure. The authors developed a functional form for evaluating growth rate at known pressure and temperature from an observed amphibole CSD and applied to a diorite from the eastern Tianshan Mountains, NW China.
Role of micropores, mass transfer, and reaction rate in the hydrothermal alteration process of plagioclase in a granitic pluton
https://doi.org/10.2138/am-2019-6786
Yuguchi et al. (page 536) investigated the role of micropores, mass transfer, and reaction rate in the hydrothermal alteration process of plagioclase from the Toki granitic pluton in central Japan. Important observations include: 1) Micropores form during the incipient stage of plagioclase alteration by dissolution of the anorthite component, and then contribute to the infiltration of hydrothermal fluid into the plagioclase. 2) The mass transfer of the components released from biotite by chloritization involves the inflow of H4SiO4, Al3+, Fe2+, Mn2+, Mg2+, K+, CO2, and F-, and the outflow of H2O, H+, and Ca2+. 3) The infiltration rate of the hydrothermal fluid and the potassium transfer rate through the micropores into the plagioclase represent the mass transfer rate of the alteration.
Lead diffusion in CaTiO3: A combined study using Rutherford backscattering and TOF-SIMS for depth profiling to reveal the role of lattice strain in diffusion processes
https://doi.org/10.2138/am-2019-6730
Beyer et al. (page 557) measured the diffusivity of Pb in CaTiO3 perovskite (commonly used for dating kimberlites and carbonatites) using Rutherford backscattering and TOF-SIMS in the depth-profiling mode. Experiments were performed on oriented CaTiO3 single crystals with (Ca0.83Pb0.07)Ti1.05O3 thin film or (Ca0.9Pb0.1)TiO3 powder as the Pb-source, which were annealed at 736—1135 °C for 2—283 h. The measured Pb profiles show two regions — a steep gradient at the diffusion interface that transitions sharply to a low concentration tail that penetrates deeper into the crystal. Moreover, Pb is trapped in the planar defects formed due to the CaTiO3/Pb-bearing perovskite lattice mismatch, and the closure temperature for Pb in CaTiO3 is found to be between 300 and 400 °C for a range of different cooling scenarios if diffusive resetting of Pb in CaTiO3 occurs. At typical cooling rates of hours to days for ascending kimberlite, the age of crystal growth is preserved, with closure temperatures similar to the magma temperature.
On growth and form of etched fission tracks in apatite: A kinetic approach
https://doi.org/10.2138/am-2019-6762
Jonckheere et al. (page 569) compare the traditional bulk etch rate (vB) and an alternative radial etch rate (vR) model for fission-track etching in apatite. A skeletal vR-model, based on the inferred orientations of the vR minima and maxima, accounts for the main geometrical features of etched fission tracks, unifies the diverse appearances of etched tracks, and embeds fission-track etching in the mainstream theories of crystal growth and dissolution. The authors suggest that the anisotropic-vB-model may be replaced with an anisotropic-vR-model based on the radial etch rate.
High-pressure behavior of liebenbergite: The most incompressible olivine-structured silicate
https://doi.org/10.2138/am-2019-6680
Zhang et al. (page 580) investigated the compressional behavior of a synthetic liebenbergite, Ni2SiO4, that has the olivine structure, up to 42.6 GPa using single-crystal synchrotron X-ray diffraction. Over this pressure range, liebenbergite retains the orthorhombic Pbnm structure. Fitting the pressure-dependent variation in its unit-cell volume to a third-order Birch-Murnaghan equation of state yielded a bulk modulus of 163(3) GPa — the most incompressible olivine-structured silicate.
Phase transition of wadsleyite-ringwoodite in the Mg2SiO4-Fe2SiO4 system
https://doi.org/10.2138/am-2019-6823
Tsujino et al. (page 588) determined the wadsleyite-ringwoodite phase transition loop in the Mg2SiO4-Fe2SiO4 system under dry conditions from 1473–1873 K using in situ high P-T synchrotron X-ray diffraction. Assuming an equilibrium composition of wadsleyite and ringwoodite coexisting with garnet in a pyrolite model and an adiabatic temperature gradient with a potential temperature of 1550–1650 K, the phase transition depth and effective width of the seismic discontinuity were found to be 500–514 and 20–22 km, respectively. Considering wet and oxidized conditions, the depth of the wadsleyite-ringwoodite phase boundary could be >520 km. Variation in the depth of seismic anomaly may be attributed to water content or oxygen fugacity of the transition zone.
Cation ordering, valence states, and symmetry breaking in the crystal-chemically complex mineral chevkinite-(Ce): X-ray diffraction and photoelectron spectroscopy studies and mechanisms of Nb enrichment
https://doi.org/10.2138/am-2019-6457
Stachowicz et al. (page 595) investigated the cation ordering, valence states, and symmetry breaking in chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, using single-crystal X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Nb-rich chevkinite-(Ce) typically possesses a space group of C2/m, though a specimen of lower, P21/α, symmetry has also been recognized. While XPS shows that both C2/m and P21/α structures contain Ti4+ and Ti3+, it also indicates that Ti2+ may occur in the P21/α phases. In addition to the substitution CFe3+ + DTi4+ ↔ CFe2+ + DNb5+, the authors propose that another substitution, 2DTi4+ ↔ DNb5+ + DTi3+, can occur, leading to substantial Nb-enrichment.
Meyrowitzite, Ca(UO2)(CO3)2·5H2O, a new mineral with a novel uranyl-carbonate sheet
https://doi.org/10.2138/am-2019-6814
Kampf et al. (page 603) describe the crystal structure, chemical composition and physical properties of meyrowitzite, Ca(UO2)(CO3)2·5H2O, a new mineral from the Markey mine, Red Canyon, San Juan County, Utah, U.S.A. The structure is monoclinic, P21/n, and contains a novel uranyl-carbonate sheet. Meyrowitzite is a secondary phase found on calcite-veined asphaltum in association with gypsum, markeyite and rozenite.
Discovery of the first natural hydride
https://doi.org/10.2138/am-2019-6949
Bindi et al. (page 611) describe the discovery of the first natural metal hydride, gamma-VH2. .It was discovered in xenoliths within volcanic rock on Mount Carmel, Israel. The hydride coexists with metallic V, which requires oxygen fugacities of ΔIW -9 or lower. The presence of VH2 is interpreted as a signature of deep-seated basaltic magmas with mantle-derived CH4+H2 at high fluid/melt ratios.
Synthesis of pigeonites for spectroscopic studies
https://doi.org/10.2138/am-2019-6869CCBYNCND
Lindsley et al. (page 615) describe the synthesis of pigeonite samples and offer them for non-destructive study. As pigeonite does not survive as a discrete phase in coarse plutonic rocks, natural samples suitable for study are difficult to come by. Eight samples of differing composition, the results from more than 125 trials, in sizes 5-50 micrometers are available for bulk analysis.
Presentation of the 2018 Roebling Medal of the Mineralogical Society of America to E. Bruce Watson
https://doi.org/10.2138/am-2019-AP10442
"Presentation of the 2018 Roebling Medal of the Mineralogical Society of America to E. Bruce Watson” by Frank Richter"
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