¹Alexander N. Krot,¹Kazuhide Nagashima,²George R. Rossman
American Mineralogist 105, 239–243 Link to Article [https://doi.org/10.2138/am-2020-7185]
¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, U.S.A.
Division of Geological and Planetary Sciences, California Institute of
²Technology, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

Machiite (IMA 2016-067), Al₂Ti₃O₉, is a new mineral that occurs as a single euhedral crystal, 4.4 μm in size, in contact with an euhedral corundum grain, 12 μm in size, in a matrix of the Murchison CM2 carbonaceous chondrite. The mean chemical composition of holotype machiite by electron probe microanalysis is (wt%) TiO₂ 59.75, Al₂O₃ 15.97, Sc₂O₃ 10.29, ZrO₂ 9.18, Y₂O₃ 2.86, FeO 1.09, CaO 0.44, SiO₂ 0.20, MgO 0.10, total 99.87, giving rise to an empirical formula (based on 9 oxygen atoms pfu) of (Al_1.17Sc_0.56Y_0.10Ti4+0.08Ti0.084+Fe_0.06Ca_0.03Mg_0.01)(⁠Ti4+2.71Ti2.714+Zr_0.28Si_0.01)O₉. The general formula is (Al,Sc)₂(Ti⁴⁺,Zr)₃O₉. The end-member formula is Al₂Ti₃O₉. Machiite has the C2/c schreyerite-type structure with a = 17.10 Å, b = 5.03 Å, c = 7.06 Å, β = 107°, V = 581 ų, and Z = 4, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 4.27 g/cm³. The machiite crystal is highly ¹⁶O-depleted relative to the coexisting corundum grain (Δ¹⁷O = –0.2 ± 2.4‰ and –24.1 ± 2.6‰, respectively; where Δ¹⁷O = δ¹⁷O – 0.52 × δ¹⁸O). Machiite is a new member of the schreyerite (V₂Ti₃O₉) group and a new Sc,Zr-rich ultrarefractory phase formed in the solar nebula, either by gas-solid condensation or as a result of crystallization from a Ca,Al-rich melt having solar-like oxygen isotopic composition (Δ¹⁷O~ –25‰) under high-temperature (~1400–1500 °C) and low-pressure (~10^-4–10^-5 bar) conditions in the CAI-forming region near the protosun. The currently observed disequilibrium oxygen isotopic composition between machiite and corundum may indicate that machiite subsequently experienced oxygen isotopic exchange with a planetary-like ¹⁶O-poor gaseous reservoir either in the solar nebula or on the CM chondrite parent body. The name machiite is in honor of Chi Ma, mineralogist at California Institute of Technology, for his contributions to meteorite mineralogy and discovery of many new minerals representing extreme conditions of formation.

¹James P. Greenwood,¹Kenichi Abe,^1,2Benjamin McKeeby
American Mineralogist 105, 255–261 Link to Article [https://doi.org/10.2138/am-2020-7180]
¹Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, U.S.A.
²Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, PA 15260, U.S.A.
Copyright: The Mineralogical Society of America

We report the occurrence of a previously unidentified mineral in lunar samples: a Cl-,F-,REE-rich silico-phosphate identified as Cl-bearing fluorcalciobritholite. This mineral is found in late-stage crystallization assemblages of slowly cooled high-Ti basalts 10044, 10047, 75035, and 75055. It occurs as rims on fluorapatite or as a solid-solution between fluorapatite and Cl-fluorcalciobritholite. The Cl-fluorcalciobritholite appears to be nominally anhydrous. The Cl and Fe²⁺ of the lunar Cl fluorcalciobritholite distinguishes it from its terrestrial analog. The textures and chemistry of the Clfluorcalciobritholite argue for growth during the last stages of igneous crystallization, rather than by later alteration/replacement by Cl-, REE-bearing metasomatic agents in the lunar crust. The igneous growth of this Cl- and F-bearing and OH-poor mineral after apatite in the samples we have studied suggests that the Lunar Apatite Paradox model (Boyce et al. 2014) may be inapplicable for high-Ti lunar magmas. This new volatile-bearing mineral has important potential as a geochemical tool for understanding Cl isotopes and REE chemistry of lunar samples.

^1,2Péter Németh,^2,3Laurence A.J. Garvie
American Mineralogist 105, 276–281 Link to Article [https://doi.org/10.2138/am-2020-7305]
¹Institute of Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, H-1117 Budapest, Magyar Tudósok Körútja 2, Hungary
²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-6004, U.S.A.
³Center for Meteorite Studies, Arizona State University, Tempe, Arizona 85287-6004, U.S.A.
Copyright: The Mineralogical Society of America

Shock caused by impacts can convert carbonaceous material to diamond. During this transition, new materials can form that depend on the structure of the starting carbonaceous materials and the shock conditions. Here we report the discovery of cage-like nanostructured carbonaceous materials, including carbon nano-onions and bucky-diamonds, formed through extraterrestrial impacts in the Gujba (CB_a) meteorite. The nano-onions are fullerene-type materials and range from 5 to 20 nm; the majority shows a graphitic core-shell structure, and some are characterized by fully curved, onion-like graphitic shells. The core is either filled with carbonaceous material or empty. We show the first, natural, 4 nm sized bucky-diamond, which is a type of carbon nano-onion consisting of multilayer graphitic shells surrounding a diamond core. We propose that the nano-onions formed during shock metamorphism, either the shock or the release wave, of the pre-existing primitive carbonaceous material that included nanodiamonds, poorly ordered graphitic material, and amorphous carbonaceous nanospheres. Bucky-diamonds could have formed either through the high-pressure transformation of nano-onions, or as an intermediate material in the high-temperature transformation of nanodiamond to nano-onion. Impact processing of planetary materials was and is a common process in our solar system, and by extension, throughout extrasolar planetary bodies. Together with our previous discovery of interstratified graphite-diamond in Gujba, our new findings extend the range of nano-structured carbonaceous materials formed in nature. Shock-formed nano-onions and bucky-diamonds are fullerene-type structures, and as such they could contribute to the astronomical 217.5 nm absorption feature.