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Machiite, Al₂Ti₃O₉, a new oxide mineral from the Murchison carbonaceous chondrite: A new ultra-refractory phase from the solar nebula

Krot, Alexander N. and Nagashima, Kazuhide and Rossman, George R. (2020) Machiite, Al₂Ti₃O₉, a new oxide mineral from the Murchison carbonaceous chondrite: A new ultra-refractory phase from the solar nebula. American Mineralogist, 105 (2). pp. 239-243. ISSN 0003-004X. doi:10.2138/am-2020-7185.

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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_(1.17)Sc_(0.56)Y_(0.10)Ti4+0.08Ti0.084+Fe_(0.06)Ca_(0.03)Mg_(0.01))(⁠Ti4+2.71Ti2.714+Zr_(0.28)Si_(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 Å3, 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.

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Krot, Alexander N.0000-0002-2278-8519
Rossman, George R.0000-0002-4571-6884
Additional Information:© 2020 Mineralogical Society of America. Manuscript received June 28, 2019; Manuscript accepted September 25, 2019; Manuscript handled by Oliver Tschauner. Comments and suggestions by J. Han, O. Tschauner, and anonymous reviewer are highly appreciated. SEM, EBSD, and EPMA were carried out at the Geological and Planetary Science Division Analytical Facility, Caltech, which is supported in part by NSF grants EAR-0318518 and DMR-0080065. SIMS was carried out at University of Hawai‘i. Raman was carried out in the Rossman lab at Caltech. This work was also supported by NASA grant NNX17AE22G (P.I., A.N. Krot). Deposit item AM-20-27185, Supplemental Material. Deposit items are free to all readers and found on the MSA website, via the specific issue's Table of Contents (go to
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Subject Keywords:Machiite, Al2Ti3O9, (Al,Sc)2(Ti4+,Zr)3O9, new mineral, Zr,Sc-rich phase, schreyerite group, ultrarefractory phase, Ca-Al-rich inclusion, Murchison meteorite, CM2 carbonaceous chondrite, oxygen isotopes
Issue or Number:2
Record Number:CaltechAUTHORS:20200203-124212439
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Official Citation:Alexander N. Krot, Kazuhide Nagashima, George R. Rossman; Machiite, Al2Ti3O9, a new oxide mineral from the Murchison carbonaceous chondrite: A new ultra-refractory phase from the solar nebula. American Mineralogist ; 105 (2): 239–243. doi:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101073
Deposited By: Tony Diaz
Deposited On:03 Feb 2020 21:17
Last Modified:16 Nov 2021 17:58

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