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

Abstract

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.