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A closer look at shocked meteorites: Discovery of new high-pressure minerals

“Discovery consists not in seeking new lands but in seeing with new eyes.”—Marcel Proust. A rock is a book from nature. When we read it closely down to micrometers and nanoscales, we often discover something new and exciting. Nanomineralogy is the study of Earth and planetary materials at nanoscales, focused on characterizing nanofeatures (such as inclusions, exsolution, zonation, coatings, pores) in minerals and rocks and revealing nanominerals and nanoparticles (Ma 2008). Nanomineralogy is a concept and also an approach that is applicable to all fields of geosciences dealing with solid materials. With advanced electron-beam and synchrotron techniques, particularly using high-resolution analytical scanning electron microscopy, we are now capable to characterize geomaterials down to nanoscales easier and faster. Nanofeatures are being identified in many common minerals and rocks that provide insights into their petrogenesis and physical properties. New minerals and new materials with important geological significance are being discovered at micrometers to nanoscales (Ma 2015), including new high-pressure minerals representing extreme conditions (e.g., Tschauner et al. 2014; Ma et al. 2015, 2016). High-pressure minerals are found in shocked meteorites and terrestrial impact rocks, which are formed by shock metamorphism during collisions of asteroids or asteroid impact events on planets (Earth, the Moon, and Mars) in the solar system (e.g., Stöffler et al. 1988; Sharp and DeCarli 2006). Most high-pressure minerals are high-pressure polymorphs of major rock-forming minerals and accessory minerals that formed via solid-state transformation or crystallized from shocked-induced melts under high-pressure and high-temperature conditions (e.g., Rubin and Ma 2017; Tomioka and Miyahara 2017). A few are brand-new to science, such as (Mg,Fe)3Si2O7 with a tetragonal spinelloid structure—a high-pressure silicate discovered in the Tenham chondrite (Ma et al. 2018a). Over the past six years, 13 new high-pressure minerals have been approved by the IMA-CNMNC, as listed in Table 1. Ice-VII is a high-pressure ice occurring as inclusions in diamonds from Earth’s deep mantle (Tschauner et al. 2018). Riesite (TiO2) and maohokite (MgFe2O4) are from the Ries and Xiuyan terrestrial impact structures, respectively (Tschauner and Ma 2017a; Chen et al. 2017). The other 10 are all identified in shocked meteorites (i.e., ordinary chondrites, shergottites, and one eucrite), including bridgmanite (MgSiO3-perovskite, the most abundant mineral in Earth; Tschauner et al. 2014), ahrensite (Fe2SiO4-spinel; Ma et al. 2016), tissintite [(Ca,Na,o)AlSi2O6-clinopyroxene; Ma et al. 2015], liebermannite (KAlSi3O8-hollandite; Ma et al. 2018), stöfflerite (CaAl2Si2O8-hollandite; Tschauner and Ma 2017b), and chenmingite (FeCr2O4-CF; Ma et al. 2018b). Pang et al. (2018, September issue) report the discovery of vestaite [(TiFe)Ti3O9]—the latest one in a shocked eucrite from asteroid Vesta. Vesta, the second largest object in the asteroid belt after Ceres, is the parent body of the howardite, eucrite, and diogenite (HED) meteorites (McSween et al. 2013). During a combined SEM-FIB-TEM-EDS-SAED-EELS investigation, Pang et al. (2018) identified vestaite as a brand-new, high-pressure mineral in the eucrite NWA 8003, crystallized from a shock melt at high pressure (≤10 GPa) in several melt pockets during an impact event on Vesta. HED meteorites usually show fewer shock features. NWA 8003 is special, containing a few high-pressure minerals in shock-induced melt veins and pockets (Pang et al. 2016). Vestaite is (TiFe)Ti3O9 with a monoclinic C2/c schreyerite-type structure, the first new mineral revealed from HED meteorites. The type vestaite has a general formula of (Ti,Fe,Al)2Ti3O9, with minor machiite (Al2Ti3O9) component (30 mol%), indicating a solid solution between vestaite and machiite (Pang et al. 2018). Vestaite is a shock-metamorphic phase, whereas machiite is an ultra-refractory phase formed at near vacuum pressure in the solar nebula (Makide et al. 2013; Krot 2016). Corundum occurs in association with both the type