^1,2M. Kimura,^3,4M. K. Weisberg,¹A. Yamaguchi
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14129]
¹National Institute of Polar Research, Tokyo, Japan
²Ibaraki University, Mito, Japan
³Kingsborough College and Graduate Center of the City University of New York, New York City, New York, USA
⁴American Museum of Natural History, New York City, New York, USA
Published by arrangement with John Wiley & Sons

Type 3 chondrites are subdivided into 3.0–3.9. Subtype 3.0 chondrites nearly preserve all of their primitive features. Many criteria have been proposed to distinguish such primitive chondrites. Here, we compiled mineral data and reconsider the petrologic classification criteria for subtype 3.0. Chondrites are classified into subtypes by the minor element distribution of olivine and textural and chemical features of Fe-Ni metal. The []Si4O8 and MgO components of feldspar also distinguish subtype 3.0 from subtypes ≥3.1. Other features, such as the occurrence of near pure chromite, are also indicators of subtype 3.0. It is difficult to distinguish between subtypes 3.0 and ≤2.9 based on mineral chemistry. Therefore, we propose the following criteria to distinguish between subtypes 3.0 and ≤2.9. In type 3.0 chondrites, major silicate (olivine, pyroxene, and plagioclase), oxide, metal, and sulfide minerals do not show aqueous alteration features. Melilite, anorthite, and glass show no or mild aqueous alteration features. Subtype 3.0 has not been identified in all chondrite groups. The absence of subtype 3.0 from some groups mainly reflects differences in the degrees of secondary parent body processes among the chondrite groups.

^1,2F. Campanale,^2,3E. Mugnaioli,^2,3L. Folco,⁴P. Parlanti,⁴M. Gemmi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14137]
¹Dipartimento di Scienze dell’Ambiente e Della Terra, Università degli Studi di Milano-Bicocca, Milan, Italy
²Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
³CISUP, Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Pisa, Italy
⁴Centre for Materials Interfaces, Electron Crystallography, Istituto Italiano di Tecnologia, Pontedera, Italy
Copyright Elsevier

TiO2II, a high-pressure polymorph of titanium dioxide, is a diagnostic indicator of shock metamorphism in impact rocks. Due to its typical micro-to-nanometer scale, there are no ab initio structure solutions of natural TiO2II, thereby generating uncertainty about its crystal structure and its known similarity with srilankite (Ti0.67,Zr0.33)O2. Nanoscale electron diffraction investigation of TiO2II from the Australasian tektite strewn field provides the first ab initio structure solution revealing a primitive orthorhombic lattice with cell parameters a = 4.547 Å, b = 5.481 Å, c = 4.891 Å, and space group Pbcn, that is, the same as srilankite and scrutinyite α-PbO2. The linear a and c decrease, and b increase with Ti content indicate TiO2II as Zr-free srilankite endmember in the binary system ZrO2-TiO2. Thereby the name srilankite should be used referring to TiO2II, according to the International Mineralogical Association recommendations. We provide the first evidence for a topotactic subsolidus rutile-to-TiO2II transition, founding their finely intermixing nanocrystals in the same TiO2 crystal, where TiO2II is within the crystal and surrounded by rutile in direct contact. They also show recurrent iso-orientation, with TiO2II [100] parallel to rutile [100], TiO2II [010] parallel to rutile [011], and TiO2II [001] parallel to rutile (0–11). The rutile-TiO2II iso-orientation suggests the compression of rutile (0–11) planes as a possible transition mechanism from rutile to TiO2II, with a consequent shortening of ~0.5 Å per cell. The presence of TiO2II in the distal (~1200 km) impact ejecta from the Australasian tektite strewn field indicates shock pressures of ~12–15 GPa and post-shock temperatures below 500°C followed by rapid quenching.