¹Melinda J. Rucks, ^1,2Matthew L. Whitaker, ¹Timothy D. Glotch,^1,2 John B. Parise, ¹Steven J. Jaret, ¹Tristan Catalano, ³M. Darby Dyar
American Mineralogist 103, 1516-1519 Link to Article [https://doi.org/10.2138/am-2018-6539]
¹Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, U.S.A.
²Mineral Physics Institute, Stony Brook University, Stony Brook, New York 11794-2100, U.S.A
³Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.
Copyright: The Mineralogical Society of America

Tissintite is a shock-induced, Ca-rich mineral, isostructural to jadeite, observed in several meteorite samples such as the martian shergottite Tissint. It may form within a “Goldilocks Zone,” indicating a potential to provide strict constraints on peak pressure and temperature conditions experienced during impact. Here we present the first laboratory synthesis of tissintite, which was synthesized using a large volume multi-anvil apparatus at conditions ranging from 6–8.5 GPa and 1000–1350 °C. For these experiments, we utilized a novel heating protocol in which we reached impact-relevant temperatures within 1 s and in doing so approximated the temperature-time conditions in a post-shock melt. We have established that heating for impact-relevant timescales is not sufficient to completely transform crystalline labradorite to tissintite at these pressures. Our findings suggest that tissintite forms from amorphous plagioclase during decompression.

^1,2Run-Lian Pang, ²Dennis Harries, ²Kilian Pollok, ¹Ai-Cheng Zhang, ^2,3Falko Langenhorst
American Mineralogist 103, 1502-1511 Link to Article [https://doi.org/10.2138/am-2018-6522]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
²Institute of Geosciences, Friedrich Schiller University Jena, D-07745 Jena, Germany
³Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, U.S.A.
Copyright: The Mineralogical Society of America

Our investigations on the shocked eucrite Northwest Africa (NWA) 8003 revealed the occurrence of a new mineral, vestaite [IMA 2017-068;(Ti4+Fe2+)Ti3 4+O9]. This mineral coexists with corundum, ilmenite, and Al-Ti-rich pyroxene in shock melt pockets. It has an empirical chemical formula of
(Ti0.73 4+ Fe0.63 2+Al0.60Mn0.03Mg0.02Cr0.01)Ti3 4+O9
and the monoclinic C2/c structure of schreyerite. The ideal vestaite structure can be considered as a modular structure with an alternate intergrowth of M3O5-type (M = Ti4+,Fe2+,Al) and Ti2O4-type slabs. Alternatively, it can also be envisaged as a crystallographic shear structure with periodically shearing of rutile or α-PbO2 units. Streaking and splitting of diffraction spots observed in selected-area electron diffraction patterns indicate planar defects in the modular structure of vestaite. Our observations reveal that vestaite crystallized at high pressure (≤10 GPa) from a melt that represents a mixture of ilmenite and silicate components. A robust constraint on its formation conditions and stability field cannot yet be provided due to the lack of experimental data for these systems. Vestaite is a new, shock-generated mineral first found in a meteorite of the howarditeeucrite-diogenite (HED) clan, the largest achondrite group. Its discovery is not only of significance to the meteoritic mineralogy, but it could also be of interest to materials science.