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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on NaCrS2 by Materials Project

NaCrS2 is Caswellsilverite structured and crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Na1+ is bonded to six equivalent S2- atoms to form distorted NaS6 octahedra that share corners with six equivalent CrS6 octahedra, edges with six equivalent NaS6 octahedra, and edges with six equivalent CrS6 octahedra. The corner-sharing octahedral tilt angles are 12°. All Na–S bond lengths are 2.86 Å. Cr3+ is bonded to six equivalent S2- atoms to form CrS6 octahedra that share corners with six equivalent NaS6 octahedra, edges with six equivalent NaS6 octahedra, and edges with six equivalent CrS6 octahedra. The corner-sharing octahedral tilt angles are 12°. All Cr–S bond lengths are 2.44 Å. S2- is bonded to three equivalent Na1+ and three equivalent Cr3+ atoms to form a mixture of edge and corner-sharing SNa3Cr3 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NaCrS2 by Materials Project

NaCrS2 is Caswellsilverite-like structured and crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Na1+ is bonded to six equivalent S2- atoms to form distorted NaS6 octahedra that share corners with twelve equivalent CrS6 octahedra, edges with six equivalent NaS6 octahedra, and faces with two equivalent CrS6 octahedra. The corner-sharing octahedra tilt angles range from 47–48°. There are two shorter (2.87 Å) and four longer (2.89 Å) Na–S bond lengths. Cr3+ is bonded to six equivalent S2- atoms to form CrS6 octahedra that share corners with twelve equivalent NaS6 octahedra, edges with six equivalent CrS6 octahedra, and faces with two equivalent NaS6 octahedra. The corner-sharing octahedra tilt angles range from 47–48°. There are two shorter (2.43 Å) and four longer (2.44 Å) Cr–S bond lengths. S2- is bonded in a 6-coordinate geometry to three equivalent Na1+ and three equivalent Cr3+ atoms.

36 MATERIALS SCIENCE↗
NASA NTRS2019

A New Look at Aubrites: Investigating 3D Modal Mineralogy with X-Ray Computed Tomography

The aubrites (approximately 30 known meteorites) are a unique group of differentiated meteorites that formed on asteroids with oxygen fugacities (ƒO2) from approximately 2 to approximately 6 log units below the iron-wustite buffer. At these highly reduced conditions, elements deviate from the geochemical behavior exhibited at terrestrial ƒO2, forming FeO-poor silicates and exotic sulfides. While previous studies have described the petrology and 2D modal abundances of aubrites, this work investigates the 3D modal mineralogies of silicate, metal, and sulfide phases in aubrite samples, which are then compared to the available 2D data. In addition to 3D modal mineralogies, we have examined the geochemistry of fourteen aubrites, including mineral major-element compositions, bulk-rock compositions, and oxygen isotopic compositions to understand their formation and evolution at extreme ƒO2 conditions. We utilize X-ray computed tomography (XCT) to non-destructively analyze the distribution and abundances of mineral phases in aubrites and locate composite clasts of sulfide grains for future analytical study. In order to better constrain elemental behavior under reduced conditions, we specifically target minerals phases that comprise moderately volatile elements (i.e. oldhamite [CaS], caswellsilverite [NaCrS2] and djerfisherite [K6Na(Fe,Cu,Ni)25S26Cl]) as it has been shown that their geochemical behavior changes as a function of ƒO2. Currently, we have produced 3D scans of the Norton County aubrite. The results of the XCT data have allowed for the determination of the abundances of silicate groundmass (i.e., enstatite, forsterite, albite, and diopside), light (based on electron density) sulfides (i.e. alabandite [MnS] and daubréelite [FeCr2S4]), heavy (based on electron density) sulfides (i.e., troilite [FeS]), and Fe,Ni metal by segmenting a density histogram in Volume Graphics Studio software. XCT scans of additional aubrites are underway. By combining the 3D representation of the exotic phases found in aubrites with existing 2D characterizations, we are able to better determine modal abundances. By integrating 3D and 2D modal abundances and geochemistry, we can ultimately better constrain aubrite petrogenesis and elemental partitioning under reduced conditions. Furthermore, application of this new 3D approach offers the opportunity to identify and select clasts for future study prior to cutting the sample, which will minimize sample loss of this precious material.

Wilbur, Zoe↗