T155 Early Career Investigators in Mineralogy and Crystallography

Tuesday, 11 October 2022
8:00 AM - 12:00 PM
601

OCCURRENCE AND CRYSTAL STRUCTURE OF INCOMMENSURATE TWO-LAYER ANTIGORITE FROM WESTERN IDAHO

Paper number: T155-3
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

  • Cody Steven Presenting Author
    University of Idaho, Moscow, ID
  • Cody Steven Presenting Author
    Planetary Science Institute, Tucson, AZ

Abstract

Two-layer polytypes of antigorite have been observed by authors by powder diffraction as early as 1954. Although the structure has routinely been observed by high resolution transmission electron microscopy since the late 1970s, no single-crystal diffraction solution has been presented for the structure. Groberty (2003) suggested that the structure likely has octahedral layer stacking that matches clinochlore IIb, given that epitaxies of two-layer antigorite form around the clinochlore. In metaperidotites from Western Idaho, antigorite epitaxies occur around clinochlore as a late-stage metamorphic texture. Antigorite samples from Western Idaho were first examined with transmission electron microscopy which revealed two-layer stacking in high resolution images. In (010) sections of antigorite, electron diffraction patterns confirmed the 14.6 angstrom (001) periodicity, though the satellite peaks appeared discordant from the main lattice reflections along a*. The discordance of the reflections comprising the reciprocal lattice requires an extra index, and is therefore an incommensurate modulated structure. Solution and refinements with single-crystal X-ray diffraction data presented here verify the alternating octahedral layer of two-layer antigorite, though the structure must be modeled as an incommensurate crystal structure. Although initial refinements have accounted for the dominant domain of the crystal structure, significant residual maxima within the tetrahedral layer is symptomatic of the usual stacking disorder observed in antigorite and other sheet silicates. Given this, correctly modeling the displacements of the tetrahedral layer could significantly reduce the R-factors of main and satellite reflections of the structure.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-381021

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.