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

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.