Crystallographically, the Egyptian herringbone calcite consists of serrate bands that are 1.25-1.45 mm thick. Bands are comprised of ~ 800 micron x 120 micron crystals that are elongate perpendicular to the banding. As in other herringbone calcite, the c-axis is oriented perpendicular to crystal elongation, and the crystals are length slow. Crystal tips are either feathery or have spearhead-like crystal faces projecting at an angle out from the main direction of crystal elongation. In oblique cuts, the elongate crystals are curved up to 45¡ along their length, suggesting a complex 3-dimensional morphology for the crystals. The unstable, feathery crystal morphology of the tops of elongate crystals is not present in previously described herringbone calcite; if originally present, feathery tips may have recrystallized to the microspar observed in some older samples.
Preliminary microprobe elemental analyses suggest that the Egyptian herringbone calcite is very high in Sr. Average concentrations are 3200±600 ppm Sr in herringbone calcite and 1800±600 ppm in associated fibrous calcite. An average of 5.5±0.4 mol% MgCO3 is present in the herringbone calcite and 3.3±0.4 mol% MgCO3 in associated fibrous calcite. Fe concentrations are less than the detection limit of 690 ppm in both, and neither luminesce demonstrating very low Mn concentrations.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616