Herringbone calcite consists of alternating light and dark crenulated bands; each light-dark pair is 0.5-1.0 mm thick. Microscopically, each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement. The bases of the crystals are optically unoriented, but upwards in each crystal, the optical c axis rotates until it is perpendicular to crystal elongation. The tops of the elongate crystal are thus optically aligned and length slow. The light bands of herringbone calcite correspond to the optically oriented parts of the elongate crystals whereas the dark bands correspond to the optically unoriented, lower parts of the elongate crystals. Microspar* crystals are also present in some dark bands. A Mg-calcite precursor for herringbone calcite, now preserved as low-Mg calcite or dolomite, is supported by the presence of microdolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed former aragonite or low-Mg calcite.
Precipitation of herringbone calcite may be consistent with a diffusionally controlled growth model involving branching growth of fibrous crystals and the diffusion of a precipitation inhibitor away from the crystallization surface. Since herringbone calcite is associated with anaerobic depositional environments, the inhibitor promoting precipitation of herringbone calcite may be present only in poorly oxygenated sea water. Thus, the stratigraphic distribution of herringbone calcite may be an important indicator of the abundance of oxygen in carbonate depositional environments through time.
(Herringbone calcite figures)
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616