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A motley assortment of low-magnesian calcite morphologies occurs in travertine deposits in Oklahoma. These morphologic variations result from (1) precipitation, (2) dissolution, and/or (3) neomorphism in this nonmarine environment; analogous morphologic variations in both marine and nonmarine strata may likewise indicate nonmarine processes.
Commonly precipitated crystals include hexagonal prisms and rhombohedrons (2-150 µm long), and bladed to fibrous forms (0.02-2 mm long, many revealing triangular cross sections). Many of these crystals contain inclusion-defined growth layers that dissolve preferentially, leaving abundant intracrystalline porosity. This porosity parallels crystal outlines, imparting a nested appearance to the crystals. Partial dissolution also creates parallel "spikes" (4-30 µm long, 1-10 µm wide), and parallel "ribbon" crystals (30-150 µm long, 3-12 µm wide) that repeatedly narrow and widen, and occasionally twist. In addition to precipitational and dissolutional forms, aggradational neomorphism produces columnar crystals, commonly exceeding 8 mm in length. These crystals orig nated as elongate spar around filamentous cyanophyte tufts and were transformed subsequently into ragged-edged columnar crystals at the expense of overlying micritic crystals.
Morphologic variations in calcite crystals often have been attributed to ionic concentrations (particularly Mg and SO4) of ambient waters. In this study, however, the low concentrations of both Mg (averaging 12 ppm) and SO4 (approximately 16 ppm) may be interpreted as supporting theories relating precipitational morphologies to growth rates rather than to "poisoning" ions. In any case, recognition of similar morphologic suites resulting from precipitation, dissolution, and/or neomorphism may aid in the identification of nonmarine processes in marine and nonmarine strata.
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