Scanning electron micrographs of skeletal carbonates (mollusks, corals, barnacles, polychaete annelids, bryozoans) from the Miami and Key Largo Limestones (south Florida) provide unequivocal evidence for in situ conversion of skeletal aragonites. Many of the petrographic criteria for such conversion have been regarded as uncertain; presence of inclusions reflecting former skeletal structures is more reliable evidence of direct conversion than solution-infill mosaics. In converted aragonites, coarse calcite mosaics contain submicroscopic remnant crystallites of the original aragonitic ultrastructure. These remnant crystallites are in original orientation, as solid inclusions in the calcite grains. Partially altered shells traversed by advancing diagenetic fronts demonstrat the continuity of orientation of the crystallites in the unaltered aragonitic shell layers with the aragonitic solid inclusions in the calcite mosaic behind the front. Mineralogy of the respective grains and crystallites was verified by means of Feigl solution, an aragonite-specific stain.

Calcite ultrastructures are commonly little affected, but in Schizoporella floridana (Bryozoa) from the Miami Limestone, the calcitic primary skeleton is recrystallized,

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although the calcite mosaic of converted superficial frontal aragonite layer still contains solid inclusions of the original aragonite.

The presence of such aragonite inclusions, or the numerous pits which result when they are dissolved (e.g., during etching or Feigl solution treatment), may be useful in recognition of older examples of in situ conversion of aragonites.

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