Calcite rims on dolomite crystals have been observed in the Little Falls Formation of east-central New York. Most rims are even, syntaxial, and 5 to 25µ thick. Some, however, are irregular, apparently resembling the "calcite envelopes" of Goldman. Electron probe microanalysis verifies the dolomitic center and the calcitic border, the latter commonly being very slightly ferroan. Dolomite preceded development of quartz overgrowths, as determined by spatial relations (e.g., enfacial junctions). The rims seem to be along crystal faces of dolomite rather than along compromise boundaries or against detrital quartz cores. Calcite beyond the rims exists as poikilotopic, centimicron-sized, void-filling spar, some of which is optically continuous with adjacent rims and dolomit crystals. Common enfacial junctions of calcite crystals against secondary quartz faces suggest a later development of the calcite. The rims are the same age as the remainder of the calcite.

Such syntaxial rims could result either from marginal dedolomitization (calcitization) or from passive precipitation on a nucleus of dolomite. Optical continuity of the rim and nucleus and the presumed post-secondary quartz age of the calcite strongly suggest dedolomitization. Solutions with high Ca⁺⁺/Mg⁺⁺ ratios provided by meteoric water passing through overlying Ordovician limestones may have been the dedolomitizing agent. This unusual fabric may constitute another piece of evidence for dedolomitization.

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