Minor quantities of ordered dolomite (δ18O = +2.0‰ PDB; composition = Ca1.22Mg0.78 [CO3] 2) have formed from near-normal seawater in a subtidal hardground as part of a modern fringing coral reef. Crystals 5 µm in diameter precipitated within the past 1.8 ka in the form of syntaxial fringes on Mg-calcite marine cements and skeletal allochems. The crystals have a fine modulated microstructure and c reflections, both apparently formed during crystal growth.

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