New type of compositional zoning in calcite: insights into crystal- growth mechanisms

Compositional zoning of trace elements is observed in time-equivalent regions within 101̄4 growth sectors of natural and synthetic calcite single crystals, and has been called "intrasectoral zoning' to distinguish it from conventional sectoral zoning. Differential interference contrast microscopy shows that {101̄4} faces grow by the spiral mechanism. Cathodoluminescence microscopy and electron probe microanalyses reveal that trace element partitioning differences on {101̄4} faces correspond exactly to crystallographically controlled orientations of growth steps. Effective partitioning of Mg and Mn varies up to a factor of four between time-correlative growth steps spreading in different directions. Effective Sr partitioning varies by approximately 20% and is inversely correlated with enrichment trends of Mg and Mn. Preferential incorporation of different aqueous species at different growth steps may provide an explanation for morphological and chemical variation in natural calcites. -Authors