Sectoral and growth rate control on elemental uptake by individual calcite crystals

Heterogeneous distribution of trace elements (impurities) within individual calcite crystals is a phenomenon commonly observed in natural and laboratory systems. Changes in thermodynamic intensive parameters (mostly chemical potential and temperature) cannot always explain the inhomogeneous impurity patterns in calcite crystals and it has been suggested that growth rate and crystallographic orientation may exert strong effects on elemental incorporation into calcite. In addition, there are a number of experimental studies on micro-scale element (E) distribution between non-equivalent pairs of calcite vicinal faces (known as sectoral zoning); however, the variability of partition coefficients (e.g., K-E = (E/Ca)(calcite)/(E/Ca)(fluid)) within individual crystals remains undetermined. In this study, we have extended the work on elemental distribution between crystal sectors to evaluation of partition coefficients of trace and minor elements (Li, B, Mg, and Sr) in calcite crystal faces (10-14) and (01 12), whose growth rates were assessed. Growth entrapment model (GEM) and lattice strain theory were applied to explain K-E heterogeneity by varying near-surface diffusivity of Mg and Sr and by varying surface enrichment factor for Li, B, Mg, and Sr. Decoupling of sectoral and growth rate effects reveals that sectoral zoning plays a key role in elemental distribution. More specifically, K-Li and K-B vary by more than one order of magnitude and K-Mg varies by a factor of two within individual crystal faces. Strontium sectoral distribution is different from those of Li, B, and Mg and K-Sr varies by up to a factor of two. These behaviors likely reflect different mechanisms of incorporation of Li, B, Mg, and Sr into calcite.

Automated summary

The distribution of trace elements within individual calcite crystals can be heterogeneous, with growth rate and crystallographic orientation playing significant roles in elemental incorporation. Experimental studies have shown variability in partition coefficients within individual crystals, with sectoral zoning influencing elemental distribution. Different mechanisms of incorporation were observed for Li, B, Mg, and Sr into calcite crystals, with significant variations in their partition coefficients within individual crystal faces.