https://eps.mcgill.ca/~courses/c644/Biomineralization%20(2011)/Growth_Apatite_Calcite/Paquette_Reeder_1995_Surface_growth_mechanism_trace_elements_in%20calcite.pdf

Relationship between surface structure, growth mechanism, and trace element incorporation in calcite

Distribution of trace elements between carbonate minerals and aqueous solutions

Calcite precipitation mechanisms and inhibition by orthophosphate: In situ observations by Scanning Force Microscopy

Crystals characterized by oscillatory zoning show evidence for a quasi-cyclic alternation in the chemical composition of discrete growth-shells from tens of nanometers to tens of micrometers in thickness. Oscillatory zoning is most commonly observed and studied in crystals of magmatic plagioclase, but sensitive imaging techniques (e.g., Nomarski interference contrast microscopy, cathodoluminescence, back-scattered electron imaging, X-ray topographs) reveal its presence in many other minerals. Through the application of these techniques and an exhaustive review of the literature, oscillatory zoning is shown to occur in at least 75 rock-forming and accessory minerals comprising most major mineral classes: silicates (hydrous and anhydrous), sulfides, oxides, halides, carbonates, phosphates, and sulfates. Such mineral zoning is a common but often well-concealed phenomenon in magmatic rocks (particularly alkaline ones), hydrothermally altered rocks, mineralized rocks, and carbonate sequences. Mechanisms of oscillatory zoning are discussed, including the less-studied effects of adsorption and elastic stress in near-surface growth-layers. Its "non-equilibrium chemistry" makes oscillatory zoning a rich subject for further research.

Oscillatory zoning in minerals; a common phenomenon

Experimental conditions of Busenberg and Plummer for 1atm p[sub CO[sub 2]] and variable saturation states were reproduced and growth mechanisms were examined in situ at these conditions. Using a pH-stat method, calcite seeds were reacted in solutions of known calcite saturation state at 25 C for 1--2 days before transferring to an SFM fluid cell for continued reaction in the same solutions. The authors observed that when solution saturations with respect to calcite were >1--2, precipitation began with the formation of surface nuclei. These nuclei spread, coalesced, and continued growing. Only after more than 1 hour was there a transition to a mechanism resembling spiral growth. At these long reaction times, migrating steps assumed individual heights of 1--3 monolayers. The influence of phosphate was examined and observations suggested two inhibition mechanisms, depending on surface history. Phosphate (6 [mu]mol PO[sub 4]) introduced during the nucleation stage results in the formation of nuclei with amorphous shapes. Phosphate introduced during layer growth disrupts the relatively straight steps produced during PO[sub 4]-free growth to form jagged steps. A comparison of their observations with kinetic interpretations reported by solution chemistry studies of calcite precipitation suggests that some rate experiments may include the early influence ofmore » a rapid nonlinear growth phase caused by surface nucleation. If true, the analysis of data and interpretations of growth mechanisms may be affected and suggests that rates of stable calcite growth in natural systems may be slower than experimentally determined values would predict. Both of the phosphate-calcite surface interactions are consistent with mechanisms proposed in previous studies.« less

Richard O. Fagioli

Papers

Oscillatory zoning of Mn in solution-grown calcite crystals