Evidence for the importance of growth-surface structure to trace element incorporation in topaz

Trace element distributions in topaz [Al₂SiO₄(F,OH)₂] single crystals display both sectoral and intrasectoral zoning of trace elements as a function of growth-surface structure. Differential interference contrast microscopy shows that the dominant {110} form exhibits large, polygonized, spiral growth hillocks with four vicinal faces, each comprising an array of parallel growth steps. Cathodoluminescence microscopy and synchrotron X-ray fluorescence microanalysis show that during growth trace elements (As, Fe, and Ti) were differentially incorporated into vicinal faces having symmetrically nonequivalent step orientations, producing intrasectoral zoning in the bulk crystal. Subsectors that formed from vicinal faces related by surface symmetry posses identical compositions. In all cases, there is precise correlation between surface microtopography and trace element distribution, demonstrating a surface-structural control of trace element incorporation. -from Authors