N. Gary Hemming

Growth-step-selective incorporation of boron on the calcite surface

Abstract Ion microprobe analyses of as-grown{10 1 4} faces of B-doped calcite single crystals show that boron oxyanions are incorporated differentially between nonequivalent vicinal faces that compose spiral growth hillocks. The step-selective incorporation pattern for boron anions is opposite to that observed for the tetrahedral anions SO42− and SeO42−. Independent work has shown that a tetrahedrally coordinated boron species is preferentially incorporated at the surface during growth, yet the boron in the bulk calcite is predominantly trigonally coordinated. Hence, there is a change in nearest-neighbor coordination of boron at the surface during incorporation. A possible explanation for the step-selective incorporation of boron lies in a surface-site control over the mechanism or rate of this coordination change. The findings demonstrate that multiple, structurally distinct surface anion sites exist on calcite {10 1 4} faces, and inherent differences in these sites influence element incorporation and possibly surface kinetics. This behavior parallels the known differences for incorporation among surface cation sites and further confirms that trace and minor element partitioning is strongly affected by the structural aspects of mineral surfaces. Such site selectivity and associated effects on potential coordination changes may be intrinsic to crystal growth and impurity incorporation.

The lithium isotopic composition of waters of the Mono Basin, California

Mono Lake, a major closed-basin alkaline salt lake in eastern California, derives its water from a mixture of creeks and springs, with the former providing in excess of 75% of the total. The Li isotopic composition of lake water has not varied significantly over a 4 year meromictic period (δ7Li ∼ +19.5). Springs are isotopically distinct: groundwater springs and seeps carry water enriched in isotopically heavy Li whereas thermal springs supply isotopically light (δ7Li 50% of the Li to the lake; (2) the Li budget is sensitively balanced on small thermal spring contributions, < 3% of the total spring inflow; and (3) the residence time of Li in the lake is 28 ka. Other Great Basin closed lakes have variable Li isotopic compositions (δ7Li from +16.7 to +23.7), all of which differ significantly from those of several major lakes and seawater (homogeneously ∼ +32).