HYDROTHERMAL SYNTHESIS OF GOLD GRAINS WITH APPARENT FIVE-FOLD SYMMETRY

CHRISTOPHER H. GAMMONS
Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7

Abstract

Crystals of gold with anomalous symmetry have been synthesized at 300 C, Psat using hydrothermal techniques. Four crystals with diameters of 50 to 60 µm display a pentagonal dipyramidal (decahedral) morphology. More typically, the gold crystals have flattened, pseudohexagonal or elongate, prismatic habits. Some of the latter also appear to possess a five-fold axis of symmetry, and look very similar to gold "fünflings" that have been reported in natural specimens. All of the crystals are gold-rich, with small amounts (<2 atomic %) of silver and silicon present in solid solution. The unusual morphology is attributed to either noncrystallographic shell packing or multiple twinning. This study is the first to illustrate that gold crystals with external five-fold symmetry can nucleate and grow to sizable dimensions in a hydrothermal environment. The occurrence of multiply-twinned particles of gold and other precious metals may be more widespread in nature then previously recognized.