HYDROTHERMAL SYNTHESIS OF GOLD GRAINS WITH APPARENT 5-FOLD SYMMETRY

Crystals of gold with anomalous symmetry have been synthesized at 300 degrees C, P-sat using hydrothermal techniques. Four crystals with dia meters of 50 to 60 mu m display a pentagonal dipyramidal (decahedral) morphology. More typically, the gold crystals have flattened, pseudohe xagonal or elongate, prismatic habits. Some of the latter also appear to possess a five-fold axis of symmetry, and look very similar to gold ''funflings'' that have been reported in natural specimens. All of th e crystals are gold-rich, with small amounts (< 2 atomic %) of silver and silicon present in solid solution. The unusual morphology is attri buted to either noncrystallographic shell packing or multiple twinning . This study is the first to illustrate that gold crystals with extern al five-fold symmetry can nucleate and grow to sizable dimensions in a hydrothermal environment. The occurrence of multiply-twinned particle s of gold and other precious metals may be more widespread in nature t hen previously recognized.