FIELD EMISSION SCANNING ELECTRON MICROSCOPY (FESEM) OF A SPINEL LAW TWINNED GOLD CRYSTAL FROM EUGENE MOUNTAIN, NEVADA: EVIDENCE FOR GROWTH COMPLEXITY AND CHEMICAL TREATMENT

The process by which large gold crystals form in nature is not well understood. This understanding is limited by the complexity of the boundary between gold crystals and the surrounding minerals such as quartz. A purchased sample (from John Betts, a well-recognized mineral dealer) of a spinel law twinned macrocrystalline gold from the Eugene Mountains, Nevada was analyzed using a Scanning Electron Microscope (SEM) to investigate textural complexity. This analysis shows a twinned crystal of gold containing many diverse micro textures that indicates a diverse range of conditions that this sample has been subjected to through its history. This sample is composed of almost entirely of gold (electrum) with small amounts of silica. The sample is very diverse in that multiple locations express euhedral to subhedral habits along the crystals preferred orientation. Adjacent to these structures are many subhedral textures indicating the continuous growth of this sample, commonly following the crystals preferred orientation. These locations on the sample are often a euhedral blocky or bladed crystal habit that exhibit new subhedral wormy or hoppered overgrowth patterns. Within the sample are anomalous hopper gaps that may indicate the sample grew around other minerals that likely dissolved away during extraction or treatment of the sample. This is based on textural associations observed at terminations of hopper gaps. Comparison with other macrocrysalling gold samples from Round Mountain Nevada indicates that that this crystal was chemically treated to enhance luster, making the specimen more attractive for collector sale. Care must be taken in future investigations to distinguish natural textures from treatment textures. This investigation forms a basis for such comparisons in the future.