群馬県みなかみ町天沼鉱山産自然金の結晶形態

─特に擬三方晶系を示す単晶及び双晶について─

抄録

This paper discusses the five distinct native gold crystal morphologies that were identified in samples collected from the Amanuma mine, Gunma Prefecture, Japan. These include: 1) Single crystals of octahedrons composed of o{111} faces, and tetradecahedron in which the a{100} faces truncate the apex of the o{111} octahedron; 2) octahedral crystals elongated along one of <110> directions; 3) triangular pyramids with d{110} sloped faces formed on an o{111} surface, and frustum of the triangular pyramids; 4) crystals elongated along the c-axis of pseudo-trigonal system; and 5) parallel intergrowths in which octahedral crystals overlap in the direction of the elongation of a crystal. These various morphologies are hypothesized to be the foundation for the growth of feathery (vein-like) aggregates, which then undergo contact twinning to finally produce a dendritic morphology in some native gold specimens.