Trapiche sapphire from some deposits in Thailand, Cambodia and Vietnam

Abstract

The number of trapiche sapphire samples from some basaltic gem fields used in this study are eleven from Chanthaburi deposit, two from Phrae deposit and one from Kanchanaburi deposit in Thailand, eight from Pailin deposit in Cambodia, and three from Di Linh deposit in Vietnam). Most samples show three distinguishable parts, namely core, arm and growth sector. Observation under a gem microscope reveals that the core and arm are occupied mainly by thin-film fluid inclusions and usually with the combination of minute particles, short needles and small black inclusions whereas the growth sector generally contains thin-film fluid inclusions and minute particles along hexagonal growth zones. Some large mineral inclusions were observed and identified as magnetite, columbite, pyrochlore, feldspar, monazite, nepheline, zircon and calcite. Magnetite appears to be related particularly to core and arm whereas the others are observed in the arm and growth sector. Trace element analysis and mapping along the different parts of trapiche sapphires significantly indicates that Fe and Ti concentrations at core are commonly higher than those of the other parts, whereas the growth sectors and arms are not clearly different in trace compositions. However in many cases arms may yield slightly higher Fe and Ti concentrations than the growth sectors or vice versa. These variations in geochemical signatures may be related to either visible or invislble micro- to-nano-sized inclusions. Regarding to the growth of trapiche sapphire it appears that the core should have been crystallized first from a unique physical and chemical condition that allowed co-precipitation and/or incorporation of many phases before the abrupt change in the growth condition that led to the formation of the arms and growth sectors simultaneously. The arms are the place in the growth sector where thin-film or multi-phase fluid inclusions were likely to form in the direction perpendicular to the {1120} faces or along the a-axes direction in which impurities in the crystal lattice could have a tendency to follow. The growth sector mainly developed as clear blue texture with lesser inclusions. These features of trapiche sapphire are quite different from trapiche ruby that usually occurred in the subsolidus metamorphic condition. In conclusion, trapiche sapphires from basaltic deposits may have been crystallized from a high volatile peraluminous felsic melt.