Mysterious Minerals and Wild West Sapphires: An Example of Scanning Electron Microscopy and Microbeam Chemical Analysis in Undergraduate Research

Abstract

Integrating electron microscopy into undergraduate education is an exciting way to engage students in serious research. An ongoing study at the Electron Microscopy Center of mineral inclusions in alluvial sapphires from Montana provides an interesting opportunity of for undergraduate research. Sample preparation, and the electron beam techniques used in the study, SEM/BSE and EDS microanalysis, are straight-forward enough for senior undergraduates to master quickly. This communication presents initial results of one of the author's (Walters) Senior Directed Study Project.Long famous for the cornflower blue Yogo dike sapphires, Montana has also produced numerous multicolored alluvial sapphires [ Fig. 1]. Since their discovery in 1865, the source for these Montana alluvial sapphires has remained a mystery; one of continuing interest to mineralogists and geologists. In this study, sapphire specimens collected from gravel bar deposits around western Montana were studied by SEM/BSE and EDS x-ray microbeam analysis. The goal: identify mineral inclusions hosted in the sapphires and use this information to provide insight into the original source-rock for the sapphires.The sapphire specimens used in this study were clear, yellow, or purple to pale blue in color with fractured, pitted, and frosted surfaces. Shapes were generally irregular with some specimens forming rough hexagonal tablets with a distinct 0001 parting. The sapphires were examined by optical microscopy to identify specimens containing inclusions for chemical analysis by the SEM/EDS. Selected specimens were mounted on glass slides utilizing the 0001 surface then ground, polished, and carbon-coated. Sapphire specimens are durable and stable under the beam thus allowing the student to work with varying accelerating voltages, beam currents, condenser lens settings etc. Specimens were imaged and analyzed on an AMRAY 1830 SEM equipped with an Oxford Link Tetra BSE detector and a Noran System Six EDS. Inclusions were mapped by SEM/BSE and then identified chemically using EDS microanalysis.Inclusions in sapphires occur as solitary phases, composite phases [i.e. lithic fragments], and multi-phase clots, and are sub-rounded to rounded with distinct contacts between inclusions and host sapphires [Fig 2]. Mineral phases identified in various specimens include biotite, apatite, Na-feldspar, K-feldspar, rutile, ilmenite, Cr-Fe spinel, and zircon [ Fig. 2]. The variety of minerals phase observed in different sapphire specimens indicates multiple potential source rocks and a complex petrologic evolution for these alluvial sapphires. High-grade metamorphic or felsic igneous rocks are both alternatives for parent/rocks for these sapphires specimens. However, one interesting composite inclusion from a single specimen stands out [ Fig 3]; a series of solitary Fe-aluminosilicate minerals-tentatively identified as almandine garnet-with a coexisting high Z-1124 CD