A STUDY OF ASTERIATED GARNETS FROM IDAHO: AN IN-DEPTH LOOK AT INCLUSIONS AND THE ROLE THEY PLAY

Garnets from Emerald Creek in Latah County Idaho are distinctive in that they are asteriated when cut en cabochon. These star garnets are found in schist that makes up the Upper Wallace Formation. Multiple studies have been done to determine the origin of the asterism. All of the studies conclude that oriented inclusions are the cause; however, there have been conflicting ideas on what those inclusions are. An early study showed through thin section analysis, X-ray powder diffraction, and magnetic separation that the oriented inclusions were magnetite. Two later studies used techniques such as transmission electron microscopy analysis, electron probe microanalysis, and Raman spectroscopy to show small, oriented inclusions of rutile were the cause of asterism. Because of these conflicting results, a study was conducted to find the origin of asterism. Petrographic analysis, scanning electron microscopy analysis, and magnetic separation are techniques used in this study. One 6-ray and two 4-ray star garnets were selected for analysis. The 6-ray and one of the 4-ray garnets were mounted on slides, ground to varying thicknesses, and polished for petrographic and SEM analysis. Because the inclusions are on different levels within the cabochon, each of the garnets were cut and ground to a level that would have a good representation of inclusions on or near the surface. The 6-ray garnet was divided into two parts, the top and lower parts of the cabochon, in order to be studied completely. Petrographic analysis of both parts of the 6-ray garnet has shown small, acicular crystals of rutile within the garnet matrix. Orientation of inclusions was difficult to see. An abundant amount of opaques were seen in thin section. SEM analysis confirmed through spot analysis and elemental mapping that these opaques were ilmenites. Results for the mineral identification of rutile via SEM analysis were inconclusive for both the 4-ray and 6-ray garnets. This was due to several factors including distance of the crystals from the surface of the garnet and the size of the crystals. The smaller rutile crystals were dwarfed by the larger and more abundant ilmenite crystals. Continuing research will include recreating the magnetic separation analysis of a powdered 4-ray garnet as was done in the earlier study.