Corundum is extremely rare in Wisconsin, being found exclusively as a redeposited sedimentary mineral. Corundum survives erosion and weathering, and can be found as a heavy mineral in sands and gravels. It can be found in silica-under saturated igneous rocks such as syenite or in certain schist with unusually high amounts of alumina and low amounts of silica. More often, it is found as drabber blue to blue-gray crystals.
(Photo by Andrew Silver.)ĭescription: Corundum is most famous for its rare colored varieties: the bright red ruby and the deep blue sapphire.
Geological Survey Denver Library Photographic Collection. From the mineral collection of Brigham Young University Department of Geology, Provo, Utah, Mineral Specimens 419. The spatial association of basalt-related gemfields in eastern Australia with a long-lived convergent margin suggests a link between corundum formation and Al-enrichment of the mantle wedge during periods of subduction.Close view of corundum. Our sapphire and ruby results reveal a continuum in trace-element compositions, an observation that raises questions regarding previous classifications that ascribe corundum from basalt-hosted gemfields to either ‘magmatic’ or ‘metamorphic’ sources. Additionally, we collected LA–ICP–MS U–Pb and traceelement data from zircon megacrysts atWeldborough, Tasmania, which is also within the gemstone belt. However, recent studies, including this one, have discovered transitional groups between these end-members that are difficult to classify.We used laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) to measure trace-element concentrations in sapphire and ruby crystals from eight alluvial deposits that span a significant length of the eastern Australian gemstone belt. Some previous studies have suggested that trace-element ratios (namely, Cr/Ga and Ga/Mg) are useful for distinguishing two types of corundum: ‘magmatic’ and ‘metamorphic’, designations that include mantle and crustal processes. Significant uncertainty surrounds the processes involved in the formation of basalt-hosted corundum, particularly the role that the mantle plays in corundum generation.
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