X-ray examination of a large number of iron disulfide samples collected from the sedimentary rocks of western New York showed the presence of only pyrite, although some of this material has long been referred to in the literature as marcasite. Grinding experiments attempting to cause an inversion of marcasite to pyrite in the manner described by Anderson and Chesley (1933) were unsuccessful and it is believed that no such inversion by grinding occurs for these polymorphs. Caution is advised in grinding heat-sensitive materials in mechanical grinding machines such as the Fisher-Spex Mixer Mill because of the high temperatures generated by friction.

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