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Importance of Cathodoluminescence Microscopy in Study of Sedimentary Ironstones

Richard D. Hagni

The iron ore study in Birmingham, Alabama, and Lorraine, France, showed that cathodoluminescence microscopy is a valuable tool for studying sedimentary ironstones, and that it is superior to other microscopic techniques for determining the character and origin of many constituent nonopaque mineral grains. Nonopaque mineral grains are largely to entirely obscured under normal transmitted-light microscopy because disseminated inclusions of fine-grained iron oxide are abundant. Reflected-light microscopy is significantly hampered because the fine-grained iron oxides have a poor polish and because the nonopaque minerals have a low reflectance.

Cathodoluminescence microscopy is especially important in detecting the various calcitic fossil fragments that occur as replacement remnants in ironstones. These remnants indicate iron oxide replacement, which is significant in the genesis of sedimentary ironstones. Internally zoned, subsequently deposited, sparry calcite cements are rapidly distinguished from the fossil fragments by cathodoluminescence.

Small amounts of collophane, which are extremely difficult to recognize by conventional microscopic techniques, are readily detected and studied by cathodoluminescence microscopy. More than 12 forms of collophane can be distinguished in the Birmingham, Alabama, sedimentary ironstones. These forms not only exhibit different morphology, but also may vary in intensity and color of cathodoluminescence and in genetic history.

Cathodoluminescence microscopy used with standard microscopic techniques can help workers better understand the character of sedimentary ironstones and their genesis.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.