ABSTRACT: Petrogenetic Significance of Expanded Mica Fabrics

D. R. Pevear, P. A. O'day, R. E. Klimentidis

Micas, especially biotite that is extensively bloated, expanded, or splayed along cleavage, and typically with intercleavage carbonate or kaolinite precipitates, are often cited as indicative of weathering, calcrete formation (wet/dry), and generally displacive cement fabric. The mica is said to be chemically altered in the process, influencing local pore fluids and supplying certain ions for the intercleavage precipitates. A petrographic study using SEM, TEM, EMP, and cathodoluminescence has identified another class of expanded micas that does not fit the genetic model described above.

These micas are in marine sands and silts, especially from deep-sea fans, which have never been exposed at the surface. They typically show tenfold expansion with many delicate septa of mica separating zones of cement. Rather than the cement-filling edge splays, these micas tend to expand in the middle. There is no evidence that they were transported in an expanded state; the expansion and cementation are clearly diagenetic. Nor are pervasive expansive fabrics seen in the rocks themselves; only the mica has expanded. Intergranular volumes of 30-35% suggest shallow formation prior to major compaction. This is supported by their occurrence in shallow Deep Sea Drilling Project cores. The micas are completely unaltered chemically; expansion appears to have been caused by progressive cemen crystallization localized by some physical or chemical property of the mica cleavage surface.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990