MICAS IN CROSS-BEDDED SANDSTONES AND THEIR ABRASIONAL TRENDS

We have noted micas (mostly muscovite) in several classic cross-bedded eolian sandstones. We have searched for micas in modern eolian settings, but have only found them near crystalline igneous sources. We set out to test the survival of muscovite flakes in the conditions known to produce cross-bedding in sandstones. Our goal was to experimentally test the survival of mica in both eolian and subaqueous environments.

Simulated eolian experiments were carried out in a 4 liter pickle jar with a remote control airplane propeller attached through the lid. Air circulation velocity was controlled so that a small dune continually migrated around the bottom of the jar. Samples were removed at various time intervals over the course of 20 days, and were made into thin sections. Results showed that micas deteriorate quickly in such conditions, within a matter of a few days. Very few pieces of visible mica could be found after 96 hours of agitation (about 483 km of travel), even when examined in microscopic thin section. In this time, most of the mica had degraded into silt- and clay-sized particles.

Simulated subaqueous experiments were carried out in a 1.5 liter pickle jar. The jar was rotated by a rock tumbler at constant speed, so that the sand continually climbed the rising side of the jar, resulting in a continuous dune. After 2853 hours of agitation, the average long axis length of mica particles had only decreased from 1.82 mm to 1.41 mm. Even after 26 weeks (about 3500 km of travel), mica flakes were still large enough to be seen with the naked eye (still > 1 mm in size). Thus, subaqueous conditions do not appear to significantly accelerate muscovite deterioration.

In the experimental conditions we tested, micas persisted for an excess of 26 weeks in a subaqueous environment, but little more than 4 days in an eolian environment. As one possible explanation, the water may be acting as a cushion, reducing the violence of grain-to-grain collisions. Air provides almost no cushioning effect and allows for more forceful collisions. Quartz has a Mohs hardness of 7, while muscovite has only a 2.5. Harsh impacts between these two minerals would account for the rapid decrease in the softer mineral’s size. Micas may be a reliable trace mineral in order to determine whether a sandstone is eolian or subaqueous in origin.