Separation of heavy minerals from mudrocks

Abstract

The non-clay-mineral fraction of mudrocks is receiving increased attention. The heavy-mineral fraction in fine-grained rocks, however, has largely been neglected, partially because of the difficulty in separating heavy minerals from the host mudrock. We propose a technique that will separate some heavy minerals from a clay matrix. This method was developed initially by soil scientists for separating quartz and feldspar from clays, and later modified for mudrocks. The technique involves fusing the rocks with sodium bisulfate, making clay minerals soluble in basic solutions. We have tested this procedure on nine common heavy minerals: andradite, almandine, grossular, zircon, magnetite, schorl, apatite, rutile, and a gem variety of tourmaline. Each mineral was fused and the mass reduction and resultant chemical changes observed. Physical grain alterations were examined by SEM and chemical changes analyzed by electron microprobe. Almandine, grossular, and zircon survived the fusion process in high percentages and showed little chemical change. Andradite, although it showed small mass loss, was highly etched by the precess. Schorl, gem tourmaline, magnetite, and ruffle showed a high mass loss with some chemical change to the residue. Apatite was totally dissolved. The chemical resistance of several heavy-mineral species makes this technique useful for provenance studies of mudrocks, especially when combined with chemical studies of heavy minerals.