GARNET COMPOSITIONS PRODUCING PROTECTIVE-SURFACE COATINGS IDENTIFIED USING THE PILLING-BEDWORTH RULE

Garnet compositions that produce a protective-surface coating can be determined by applying the Pilling-Bedworth Rule. Application of the Pilling-Bedworth Rule to each of 251 published unique garnet chemical compositions found that only ~23% (11 out of 48) almandine garnet compositions have adequate iron and aluminum stoichiometries to produce a protective-surface coating of goethite and gibbsite. The minimum almandine garnet stoichiometries needed to produce a goethite and gibbsite protective-surface coating (based on an anion basis of O₂₄) are ~3.5 for iron and ~3.8 for aluminum. Substituting kaolinite for gibbsite lowers the stoichiometries for iron to ~2.2 and aluminum to ~3.6 and increases the almandine garnets producing a protective surface coating to ~88% (42 out of 48).

The chemical compositions of four almandine garnet samples from western North Carolina and one almandine garnet sample from southern Lancaster County, Pennsylvania where obtained. Microscopic observations reveal that the garnets from western North Carolina form protective-surface coatings while the southern Lancaster County, Pennsylvania garnet does not. The values obtained by applying the Pilling-Bedworth rule to all five garnets shows that none of them should have produced a protective-surface coating of goethite and gibbsite for their respective chemical compositions, while only one of the western North Carolina garnets would not produce a protective-surface coating of goethite and kaolinite. Therefore, the likely weathering products for the Lancaster County, Pennsylvania garnets are goethite and gibbsite, while for the North Carolina garnets, goethite and kaolinite are the likely secondary minerals.

These results underscore the importance of proper secondary mineral characterization when applying the Pilling-Bedworth Rule to almandine garnet weathering. Substituting kaolinite for gibbsite demonstrates how changing a secondary mineral can impact the number of garnets producing a protective-surface coating. Microscopic confirmation of the presence of a protective-surface coating, along with XRD and other microanalytical methods is necessary to support that the proper secondary minerals have been identified.