DIFFERENTIAL WEATHERING OF HOST CHAIN-SILICATE MINERALS AND INTERGROWN EXSOLVED PHASES IN THE HEAVY-MINERAL FRACTION OF SOILS DEVELOPED ON GLACIAL PARENT MATERIALS IN MICHIGAN

Chain silicates (pyroxenes and amphiboles) are common constituents of the heavy-mineral fractions of glacial deposits. Specific varieties are indicators of the source rocks for the glacially transported minerals, and vary in relative abundances among different glacial lobes. The entire ensemble of chain-silicate minerals in soils on glacial parent materials is a mechanical mixture of pyroxenes and amphiboles of many different compositions from different source-rock types. The heavy-mineral fraction of any individual sample of such soil consequently samples a variety of different chain-silicates weathered under identical conditions. Studies of such ensembles of naturally weathered chain-silicates provide insight into the relative persistence of different compositional varieties, and into parent-mineral-controlled variations in mineral-water reactions involved during natural weathering.

Most chain-silicates appear to weather by a single ubiquitous mechanism, forming elongate lenticular etch pits and denticulated terminations. Environmental factors appear not to vary sufficiently among sites to produce different surface textures. Among other things, this uniformity of process allows corrosion of hornblende to be used as a relative-age and soil development-environmental indicator. More variability in parent-mineral controlled corrosion textures is observed on other naturally weathered chain-silicates. Scanning electron microscope (SEM) imaging has revealed occurrences of weathering-resistant lamellae (exposed by preferential corrosion of the host chain-silicate) on the surfaces of some chain-silicate grains in soils developed from glacial parent materials in Michigan. More extensive weathering produces transverse grooves and truncated denticulated margins, with the squared-off ends of denticles and etch-pits appearing to correspond with less-weathered lamellae. Energy dispersive spectroscopy (EDS) allows characterization of the host chain-silicate, and shows variations in chemical composition between the corroded host chain-silicates and the resistant material. The most weathering-resistant lamellae identified so far appear to consist of ilmenite exsolved from pyroxene.