The Fate of Asbestiform Minerals Across a Development Sequence In Serpentinitic Landscapes.

Naturally occurring asbestos (NOA) can be found in serpentinized ultramafic parent material and associated soils, which appear throughout California and Oregon.  These landscapes pose a potential danger to human health if particles are liberated from the soil and inhaled.  The objective of this study was to examine the weathering patterns of NOA minerals in soil horizons and dust across a developmental sequence in serpentinitic landscapes.  We have examined NOA mineral (chrysotile) abundance and location in the soil profiles using x-ray diffraction, polarized light microscopy, and transmission electron microscopy.  We have found that there is a negative relationship between abundance of chrysotile in the solum and weathering intensity.  In soils with a low weathering index (< 30), the abundance of chrysotile bundles in the fine sand, clay, and dust fractions, generally decreased with proximity to the soil surface, while needles increased.  In soils with a moderate weathering index (>30), chrysotile needles and bundles increased with proximity to the soil surface in the fine sand and clay fractions, but decreased in the dust fractions.  The results indicated that during initial stages of development, chrysotile bundles and needles were liberated from parent material through physical and chemical weathering processes.  In the moderately weathered soils, bundles continued to weather further into smaller size fractions, resulting in more needles in the clay fractions.   In the dust fractions, numerous chrysotile needles were found in the weakly weathered soils, while the moderately weathered soils contained few to no needles.  The A horizons of the moderately weathered soils produced the largest quantities of dust, contained the lowest abundance of chrysotile, pedogenic iron and clay, but also the highest percent carbon.  This indicated that soils containing NOA in intensely weathered environments have a lower potential to release chrysotile needles in dust than soils in mild weathering environments.