CHARACTERIZATION OF MONTMORILLONITE SURFACE COATINGS ON RESPIRABLE QUARTZ

In 1997, the International Agency for Research on Cancer recognized that silica carcinogenicity may be dependent on the physical and chemical characteristics of external grain surfaces. Accordingly, detailed surface characterizations of silica dusts are essential precursors to both in vitro and in vivo investigations of silica toxicity. Surface coatings on respirable quartz grains are of particular interest as they have been implicated in modifications to cytotoxic reactivity in lung tissue (e.g., Fubini, 1998, Ann. Occup. Hyg., 42(8)). Our investigations utilize SEM and ESEM analyses to characterize the mineralogy, habit, and thickness of surface coatings on quartz grains in bentonites collected from Alabama and South Dakota.

Surface coatings are pervasive on all quartz grains in the observed bentonites and consist of either montmorillite or opaline silica, or a mixture of both. Montmorillonite coatings may be so thin that underlying pre-existing conchoidal fractures are clearly visible, but may also be more than 10 microns thick. Aggressive treatments to remove the montmorillonite coatings from the quartz surfaces, involving repeated washings and reaction with HCl, were unsuccessful. Other grains in the bentonite, including K-feldspar and plagioclase, rarely show well-developed montmorillonite surface coatings. Both K-feldspar and plagioclase have been observed with opaline silica that can have growth morphology similar to that of opal-CT. Biotite and muscovite grains never have surface coatings.

A method has been developed to map variations in thickness of montmorillonite coatings on quartz grains using the SEM-EDS X-ray intensity ratio, I_Si/(I_Al + I_Si), which is obtained for multiple accelerating potentials and calibrated to thickness using the theoretical treatment of Wallace et al. (in: Silica and Silica-induced Lung Disease, Castranova et al., eds., 1996). The resulting maps reveal subtle variations in coating thickness and mineralogy that are not evident in BSE images and have potential to influence silica toxicity investigations.