Scanning Electron Microscopy of Cherts in Relation to the Oxygen Isotopic Variation of Soil Quartz

Mahmut Sayin and M. L. Jackson*
Department of Soil Science, University of Wisconsin, Madison, Wisconsin 53706, U.S.A.
* Reseach Assistant and Franklin Hiram King Professor of Soil Science, respectively.

Abstract: Quartz isolated from soils by the pyrosulfate-H2SiF6 method and chert samples of various origins were examined with the scanning electron microscope. Quartz isolates of the 20–50 µm from the A2 and B2lt horizons of the Baxter soil (AR), with quartz δ18O of 18·2 and 19·‰, respectively, showed a mixture of detrital quartz particles and chert clusters of aggregates of fine euhedral quartz crystals. The 2–5 µm fractions of both horizons consisted mainly of euhedral quartz particles. The 21–50 µm fraction from the underlying chert, with a δ18O = 29·6‰, consisted of aggregates of euhedral quartz particles 1–10 µm dia. and of subhedral particles 0·1–0·5 µm dia. In the soil fractions, the size and shape of the quartz particles as well as oxygen isotope data indicated that the aggregates were from the underlying chert but that irregular, unaggregated grains were detritally admixed loess, particularly in the medium and coarse silt fractions. This mixing of chert (of low temperature origin and heavy oxygen) with detrital quartz (of high temperature origin and light oxygen) gave rise to the intermediate δ18O values in the soil quartz. The SEM of cherts of different geological ages showed different morphologies. Prismatic, polyhedral microcrystalline quartz of 1–10 µm size as well as submicron, euhedral particles were observed in cavities. Submicron, subhedral particles and interlocking quartz grains were characteristic of Precambrian chert. Quartz grains more than 100 µm in size isolated by HCl from Ordovician dolomite (WI) had large (many microns) subhedral overgrowths and attached clusters of 0·1–0·5 µm microcrystalline quartz. A Danish flint formed in chalk had calcite-lined cavities (x-ray emission determined) in which spheroidal fibrous chalcedony occurred.

Clays and Clay Minerals; November 1975 v. 23; no. 5; p. 365-368; DOI: 10.1346/CCMN.1975.0230507
© 1975, The Clay Minerals Society
Clay Minerals Society (www.clays.org)