Biogenic Opal in Ohio Soils

Abstract

Abstract Soils in the eastern extremity of the Prairie Peninsula have developed during most of their history under forested conditions. Biogenic opal isolated from the 50‐20µ fraction of 55 profiles representing different topo‐ and biosequences in western Ohio indicates that most Mollisols (Brunizems) of this area have similar total opal accumulations and depth‐distributions as their Alfisol (Gray‐Brown Podzolic) analogues. The prairie influence on both groups of soils has been calculated to be about 150 to 500 yars. Opal depth distributions decrease from 0.1 to 0.3% in surficial horizons to <0.03% below 50 cm. No evident differences in opal concentration occur among toposequence members. Opal isolated from leaves of deciduous trees common to Ohio is optically isotropic, has a specific gravity <2.3, and a refractive index between 1.44 and 1.48. With few exceptions, <2% of the forest opal occurs in the 50‐20µ size range with >75% in finer fractions. Scanning electron microscopy, reveals that opal in >20µ sizes isolated from tree leaves is sufficiently distinct morphologically that it would not normally be confused with that of grass origin. Opaline constituents of forest origin are generally 1 to 2µ fragile incrustations of the cell walls and seldom form solid rectangular or polyhedral structures characteristic of opalized cells of grass origin.