Thin Sections of Soils and Friable Materials Prepared by Impregnation With the Plastic “Castolite”

T study of thin sections of soils is important because it gives a two dimensional insight into the microscopic morphology, mineralogy, and biology of the soil. Unfortunately, satisfactory procedures for making thin sections of soils have not been available. The mineralogical study of soils or friable sediments has been limited to the determination of the varieties and the relative abundance of minerals in these materials, usually after cleaning and mounting the grains in a medium of a selected refractive index. Methods and procedures for such work, as given by Marshall and Jeffries (4), remove the particles from their original positions and therefore reveal nothing of the relative positions and patterns of arrangement of the minerals. Thin sections of soils, as described in this paper, are prepared by impregnation of the natural soil with a plastic material, "Castolite," which, upon hardening, consolidates the soil into a rock-like mass. The treated and hardened soil is sliced and ground to sections 30 microns thick for mineralogical investigations and 50 to 70 microns thick for fabric studies. Thin sections of friable materials have been made by Kubiena (3) and Ross (5) with Kollolith. However, this resin will not penetrate readily into compact soils. Bakelite has also been used (5) to make thin sections of friable materials, but its refractive index is too high for distinguishing quartz from orthoclase by the relief method, also, because ether is used as a solvent, there is a possibility that part of the organic matter might be dissolved. Although the refractive index of Canada balsam is ideal for mineral identification, it must be heated to lower its viscosity sufficiently to impregnate soils, and this seriously disturbs the soil. Furthermore Canada balsam deteriorates with age and is brittle. Weatherhead (6) used pyroxylin diluted with amyl acetate to coat the surface of pure clay specimens. With this procedure, very thin films but not thin sections were obtained. Castolite, used in the procedure which follows, has numerous advantages over materials previously used to make thin sections.