Use of Circular Polarization on Soil Thin Sections to Distinguish Voids from Mineral Grains

Application of crossed‐polarized (XP) light to thin sections of soil often results in images where the assessment of patterns of void distribution is complicated by the extinction of anisotropic mineral grains which have their optical axes exactly parallel to the plane of polarized light. A low cost system utilizing circularly polarized (CP) light rectifies this problem and is described in detail. Thirty‐three thin sections were photographed under both XP and CP light. The resulting 35‐mm color slides were examined with a Quantimet 720 Image Analyzer. A paired t ‐test showed a highly significant reduction in void space estimates using CP light indicating a correction for mineral grain extinction. The photographs resulting from this method give consistent and accurate representation of parameters of voids regardless of sample orientation.