Effect of matrix composition on carbonate nodule crystallization

The mechanism of carbonate nodule formation in three soil profiles of different texture was elucidated by the application of several methods: petrographic microscopy, electron microprobe, and scanning electron microscope analysis. The profiles chosen for detailed investigation included: a loessial Serozem overlying a buried soil developed on calcareous sand in the semiarid climate of the northern Negev, Israel (EH); a polygenetic Husmas soil with secondary carbonate enrichment in a leached red mediterranean sandy clay loam from the southern Pleshet, Israel (GA); a brown Grumusol with carbonate nodules in the lower horizon from the Zebulon valley, Israel (GR).Three kinds of carbonate nodules were distinguished according to their morphology and origin: (1) orthic nodules, which have skeleton grains similar to the surrounding soil and a gradual transition to the soil matrix — these are formed in situ; (2) disorthic nodules, which on the basis of their sharp boundaries can be judged as having been subjected to some pedoturbation but have a fabric resembling the surrounding matrix; and (3) allothic nodules, which have a fabric that differs in composition from the soil in which they are incorporated and are thus judged to have been transported into the soil.This study indicates that the orthic nodules in the loessial Serozem and in the buried soil have been formed by gradual precipitation of carbonate in the microvoids of the matrix resulting in greater density and a partial expulsion of the non-carbonate clay to the fringes. X-ray spectroscopy traverses and cathode-ray distributions of the Al, Mg and Fe by microprobe indicate the presence of clay in the nodule and its gradual increase towards the still active fringes. In disorthic nodules no such increase toward the fringes was observed.Observations with the scanning electron microscope (SEM) indicate a diameter of 1.5–4 μ for the microcalcite in the Serozem and 4–7 μ for that in the buried sandy soil. Both are layered and built up from oval subhedral crystals 0.2–0.6 μ in size, a morphological type which has not been reported previously.From a detailed study of these profiles and from comparisons with some other soils, the authors conclude that the size and growth of the calcite crystals in the nodule is determined by the matrix composition, in particular by the presence of clay minerals. In a coarse-grained sandy matrix, microsparite and sparite precipitate, thus filling the voids. Similarly the vughs between the carbonate-free stable and compact peds of the Grumusol serve as good sites for the crystallization of a sparite fabric. On the other hand, the presence of dispersed clay minerals in the calcareous Serozem soil offers a large number of nucleation points for the formation of a micritic fabric, which then occludes part of the clay minerals. The presence of the clay retards and possibly even prevents a subsequent growth and recrystallization of the calcite crystallites.