Micromorphological fabrics and developmental stages of carbonate nodular forms related to soil characteristics

Abstract

Carbonate nodules often constitute the first stage of calcrete formation. Micromorphological analysis of carbonate nodules and nodular fabrics in different soil materials shows that their development is a function of several factors: nature of the host matrix (texture, porosity), carbonate and non-carbonate clay distribution, bulk density and the interactions among them. Different micromorphological stages, therefore, exist in different soil materials.In calcareous, medium-textured soil materials, the stages of nodule formation are related to the increase of the density of the nodule caused by the accumulation of microcalcites. In such a soil material the following stages occur: (1) microcalcites within the low-to-moderate density matrix; (2) microcalcites of moderate to high density and moderately dense, diffuse nodules; parallel calcans may occur; (3) microcalcites of high density with dense microcalcitic nodules. During these stages the amount of non-carbonate clay decreases but is homogeneously dispersed and disseminated with the microcalcites. In general, the size of microcalcites, 1–8 μm, is inversely related to the clay content.In non-calcareous, medium-to-fine textured soil materials, secondary carbonate precipitates in intrapedal and/or interpedal voids as large sparry crystals. Due to pedoturbation processes, the nodules thus formed are subjected to recrystallization, resulting in a microsparry pattern. The development stages are: (1) few calcans and crystal chambers; (2) crystic nodules mainly of sparry crystals and an increase of calcans and crystic chambers; (3) recrystallized nodules composed mainly of microspars.In coarse-grained materials, secondary carbonates precipitate directly in voids as sparry crystals and the resulting nodules are of the crystic type. The large crystals are often subjected to recrystallization.In partly calcareous, coarse-grained materials with additions of calcareous dust, particular micromorphological carbonate forms occur. In the C horizon, cutanic calciasepic fabric forms besides zones of crystic fabric which gradually become calciasepic in the B horizon and include the typical nodular forms of the calcareous medium-textured soil material, together with appreciable amounts of glaebular halos.The processes leading to the formation of these stages occur essentially without a significant biogenic influence. Where carbonate segregation occurs as rhizomorphs surrounding active roots, the fabric is always micritic even in coarse textural soil material.