Chapter 9 - Calcium Carbonate Features

Abstract

The precipitation of calcium carbonate is widespread in soils and regoliths and common under different environmental settings, especially in soils of arid environments. This topic is of particular interest as calcareous features are characterised by a wide variety of forms, from isolated crystals and discrete elementary features to differentiated calcrete profiles. Their development in interaction with the biosphere, atmosphere, and hydrosphere makes them useful proxies to identify the processes affecting soils. An extensive literature exists on calcite features in soils, but only few systematic and comprehensive reviews have been made from a micromorphological point of view. This chapter deals with the different morphologies of calcium carbonate accumulation in soils and regoliths, as observed in thin sections. The main calcareous pedofeatures are presented here, including coatings, hypo- and quasicoatings, infillings and nodules, which comprise specific features such as needle-fibre calcite and pisoliths. Also considered are root-related features, calcified filaments and biogenic crystal intergrowths. The microscopic characteristics of recrystallisation and dissolution features are described, which is an important topic because carbonates are frequently subjected to transformation, increasing with age of the soil. Calcareous features associated with palustrine carbonates are also reported. Each feature is reviewed considering the main earliest studies and views expressed by various later authors, on features whose origin often remains controversial. Finally, the potential of the micromorphological study of recent or inherited features to (palaeo) environmental reconstructions is also discussed.