Mineral replacement reactions take place primarily by dissolution-reprecipitation processes. Processes such as cation exchange, chemical weathering, deuteric alteration, leaching, pseudomorphism, metasomatism, diagenesis and metamorphism are all linked by common features in which one mineral or mineral assemblage is replaced by a more stable assemblage. The aim of this paper is to review some of these aspects of mineral replacement and to demonstrate the textural features they have in common, in order to emphasize the similarities in the underlying microscopic mechanisms. The role of volume change and evolution of porosity is explored both from natural microtextures and new experiments on model replacement reactions in simple salts. It is shown that the development of porosity is often a consequence of mineral replacement processes, irrespective of the relative molar volumes of parent and product solid phases. The key issue is the relative solubility of the phases in the fluid phase. Concepts such as coupled dissolution-precipitation, and autocatalysis are important in understanding these processes. Some consequences of porosity generation for metamorphic fluid flow as well as subsequent crystal growth are also discussed.

The Viséan sequence at East Kirkton contains abundant plant fossils which show a wide range of preservation states. Most of the plant fragments are allochthonous, but stigmarian rootlets are found in situ at the top of the sequence and form mats near the base, where they are preserved in cherts. Axes have commonly been reworked within tuffs at the base. The majority of the plants are preserved as fragmentary compressions, although mineral replacement of the organic matter has occurred in some places. A number of woody axes which have been permineralised by phases of calcite and silica are found in the limestone and tuffs of the sequence. These axes often show complex mineralisation patterns and can occur at the centre of stromatolitic nodules. Some are well preserved and appear to have been permineralised rapidly whereas others show evidence of decay prior to calcite growth or degradation after stromatolite developed. Fusain (fossil charcoal) is abundant in the sequence, in addition to fusain transition fossils caused by partial charring, which suggests the occurrence of palaeowildfires in the area. The plant fossils have been assigned to a number of categories (some of which are new), based on their varied preservation states.