Mineral replacement reactions: from macroscopic observations to microscopic mechanisms

TLDR

Mineral replacement reactions occur mainly through dissolution–reprecipitation, involving processes such as cation exchange, weathering, and metasomatism, and are governed by relative solubility, coupled dissolution–precipitation, autocatalysis, and porosity generation that influences fluid flow and crystal growth. The paper reviews mineral replacement aspects and demonstrates common textural features to highlight similarities in their underlying microscopic mechanisms. It examines how volume change and porosity evolution, observed in natural microtextures and model salt replacement experiments, drive mineral replacement. Porosity development is a common consequence of mineral replacement, independent of the relative molar volumes of parent and product phases.

Abstract

Abstract 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.