Mixed-Layer Clays in Geothermal Systems and Their Effectiveness as Mineral Geothermometers

Abstract

Mixed-layer clay structures have proved useful mineral geothermometers in the study of many geothermal systems around the world. The earliest work was built on the dioctahedral smectite to illite transformation but it was subsequently extended to other transformations such as trioctahedral smectite to chlorite. More recently, numerous other mixed-layer structures which may have significance as sensitive indicators of changes in either temperature or chemical environment have been recognised.

In this paper, the development of mixed-layer clay geothermometry is followed from its first applications in sedimentary sequences, through its early applications at the Wairakei geothermal system and then its general application to mixed-layer studies of other geothermal systems. The technique is successful in many fields but less so in others. The possible reasons for these successes and failures almost always is directly associated with permeability, and the ability of a system to achieve equilibrium between fluid and rocks. However, the chemical environment and the sites of substitution within the smectites (octahedral versus tetrahedral) will also impact the transformation pathways for the smectite to illite transition.