Clay Mineral Thermometry—A Critical Perspective

Abstract

Abstract--Diagenetic clay minerals usually occur as heterogeneous assemblages of submicroscopic layers consisting of different structure types such as illite, smectite and chlorite, with variable composition within a given structure type, and with highly variable concentrations of imperfections. The dimensions of mixed-layering, the semi-coherent to coherent nature of the structures across the layering, and compositional heterogeneity occur at a scale well below that of an individual thermodynamic phase. These relations imply that most clays are not distinct minerals or phases, and that assemblages of clays in shales and mudstones are incompatible with the phase rule. Such relations are better evaluated in terms of the formation of metastable materials with each small unit having unique chemical properties, rather than as a small number of stable homogeneous phases. Consequently, treatment of most clay minerals in terms of equilibrium stability with either a thermodynamic or experimental approach is subject to error.Chemical reactions involving most clay minerals are best understood with kinetic models. These involve a great variety of parameters such as time, fluid/rock ratio, deformation history, nature of starting materials and transformation mechanisms, as well as the variables, such as temperature, pressure and composition, that are commonly used to define equilibrium. Solubility experiments on the stabilities of clay minerals are unlikely to attain equilibrium at low temperatures. Moreover, the activity of soluble species may be controlled by surface equilibria, or by absorbed or exchangeable cations. Interpretations of available experiments on the solubility of illite vs. other mineral assemblages are in violation of Schreinemakers' rules and indicate lack of equilibrium.Predictable sequences of clay minerals as a function of temperature are best understood through the Ostwald step rule, in which clay mineral assemblages undergo reactions in response to kinetic factors that represent reaction progress rather than an approach to equilibrium. Currently used clay mineral thermometers (illite crystallinity, smectite/illite reaction, chlorite composition) are not based on equilibrium reactions. Such systems are not accurate thermometers and therefore have questionable utility.

Post‐impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?

“…Smectite is thermally a metastable phase and its conversion to illite through mixed-layer illite-smectite takes place over a broad temperature interval of 70-150 °C. The temperature interval varies as function of kinetically controlled processes of the reaction progress (Inoue et al 1992;Essene and Peacor 1995). In addition, a low temperature (<100 °C) origin of this mineral is indicated by its composition-octahedrally substituted Al-Fe montmorillonite-whereas, the high temperature smectites are typically tetrahedrally substituted beidellites (Inoue et al 1992).…”

Genesis of Dioctahedral Phyllosilicates During Hydrothermal Alteration of Volcanic Rocks: I. The Golden Cross Epithermal Ore Deposit, New Zealand

“…Much evidence suggests that the reaction from smectite to illite in diagenetic environments proceeds through a series of Ostwald steps (e.g., reviews of Morse and Casey, 1988;Essene and Peacor, 1995), involving metastable intermediates that result in an end-member mineral assemblage containing muscovite as the dioctahedral phyllosilicate under epizonal (greenschist facies) conditions. The series of steps thus represents a natural progression from metastable phases to an assemblage of thermodynamically stable phases.…”

Comparison of Evolution of Trioctahedral Chlorite/Berthierine/Smectite in Coeval Metabasites and Metapelites from Diagenetic to Epizonal Grades

“…Such mixed layering has been interpreted to be the principal cause of variations in so-called chlorite which have been correlated with temperature of formation (Jiang and Peacor, 1994a). Whether the mixed layering involves berthierine or smectite, however, such phases are thermodynamically metastable, and their occurrence can be ascribed only to reaction progress, and are not a quantitative function of temperature (Essene and Peacor, 1995). Arkai et al (2000) pointed out that reaction progress in metapelites, measured in part by chlorite crystallinity, paralleled that in metabasites.…”

“…The prograde changes involve metastable states (e.g. mixed layering, small crystal size; see Essene and Peacor, 1995) that depend on kinetic factors which influence reactions between starting materials and more stable states. The bulk compositions, textures and original mineral assemblages of sediments and basaltic rocks are very different.…”