Basics for the Study of Soil and Weathered Rock Geochemical Systems

Abstract

Clays minerals form and transform in many different environments at the Earth’s sur- face or sub-surface such as soils, weathered rocks, oceanic and continental hydrother- mal systems, sedimentary layers or diagenetic series. Whatever the environments, the common point is that aqueous solutions are always present and interact with rocks where clay minerals form. These solutions may be cold (few degrees at the ocean bot- toms) or very hot (above 300 °C in the deep zones of hydrothermal systems). They may be diluted (meteoric water) or highly concentrated (brines). In all the cases, clay min- erals adjust their composition and their crystal structure to the local conditions where they form or transform. At present, the numerous observations of their natural oc- currences make the predominance of two factors obvious: the temperature and the chemical composition of both solids and solutions. Pressure seems to be negligible in the range of several kilobars since rather similar clay minerals may exist at the sur- face as well as deep in the oceanic or continental crusts. Consequently, before presenting the ways that clay minerals form or transform in different geological environments, the present chapter will focus on the definition of the driving forces at work. The principal phenomena leading to the formation of clay minerals are the alter- ation processes (replacement of a pre-existing mineral) or the direct precipitation from solutions (homogeneous or heterogeneous nucleation and crystal growth, epitaxy, …). Those mineral reactions are an exchange of matter and energy, they are typically in the scope of the chemical analyse of rocks and fluids and they can be studied through thermodynamic estimations. As for mineral reactions, if thermodynamic equilibrium is reached, the products must have specific compositions and be associated in adequate assemblages. If not, the departure from estimated equilibrium indicates that the reac- tion is incomplete and the presence of phases is governed by the kinetics of the incompleted reaction. This shows the importance of understanding the thermody- namic basics in clay mineral study. Clays are very reactive minerals because of their huge outer and inner surfaces per mass unit. This characteristic is the source of many chemical and physical properties and enhance their capacity to adjust their composition and crystal structure when the local physicochemical conditions change. We will consider here that a clay mineral is transformed if the silicate layer (1:1, 2:1, or 2:1:1 type) is changed. This excludes the modifications in the interlayer zone related to the exchange capacity. One of the most documented transformation is the smectite to illite conversion in diagenetic series and geothermal fields. It has shown the importance of kinetic effects that is to say the in- fluence of time. Summarizing, the main factors to be considered in the clay mineral formation and transformation at the surface are: temperature, chemical compositions of solids and solutions, and time. The missing dimension is the scale of the geological object under consideration. This is the first step in our quest of the driving forces for clay mineral formation because the predominant factor changes with the size of the object. Indeed, at the scale of a continent, the thermodynamics of equilibrium may depict correctly the formation of the dominating mineral species in a weathered mantle for instance while at the scale of a crystal, electrical charges overcome short-distance interactions. This leads us to define the “system” under consideration that is to say the solids and the solu- tions interacting at specific temperature conditions in a given volume of the Earth’s sur- face. Then, the reasons why clay minerals are typically formed at the Earth’s surface con- ditions will be analysed. In alteration, the reactions are most often governed by the migration of elements in solution into or out of the system under consideration. This distinguishes alteration from diagenesis, metamorphic or igneous geologic systems.