Introduction

Abstract

The idea that the present character of a rock expresses its conditions of formation is called the facies concept in geology and is used in different fields of the science for classification and interpretation. For example, it is generally accepted by stratigraphers and sedimentologists that the present aspect of a sedimentary rock (its particle size and shape, degree of sorting, bedding characteristics, etc.) is indicative of a particular depositional environment, and that sedimentary rocks displaying the same characteristics were deposited under similar environmental conditions. Another example is the mineral facies concept in metamorphic petrology. It has been shown from thermodynamic considerations that intensive parameters like temperature, total pressure, and oxygen fugacity determine the equilibrium mineral assemblage displayed by any rock of a given bulk chemical composition; conversely, rocks displaying the same mineral assemblage are interpreted to have attained equilibrium by recrystallization under the same ranges of these environmental parameters.