Question

State (2), explain (2), and derive (6) Gibbs’ Phase rule for a system with C components occurring in P different phases.

Answer 1

Gibbs Phase Rule is expressed by the simple formulation:

P + F = C + 2, where

P is the number of phases in the system

A phase is any physically separable material in the system. Every unique mineral is a phase (including polymorphs); igneous melts, liquids (aqueous solutions), and vapor are also considered unique phases. It is possible to have two or more phases in the same state of matter (e.g. solid mineral assemblages, immiscible silicate and sulfide melts, immiscible liquids such as water and hydrocarbons, etc.) Phases may either be pure compounds or mixtures such as solid or aqueous solutions–but they must "behave" as a coherent substance with fixed chemical and physical properties.

C is the minimum number of chemical components required to constitute all the phases in the system

For historical reasons, geologists normally define components in terms of the simple oxides (e.g. SiO₂, Al₂O₃, CaO, etc.). If two possible components always occur in the same proportions in multiple phases in a system, these can be combined into a single component (remember, we are always trying to define the minimum number of components required to make all the phases in the system)