# Practical Aspects of Mineral Thermobarometry

## Solid Solutions

This section deals with the thermodynamic treatment of mineral
solid solutions, from the practical viewpoint of finding simple but
effective expressions for estimating the activities of end members
(phase components) to use in P-T calculations. The topic is treated
in chapter 7 of Spear's *Metamorphic Phase Equilibria*. There are several
sub-sections:

Energy of mixing, chemical
potential, and activity

A brief introduction to the thermodynamics of solid solutions, and the
definition of activity.

Mixing on sites

Ionic model, one site, multi-site, end members with part-filled
sites (normalisation)

Non-ideal solutions

Simple energetics of non-ideal solutions. Regular and sub-regular
models. Multicomponent solutions.

Reciprocal solutions

Interactions between sites

Simplified models, and Darken's
quadratic formalism (DQF)

Useful simplified relationships for a-X relations in complex
phases

Activity calculations

Recommended models, implemented in Holland's AX program

[ Work is in progress on some of these pages ]

#### Sources and further reading

This topic is treated quite thoroughly by

- Spear (1993) Ch 7, pp. 175-190.
- Powell (1978)
*Equilibrium Thermodynamics in Petrology*

This page last modified 12 October 2004