Mixing on crystallographic sites

An activity model in which distinct atomic positions in a crystal structure are occupied independently by specified species. For ideal random mixing, a pure mineral endmember requiring ${\nu }_s$ specified atoms on site type $s$ has thermodynamic activity $a={\prod }_s{X}_s^{{\nu }_s}$ relative to that pure endmember, with other occupations held fixed. Its mixing contribution to the chemical potential is $RT\ln a$. The powers count occupied positions per formula unit; they are not always the mole fraction of whole endmember formulae.