= 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> $a=\prod_sX_s^{\nu_s}$ 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.
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