The Ca contents of coexisting augite and olivine are strongly correlated with the activity of silica through the equilibrium:
Ca2SiO4La in olivine+SiO2=Ca2Si2O6Wo in augite.

In turn, the Ca contents affect the distribution of Mg and Fe between augite (clinopyroxene, cpx) and olivine (ol). Thus any attempt to calibrate the Ca content of olivine for barometry or the Fe-Mg exchange for thermometry must take this relationship into account. For example, simple Fe-Mg exchange experiments between olivine and augite cannot give unique results if aSiO2 is not fixed, either explicitly or implicitly, through fixing a sufficient number of intensive parameters (four of six from a set equivalent to XFe,cpx, XCa,cpx, XFe,ol, XCa,ol, P, and T). Details of the relationship depend on the set of solution models chosen; we illustrate it for the Davidson and Lindsley (1989) model that has been incorporated into the QUILF program (Andersen et al., 1993; Lindsley and Frost, 1992; Frost and Lindsley, 1992).

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