Unit-cell parameters as functions of mole fraction fluoro-endmember have been determined for clinohumite, chondrodite, norbergite, brucite, and sellaite. In addition, the d spacing for the (060) peak of talc was determined as a function of mole fraction fluoro-talc. Unitcell parameters for the phase intermediate sellaite are

a=10.123,b=4.6861,c=3.0780A when coexisting with periclase anda=10.097,b=4.6812,c=3.0738A when coexisting with sellaite.

From these X-ray data the compositions of coexisting phases have been determined in forty hydrothermal experiments that yielded information on eighteen different chemical equilibria. These data, combined With phase equilibrium and calorimetrie data from the literature, have been treated by the method of least squares to produce a thermodynamic model for the system. The derived endmember Gibbs energies of formation from the components MgO, MgF2, SiO2, H2O at 1023 K and 1 bar are in cal mol−1ΔGHBo=15538,ΔGFBo=8466,ΔGHSo=44092,ΔGHNo=5333,ΔGFNo=15721,ΔGHCho=12031,ΔGFCho=31161,ΔGHClo=41347,ΔGFClo=58853,ΔGHTco=8509,ΔGFTco=2086,ΔGFoo=14249,ΔGEno=7566. Computed equilibria based upon these Gibbs energies and related excess parameters combined with entropies and heat capacities are in good agreement with data on natural assemblages.

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