The stability field of anthophyllite : the effect of experimental uncertainty on pennissible phase diagram topologies

We have examined experimental data for reactions among anthophyllite, enstatite, forsterite, quartz, talc, and water, using linear programming methods, in order to examine the range of internally consistent thermodynamic data sets that is permitted by the experiments. Phase diagrams calculated using the derived thermodynamic parameters may have all topologies previously proposed to describe the anthophyllite stabitty field as well as others. Depending upon the experimental data analyzed, phase diagrams in close agreement with calorimetrically measured thermodynamic parameters may have the topology originally proposed by Greenwood (1963) or the topology he sketched (Greenwood, l97l) in revising his original ideas. The experimental data of Chernosky (1976) and Greenwood (1963) appear not to be consistent with the available calorimetric data for the above minerals within their stated uncertainties. This discrepancy is most easily explained if there exists a major difference between the properties of talcs used in the experiments and that used for calorimetric measurements or if the calorimetric measurements are incorrect. A similar di-fference in the properties of anthophyllite may also be indicated. Introduction ent inconsistencies between his experiments and Greenwood's, and suggested that a phase diagram Greenwood (1963) demonstrated for the first time topology in which anthophyllite stability terminated that the orthorhombic amphibole, anthophyllite, has at low pressures might be required in order to resolve an extensive stability field in the system magnesiathese discrepancies. Hemley et al. (19774 b) reported silica-water. Using thermodynamic data and the reequilibrium temperatures at one kbar for several results of his experinents, he suggested that the stabilactions in the system magnesia-silica-water. They ity of anthophyllite was limited at both low and high calculated a phase diagram using these temperatures temperatures by dehydration reactions and that the and thermodynamic data, some of which were destability field terminated at about twenty kbar by the rived from the experiments. This diagram, Figure 2, anhydrous decomposition of anthophyllite to enstaappears to contradict that favored by Chernosky tite plus talc (Fig. l). (1976) and Greenwood (1963). Chernosky (1976) and Hemley et al. (1977a, b) Chernosky and Knapp (1977) have reported addihave made new experiments in the system rragnesiational experiments and have concluded that the ansilica-water. Chernosky (1976) combined his data thophyllite field is best described by the phase diawith experiments by Skippen (1971) and Greenwood gram proposed by Hemley et al. (1977b). (1963), and favored the topology illustrated by Any attempt to decide between alternatives should Greenwood (1963); he suggested, however, that the proceed with a clear idea of the uncertainties inanthophyllite field might terminate at pressures as volved. Toward this end, we have investigated the low as five kbar. Chernosky recognized some apparpossible thermodynamically consistent phase diam3 _00,4X / 7 9 / 0708-0809$02. 0 810 DAY AND HALBACH: ANTHOPHYLLITE