Computer Science
Scientific paper
Sep 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44.1363w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 44, Issue 9, pp.1363-1371
Computer Science
13
Scientific paper
The enthalpies of solution of seven synthetic clinopyroxenes on the join CaMg 2 Si 2 O 6 (diopside)-NaAlSi 2 O 6 (jadeite), of two natural low-Fe ordered omphacites near the 1:1 composition, and of a nearly pure natural jadeite, were measured in molten Pb 2 B 2 O 5 at 970 K. Enthalpies of solution of the natural omphacites experimentally disordered at 1350°C and 30 kbar were also measured. The synthetic clinopyroxenes have positive excess enthalpies of mixing, which can be expressed by a symmetrical function H mix = W H X Jd X Di , with W H = 7250 ±950 calories. The enthalpy of disordering of the two natural omphacites averages 1.8 kcal, which is nearly the same as the excess enthalpy of mixing of a 1:1 disordered pyroxene. The interaction parameter, W H , can be shown to be essentially equivalent to G ° of the reciprocal reaction: CaMgSi 2 O 6 + NaAlSi 2 O 6 = CaAlSi 2 O + 6 + NaMgSi 2 O - 6 M-site cation distribution data of natural omphacites heat-treated at 1000°C ( et al ., 1978) lead to G ° = 7200 cal for the above reaction, in good agreement with the calorimetric W H . The reciprocal solution theory with G ° = 7200 cal predicts closely the activities of NaAlSi 2 OP 6 in jadeite-diopsides found from phase equilibrium measurements at 600°C ( , 1979a) and is nearly equivalent to an entropically ideal two site mixing model with a (fictive) W H of 5800 cal. Jadeite-diopside solid solutions near the 1:1 composition at temperatures of 1000-1500 K are `pseudoideal'; that is, they have nearly the free energies of ideal one-site mixtures ( , 1973). If the order-disorder transition is nearly first-order at about 1000 K, as suggested by et al . (1978), the pseudo-ideality holds also for ordered omphacites at least somewhat below 1000 K.
Holland Tim J. B.
Kleppa O. J.
Newton Robert C.
Wood Bernard J.
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