Mathematics – Logic
Scientific paper
Sep 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983gecoa..47.1597c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 47, Issue 9, pp.1597-1603
Mathematics
Logic
19
Scientific paper
Enthalpies of solution in eutectic (Li, Na) 2 B 2 O 4 melts at 1023 K were measured for five synthetic orthopyroxenes on the join MgSiO 3 -FeSiO 3 . The pyroxenes were synthesized at 1120°C and 20 kbar and thus were presumed to be highly disordered. The measurements indicate a small positive enthalpy of mixing, with W H = 950 cal / MSiO 3 . Enthalpy of solution measurements were made on a natural, well-ordered orthopyroxene near the composition En 52.5 Fs 47.5 and on this material after heat-treatment at 1150°C and 20 kbar. Irreversible expansion of the unit-cell constants of the natural pyroxene after heat-treatment at various temperatures was used to characterize the degree of M-site disorder. The observed enthalpy of solution decrement of 0.85 kcal/MSiO 3 between the natural En 52.5 and the same material heated at 1150° corresponds to about half of the maximum possible disordering, or X Fe M 1 0.25, which leads to a H of 7.5 kcal/M 2 Si 2 O 6 , for the exchange reaction: Fe ( M 2) + Mg ( Ml ) = Fe ( Ml ) + Mg ( M 2) if M-site interaction energy terms are ignored. This H is larger than inferred from any of the analyses of site-occupancy data except that of Besancon (1981), who gave a very similar value. The measured H of disorder and the W H of mixing together indicate a large H as great as 3.2 kcal for the reciprocal reaction: Fe 2 Si 2 O 6 + Mg 2 Si 2 O 6 = Fe ( M 2) Mg ( M 1) Si 2 O 6 + Fe ( M 1) Mg ( M 2) Si 2 O 6 as anticipated by Sack (1980). As a consequence of the inferred magnitudes of H of the exchange and reciprocal reactions, departures from ideality of Gibbs energy of mixing of orthopyroxene are very small at 700°-1000°C. Activities of MgSiO 3 and FeSiO 3 may be replaced by their mol fractions at all temperatures in most petrologic calculations.
Chatillon-Colinet C.
Kleppa O. J.
Newton Robert C.
Perkins Dexter III
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