Mathematics – Logic
Scientific paper
Mar 1960
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1960gecoa..19...10e&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 19, Issue 1, pp.10-40
Mathematics
Logic
8
Scientific paper
Magnesioriebeckite, o Na 2 Mg 3 Fe 2 3+ Si 8 O 22 (OH) 2 , has been synthesized hydrothermally and its stability field determined. The physical parameters which determine experimental phase equilibria include the partial pressure of oxygen as well as the temperature and vapour (= total, largely H 2 O) pressure. The stability field of magnesioriebeckite is bounded by surfaces within a P o 2 - P vapour - T volume. P vapour - T diagrams represent projections along the P o 2 axis. Three P vapour - T diagrams are presented for magnesioriebeckite, each determined using a different buffer. At low vapour pressure, magnesioriebeckite breaks down to a high-temperature assemblage consisting of one or two iron (+ Mg) oxides, an olivine, Na 2 O·5(Mg,Fe)O·12SiO 2 , acmite or Na 2 O·2(Mg,Fe)O·6SiO 2 , and vapour. At vapour pressures greater than about 200 bars, magnesioriebeckite melts incongruently to one or two iron (+ Mg) oxides, an olivine, Na 2 O·5(Mg,Fe)O·12SiO 2 , liquid and vapour. Above 300 to 500 bars vapour pressure, magnesioriebeckite melts incongruently to one or two iron (+ Mg) oxides, an olivine, an orthopyroxene, liquid and vapour. Values of H for reactions involving magnesioriebeckite have been calculated; heats of reaction are similar to those for the breakdown of other hydrous minerals. Optical properties of synthetic magnesioriebeckite vary in response to P o 2 and T . The effect of the partial oxygen pressure on the composition and stability relations of minerals synthesized in this study again illustrates the petrologic importance of this variable. Results of the investigation indicate that magnesioriebeckite is stable at magmatic temperatures; the existence of an intermediate member of the magnesioriebeckite-riebeckite series as an authigenic mineral suggests that magnesioriebeckite is also stable at very low temperatures. Since it has a wide P vapour - T range, the restricted natural occurrence of magnesioriebeckite results chiefly from the rare fulfillment of hypersodic chemical conditions.
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