Other
Scientific paper
Jun 1955
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1955gecoa...7..212g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 7, Issue 5-6, pp.212-228
Other
25
Scientific paper
Diagrams correlating X-ray spacing and Mg substitution in calcite, based upon spectrochemical analysis and powder diffraction data, are presented for several X-ray reflections. The Mg content of natural magnesian calcites is correlated with the P oo 2 , T relations in the system CaO--MgO--CO 2 . Sedimentary and other low-temperature calcites are shown to be very low in Mg, with the exception of certain metastable materials. Metamorphic carbonates commonly contain dolomite associated with calcite containing significant amounts of MgCO 3 in solid solution. MgCO 3 contents up to ~9 mol per cent have been observed in granulite-facies rooks and in predazzite from the Tyrol, corresponding to a minimum equilibrium temperature of ~600°C. Blebs of dolomite within single calcite grains of some metamorphic carbonates are believed to be exsolution textures, on the basis of optical and X-ray examination. Low Mg contents of calcites that may once have been at high temperatures may be a consequence of such factors as: (1) exsolution during cooling; (2) CO 2 pressures insufficient to maintain significant Mg in solid solution; (3) postmetamorphic retrograde processes, including low-temperature hydro thermal action and weathering; and (4) an environment poor in Mg.
Goldsmith Julian R.
Graf Donal L.
Joensuu Oiva I.
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