Stability of scapolite in the system Ab-An-NaCl-CaCO3 at 4 kb and 750°C |
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| Authors: | Philip M Orville |
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| Institution: | Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, U.S.A. |
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| Abstract: | Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab85An15 to Ab70An30, although albite + halite is stable relative to marialite, Na4Al3Si9O24Cl, and anorthite + calcite is stable relative to meionite, Ca4Al6Si6O24CO3. A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa3Al5Si7O24CO3 (Ab. 2An. CaCO3). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab60An40) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab60An40-Ab35An65 and another three-phase invariant assemblage, calcic plagioclase (~Ab15An85) + scapolite + calcite is stable over the approximate range Ab30An70-An15An85.Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites.Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO3 ratio in the large anion site can vary as a function of NaCl and CaCO3 activities. |
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