Leucite-Na-feldspar incompatibility: an experimental study
نویسنده
چکیده
Phase relations at atmospheric pressure in the pseudobinary join KA1Si206 (Lc)NaA1Si3Os(Ab) and in the pseudoternary join Lc-Ab-CaA12Si2Os(An) indicate that leucite is incompatible with Na-feldspar. In the former join leucite can exist with an alkali feldspar of maximum albite content Ab54. In the Lc-Ab--An join, leucite only coexists with ternary feldspars with high An contents (approximately Ans0). Under PH~O conditions leucites may only coexist with alkali feldspars even poorer in Ab than those found at atmospheric pressure. Rare occurrences of coexisting leucite and Na-feldspar in nature have probably not crystallized directly from a melt but may have formed by a process of alkali ion exchange; or they may be unstable assemblages. No support can be found for the suggestions based on thermochemical calculations that albite and leucite are compatible at high temperatures.
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