Abstract
SummaryPhase relations at atmospheric pressure in the pseudobinary join KAlSi2O6 (Lc)-NaAlSi3O8(Ab) and in the pseudoternary join Lc-Ab-CaAl2Si2O8(An) indicate that leucite is in-compatible 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 An50). Under PH2O 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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