Paragonite has been identified by electron microprobe and X-ray diffraction analysis in four specimens of metamorphic rock from northern New Mexico. The sodic mica coexists with quartz in three of these rocks. One comes from an area where kyanite, andalusite and sillimanite coexist. The other two come from widely separated areas where sillimanite is the only polymorph of Al2SiO5 to be found, and sillimanite coexists with paragonite and quartz in each. The paragonite–quartz–sillimanite assemblages appear to be stable. These are the first reported occurrences of this assemblage, and its presence supports the Al2SiO5 triple point of Holdaway (1971).

Muscovite occurs in two of the paragonite–sillimanite quartzites. Because these two samples crystallized at similar temperatures, and define a solvus, K/(K+Na) ratios should be similar for the two muscovites and for the two paragonites. However, large differences in K/(K+Na) of muscovite exist between the two samples and smaller differences in that ratio occur between the two paragonites. These variations may be related to differences in the celadonite content of muscovite in the two rocks. Consistent with tentative suggestions of previous workers, data presented here suggest that, as Fe and Mg are added to muscovite–paragonite pairs, K/(K+Na) increases in muscovite and may decrease slightly in paragonite. This relationship has serious implications for muscovite–paragonite solvus geothermometry since most natural muscovites and paragonites have small amounts of Fe and Mg. Generally, muscovite–paragonite solvus geothermometry seems to yield inconsistent and unreasonably high temperatures.

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