Feldspars from pegmatites in Precambrian gneiss within 2770 feet of a Tertiary quartz monzonite stock near Eldora, Colorado, were transformed from maximum microcline to orthoclase by contact metamorphism. In the range 2770 to 1130 feet a monoclinic and a triclinic potassic phase coexist in perthitic intergrowth with albite of low-intermediate structural state. At greater distances perthites consisting of maximum microcline and low albite were unaffected, and at shorter distances the samples are composed of orthoclase and intermediate albite.

Data of this study combined with the work of Steiger and Hart place the upper stability limit of maximum microcline on the binary Or-Ab solvus at 375° ± 50°C. A first-order transformation is suggested by the absence of microcline of intermediate obliquity but a higher order transformation occurring over a small range of temperature is also possible.

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