Circular 62 — Alkali Feldspars from Some Intrusive Porphyries of Southwestern United States

By Frederick J. Kuellmer, 1962, 15 pages.

Powder X-ray diffraction analysis of the (i01) reflections of many perthitic alkali feldspars at room temperature, 750"C., and 10500C. shows considerable differences which appear to be a function of the relative temperature-structural states of the feldspars. Sanidines remix more rapidly and completely than microclines, whereas orthoclases show all intermediate stages. Variation in rate and completeness of remixing appears to be independent of bulk composition, although the ultimate cause may well be affected by impurity elements and/or lattice defects.

The average bulk composition of the feldspars studied (225) is about Or,, (Ab + An)., weight percent, and the compositional range for any porphyry is 20 to 40 + weight percent. Perthitic remixing is accomplished faster and more completely in feldspar specimens from those parts of an igneous sequence which are more porphyritic (Globe-Miami, Arizona; Rabb Canyon, and Hillsboro, New Mexico), more pegmatitic (Rabb Canyon), more eruptive (La Sal Mountains, Utah), and/or more orebeariqg (Ajo, and Globe-Miami, Arizona; Questa, New Mexico). Thus the perthitic alkali feldspars showing the relatively highest temperature-structural states appear to be found in those parts of an igneous sequence which show the most hydrothermal features.

Such a method of comparing alkali feldspars from an intrusive appears to have value for igneous petrogenesis, as well as ore distribution. The results provide additional evidence of a genetic association between igneous crystallization and ore deposition.