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THE MICROCLINE-SANIDINE STABILITY RELATIONS,
CHICAGO UNIV ILL DEPT OF GEOLOGY
States that are structurally intermediate between the triclinic and monoclinic modifications of KAlSi3O8 are described and discussed with relation to order-disorder in alloys. X-ray powder diffraction data on microcline are given. Intermediate states were produced by heating microcline well above the temperature of its inversion to sanidine. These states could not be produced in the approximate range of the transition, either by dry heating, or hydrothermally. Microcline is readily converted to sanidine, however, under hydrothermal conditions at temperatures well below those necessary in the dry state. The lowest temperature observed for the hydrothermal conversion of a microcline to sanidine was 525C. The hydrothermal conversion takes place by a mechanism that differs from that of the simple diffusive transformation that occurs when microcline is heated dry above ca. 1000C. The mechanism of this conversion, which involves solution of microcline and re-precipitation of sanidine is discussed. Some doubt is cast on the validity of using this type of conversion in placing transformation temperatures in the alkali feldspar system. Author
Pub. in Geochimica et Cosmochimica Acta (Gt. Brit). v5 n1 p1-19 1954 (Copies available only to DDC users).