A Model for the Irradiative Coloration of Smoky Feldspar and the Inhibiting Influence of Water

Anne M. Hofmeister and George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA  91125

 Radiation-induced smoky color and associated electron paramagnetic resonance (EPR) signals develop only in potassium feldspar (KAlSi₃O₈) free of structurally bound molecular water. Fluid inclusion water does not influence coloration. The integrated intensity of each of the four bands (11,600, 16,200, 19,100, and 27,200 cm^-1) in the optical absorption spectra are lineraly correlated with the  doubly-integrated intensity of a broad, asymmetric first derivative at g_eff = 2.027 in, EPR spectra. In microcline, the EPR pattern is resollved into an asymmetric six-line pattern at g_eff = 2.024 and a single derivative at  g_eff - 2.009 which, based on analogy to alkali-silicate glass, are due respectively to [SiO₄/K⁺]²⁺ and a hole shared between the two nonbonding oxygens on Si. We propose that structural water inhibits formation of smoky centers in feldspar by releasing atomic hydrogen during irradiation which des troys centers while diffusing towards a stable site.