REINVESTIGATION OF THE ANNITE EQUALS SANIDINE PLUS MAGNETITE PLUS H-2REACTION USING F(H-2)-SENSOR TECHNIQUE

The decomposition of the iron mica, annite, to sanidine plus magnetite and vapor, KFe3AlSi3O10(OH)(2) = KAlSi3O8 + Fe3O4 + H-2, has been ree xamined experimentally with the use of a variety of buffers coupled wi th f(H2) sensors at 2 kbar and between 400 and 840 degrees C. Various capsule configurations were used in this study to delineate the equili brium constant for this reaction in conjunction with Fe-57 Mossbauer s pectroscopy measurements to monitor the oxy-annite component in mica i n selected experiments. Results at the most reducing and highest tempe rature conditions of this study extend the annite stability field to h igher temperature and f(O2) values than those defined in previous work . Lower temperature results indicate that the annite-sanidine-magnetit e stability boundary does not intersect the hematite + magnetite + H2O buffer at 400 degrees C as previously reported but rather is subparal lel to the buffer curve at lower f(O2) values. The equilibrium f(H2) ( in bars) for the assemblage annite + sanidine + magnetite + vapor at 2 kbar and between 400 and 840 degrees C can be described by the relati on log f(H2) (+/-0.08) = 13.644 - (17368/T) + (5.168 x 10(6))/T-2, whe re T is temperature in kelvins.