Resumen de Estudio por SEM y TEM de interestratificados cookeita-paragonita en metapelitas de bajo grado de la Cuenca de Cameros (Soria- La Rioja)
María Pilar Mata Campo, Fernando Javier Nieto García, Francisco López Aguayo
Cookeite is a common phyllosilicate in low-grade metapelites from Cameros Basin (NW Spain).
It occurs as a thin film on poikiloblastic pyrite crystals. Mineral assemblage of cookeite-pyrite bearing rocks is made of i) chamosite, Fe-rich chloritoid, muscovite, paragonite, rectorite, kaolinite, calcite, and quartz and ii) chamosite, muscovite, paragonite, rectorite, kaolinite, calcite, pyrophyllite, albite and quartz with minor amphibole, anhidrite, gypsum and monazite.
BSE and TEM images show a transitional area between the cokeite and the pyrite matrix. 23.6 to 24 A units occurs in thick packets with rectorite, paragonite and cookeite. EDS analysis show an average composition of Na027 K0M Cao u (Si326 AlQ74) Al25B Mg0 23 Feoos Ow (OH)5. Combined SEM-TEM and EDS data suggest that interstratified cookeite-paragonite and cookeite-smectite occurs. Although the regular interstratificed cookeite-paragonite has been interpreted as a highpressure mineral, lack of any suitable high pressure indicator in these rocks allow us to conclude that both kinds of interstratification have a low-T, low-P origin.
