Estudio por SEM y TEM de interestratificados cookeita-paragonita en metapelitas de bajo grado de la Cuenca de Cameros (Soria-La Rioja)

Abstract

Cookeite is a common phyllosilicate in low-grade metapelites from Cameros Basin (NW Spain). It occurs as a thin film on poikiloblastic pyryte 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, pyrophillite, albite and quartz with minor amphibole, anhidrite, gypsum and monazite. BSF and TEM images show a transitional area between the cokeite and the pyrite matrix. 23.6 to 24 Å units occurs in thick packets with rectorite, paragonite and cookeite. EDS analysis show an average composition of Na0.27 k0.09 Ca0.14 (Si3.26 AI0.74) AI2.58 Mg0.23 Fe0.05 O10 (OH)5. Combined occurs. Although the regular interstratificed cookeite-paragonite has been interpreted as a high-pressure 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.