Controls on ferric iron in chlorite

Abstract

Fe[sup +3](Fe+3) contents have been studied in a suite of over 30 chlorites from a variety of provenances representing a range of F[sub O[sub 2]](FO2) conditions, including: (a) metapelites in western Maine, ranging from lower garnet to upper staurolite grade rocks, coexisting with graphite and Fe+3-poor ilmenite at 375--575 C; (b) extension veins cutting the Josephine ophiolite, formed at [approximately]430 C during regional metamorphism, associated with pyrite and methane-bearing fluid inclusions; (c) chlorites from metasediments in the Salton Sea geothermal system, sampled from depths of 2,985--6,120 coexisting with hydrothermal veins containing epidote, hermatite, and pyrite and (d) chlorites from Calveras County, CA and Burke Mountain, VT which were treated in hydrothermal cells for up to 4 months at temperatures of 300C at FO2 = 10[sup [minus]37] to 400C at 10[sup [minus]26.8]. All samples were analyzed using Moessbauer spectroscopy. All except the Calveras County specimens contain 10[+-]5% of the total Fe present as Fe+3, regardless of the FO2 of the system. The Calveras starting material and treated samples contain 23--25% of the total Fe as Fe+3; no change is observed as a function of the temperature and FO2 at which they were treated. Some samples were also analyzed by wet chemistry;more » Moessbauer Fe+3 results were 1--2% higher (consistently) than wet chemical data, but the techniques agree within analytical error. Crystal chemistry and composition of the chlorites, rather than FO2, control the Fe+3 contents of chlorite. This contrasts with the case for micas and supports the conclusion of Guidotti and coworkers that site substitutions in chlorite are simpler and more constrained than in the micas. Thus, limits on the amount of Fe+3 in chlorite may reflect the inability of the structure to substitute highly-charged cations, owing to charge balance constraints.« less