Mg^2^+-Fe^2^+ order-disorder in cummingtonite, (Mg, Fe)"7Si"8O"2"2(OH)"2: A new geothermometer - Ghose, S.; Weidner, J.R. (Elsevier)
Using the ^5^7Fe Mossbauer resonance spectroscopy the temperature dependence of the Mg^2^+ - Fe^2^+ distribution between the M4 site and M1, M2, M3 sites in magnesium-rich cummingtonites has been determined. Assuming the ideal solution model at each set of sites, the Gibbs free energy, ΔG^o for the ion-exchange reaction: Fe^2^+(M4) + Mg^2 (M1, M2, M3) Mg^2 (M4) + Fe^2^+ (M1, M2, M3) has been estimated to be 3.6 - 4.1 kcal/mole. The temperature T"q, below which no cation migration takes place, lies between 400^o and 500^oC. The temperature of crystallization of two chlorite-amphibolite (metamorphosed ultrabasic) rocks crystallizing below T"q have been estimated to be 265^o and 290^oC based on the observed Mg^2^+ - Fe^2^+ distribution.
http://linkinghub.elsevier.com/retrieve/pii/0012-821X(72)90152-5
Keywords: TE-TZ; U