MG-FE ORDER-DISORDER REACTION IN FE-RICH ORTHO-PYROXENE - STRUCTURAL VARIATIONS AND KINETICS

Single-crystal X-ray techniques were used to study isothermal Fe2+-Mg disordering in two orthopyroxene samples, Opx 4 (Fs61) and Opx 5 (Fs85 ), at temperatures of 625, 675, 725-degrees-C and 525, 575, and 625-de grees-C, respectively. Heating was continued until cation distribution s reached steady state, which was considered equilibrium. The structur e data obtained correlate very well with all other work on orthopyroxe ne, including natural (heated and unheated) crystals and synthetic end -members. These crystallographic variations also provide an internal c onstraint on the disordering process. Instability of the orthopyroxene crystal structure was noted for some crystals where heating was conti nued after cation distribution equilibrium was attained. This was indi cated by a decrease in Fe2+ in the M2 site, with no corresponding incr ease in Fe2+ in the M1 site. In addition, there was a small decrease i n the size of the M1 site, which is thought to be caused by vacancy in troduction in the M2 site, with Fe2+-Fe3+ substitution in the M1 site and the Fe sublimating during heating. The Mueller chemical rate law w as used to calculate rate constants of 4.70 x 10(9) and (1.97 x 10(9)) /min and activation energies of 47.1 and 41.3 kcal/mol for Opx 4 and O px 5, respectively. From these results, together with values in the li terature, we have shown that a distinct step is found at Fs50-Fs60 in the activation energies, with higher activation energies of approximat ely 60 kcal/mol for Mg-rich samples and lower activation energies of a pproximately 50 kcal/mol for the Fe-rich samples. The rate constants o f Opx 4 and Opx 5 are comparable with previously determined disorderin g rates.