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Order and anti-order in olivine III: Variation of the cation distribution in the Fe,Mg olivine solid solution series with temperature and composition

¹ Institut für Mineralogie, Westfälische Wilhelms-Universität, Corrensstr. 24, 48149 Münster, Germany
² Mineralogisch-Petrologisches Institut, Poppelsdorfer Schloss, 53115 Bonn, Germany

^* Corresponding author, e-mail: kroll{at}nwz.uni-muenster.de

The structures of four Fe,Mg olivine crystals Fa11.6, Fa22.3, Fa27.6 and Fa27.8 have been investigated by ex situ and in situ experiments using single-crystal X-ray diffractometry. For the ex situ experiments, the crystals were quenched after equilibration at temperatures between 500 °C and 900 °C. Resetting effects were only observed for samples quenched from 900 °C. The in situ experiments were performed at temperatures up to 750 °C. With increasing equilibration temperature, Fe²⁺ was found to progressively segregate into the octahedral M1 site, i.e. the degree of anti-order increases with rising temperature. Thus, contrary to the results of Rinaldi et al. (2000) and Redfern et al. (2000), no indication of an ordering reversal at high temperatures is found. Incorporation of the results of Heinemann et al. (2006) on olivine Fa47.9 into the present study allows for the first time to systematically investigate the compositional variation of the temperature dependence of the Fe²⁺,Mg site occupancies. The formulation of Thompson (1969, 1970) for solid solutions undergoing non-convergent disordering processes,

(1)

yielded

H_exch⁰ and S_exch⁰ are the exchange enthalpy and entropy, respectively, related to the Fe²⁺,Mg site exchange, L_M1,M2^H and L_M1,M2^S denote enthalpic and entropic intrasite interaction parameters. The compositional parameter X varies between –1 for forsterite (Fa0) and +1 for fayalite (Fa100). The magnitudes and signs of the four refined quantities can be rationalized in terms of thermodynamic and crystal-chemical considerations. (L_M1^H – L_M2^H) > 0 indicates that the interactions between the M1 sites are stronger than those between the M2 sites, conforming with the M1-M1 distances being shorter than the M2-M2 distances. (L_M1^S – L_M2^S) < 0 relates to the electronic entropy which is proportional to the Fa-content. According to equation (1), H_exch⁰ increases towards the Mg endmember while S_exch⁰ decreases. Since H_exch⁰ > 0 stabilizes the ordered state whereas S_exch⁰ >0 stabilizes the anti-ordered state, it follows that the ordered state is progressively favoured with increasing Mg content. Consequently, ordered site distributions should occur only in slowly cooled Mg-rich olivines, whereas olivines with Fa > 25 mol% should be found in their anti-ordered states frozen during cooling. This conclusion is supported by structure refinements and ordering path simulations for a metamorphic olivine Fa12.4 as well as two volcanic olivines Fa25.6 and Fa27.8.

Key-words: olivine, order, anti-order, cation distribution, non-convergent disordering, solid solution, geospeedometry.