Octahedral cation ordering in olivine at high temperature.: II:: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50)

被引:84
作者
Redfern, SAT
Artioli, G
Rinaldi, R
Henderson, CMB
Knight, KS
Wood, BJ
机构
[1] Univ Cambridge, Dept Earth Sci, Cambridge CB2 3EQ, England
[2] Univ Milan, Dipartimento Sci Terra, I-20133 Milan, Italy
[3] Univ Perugia, Dipartimento Sci Terra, I-06100 Perugia, Italy
[4] Univ Manchester, Dept Earth Sci, Manchester M13 9PL, Lancs, England
[5] Rutherford Appleton Lab, ISIS, Didcot OX11 0QX, Oxon, England
[6] Univ Bristol, Dept Geol, Bristol BS8 1RJ, Avon, England
关键词
olivine; order/disorder; high temperature cation partitioning; neutron diffraction (powder);
D O I
10.1007/s002690000109
中图分类号
T [工业技术];
学科分类号
08 ;
摘要
The partitioning of Fe and Mg between the M1 and M2 octahedral sites of olivine has been investigated by in situ time-of-flight neutron powder diffraction. The degree of M-cation order was determined from direct measurements of site occupancies in a synthetic sample of Fo50Fa50 heated to 1250 degreesC at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 degreesC, progressively disordering on heating to this temperature. Above 630 degreesC, the temperature at which site preferences cross over (T-cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering behaviour is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that vibrational entropy, crystal field effects and changes in bond characteristics play a part in the cross-over of partitioning behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering of the type DeltaG = -hQ + gTQ + a/2 (T - T-c)Q(2) + b/4 Q(4), with a/h = 0.00406 K-1, b/h = 2.3, T-c = 572 K and g/h = 0.00106 K-1. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite join will have a bearing on the activity-composition relations of this important rock-forming mineral, and indicate that Fe-Mg olivine solid solutions become less ideal as temperature increases.
引用
收藏
页码:630 / 637
页数:8
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