Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50)
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 °C at...
| Main Authors: | , , , , , |
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| Format: | Journal article |
| Language: | English |
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2000
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| _version_ | 1797094592500203520 |
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| author | Redfern, S Artioli, G Rinaldi, R Henderson, C Knight, K Wood, B |
| author_facet | Redfern, S Artioli, G Rinaldi, R Henderson, C Knight, K Wood, B |
| author_sort | Redfern, S |
| collection | OXFORD |
| description | 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 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site preferences cross over (Tcr), 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 ΔG = -hQ + gTQ + 2-a (T- Tc)Q2 + 4-b Q4, with a/h = 0.00406 K-1, b/h = 2.3, Tc = 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. |
| first_indexed | 2024-03-07T04:16:08Z |
| format | Journal article |
| id | oxford-uuid:c97cef9b-d0e0-4227-9271-cb46b06e2c21 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T04:16:08Z |
| publishDate | 2000 |
| record_format | dspace |
| spelling | oxford-uuid:c97cef9b-d0e0-4227-9271-cb46b06e2c212022-03-27T06:59:27ZOctahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50)Journal articlehttp://purl.org/coar/resource_type/c_dcae04bcuuid:c97cef9b-d0e0-4227-9271-cb46b06e2c21EnglishSymplectic Elements at Oxford2000Redfern, SArtioli, GRinaldi, RHenderson, CKnight, KWood, BThe 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 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site preferences cross over (Tcr), 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 ΔG = -hQ + gTQ + 2-a (T- Tc)Q2 + 4-b Q4, with a/h = 0.00406 K-1, b/h = 2.3, Tc = 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. |
| spellingShingle | Redfern, S Artioli, G Rinaldi, R Henderson, C Knight, K Wood, B Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50) |
| title | Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50) |
| title_full | Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50) |
| title_fullStr | Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50) |
| title_full_unstemmed | Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50) |
| title_short | Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50) |
| title_sort | octahedral cation ordering in olivine at high temperature ii an in situ neutron powder diffraction study on synthetic mgfesio4 fa50 |
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