Abstract
The melting curves of the fluorides ZnF 2 and NiF 2 (rutile structure), CaF 2, SrF 2 and BaF 2 (fluorite structure), and of the fluoroberyllates Na 2BeF 4 and Li 2BeF 4 have been studied at pressures ⩽ 40 kbar by differential thermal analysis in a piston-cylinder high-pressure device. The initial slopes (d T m/d P) 0 of these melting curves are respectively 7.2, 5.8, 16.7, 15.2, 15.7, 15.1 and <0°C/kbar. A new Li 2BeF 4 polymorph, apparently of the olivine structure type, is stable at pressures > 10 kbar and its melting curve has an average slope of ∼6.7°C/kbar. These new data and those for SiO 2, BeF 2, GeO 2, LiF and MgF 2, recently studied by Jackson, are combined with existing data for elements, ionic compounds and silicates to assess the influence of crystal structure, molar volume and the nature of interatomic bonding on the initial slopes of melting curves. It is found that the entropy of fusion ( ΔS m) is primarily a function of crystal structure while the volume change on fusion ( ΔV m) is controlled by crystal molar volume within each isostructural series. Such systematics have recently facilitated estimation of the initial slopes of the melting curves of periclase and stishovite. New and existing melting data for silicates and their analogues have been analysed and a systematic dependence of (d T m/d P) 0 on SiO 2 concentration has been demonstrated. Possible implications of this trend for partial melting of upper-mantle garnet lherzolite are illustrated. Finally, the use of the traditional silicate-germanate and oxide-fluoride modelling schemes is reviewed in the light of information from this present study.
Published Version
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