Melting of some alkaline-earth and transition-metal fluorides and alkali fluoroberyllates at elevated pressures: A search for melting systematics

Details Melting of some alkaline-earth and transition-metal fluorides and alkali fluoroberyllates at elevated pressures: A search for melting systematics Melting of some alkaline-earth and transition-metal fluorides and alkali fluoroberyllates at elevated pressures: A search for melting systematics

Apr 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977pepi...14..143j&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 14, Issue 2, p. 143-164.

Physics

1

Scientific paper

The melting curves of the fluorides ZnF2 and NiF2 (rutile structure), CaF2, SrF2 and BaF2 (fluorite structure), and of the fluoroberyllates Na2BeF4 and Li2BeF4 have been studied at pressures <= 40 kbar by differential thermal analysis in a piston-cylinder high-pressure device. The initial slopes (dTm/dP)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 Li2BeF4 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 SiO2, BeF2, GeO2, LiF and MgF2, 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 (ΔSm) is primarily a function of crystal structure while the volume change on fusion (ΔVm) 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 (dTm/dP)0 on SiO2 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. Present address: Seismological Laboratory, California Institute of Technology, Pasadena, Calif., 91125, U.S.A.

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