Dingwell, Donald B.; Knoche, Ruth; Webb, Sharon L.
The effect of F on the density of haplogranite melt.
In: American Mineralogist, Vol. 78, No. 3-4: pp. 325-330
The densities and thermal expansivities of F-bearing haplogranitic glasses and liquids
have been investigated using a combination of scanning calorimetry and dilatometry.
F2O-1 reduces the density of haplogranitic liquids (at 750 °C) from 2.295 + 0.006 g/cm3
to 2.261 + 0.005 g/cm3 with the addition of 4.55 wt% F (0.33% per wt% of F added). The
expansivities of the liquids increase with the addition of F2O-1 from 29.9 +- 3.0 x l0 -6/°C to 53.1 +- 1.4 x l0 -6/°C (at 750°C).
Densities have been converted into molar volumes based on the haplogranite and F2O-1
components. The partial molar volume of F2O-1 has been calculated at 750°C to be 14.2
+- 1.3 cm3/mol in these melts. This value is close to the molar volume per O for several
components of silicate melts. F and O have similar ionic and covalent radii, and thus the
substitution of two F for one O yields approximately the volume change expected, assuming
no secondaryc onsequencesfo r the averagec oordination number of cations. This is
despite evidence from quenched melts that Al exists in these compositions.
F is significantly more effective (per wt% added) than B2O3 in reducing the density of
haplogranitic melt. The effect of F on density reported here should complement the viscosity-
reducing effect of F2O-1 on granitic melts in significantly acceleratingg ravity-driven
processes of crystal-melt fractionation in F-rich igneous systems.