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 × 10−6/°C to 53.1 ± 1.4 × 10−6/°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 secondary consequences for the average coordination number of cations. This is despite evidence from quenched melts that [6]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 accelerating gravity-driven processes of crystal-melt fractionation in F-rich igneous systems.

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