Knoche, Ruth and Webb, Sharon L. and Dingwell, Donald B.
A partial molar volume for B 2 O 3 in haplogranitic melt.
In: The Canadian Mineralogist, Vol. 30, No. 3: pp. 561-569
The densitiesa nd thermal expansivitieso f boron-bearingh aplogranitic glassesa nd Iiquids have been determined
using a combination of scanning .florimetry and dilatomelry. B2O3 reduces the density of haplogranitic liquids (at
750'C) from 2.295 t 0.006 g cm-r to 2.237 + 0.005 g cm-3 wirh the addition of 8.92 wt. Vo 82o,. These densities
have been converted into molar volumes in the binary system haplogranite - BrO3. The partial molar volume of 8203,
calculated from a linear fit to the data at 750oC, is ,10.30 + 0.77 cmr mole-r in these melts. This value compares
with a molar volume of pure B2O3 at this temperature of M.36 x. 0.22 cm3 mole-l (Napolitano et ol. 1965), indicating
a negative excess volume of mixing along the haplogranite - B2O3 join. In comparison, at l3moc, the addition ot
Na2O to B2O3 reduces the panial molar volume of B2O3 from 46.6 to 32.3 cm3 mole-r ar 45 molego Na2O (Riebling
1966).T he densityr esultsr eported here, along with the viscosity-reducinge ffect of B2O3o n granitic melts (Dingwell
et al, 1992),s hould both significantlya cceleratep rocesseso f crystal-melt fractionation and facilitate the evolution of
extremely fractionated igneous systems.