Statistical thermodynamics was applied to the quantitative interpretation of the immiscibility of the binary silicate systems. The author has established the model of molten MO-SiO
2 systems composed of
c mol MO and (1−
c) mol SiO
2, as follows: (1) When
c mol MO dissolves into (1−
c) mol SiO
2,
Nc in the 2(1−
c)
N Si-O-Si chains decompose into Si-O
* *O-Si by the following reaction: MO+Si-O-Si→Si-O
* MO
*-Si. (2) The binding energies of Si-O, Si-O
*, M-O and M-O
*. vary in proportion to the Si-O-Si decomposition ratio
c⁄2(1−
c). From this model, the Gibbs’ free energy of the molten system
Gl can be written, as follows: G_l=(1-c)G_Sio_2^l+Ac+Bc^2/1-c+Cc^3/(1-c)^2+RT{c lnc+(2-3c) ln(2-3c)-2(1-c) ln2(1-c)}where
GSio2l is the Gibbs’ free energy of SiO
2 liquid and A, B and C are constants. From this formula, the immiscibility of MO-SiO
2 systems can be explained systematically and the activities of SiO
2 in molten binary silicate systems have been calculated.
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