Effects of melt structure on liquidus boundaries

It has been shown above that silicate melts contain distributions of silicate polyanions of different sizes. These polymeric anions can all be constructed from five structural units  

When silicate minerals crystallize from these melts, only particular structural units can be accommodated by the growing mineral. For example pyroxene chains contain only middle groups ( Si) and olivines contain only orthosilicate ( Osi) anions. 

Masson (I965) assumed that the discrete silicate polyanions of different sizes mix ideally. However the activity coefficients (y) in equation (9) will also disappear if they lie in a geometrical series and cancel out (Masson, 1972). This is equivalent to allowing the ideal mixing of the structural units of the polymers. It should be of interest, therefore, to treat silicate melts as ideal solutions of structural units or structons (Huggins, 195 ) and cations. 

This concept may be used to derive simple solubility expressions for silicate minerals in melts. Consider, for example, the crystallization of enstatite from a melt. Since the structvire of enstatite can only accommodate middle groups the solubility product for enstatite in a silicate melt can be written in the form 

Referring to equations (2) and (3) it can be seen that ( O) and (i+l) suggest in the melt that the activities of the MgSiO and Mg2Si01 components are given by the expressions 

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