A method is described for approximate evaluation of thermodynamic oxygen isotope fractionations between different paragenetic silicates or between silicate and water, respectively. The evaluation uses a newly introduced18O index. It can be calculated for any silicate by means of isotopic increments of the different oxygen bonds within the silicate. Multiplying the18O index of any silicate by the expression in βQuartz(T) gives an adequate expression of the corresponding silicate and opens the possibility to calculate approximately isotope fractionations in dependence on the temperature.