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For many oxides and silicates, heat capacities are approximately additive at room temperature. Thus, for example, the heat capacity of enstatite, MgSiO_3,, may be approximated by adding the heat capacities of its oxide components, quartz (SiO₂) and periclase (MgO). In other words, since:

then

Substituting values:

The observed value for the heat capacity of enstatite at 300 K is 82.09 J/mol-K, which differs from our estimate by only 0.1%. For most silicates and oxides, this approach will yield estimates of heat capacities that are within 5% of the observed values. However, this is not true at low temperature. The same calculation for C_p-En carried out using heat capacities at 50 K differs from the observed value by 20%.

The explanation for the additive nature of oxide and silicate heat capacities has to do with the nature of bonding and atomic vibrations. The vibrations that are not fully activated at room temperature are largely dependent on the nature of the individual cation–oxygen bonds and not on the atomic arrangement in complex solids.