Читать книгу Geochemistry - William M. White - Страница 63

Olivine is an example of a solid solution, which we will discuss at length in Chapter 3. Fe and Mg may substitute for each other in the octahedral site. Assuming that the distribution of Fe and Mg within this site is purely random, what is the configurational entropy of olivine of the composition (Mg_0.8,Fe_0.2)₂SiO₄?

Answer: To solve this problem, we need to apply eqn. 2.110. We need only consider the octahedral site containing Fe and Mg, because O and Si are the only kinds of atoms occupying the tetrahedral and anion sites. The values for X for these two sites will therefore be 1, and ln(1) = 0, so there is no contribution to configurational entropy.

For the octahedral site, , , and . Therefore, the configurational entropy will be: