Unlike the aluminosilicate minerals, most minerals have chemical compositions that can vary. When in equilibrium with other phases, the chemical composition of solid solution minerals can depend directly on temperature. An everyday example of a phase whose composition depends directly on temperature is sugar-saturated hot tea. The maximum amount of sugar that can be dissolved in tea increases with temperature. At 20°C sucrose-saturated tea (water) is 69 weight percent C12H22O11. At 100°C sucrose-saturated tea (water) is 83 weight percent C12H22O11 (see Figure 6.05). So if you measured the weight percent C12H22O11 in a sample of sucrose-saturated tea, you could determine the temperature of the tea.
Similarly, the chemical composition of olivine in equilibrium with an Mg2SiO4-Fe2SiO4 liquid depends on temperature, as shown in Figure 6.06. If you measure the composition of an olivine crystal that crystallized at equilibrium from an Mg2SiO4-Fe2SiO4 liquid, Figure 6.06 can be used to read the temperature of growth. To confirm that you understand the direct relationship between olivine composition and temperature, answer the following question.
Yes. 1431°C is the temperature at which an olivine with composition 50% Fe2SiO4 grows from an Mg2SiO4-Fe2SiO4 liquid.
No. Open Figure 06, click on the "Coordinates" and "Show Phase %" buttons, then mouse over the pattern region to see the phase compositions and proportions of both the olivine and the liquid at equilibrium (all in weight percent). Try again.
Yes. 1431°C is the temperature at which an olivine with composition 50% Fe2SiO4 grows from an Mg2SiO4-Fe2SiO4 liquid.
No. 1431°C is the temperature at which an olivine with composition 50% Fe2SiO4 grows from an Mg2SiO4-Fe2SiO4 liquid.
At equilibrium, solid solution minerals in metamorphic rocks will have compositions that depend on temperature, pressure, and the compositions of the other minerals in their assemblage, just as olivine compositions depend on the temperature and composition of the magma from which they crystallize. Compositions of the minerals can be used, along with the results of equilibrium experiments in the lab, to identify the equilibrium conditions for the metamorphic mineral assemblage.
There are a number of common metamorphic minerals that, like olivine, are solid solutions between Mg-rich and Fe-rich varieties. When two Mg-Fe minerals occur together in equilibrium, the Mg/Fe values for the two minerals will vary with temperature. As the temperature is changed, the two minerals exchange Mg and Fe. Because Mg+2 and Fe+2 have a similar size, Mg-Fe exchange equilibrium is not affected much by pressure and depends principally on temperature. Therefore, several "geothermometers" have been calibrated using Mg = Fe exchange.
