Description:
<jats:p>From the ternary phase diagrams of Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>–SiO<jats:sub>2</jats:sub> with CaO, MgO, or FeO, it can be concluded that the free energies of formation of kyanite, andalusite, and sillimanite, according to the reaction Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>+ SiO<jats:sub>2</jats:sub>= Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>.SiO<jats:sub>2</jats:sub>, are of the order of magnitude of 0 to –10 kcal. rather than the previously accepted value of –40 to –45 kcal. However, this result may be expected from the general variation of free energies of formation with the ionic potential of the silicate‐forming cation; this conclusion is supported by a plot for some silicates, carbonates, and sulfates.</jats:p>