A mineral crystallizes in a cubic closest packed array of oxygen ions with aluminum ions in some of the octahedral holes and magnesium ions in some...

A mineral crystallizes in a cubic closest packed array of oxygen ions with aluminum ions in some of the octahedral holes and magnesium ions in some of the tetrahedral holes. (a) Deduce the formula of this mineral.

(b) Predict the fraction of octahedral holes and tetrahedral holes that are filled by the various cations.