Chapter 12, Problem 53GQ

Spinels are solids with the general formula AB₂O₄ (where A²⁺ and B³⁺ are metal cations of the same or different metals). The best-known example is common magnetite, Fe₃O₄ [which you can formulate as (Fe²⁺)(Fe³⁺)₂O₄]. Another example is the mineral often referred to as spinel, MgAl₂O₄.

A crystal of the spinel MgAl₂O₄

The oxide ions of spinels form a face-centered cubic lattice. In a normal spinel, cations occupy ⅛ of the tetrahedral sites and ½ of the octahedral sites.

1. (a) In MgAl₂O₄, in what types of holes are the magnesium and aluminum ions found?
2. (b) The mineral chromite has the formula FeCr₂O₄. What ions are involved, and in what types of holes are they found?