**Types of Interstitial Voids**

In crystal structures, interstitial voids are empty spaces or cavities between atoms or ions. These voids can be classified into different types based on their shape and arrangement. The most common types of interstitial voids are:

1. **Tetrahedral Voids**: These voids are formed when four atoms or ions arrange themselves in a tetrahedral shape. The centers of these voids are located at the midpoints of the edges of a regular tetrahedron.

2. **Octahedral Voids**: These voids are formed when six atoms or ions arrange themselves in an octahedral shape. The centers of these voids are located at the midpoints of the edges of an octahedron.

3. **Cubic Voids**: These voids are formed when eight atoms or ions arrange themselves in a cubic shape. The centers of these voids are located at the midpoints of the edges of a cube.

**Occupation of Mg2+**

In the given case, we have a Mg2+ ion with an ionic radius of 66 pm. The Mg2+ ion is a cation, which means it has lost two electrons and has a +2 charge. Since it is a cation, it will occupy an interstitial void in the crystal lattice.

**Coordination Number**

The coordination number of an ion is the number of ions of opposite charge that surround it in a crystal lattice. It indicates the number of nearest neighbors of the ion. The coordination number is determined by the size and charge of the ions present.

In the case of Mg2+, the coordination number depends on the size and arrangement of the anions (O2-) surrounding it. Since the ionic radius of O2- is 140 pm, it is significantly larger than the ionic radius of Mg2+ (66 pm).

Given that the O2- ions are much larger than the Mg2+ ions, it is likely that Mg2+ will occupy a smaller interstitial void, such as a tetrahedral void or an octahedral void.

**Probable Coordination Number of Mg2+**

1. **Tetrahedral Coordination**: In a tetrahedral coordination, the Mg2+ ion is surrounded by four O2- ions arranged in a tetrahedral shape. The coordination number in this case would be 4.

2. **Octahedral Coordination**: In an octahedral coordination, the Mg2+ ion is surrounded by six O2- ions arranged in an octahedral shape. The coordination number in this case would be 6.

The exact coordination number of Mg2+ can be determined by considering the size and charge of the ions involved, as well as the crystal structure of the compound. Additional information about the crystal structure would be required to determine the specific coordination number in this case.