How would a mineral's specific gravity be affected if the mineral contained impurities (other elements not normally part of a mineral's atomic structure)?

The specific gravity of minerals will vary because of the presence of impurities in the minerals structure.

Explanation:

The specific gravity or relative density of a substance is a measure of the ratio of the density of a substance to that of the density of a reference substance, in this case, water is the reference substance.

Specific gravity measures how much heavier as substance is relative to water. Therefore, the specific gravity of a mineral is a measure of how much heavier it compared to the weight to an equal amount of water. The specific gravity of water is 1.0. If a mineral has a specific gravity of 4.5, it means that it is 4.5 times heavier than water. Different minerals have different specific gravities. Most minerals with metallic luster have high specific gravities and are known as heavy minerals. Some have average weights while some minerals are light minerals.

The specific gravity of minerals will vary because of the presence of impurities in the minerals structure.  If a light mineral is present as an impurity in a heavy mineral, the specific gravity of that mineral will be less than the pure mineral and vice versa.