Which two rocks usually consist of only one mineral, but may contain additional minerals?

Anorthosite and Marble

Explanation:

The rocks that are comprised of only one single mineral is often considered as mono-mineralic rocks. These rocks can be purely or partially mono-mineralic, depending on the number of mineral types present in it. A purely mono-mineralic rock is composed of only one mineral type. But in nature, it is very rare, as rocks are formed from the aggregates of different minerals.

Two examples of rocks that are made up of only one mineral includes "Anorthosite'' which is an igneous rock and is comprised entirely of mineral plagioclase feldspar, and "Marble" which is a metamorphic rock which is comprised entirely of mineral calcite.