Here's why:

* Composition: Slate is made up of various minerals, primarily clay minerals, mica, and quartz. These minerals are not chemically bonded together.

* Variable Composition: The proportions of these minerals can vary depending on the specific source of the slate.

* Physical Separation: The minerals in slate can be physically separated by grinding and other methods.

In contrast, a compound would be formed by chemically combining two or more elements in a fixed ratio. For example, water (H₂O) is a compound because it always contains two hydrogen atoms and one oxygen atom.