Here's why:

* Elements are the simplest pure substances that cannot be broken down further (like carbon, oxygen, or gold).

* Compounds are formed when two or more elements chemically combine in a fixed ratio (like water, H₂O).

* Mixtures are combinations of two or more substances where each substance retains its own properties and can be separated by physical means.

Shale is made up of various minerals (like clay, quartz, and mica) and organic matter. These components are not chemically bonded together, and their proportions can vary within a shale sample. You can even physically separate some of these components (like the sand grains) from the shale.