Here's why:

* Glauconite: This green mineral is formed in marine environments and is a key indicator of low-energy, oxygen-poor conditions. It is often found in shales.

* Limonite: This iron oxide is a common weathering product and can be transported into sedimentary environments. It can stain shales and contribute to their color.

* Biotite: This black mica mineral is a common constituent of many igneous and metamorphic rocks. It can be weathered and transported into sedimentary environments, often ending up in shales.

While these minerals can be present in other sedimentary rocks like mudstone and sandstone, shale is the most common rock type where they are found together.