Shale doesn't have a single chemical formula. It's a sedimentary rock composed of various minerals, mainly:

* Clay minerals: like kaolinite, illite, smectite.

* Quartz: (SiO2)

* Feldspar: (e.g., KAlSi3O8)

* Carbonates: like calcite (CaCO3)

* Iron oxides: like hematite (Fe2O3)

The specific composition of shale varies depending on the environment it formed in and the minerals present in the surrounding area.

So, instead of a single formula, we describe shale by its mineralogical composition and sometimes its chemical analysis.