Here's why:

* Porphyry is a rock type, not a specific mineral. It's a volcanic rock that contains larger crystals (phenocrysts) embedded in a finer-grained matrix.

* The composition of porphyry varies greatly. It can be made up of various minerals, including quartz, feldspar, mica, and amphiboles. The specific minerals present determine the chemical composition of the porphyry.

Therefore, it's impossible to give a single chemical formula for porphyry. Instead, you would need to specify the specific minerals present in a particular porphyry sample to determine its chemical composition.