Here's why:

* Compounds are formed when two or more different elements chemically combine in a fixed ratio.

* Mixtures are combinations of substances where each substance retains its own properties.

Feldspar is made up of specific elements (like aluminum, silicon, oxygen, and sometimes potassium, sodium, or calcium) bonded together in a defined crystal structure. This chemical bonding defines it as a compound.

While there are different types of feldspar (orthoclase, plagioclase, etc.) these are all variations on the same basic compound structure, differing primarily in their chemical composition.