Actual matter must have at least elemental form. It can, in addition, get other forms imposed on it, as when a bunch of matter gets formed into a statue. Mixta are not simply one element next to another. Rather, the elements are completely mixed. None of these things is true of minima.
Gold is the most perfect metal.
Less perfect metals like iron or copper are 'base' metals. Alchemists tried to figure out how to make one. This was the beginning of chemistry later on. Medievals had a whole different conceptual scheme from classical physics, which is complex and hard to get into. We'll just look at a few parts of it.