Feldspar is one of the most common rock forming minerals and is present in most rocks. It is further broken down into two main groups : K-feldspar and plagioclase feldspar. These groups are both solid solution series. K-feldspar consist of some proportion of sodium and potassium, while plagioclase feldspars have some ratio of sodium and calcium. The structure of K-feldspars are very responsive to temperature and pressure and there are several different members of the group.