The feldspars are a family of silicate minerals which occur in igneous rocks. There are many different members to the feldspar group. Obviously, silicon and oxygen form the foundation for the group, but calcium, sodium, and potassium are also present. One of these elements is usually dominant, but most of the feldspars contain all 3 in varying amounts. It is the proportions of these 3 elements which help determine which specific feldspar is formed.
The feldspars are divided into 2 broad categories: plagioclase, which contains calcium and sodium; and orthoclase, which contains potassium. The plagioclase feldspars represent the "continuous branch" of Bowen's Reaction Series, and form a complete series between anorthite (the pure calcium member), and albite (the sodium-rich variety).