A solid-solution of two types of this mineral can exsolve to form perthites. The continuous branch of Bowen's reaction series consists of this mineral's forms as various solid-solutions of albite and anorthite. Those forms characterized by the presence of sodium or calcium comprise the plagioclase series. Potassium characterizes microcline and orthoclase, members of the alkali type of this mineral. A six on the Mohs scale, For 10 points, name this aluminosilicate mineral, the most common in the Earth's crust.