Solid solution - A series formed when there are minerals in-between two isomorphous minerals. An example is the Plagioclase Feldspar group, which contains Albite and Anorthite as the anchor minerals, and minerals varying in amounts of those two minerals. Solid solutions may also be composed of minerals with a definite structure where one element may be minutely replace by another, but is too insignificant to be mentioned in the structure of that mineral.