Feldspar is a name given to a class of aluminum-containing silicates which form with no cleavage planes. The feldspars include orthoclase (KAlSi₃O₈) and plagioclase ((Ca,Na)AlSi₃O₈). Note that the plagioclase differs from the orthoclase in terms of the additional metals, plagioclase having calcium or sodium and orthoclase having potassium.

Other members of the feldspar series are anorthite (CaAl₂Si₂O₈) and albite (NaAlSi₃O₈). These are extreme members of the feldspar series.

The feldspars are the most abundant minerals on the earth.

Plagioclase ((Ca,Na)AlSi₃O₈) is one of the forms of feldspar. It is an aluminum-containing silicate which forms with no cleavage planes. This feldspar is distinguished from the orthoclase form by its content of calcium and sodium.

The feldspars are the most abundant minerals on the earth. In the Bowen reaction series, plagioclase can be seen to form throughout the process of magma crystallization in what is called the continuous reaction series. As the cooling continues, the calcium content of plagioclase diminshes while the sodium content increases.

Orthoclase (KAlSi₃O₈) is one of the forms of feldspar. It is an aluminum-containing silicate which forms with no cleavage planes. This feldspar is distinguished from the plagioclase form by its content of potassium .

Orthoclase can form in single crystals like that shown at right, although the polycrystalline form above is the common occurrence. This crystal sample is about 5x3 cm and the gem is 61 carats. The sample is from Madagascar.

The orthoclase sample above left is a single crystal about 4x4 cm from Binnatal, Valais, Switzerland. The sample above right is about 8x5 cm and is from Baveno, Italy. At left is a sample about 15x10 cm from Rhone Glacier, Aar Massif, Switzerland.

This orthoclase sample is of variety adularia and is described as orthoclase with chlorite. It is about 20 cm wide and is from Grissons, Switzerland.