Feldspar in Sediments and Sedimentary Rocks It is a sandstone that contains at least 25% feldspar. They include albite (NaAlSi 3O 8), anorthoclase ((Na,K)AlSi 3O 8), sanidine ((K,Na)AlSi 3O 8), orthoclase (KAlSi 3O 8), and microcline (KAlSi 3O 8).Īrkose is a sedimentary rock that forms from the weathering of feldspar-rich igneous and metamorphic rocks. They can also be seen forming the plagioclase feldspar series along the base of the triangular diagram described above.įeldspar minerals with compositions that range between NaAlSi 3O 8 and KAlSi 3O 8 are known as alkali feldspars. The names of these plagioclase minerals with intermediate compositions are summarized in the accompanying table. These names are arbitrary and based upon the relative amounts of albite and anorthite in their composition. To facilitate communication, names are given to the feldspar minerals at different positions in the plagioclase solid solution. This continuum of compositions is known as a solid solution series because it is analogous to a melt with dissolved sodium and calcium ions suspended in various positions throughout the solution that we refer to as a melt.Īlthough the range of mineral compositions between pure albite and pure anorthite is made up of very similar minerals, there are differences in their chemistry and physical properties. The relative abundance of sodium and calcium in melts varies greatly, and the full series of mineral compositions between pure sodium plagioclase and pure calcium plagioclase does occur. If a sodium ion with a 1+ charge substitutes for a calcium ion with a 2+ charge, a balancing substitution of an aluminum ion with a 3- charge for a silicon ion with a 4- charge will also occur. As a result, most albite will contain some calcium substituting for sodium in its crystalline structure, and most anorthite will contain some substitution of sodium for calcium in its crystalline structure. In nature, many melts contain abundant sodium or calcium, along with abundant aluminum, silicon and oxygen. Both of them form by crystallization from a melt. Albite is a sodium feldspar, and anorthite is a calcium feldspar. Albite and anorthite have an interesting relationship.

Ternary diagram of feldspars