NaKAlSi3O8nH2O

NaKAlSi3O8nH2O is a generalized chemical formula used in mineralogy to describe a hydrated alkali feldspar composition that spans the albite-sanidine solid solution. It represents a framework silicate where sodium (Na) and potassium (K) occupy the alkali sites, aluminum (Al) substitutes for silicon (Si) in the tetrahedral network, and water is incorporated as nH2O per formula unit. The value of n denotes the amount of water present and can vary with formation conditions, making the hydration state of the mineral variable.

In terms of structure, these minerals are based on a three-dimensional framework of corner-sharing SiO4 and AlO4 tetrahedra, with Na+ and/or K+ balancing the framework charge. Water can occur within interstitial sites or as discrete molecules associated with defects, though in most natural feldspars true, fully hydrated formula units are uncommon. Consequently, NaKAlSi3O8nH2O is primarily used as a model for hydrated or hydrothermally altered feldspars rather than as a distinct, well-established mineral species.

Occurrence and application: The NaKAlSi3O8nH2O expression is most useful in geochemical modeling and petrology to describe hydration states and solid-solution behavior within alkali feldspars, particularly under low-temperature, hydrothermal, or weathering environments. It helps account for water storage and release during alteration and cooling, but its exact existence as a discrete mineral is not widely recognized; rather, it serves as a representative state for feldspar hydration.

See also: Feldspar, Albite, Sanidine, Alkali feldspars, Hydration in minerals.