NaAlSi3O8CaAl2Si2O8

NaAlSi3O8CaAl2Si2O8 is not a single compound but a representation of the end-member compositions of the plagioclase feldspar solid-solution series, spanning from albite to anorthite. The Na-rich end-member is albite, with the formula NaAlSi3O8, while the Ca-rich end-member is anorthite, with CaAl2Si2O8. The plagioclase series forms a continuous range of compositions between these two extremes.

In plagioclase, the chemical composition is commonly described by the empirical formula (Na,Ca)(Al,Si)4O8, reflecting the substitution of Na for Ca and Al for Si in the tetrahedral framework. The series includes many intermediate minerals such as oligoclase, andesine, labradorite, and bytownite, each with varying proportions of Na and Ca.

Occurrence and significance are central to igneous and metamorphic petrology. Plagioclase feldspars are major rock-forming minerals in continental crust, present in granitic to basaltic rocks. The Na/Ca ratio in plagioclase helps classify rocks and interpret magmatic differentiation and crustal evolution. Temperature, pressure, and fluid conditions influence the exact composition and zoning within individual crystals.

Crystallography and physical properties: plagioclase feldspars crystallize in the triclinic system and commonly exhibit two good cleavages at near 90 degrees with characteristic polysynthetic twinning, including albite and Carlsbad twin patterns. Cleavage surfaces often show striations that aid optical identification in hand specimens and thin sections.