Banalsite

Banalsite is a relatively rare barium sodium feldspar mineral with the chemical formula BaNa₂Al₄Si₄O₁₆. It crystallizes in the orthorhombic crystal system, typically forming as white to colorless tabular crystals or massive aggregates.

It is an aluminum-rich feldspar and is chemically related to both albite (NaAlSi₃O₈) and celsian (BaAl₂Si₂O₈), representing a solid solution series between these two end-members. However, banalsite is a distinct mineral species due to its specific stoichiometry and structure.

Banalsite is found in metamorphic rocks, specifically those that have undergone high-pressure/low-temperature metamorphism, such as blueschists. Its formation is often associated with the presence of barium and sodium in the metamorphic environment.

The mineral was first described in 1967 for an occurrence in Benallt Mine, Rhiw, Wales, hence the name "banalsite". Other occurrences have been reported worldwide, although it remains a relatively uncommon mineral.

Optical properties of banalsite include its biaxial optic character and its refractive indices, which can be used for identification under a petrographic microscope. It typically exhibits low birefringence.

Due to its rarity, banalsite has limited economic importance and is primarily of interest to mineralogists and geologists studying metamorphic petrology and the behavior of barium and sodium in geological systems.