BaAl2Si2O8

BaAl₂Si₂O₈, commonly known as **albite**, is a tectosilicate mineral belonging to the plagioclase feldspar group. It is a sodium-rich endmember of the plagioclase solid solution series, with the chemical formula NaAlSi₃O₈ being its ideal composition. However, in natural occurrences, albite often contains varying amounts of calcium, forming a mix with anorthite (CaAl₂Si₂O₈), which is why its formula is sometimes written as NaAlSi₃O₈–CaAl₂Si₂O₈.

Albite typically forms as white, gray, or pinkish crystals, often occurring in tabular or prismatic shapes with striations parallel to the length of the crystal. It has a glassy to pearly luster and a hardness of 6 to 6.5 on the Mohs scale. The mineral is commonly found in igneous, metamorphic, and sedimentary rocks, including granite, pegmatites, and some types of schist. It is also a significant component of some volcanic rocks, such as rhyolite and dacite.

Albite plays a crucial role in geological processes, particularly in the formation and alteration of rocks. It is stable under a wide range of conditions and can undergo exsolution, where calcium-rich and sodium-rich phases separate over time. This process is important in the study of metamorphic and igneous petrogenesis.

In addition to its geological significance, albite has been used as a gemstone in some cases, particularly when it exhibits clear, well-formed crystals. However, its value is generally lower compared to other feldspars like orthoclase or labradorite. Synthetic albite is also produced for use in ceramics and as a flux in glassmaking due to its low melting point and stability.

The mineral was first described in the early 19th century and named after the Greek word *albos*, meaning "white," due to its common color. Albite is widely distributed and can be found in various locations worldwide, including Norway, Madagascar, the United States, and Brazil. Its occurrence in pegmatites often allows for the formation of large, well-developed crystals, making it a notable specimen for collectors.