Albite

Lore & Background

Albite is a plagioclase feldspar mineral that serves as the sodium endmember of the plagioclase solid solution series, defined by having less than ten percent anorthite content. Its chemical formula corresponds to a pure sodium tectosilicate. The mineral typically appears pure white, which inspired its Latin-derived name. It is a common constituent of felsic rocks. Albite crystallizes in triclinic pinacoidal forms, with a specific gravity near 2.62 and a Mohs hardness ranging from 6 to 6.5. A defining characteristic is its almost ubiquitous crystal twinning, which often manifests as minute parallel striations on crystal faces. It also frequently occurs as fine parallel segregations alternating with pink microcline in perthite, a texture resulting from exsolution upon cooling. Two variants exist: low albite and high albite (also called analbite). Both are triclinic, but the high form has a slightly larger unit cell volume and can be produced by heating the low form. High albite is found in meteor impact craters, such as at Winslow, Arizona. Further heating changes the crystal symmetry from triclinic to monoclinic, producing a variant known as monalbite. Potassium can replace sodium in albite up to about ten percent; beyond this, the mineral is classified as anorthoclase. Albite occurs in granitic and pegmatite masses (including the variety cleavelandite), in hydrothermal vein deposits, and as part of the greenschist metamorphic facies for rocks of originally basaltic composition. Associated minerals include biotite, hornblende, orthoclase, muscovite, and quartz. First reported in 1815 from Finnbo, Falun, Dalarna, Sweden, albite is used as a semiprecious gemstone, as an important rock-forming mineral for geologists, and industrially in glass and ceramics. An iridescent variety, discovered in 1925 near the White Sea coast by academician Alexander Fersman, is known under the trade name belomorite.

Reader's Guide

Albite, the sodium endmember of the plagioclase feldspar series, is a tectosilicate mineral that typically appears pure white, a trait reflected in its Latin-derived name. It crystallizes in triclinic pinacoidal forms, with a specific gravity of about 2.62 and a Mohs hardness of 6 to 6.5. A defining characteristic is its almost ubiquitous crystal twinning, which often manifests as minute parallel striations on crystal faces. In perthite, albite forms fine parallel segregations alternating with pink microcline, a result of exsolution upon cooling. Two structural variants exist: low albite and high albite (also called analbite), both triclinic but with the high form having a slightly larger unit cell volume. High albite can be produced by heating low albite and is found naturally in meteor impact craters, such as those near Winslow, Arizona. Further heating above a certain temperature shifts the symmetry to monoclinic, producing monalbite. Potassium can substitute for sodium in the structure up to about 10%; beyond this threshold, the mineral is classified as anorthoclase. First reported in 1815 from Finnbo, Falun, Sweden, albite is a common constituent of felsic rocks, occurring in granitic and pegmatite masses (often as the variety cleavelandite), hydrothermal veins, and the greenschist metamorphic facies of basaltic rocks. It is frequently associated with biotite, hornblende, orthoclase, muscovite, and quartz. Industrially, it is used in glass and ceramics, and as a semiprecious gemstone. A notable iridescent variety, discovered in 1925 near the White Sea coast by academician Alexander Fersman, is known under the trade name belomorite.

Did You Know?

• Albite's name comes from Latin 'albus' meaning white, due to its usual pure white color.
• High albite can be found in meteor impact craters such as the Ries crater in Germany.

Chemical Identity & Position in the Feldspar Family

Albite occupies the sodium-rich extreme of the plagioclase feldspar solid solution series, representing any plagioclase composition in which anorthite content falls below ten percent. In its purest endmember form, the mineral carries the chemical formula NaAlSi3O8 and is classified as a tectosilicate. Its name is a direct reference to its most recognizable visual characteristic: a clean, pure white color, derived from the Latin word albus, meaning white. Because of its sodium-dominant chemistry, albite is a frequent and dependable constituent of felsic rocks, where the geochemical conditions favor its formation over the more calcium-rich members of the plagioclase family. For petrologists and geologists, recognizing albite at this sodium endmember is a key step in reading the cooling history and source composition of the igneous and metamorphic rocks in which it appears. Its position as the sodium pole of the series also makes it a critical reference point when geologists trace compositional trends across the full plagioclase range.

Physical Character & Thermal Polymorphism

Albite crystallizes in triclinic pinacoidal forms, with a specific gravity hovering around 2.62 and a Mohs hardness between 6 and 6.5. One of its most diagnostic field features is the near-universal presence of crystal twinning, which typically manifests as fine parallel striations etched across the crystal face. In perthite, albite commonly appears as thin parallel lamellae intergrown with pink microcline, a texture produced by exsolution as the rock cools. The mineral exists in two structural variants: low albite and high albite, the latter also called analbite. Both are triclinic, yet the high form possesses a slightly larger unit cell volume. Additionally, potassium can substitute for sodium in the structure up to roughly ten percent; beyond that threshold the mineral is reclassified as anorthoclase.

Where Albite Is Found in Nature

Albite is encountered across a variety of geological settings, reflecting its role as a common rock-forming mineral. It is a frequent component of granitic and pegmatite masses, where it sometimes appears under the specific variety name cleavelandite. Hydrothermal vein deposits also host albite, and in metamorphic environments it forms part of the characteristic greenschist facies assemblage in rocks that originally had a basaltic composition. In hand specimen and thin section, albite is typically found in association with a recognizable suite of companion minerals: biotite, hornblende, orthoclase, muscovite, and quartz. This consistent mineralogical neighborhood helps field geologists and petrographers identify the rock type and infer the pressure-temperature conditions under which it formed. The breadth of settings—from granitic intrusions to hydrothermal veins to greenschist-facies schists—underscores how versatile albite is as a sodium-bearing feldspar, capable of crystallizing across a wide range of geological processes and rock families.

Discovery, Uses & the Belomorite Story

Since then, its practical and scientific applications have broadened considerably. In the gem trade, albite is valued as a semiprecious stone, prized for its clean white appearance. For geologists, its status as an important rock-forming mineral makes it indispensable in petrographic analysis. On the industrial side, the mineral finds use in the manufacture of glass and ceramics. The striking iridescence of belomorite quickly gave it wide recognition under its trade name, distinguishing it from the plain white albite typically encountered in granitic rocks. Together, these diverse roles—from laboratory reference mineral to industrial feedstock to a sought-after gemstone—illustrate how a single sodium feldspar endmember can occupy multiple niches across science, industry, and the decorative arts.