Bytownite - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal System : Triclinic
    Chemistry : (Na,Ca)(Al,Si)4O8
    Rarity : Common

Bytownite belongs to the group of plagioclase feldspars of which it represents a calcic term of composition (Ab30-An70 to Ab10-An90). It is a very widespread constituent of basic magmatic rocks (diorites, basalts, andesites, norites...) as well as in anorthosites and certain amphibolites. Its name is linked to its locality of discovery : Bytown (former name of Ottawa, Canada). Bytownite sometimes occurs as polysynthetic twinned tabular crystals in the pegmatoids of these basic rocks, but mostly as lamellar masses of greyish-white to pale brownish-purple hue. It is occasionally used as a gemstone and as an ornamental stone.

Main photo : Bytownite from Dorado Mine, Nueva Casas Grandes, Chihuahua, Mexico © Rob Lavinsky

Bytownite in the World

The most beautiful masses known, magnificently iridescent, come from Ylämaa (Finland). The finest known bytownite crystals are light yellow gemstone crystals, up to 8 cm, from Nueva Casas Grandes (Chihuhua, Mexico) and Lakeview (Oregon). The lavas of Vesuvius (Italy) also show very beautiful crystals.

Bytownite in France

In France, bytownite is present in some lava from Guadeloupe and Martinique.
Photo : Rough and gemstones of bytownites from Dorado Mine, Nueva Casas Grandes, Chihuahua, Mexico © Russ Rizzo & Cal Neva


The twins are common around [010] or perpendicular to {010}, giving polysynthetic streaks on {001} or {010} ; many other single and multiple contact twins exist.

Fakes and treatments

No fake identified for this mineral species. But can potentially be diffused just like andesine.

Hardness :  6 to 6.5
Density : 2.6
Fracture : Irregular to conchoidal
Trace : White

TP : Translucent to transparent
RI : 1.563 to 1.583
Birefringence : 0.010 to 0.11
Optical character : Biaxial (+/-)
Pleochroism : Low
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : None
Radioactivity : None