Group: Tectosilicates without zeolitic H2O. Classed as Sodalite Group but also classed as being in the feldspathoid group.
Hardness: 5.5-6.0 mohs
Crystal: Cubic, rare in crystal form but common in massive forms.
Color: Rich royal blue, green, yellow, violet, white veining common
Fracture: Conchoidal to uneven
Density: 2.73g/cm3 (measured) 2.31g/cm3 (calculated)
Although somewhat similar to lazurite and lapis lauzuli, sodalite rarely contains pyrite (a common inclusion in lapis) and its blue color is more like traditional royal blue rather than ultramarine. It is further distinguished from similar minerals by its white (rather than blue) streak. Sodalite’s six directions of poor cleavage may be seen as incipient cracks running through the stone.
It is sometimes referred to as “poor man’s lapis” due to its similar color and the fact that is much less expensive. Its name comes from its high sodium content. Most sodalite will fluoresce orange under ultraviolet light.
Sodalite occurs in igneous rocks that crystallized from sodium-rich magmas. This is the origin of the name “sodalite.” These magmas also contained so little silicon and aluminum that quartz and feldspar minerals are often absent. Sodalite-bearing rocks include: nepheline syenite, trachyte, and phonolite. These types of rocks are so rare that most geologists never see them in the field.
Hackmanite is an important variety of sodalite exhibiting tenebrescence. When hackmanite from Mont Saint-Hilaire (Quebec) or Ilímaussaq (Greenland) is freshly quarried, it is generally pale to deep violet but the color fades quickly to greyish or greenish white. Conversely, hackmanite from Afghanistan and the Myanmar Republic (Burma) starts off creamy white but develops a violet to pink-red color in sunlight. If left in a dark environment for some time, the violet will fade again.
Tenebrescence is the ability of minerals to change color when exposed to sunlight. The effect can be repeated indefinitely, but is destroyed by heating.