方钠石

Sodalite is a rich royal-blue mineral of the feldspathoid group — a sodium aluminium silicate containing chlorine. Usually massive rather than in crystals, it is recognised by its deep blue colour streaked with white, and it is one of the components of the gem lapis lazuli.

Identifying sodalite

Sodalite is moderately soft (5.5–6), with a blue colour more uniform than lapis (it lacks lapis's brassy pyrite flecks). A related variety, hackmanite, is tenebrescent — it temporarily changes colour in sunlight or UV. Sodalite forms in silica-poor igneous rocks such as nepheline syenites.

Where it is found

Major sources include Brazil (Bahia), the Bancroft area of Ontario in Canada, and Namibia, with classic occurrences in Greenland and elsewhere. Most sodalite reaches the market as polished slabs, spheres and cabochons.

For collectors

Deep, even blue with attractive white veining is the most desirable; sharp crystallised sodalite is uncommon and prized by mineral collectors.