Sodalite

Lore & Background

Sodalite is a member of the sodalite group, which includes hauyne, nosean, lazurite, and tugtupite. It is a cubic mineral consisting of an aluminosilicate cage network with Na+ cations and chloride anions in the interframework, forming a zeolite cage structure. Each unit cell has two cavities, one around each chloride ion, with chiral tetrahedral symmetry. The silicon and aluminum atoms are located at the corners of a truncated octahedron. The characteristic blue color arises mainly from caged S−3 and S4 clusters. Natural sodalite holds primarily chloride anions in the cages, but they can be substituted by other anions such as sulfate, sulfide, hydroxide, or trisulfur. The sodium can be replaced by other alkali elements, and the chloride by other halides. Most sodalite will fluoresce orange under ultraviolet light. Hackmanite, a variety of sodalite, exhibits tenebrescence—a change in color intensity when exposed to light or left in the dark.

Reader's Guide

Sodalite is significant as an ornamental gemstone, particularly its royal blue varieties, and as a mineral of scientific interest due to its zeolite-like cage structure. Its mesoporous cage structure makes it useful as a container material for many anions, including nitrate, iodide, iodate, permanganate, perchlorate, and perrhenate. The mineral is named after its sodium content and is classed as a feldspathoid. It is distinguished from similar minerals like lazurite and lapis lazuli by its white streak and rarity of pyrite inclusions. Significant deposits of fine material are found in Canada (Bancroft, Ontario; Mont-Saint-Hilaire, Quebec), the United States (Litchfield, Maine; Magnet Cove, Arkansas), and smaller deposits in South America, Portugal, Romania, Burma, and Russia. Euhedral, transparent crystals occur in northern Namibia and in the lavas of Vesuvius, Italy. The mineral's structure and thermal expansion properties have been studied in relation to substitutions of chloride by sulfate or iodide.

Did You Know?

• The people of the Caral culture traded for sodalite from the Collao altiplano.
• Most sodalite will fluoresce orange under ultraviolet light.
• Hackmanite, a variety of sodalite, exhibits tenebrescence, a form of photochromism.