Sodalite Group

Sodalite Group 

a group of silicate minerals that includes sodalite, Na₈[AlSi0₄]₆Cl₂, nosean, Na₈[AlSi0₄]₆S0₄, haüyne, Na₆Ca₂[AlSi0₄]₆(S0₄) ₂, and lazurite, Na₆Ca₂[AlSi0₄]₆(S0₄)S. The crystal structure of this group features an AlSi0₄ aluminum silicate framework whose large cavities are occupied by Cl^– and (sometimes S²~) anions surrounded by four Na⁺ and Ca²⁺ions occupying small cavities. The minerals crystallize in the isometric system to form rhombicdodecahedral (sodalite and lazurite) and octahedral (haüyne) crystals; they are usually encountered as irregular grains in rocks. The hardness on Mohs’ scale is 5.5–6, and the density varies from 2,300 kg/m³ (lazurite) to 2,500 kg/m³ (haüyne). The minerals can be blue, light blue (lazurite and haüyne), yellow, greenish, white, or gray. The variety of sodalite having a pink color upon fracture, which fades with exposure to light but is restored with irradiation (photochromism), is called hackmanite.

Certain minerals of the sodalite group are rock-forming minerals, for example, sodalite in nepheline syenites and nosean and haüyne in phonolites and other alkaline extrusive rocks. Lazurite and sodalite are also formed in limestones that have undergone contact metamorphism. In the USSR, sodalite is found in nepheline syenites on the Kola Peninsula, in the Il’men’ Mountains in the Urals, and in the Ukrainian SSR, the Tadzhik SSR, and the Saians. In the USSR, haüyne has been found in the Baikal Region, and abroad, in the vicinity of Mount Vesuvius and elsewhere. Nosean has been found in the USSR (Minusinsk) and in Italy.

Blue varieties of sodalite and lazurite are gems (Malobystrinsk deposit in the Baikal Region, Laushuardary deposit in the Tadzhik SSR, Badakhshan deposit in Afghanistan). Minerals of the sodalite group are used in ultramarine, a valuable pigment. The photochromic properties of synthetic minerals of the sodalite group with various substitutes for the sodium cations are made use of in radio electronics and television.

A. S. MARFUNIN