SODALITE [ Silicates : Tectosilicates : Sodalite ]
Na4Al3(SiO4)3Cl, sodium aluminium Silicate Chloride
Sodalite is a scarce mineral that can be rock forming. Sodalite is named in reference to its sodium content. It is used for carvings and some jewellery pieces. Its light to dark pure blue colour is well known in the semi-precious stone trade. Sodalite is a member of the feldspathoid group of minerals. Minerals whose chemistries are close to that of the alkali feldspars but are poor in silica (SiO2) content, are called feldspathoids. As a result or more correctly as a function of the fact, they are found in silica poor rocks containing other silica poor minerals and no quartz. If quartz were present when the melt was crystallizing, it would react with any feldspathoids and form a feldspar.. Localities that have feldspathoids are few but some produce large quantities of sodalite. Sodalite, when not blue, is hard to distinguish from other feldspathoids. It is the only feldspathoid that contains chlorine. Sodalite dissolved in a dilute solution of HNO3 gives a positive chlorine test obtained from some swimming pool test kits.
Mineral specimens and ornamental stone
Sodalite is a scarce mineral that can be rock forming. Sodalite is named in reference to its sodium content. It is used for carvings and some jewellery pieces. Its light to dark pure blue colour is well known in the semi-precious stone trade. Sodalite is a member of the feldspathoid group of minerals. Minerals whose chemistries are close to that of the alkali feldspars but are poor in silica (SiO2) content, are called feldspathoids. As a result or more correctly as a function of the fact, they are found in silica poor rocks containing other silica poor minerals and no quartz. If quartz were present when the melt was crystallizing, it would react with any feldspathoids and form a feldspar.. Localities that have feldspathoids are few but some produce large quantities of sodalite. Sodalite, when not blue, is hard to distinguish from other feldspathoids. It is the only feldspathoid that contains chlorine. Sodalite dissolved in a dilute solution of HNO3 gives a positive chlorine test obtained from some swimming pool test kits. Physical Characteristics
- Colour : blue, white, gray, or even green
- Luster : vitreous or greasy
- Transparency : Crystals are transparent to translucent, massive specimens are opaque
- Crystal System : Isometric; bar 4 3/m
- Crystal Habits : Dodecahedral crystals have been found, usually massive as a rock forming mineral
- Cleavage : poor, in six directions, but rarely seen
- Fracture : uneven
- Hardness : 5.5 - 6.0
- Specific Gravity : 2.1 - 2.3
- Streak : white
- Other : it is the only feldspathoid to give a positive chlorine test when dissolved in a HNO2 dilute solution
- Associated Minerals : calcite, nepheline, cancrinite and other feldspathoids
- Major Occurrences : include Bancroft, Ontario; Mt. Vesuvius, Italy; Brazil; ice River area, British Columbia and Maine, USA
- Best Indicators : colour if blue, lack of pyrite association (as in lazurite), hardness and associations
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