THE MINERAL SODALITE

Chemistry: Na4Al3(SiO4)3Cl, Sodium Aluminum Silicate Chloride.
Class: Silicates
Subclass: Tectosilicates
Group: Sodalite and feldspathoid groups.
Uses: mineral specimens and ornamental stone
Specimens

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 jewelry pieces. Its light to dark pure blue color 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:

Color is blue, white, gray, or even green.
Luster is 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 is poor, in six directions, but rarely seen.
Fracture is uneven
Hardness is 5.5 - 6.0
Specific Gravity is 2.1 - 2.3
Streak white
Associated Minerals calcite, nepheline, cancrinite and other feldspathoids.
Other Characteristics: it is the only feldspathoid to give a positive chlorine test when dissolved in a HNO2 dilute solution.
Notable Occurrences include Bancroft, Ontario; Mt. Vesuvius, Italy; Brazil; Ice River area, British Columbia and Maine, USA.
Best Field Indicators are color if blue, lack of pyrite association (as in lazurite), hardness and associations.

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