Sodalite

Hand Specimen identification: Color (if blue) and association are usually diagnostic for sodalite. If not blue, identification may require chemical tests to tell it from zeolites or analcime. It is sometimes confused with lazulite but has different associations.

Chemical Composition : Na3Al3Si3O12.NaCl

Hardness : 51 2 to 6

Specific Gravity2.3

Cleavage/Fracture : six poor {110} at 60°
angles/conchoidal

Luster/Transparency : vitreous/transparent
to translucent

Color : blue, white

Streak : white

Optical Properties : In thin section, sodalite is distinguished by being isotropic and having a low index of refraction and, sometimes, a hexagonal outline. Isotropic, n = 1.485.

Crystallography : Cubic, a = 8.87, Z = 2; space group P43m; point group 43m.

Habit : Often massive or in embedded grains, sodalite forms rare dodecahedral crystals.

Structure and composition : The structure of sodalite is similar to many zeolites, containing 4- and 6-member tetrahedral rings but, un-like true zeolites, it contains no molecular water that can be driven off easily. The Al and Si tetrahedral rings are linked to form a framework with cagelike openings that hold Cl and sometimes S or SO4. Sodalite is usually close to end member composition. Small amounts of K or Ca may also be present.

Occurrence and associations : Sodalite is associated with nepheline, 1Na,K2AlSiO4,cancrinite, 1Na3Ca221Al3Si3O122CO3 .2H2O, leucite,KAlSi2O6, and with feldspars in Si-poor, alkali-rich eous rocks.

Varieties : Hackmanite is a sulfurous form of sodalite.

Related Minerals : Sodalite is structurally and chemically related to other zeo-lites and to cancrinite, 1Na3Ca2221Al3Si3O122CO3 . 2H2O.

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