Zeolites

The zeolite group of minerals are hydrous Ca-Na aluminum silicates. Only four of the group are listed here; there are many more varieties, and many look similar in thin section. The zeolites are listed here as secondary minerals in igneous rocks, a common environment for them, but they are widespread in some marine and fresh water deposits as well.

Analcite NaAl(SiO3)2.H2O

Isometric, therefore extinct under crossed polarizers. Occurs as vesicle fillings and replacement in some lavas, and deposited in some lake beds. Shown as vesicle filling upper left (crossed polarizers).

Heulandite H4CaAl2(SiO3)6.3H2O

Biaxial +; first order gray interference colors; 2V 0-48 degrees; parallel extinction; 1 excellent cleavage. Occurs as a secondary mineral in volcanic rocks. Shown as vesicle filling in upper right photo.

Thomsonite NaCa2Al5(SiO4)5.6H2O

Biaxial +; colorless in plane polarized light; low birefringence; fibrous to bladed or columnar aggregates. Occurs as cavity filling in igneous rocks, as shown under crossed polarizers at upper left.

Stilbite H4(Ca,Na2)Al2(SiO3)6.4H2O

Biaxial-; first order gray birefringence; colorless in plane polarized light; 1 good direction of cleavage; wavy extinction. Occurs as cavity fillings in igneous rocks and as hydrothermal alteration.

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