Hand Specimen identification: Form, color, and association help identify chabazite but it may be difficult to distinguish from other zeolites. Crystals are transparent to translucent pseudocubic rhombs, similar to calcite. Chabazite is distinguished from calcite by its poorer cleavage and lack of reaction to HCl. Figure 6a shows chabazite crystals on basalt.
Chemical Composition : CaAl2Si4O12.6H2O
Hardness : 4 to 5
Cleavage/Fracture : poor rhombohedral
Luster/Transparency : vitreous/transparent
Color : colorless, red
Streak : white
Optical Properties : Uniaxial 1- 2, v = 1.484, e = 1.481, d = 0.003.
Crystallography : Trigonal, a = 13.17, c = 15.06, Z = 6; space group R32/m; point group 32/m.
Habit : Chabazite usually forms simple rhombohedral crystals that may, at first glance, appear cubic. Some crystals are more complicated, showing more than one rhom-bohedral form or exhibiting penetration twins.
Structure and composition : Chabazite is similar in structure to natrolite and other zeolites. Tetrahedra form large cagelike open-ings, which can hold a variety of loosely bonded ions and molecules. The large openings allow diffu-sion of some small molecules through the structure. Considerable substitution of Na and K for Ca is common.
Occurrence and associations : Chabazite is a secondary mineral that forms in cracks or on cavity walls in mafic igneous rocks and as an alteration product in silicic igneous rocks. It is found with calcite and with other zeolites.
Related Minerals : Several other rare zeolites have structures similar to chabazites.