Hand Specimen identification: Sheaflike aggregates of twinned crystals, one excel-lent cleavage, pearly luster, color, and association identify stilbite. See Figure 6b.
Chemical Composition :CaAl2Si7O18 . 7H2O
Hardness : 31 2 to 4
Cleavage/Fracture : perfect (010)/subconchoidal
Luster/Transparency : pearly/transparent to
Color : gray
Streak : gray
Optical Properties : Cruciform twins and parallel or near-parallel extinc-tion help identify stilbite in thin section. Biaxial 1- 2, a = 1.49, b = 1.50, g = 1.50, d = 0.010, 2V = 30° to 50°.
Crystallography : Monoclinic, a = 13.64, b = 18.24, c = 11.27, b = 129.16°, Z = 8; space group C2/m; point group 2/m.
Habit : Simple crystals are extremely rare. Aggregates of twinned crystals, having the appearance of sheaves of grain, are common. Sometimes aggregates appear fibrous. More rarely, stilbite forms crystals displaying cruciform twinning.
Structure and composition : Stilbites structure is similar to heulandites. Consid-erable substitution of Na and K for Ca is common.
Occurrence and associations : Stilbite is one of the more common zeolites. It is a sec-ondary mineral, found with calcite and with other zeo-lites, that forms in cracks or on cavity walls in igneous rocks and in some schists associated with hydrother-mal ore bodies.
Related Minerals : Stilbite is grouped with heulandite, CaAl2Si7O18 . 6H2O, and several others because of its chemical and structural similarity.