Abstract
Scolecite crystals consist of {110}, {010}, and {111} growth sectors with triclinic symmetry, although scolecite appears to be monoclinic by X-ray analysis. The crystals have twinning caused by an asymmetric arrangement of Ca and H2O in the channel, and the (100) twin plane does not coincide with the sector boundary. The crystal has partly ordered (Al,Si) arrangements that are symmetrical with respect to the boundaries between the four {111} sectors; that is, the (Al,Si) arrangement allows sectoral (100) and (010) twins.
Natrolite crystals consist of {111} and {110} growth sectors, which correspond to smooth (111) and striated (110) faces. The optical extinctions are oblique to the b axis and parallel to the c axis, suggesting monoclinic symmetry, which can be attributed to (Al,Si) ordering. The {110} sectors show parallel intergrowths akin to twinning: one part is rotated 180° about the normal to (110) relative to the other. Sectors that grow in the opposite directions along the +c and −c axes show parallel intergrowths. Fine striations on the (110) face grow in the two directions corresponding to the +c and −c axes, producing the intergrowths. Mesolite crystals consist of only {111} sectoral twins, without {110} growth sectors, and are monoclinic.
