are pseudotetragonal, and this can be represented by taking a different unit cell, with twice as many formula units (Z = 8) and two long sides and one short one. The axes are redefined, a and b are very nearly equal and the angle β between the new a and c axes is very nearly equal to 90° (a truly tetragonal crystal would have a = b and β = 90° exactly). The sources give the following values: a=18.488 to 18.508 Å, b=18.891 to 18.96 Å, c=6.527 to 6.548 Å, β = 90.64 to 90.75° a = 18.508(5) Å, b = 18.981(5) Å, c = 6.527(2) Å β = 90.64° a = 18.508(5) Å, b = 18.981(5) Å, c = 6.527(2) Å, β = 90:64(1)° a = 18.51 Å, b = 18.97 Å, c = 6.53 Å, β = 90.6° Crystal habit Scolecite commonly occurs as sprays of thin, prismatic needles, frequently flattened on one side, with slanted terminations and striated parallel to the length of the needles. The crystals appear to be pseudo-orthorhombic or pseudo-tetragonal, and may be square in cross section. It also occurs as radiating groups and fibrous masses. Epitaxial intergrowths (intergrowths of two different crystalline substances in a non-random way) with mesolite, Na2Ca2Al6Si9O30·8H2O, are common. (Natrolite, Na2Al2Si3O10·2H2O, does not directly form epitaxial overgrowths on scolecite). All three minerals may be developed in the same crystal. Scolecite crystals appear to be monoclinic by X-ray analysis. Common forms include , , , and . Physical properties Scolecite is usually colorless or white, but can also be pink, salmon, red or green. It is transparent to translucent, with a white streak and a luster which is vitreous, or silky for fibrous specimens. It has a Mohs hardness of 5 to and a specific gravity in the range 2.16 to 2.40. (2.24 to 2.31 2.25 to 2.29 2.16 to 2.4 2.25 to 2.31). Cleavage is perfect in two directions parallel to the length of the crystals; the mineral is brittle with an irregular fracture. Twinning is common on , twin axis [001], as penetration or contact twins producing V-shaped or fishtail terminations. Scolecite is pyroelectric and piezoelectric, sometimes fluorescent yellow to brown in longwave and shortwave ultraviolet light. It is soluble in common acids. Not radioactive. Optical properties Biaxial (-) with refractive indices: Nx = 1.507 to 1.513, Ny = 1.516 to 1.520, Nz = 1.517 to 1.521. Pleochroism has