A redetermination of the unit-cell geometry of scolecite

S COLECITE (Ca(A12Si3Oa0).3H20) is a naturally occurring zeolite closely related to natrolite and mesolite. Hey and Bannister (I936) examined scolecite in detail and derived from Laue and rotation photographs a nearly orthogonal monoclinic cell with parameters a I8"48 A, b I8"95 A, c 6"54 A,/3 9 ~ 3~'. They noted that scolecite belonged to class m because of its pyroand piezoelectric properties and concluded that its space group is C~ with a close approximation to the orthorhombic space group C~ (Fdd2), which is the space group possessed by natrolite. Their best determination of the crystal density was taken to be 2"274 g/cm ~ although other reported values were in the range 2"24-2"3I g/cm a. Taylor, Meek, and Jackson (1933) have discussed all three minerals and have proposed a structure for scolecite based upon their structure for natrolite whose orthorhombie cell (a I8"3 A, b I8.6 A, c 6"57/~) is very close to Hey and Bannister's cell for seolecite. We recently examined a specimen from Thailand consisting of radiating groups of white or colourless needles up to I o mm long, whose optical properties (a I'5 t 4,/3 I'5 t 7, y t'519; 2V~ 37~ a:[oot]I9 ~ suggested its identity with scolecite. The radiating needles enclose small fresh or limonitized crystals of pyrite and are associated with a wollastonite-diopside rock. The material was collected from a poorly exposed area underlain chiefly by quartz-veined sandstone of Triassic (?) age, some ~ 5 km south-west of Surat Thani, at approximately 99 ~ ~5' o" E, 9 ~ 3' 30" N. Single-crystal photographs taken about the needle axis revealed an axial repeat distance of about 6"53 A whilst zero and upper layer equi-inclination Weissenberg photographs showed monoclinic symmetry with a/3 angle close to 90 ~ After mounting the crystal about the b axis a unit cell could be derived with parameters a ~8'5o6 A, b I8"978/~, c 6"522 A, fl 9 O~ 4I', which are in very good agreement with those reported by Hey and Bannister. The twinning across (IOO) was readily observed in these photographs. However the lattice and space group absences were clearly those of a non-standard cell. The following presences were noted: hkl, all odd, all even; hko, (h -2n, k = 2n);