The search for a crystal suitable for X-ray data collection was laborious, because all the crystals tested were multiple crystals. After many trials a very thin (100) tabular crystal was chosen. Preliminary X-ray Weissenberg photographs showed that it consists of two individuals, almost in the same crystallographic orientation. Furthermore these individuals are affected by polysynthetic twinning with (100) as the twin plane. The larger individual was used for intensity data collection on a Philips PW-1100 four-circle diffractometer (Centro di Cristallografia Strutturale del CNR, Pavia, Italy). A least-squares refinement of 25 high-angle reflections yielded for the unit cell a = 21.725(8), b = 6.118(6), c = 11.233(7)Å and β = 100.40(5)° in good agreement with the values obtained by the powder method. Diffraction intensities were collected with graphite monochromated MoKα radiation, in the range 2° < θ < 25°, ω − 2θ scan technique, integration width of 1.3° and scan speed of 0.025°/sec. Intensities were corrected for Lorentz and polarization effects. Because of the small size of the crystal the absorption correction was not applied; however, a partial compensation was achieved by averaging the intensities of equivalent reflections.