Crystals, free from twinning, of the monoclinic phase of tricalcium silicate exhibiting superstructure reflections have been obtained. Crystal data are: a = 33.083(8), b = 7.027(2), c = 18.499(4) Ä, β = 94.12(2)°, Ζ = 36 χ Ca2.89Mgo.i ι 0Si0 4 , V = 4289 Ä 3 . The structure analysis was carried out using 3886 independent reflection intensities measured on a single-crystal diffractometer. The unit cell (a = 12.242, b = 7.027, c = 9.250 Ä, β = 116.04°) of the monoclinic expression of the rhombohedral average structure, contains six Ca's and a triplet of silicate tetrahedra which are successively arranged on the pseudothreefold axis. The mode of orientational disorder of these tetrahedra is found to be different from that of the corresponding tetrahedra in the rhombohedral phase R. The average structure has been successfully deconvoluted to yield the superstructure containing six triplets of tetrahedra in its cell; the structure has been refined to R = 9.9% (13.6% for 2818 superstructure reflections, and 5.9% for average structure). With regard to the mode of orientational disorder, the tetrahedra of certain triplets of the superstructure determined are similar with those of R, while others share features with those of the triclinic phase Τ (Golovastikov et al., 1975). Thus the orientational disorder of the tetrahedra that characterizes the superstructure has features intermediate between those of R and T. The coordination number of the Ca atoms (mean Ca -Ο = 2.41 Ä) is in the range 6.0 ~ 7.12, giving an overall average 6.15 which is also intermediate between those of R and T.

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