The crystal structure of ferrierite from Monastir, Sardinia, (Nao.56K,.,9Mg2.o2Cao.52Sro. 14 )(AI 6 . 8 9Si29.o4)072 · 17.86 H z O was refined to an R value of 4.6%. Systematic absences were consistent with the Immm space group but the orientation of the Mg(H 2 0) 6 octahedron indicates that the true symmetry is consistent with the subgroup Pnnm. Consequently the set of Τ at 1/4,1/4,1/4, on which the oxygen atom 05 lies in the case of Immm symmetry, disappears and the Τ-05-Τ angle is not constrained to 180°. The Si/Al distribution is partially ordered, with an enrichment of Al in the tetrahedra of the six-membered ring. The Mg(H 2 0) 6 octahedron assumes a statistical orientation around one of its axes with two configurations more frequently occupied. Another extra-framework cation site is coordinated to four framework oxygens and to four water molecules.
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