The crystal structure of pyrophyllite from Ibitiara, Bahia, Brazil has been refined by least-squares from single crystal X-ray data to an unweighted R value of 0.060 and a weighted R value of 0.070. The Ibitiara pyrophyllite is a 1Tc polytype having space group symmetry of C1. Average bond lengths are Al–O 1.912, Si(1)–O 1.617, and Si(2)–O 1.618Å. The tetrahedral rotation angle (α) is 10.2°, the octahedral flattening angle (ψ) is 57.1° and the tetrahedral thickness angle (τ) is 109.4°. The results agree closely with the structural determination from X-ray powder data of Wardle and Brindley (1972).

Formulae to calculate the apical oxygen separation are derived in order to determine the magnitude of the effects of four geometric parameters: (a) the tetrahedral rotation angle, (b) the octahedral flattening angle, (c) the deviation of the apical oxygen from a mean OApical–T–Obasal angle and (d) the corrugation parameter, Δz. Of these, the corrugation is more effective in adjusting the distance between apical oxygens to provide linkage between the tetrahedral and octahedral sheets and to allow readjustments around the larger vacant site.

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