The crystal structure of the Mn3+-bearing epidote, piemontite, has been refined. For a crystal of composition, Cz65Pm24Ps11, refinement of the occupancies cf the two large, nominally calcium sites and the three non-equivalent octahedral sites, gives:

where M = Mn0.7Fe0.3. All of the (Mn, Fe) substitution for A1 occurs in one of the two nonequivalent chains of edge-sharing octahedra. In the substituted chain, which is topologically identical with that found in the olivine structure, the larger M(3) site (symmetry-m) lying on the edge of the chain, contains most of the (Mn,Fe) with lesser amounts in the smaller M(l) site (symmetry-T) which forms the core of the chain.

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