Natural hardystonite, (Ca1.85Na0.14Pb0.01)(Zn0.85Al0.07Mg0.03Mn3+0.04Fe3+0.02)Si2.00O7.00, from the type locality was reinvestigated with single crystal X-ray diffraction, microprobe analysis, and electron diffraction. The average structure, space group P4̅21m, a = 7.800(1) Å, c = 5.000(1) Å, was refined to R = 1.91% using 331 independent reflections. Hardystonite exhibits an incommensurate modulated structure. As in synthetic melilite-type compounds, modulation is two-dimensional, with tartan-like appearance; modulation vectors are q1 = α(a1* + a2*) and q2 = α(–a1* + a2*). A modulation wavelength λ = 19.0(4) Å was estimated by centering satellite reflections using a single-crystal diffractometer. TEM-EDX investigations proved the chemical composition of the sample to be slightly inhomogeneous, with stronger and sharper satellites in the regions where the composition approaches the Ca2ZnSi2O7 end-member. Geometrical parameters as well as anisotropic displacement ellipsoids of hardystonite are consistent with those of the other melilite-type compounds having a modulated character.

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