Hardystonite from Franklin furnace: A natural modulated melilite

Natural hardystonite, (Ca3.85Na0.14Pb0.01)(Zn0.85Al0.07Mg0.03Mn0.043+Fe0.02 3+)Si2.00O7.00, from the type locality was reinvestigated with single cryst al X-ray diffraction, microprobe analysis, and electron diffraction. The av erage structure, space group P (4) over bar2(1)m, a = 7.800(1) Angstrom. c = 5.000(1) Angstrom, was refined to R = 1.91% using 331 independent reflect ions. Hardystonite exhibits an incommensurate modulated structure. As in sy nthetic melilite-type compounds, modulation is two-dimensional, with tartan -like appearance: modulation vectors are q(1) = alpha (a(1)* + a(2)*) and q (2) = alpha(-a(1)* + a(2)*). A modulation wavelength lambda = 19.0(4) Angst rom 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 satellit es in the regions where the composition approaches the Ca2ZnSi2O7 end-membe r. Geometrical parameters as well as anisotropic displacement ellipsoids of hardystonite are consistent with those of the other melilite-type compound s having a modulated character.