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Hardystonite from Franklin Furnace: A natural modulated melilite

Luca Bindi¹, Michael Czank², François Röthlisberger^3, and Paola Bonazzi^1,*

¹ Dipartimento di Scienze della Terra, University of Florence, via La Pira 4 I-50121 Florence, Italy
² Institut für Geowissenschafte-Mineralogie-Olshausenstrasse 40 D-24098 Kiel, Germany
³ Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

Correspondence: ^* E-mail: pbcry{at}steno.geo.unifi.it

Natural hardystonite, (Ca_1.85Na_0.14Pb_0.01)(Zn_0.85Al_0.07Mg_0.03Mn³⁺_0.04Fe³⁺_0.02)Si_2.00O_7.00, from the type locality was reinvestigated with single crystal X-ray diffraction, microprobe analysis, and electron diffraction. The average structure, space group P2₁m, 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 = (a₁* + a₂*) and q₂ = (–a₁* + a₂*). A modulation wavelength = 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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