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Towards a revisitation of vesuvianite-group nomenclature: the crystal structure of Ti-rich vesuvianite from Alchuri, Shigar Valley, Pakistan.

Vesuvianite containing 5.85 wt% TiO2 from an Alpine-cleft-type assemblage outcropped near Alchuri, Shigar Valley, Northern Areas, Pakistan, has been investigated by means of electron microprobe analyses, gas-chromatographic analysis of H2 O, X-ray powder diffraction, single-crystal X-ray structure refinement,27 Al NMR,57 Fe Mössbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are: a = 15.5326 (2), c = 11.8040 (2) Å, space group P4/nnc. The structure was refined to final R1 = 0.031, wR2 = 0.057 for 11247 I > 2σ(I). A general crystal-chemical formula of studied sample can be written as follows (Z = 2):[8-9] (Ca17.1 Na0.9 )[8] Ca1.0 [5] (Fe2+ 0.44 Fe3+ 0.34 Mg0.22 )[6] (Al3.59 Mg0.41 )[6] (Al4.03 Ti2.20 Fe3+ 1.37 Fe2+ 0.40 ) (Si18 O68 ) [(OH)5.84 O2.83 F1.33 ]. The octahedral site Y2 is Al-dominant and does not contain transition elements. Another octahedral site Y3 is also Al-dominant and contains Fe2+ , Fe3+ and Ti. The site Y1 is split into Y1a and Y1b predominantly occupied by Fe2+ and Fe3+ , respectively. The role of the Y1 site in the diversity of vesuvianite-group minerals is discussed.

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