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 H2O, X-ray powder diffraction, single-crystal X-ray structure refinement, 27 Al NMR, 57 Fe Mossbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are: a = 15.5326 (2), c = 11.8040 (2) angstrom, space group P4/nnc. The structure was refined to final R1 = 0.031, wR(2) = 0.057 for 11247 I > 2 sigma(I). A general crystal-chemical formula of studied sample can be written as follows (Z = 2): ([8-9])(Ca17.1Na0.9)([8]) Ca-1.0 [5](Fe0.442+Fe0.343+ Mg-0.22) ([6])(Al3.59Mg0.41)([6])(Al4.03Ti2.20Fe3+ Fe-1.37(2+) (0.40)) (Si18O68) [(OH)(5.84)O2.83F1.33]. 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.