Wiluite from Ariccia, Latium, Italy : occurrence and crystal structure

We report a new occurrence of the rare mineral wilnite, the B-rich equivalent of vesuvianite, from Ariccia, Alban Hills volcano, Rome, Italy. The specimen studied was found in the collection of the Museum of Mineralogy of the University of Rome (label MMUR22496/482). Wilnite from Ariccia was sampled at the Parco Chigi quarry, within the phreatomagmatic unit emitted by the Albano maar known locally as "Peperino di Marino". It occurs as dark brown to black euhedral, well-formed prismatic crystals, up to 1.0 cm in length and 0.5 cm across. Optical observations show a weak pleochroism and an imperfect extinction. The mineral is uniaxial (+) with ω 1.722(2) and ε 1.727(2). It is tetragonal P4/nnc, a 15.716(2), c 11.704(2) Å, V 2890.8(7) Å3. The crystal-chemical formula, obtained by combining EMP, SIMS and XREF data and calculated on the basis of 18 Si atoms, is: X(Ca18.72Mg0.15 Sr0.02La0.05Ce0.04Nd0.01) Y(1)(Fe3+0.36Mg0.26 Ti0.27Mn0.11) Y(2)(Al3.67 Fe3+0.19Mg0.14) Y(3) (Al3.29Fe3+1.36Mg3.35) T(1)B2.18Al0.02Be0.02 H0.47□1.31) T(2)(B0.68 H0.32) ZSi18O68O(11)(F1.21O6.79) [O(10)+O(12)]O2.68. The micro OH-stretching FTIR spectrum consists of a defined triplet of broad bands at 3639, 3573 and 3488 cm-1, respectively, and a very broad, composite absorption at 3300 cm-1. All bands are strongly polarized, with maximum absorption for E // c and minimum absorption for E ⊥ c. The integrated molar absorption coefficient for wiluite is εi = 90.000 ± 1500 1 mol-1 cm-2. In the 1600-1100 cm-1 region, absorptions due to both [3] B-O and [4]B-O bonds areobserved. The Mg and Al K-edge X-ray-absorption spectra are consistent with the presence of two octahedral coordination sites for both atoms, as determined by XREF, with the addition of a third, highly unusual, five-fold pyramidal site for the divalent cation.