Magnesio-riebeckite from the Varenche mine (Aosta Valley, Italy): crystal-chemical characterization of a grandfathered end-member

Magnesio-riebeckite from the dumps of the abandoned mine of Varenche (45 degrees 47' 22 '' N, 7 degrees 29' 17''' E), Saint-Barthelemy, Nus, Aosta Valley (Italy), was studied to provide the complete mineral description (including crystal structure) and insights into the crystal-chemistry of riebeckite. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is (A)(Na0.09K0.01)(Sigma-0.10) (B)(Na1.77Ca0.11Mg0.08Mn0.042+)(Sigma-2.00) (C)(Mg2.93Mn0.132+Fe0.0072+Zn0.01Ni0.12Fe1.253+ Al0.48Ti0.01)(Sigma-5.00) (T)(Si7.92Al0.08)(Sigma-8.00) O-22 (W)(OH1.88F0.12)(Sigma-2.00). Magnesio-riebeckite is biaxial (+), with alpha = 1.678(2), beta = 1.682(2),gamma = 1.688(2) and 2V (meas.) = 80.2(1.7)degrees, 2V (calc.) = 78.7 degrees. The unit-cell parameters are a = 9.6481(14), b = 17.873(3), c = 5.3013(7) angstrom, beta = 103.630(2)degrees, V = 888.4 (2) angstrom(3), Z = 2, space group C2/m. The strongest ten reflections in the powder X-ray pattern [d values (in angstrom), I, (hkl)] are: 2.701, 100, (151); 8.303, 83, (110); 3.079, 62, (310); 3.391, 53, (131); 4.467, 50, (040,021); 2.522, 50, ((2) over bar 02); 2.578, 35, (061); 2.155, 30, (261), 4.855, 30, ((1) over bar 11), 2.300, 29, ((3) over bar 51).