Ferro-tschermakite from the Ploumanac'h granitic complex, Brittany, France: mineral description

The mineral description is provided for ferro-tschermakite, ideally (A)square Ca-B(2)C(Fe32+Al2)(T)(Si6Al2)O-22(W)(OH)(2). The type specimen has been found in the dump of the Batiment et Granit de Ploumanac'h northern granite quarry, La Clarte, Perros-Guirec, Ploumanac'h granitic complex, Brittany, France. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is: (A)(Na0.29K0.08)(Sigma= 0.37)(B)(Ca1. 69Fe0. 112+Mn0.022+Na0.18)(Sigma=2.00)(C)(Fe1. 842+Mg1.54Al1.33Fe0.243+V0.013+ Ti-0.04)(Sigma= 5.00)(T)(Si6.15Al1.85)(Sigma= 8.00)O-22(W)(OH1.94F0.06)(Sigma= 2.00). Ferro-tschermakite is biaxial (-), with alpha = 1.666(2), beta = 1.680(2), gamma = 1.690(2) and 2V (meas.) = 84(1)degrees, 2V (calc.) = 79.8. The dispersion is medium (r> v), and the orientation is: X <^> a = 9.5 degrees (in beta acute), Y || b, Z <^> c = 24.3 degrees (in beta obtuse). The unit-cell parameters are a = 9.7598(6), b = 18.0220(11), c = 5.3299(3) angstrom, beta = 104.826(1)degrees, V = 906.27 (9) angstrom, Z = 2, space group C2/m. The strongest ten reflections in the X-ray powder pattern obtained from single-crystal data [d values (in angstrom), I, (h k l)] are: 8.359, 100, (1 1 0); 2.708, 84, (1 5 1); 3.098, 55, (3 1 0); 2.552, 43, ((2) over bar 0 2); 2.595, 41, (0 6 1); 2.330, 33, ((3) over bar 5 1); 2.159, 27, (2 6 1); 2.936, 27, (2 2 1); 3.338, 27, (1 3 1); 2.012, 24, ((4) over bar 0 2; 3 5 1).