The mineral description is provided for ferro-tschermakite, ideally ABCa2C(Fe32+Al2)T(Si6Al2)O22W(OH)2. The type specimen has been found in the dump of the Bâtiment et Granit de Ploumanac'h northern granite quarry, La Clarté, 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)Σ=0.37B(Ca1.69Fe0.112+Mn0.022+Na0.18)Σ=2.00C(Fe1.842+Mg1.54Al1.33Fe0.243+V0.013+Ti0.04)Σ=5.00T(Si6.15Al1.85)Σ=8.00O22W(OH1.94F0.06)Σ=2.00. Ferro-tschermakite is biaxial (–), with α = 1.666(2), β = 1.680(2), γ = 1.690(2) and 2V (meas.) = 84(1)°, 2V (calc.) = 79.8. The dispersion is medium (r > v), and the orientation is: X ^ a = 9.5° (in β acute), Y || b, Z ^ c = 24.3° (in β obtuse). The unit-cell parameters are a = 9.7598(6), b = 18.0220(11), c = 5.3299(3) Å, β = 104.826(1)°, V = 906.27 (9) Å3, Z = 2, space group C2/m. The strongest ten reflections in the X-ray powder pattern obtained from single-crystal data [d values (in Å), I, (hkl)] are: 8.359, 100, (1 1 0); 2.708, 84, (1 5 1); 3.098, 55, (3 1 0); 2.552, 43, (2¯ 0 2); 2.595, 41, (0 6 1); 2.330, 33, (3¯ 5 1); 2.159, 27, (2 6 1); 2.936, 27, (2 2 1); 3.338, 27, (1 3 1); 2.012, 24, (4¯ 0 2; 3 5 1).

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