Mangani-eckermannite, NaNa (sub 2) (Mg (sub 4) Mn (super 3+) )Si (sub 8) O (sub 22) (OH) (sub 2) , a new amphibole from Tanohata Mine, Iwate Prefecture, Japan

Mangani-eckermannite, ideally NaNa (sub 2) (Mg (sub 4) Mn (super 3+) )Si (sub 8) O (sub 22) (OH) (sub 2) , is a new member of the amphibole supergroup found at Tanohata Mine, Shimohei District, Iwate Prefecture, Japan. It occurs as prismatic crystals up to 0.3 X 0.2 mm and their aggregates up to 1 mm intergrown with braunite, vittinkiite and quartz. Mangani-eckermannite is cherry-red to very dark red and reddish-brown in thicker grains. It is translucent with a pinkish white streak and vitreous lustre. It is brittle, fracture is stepped along crystal elongation and uneven across a crystal. Cleavage is perfect on {110}. Mohs hardness is 6. D (sub meas) = 3.16(2) and D (sub calc) = 3.186 g/cm (super 3) . The mineral is optically biaxial (-), with alpha = 1.645(3), beta = 1.668(2), gamma = 1.675(3) (589 nm); 2V (sub meas) = 60(10) degrees , 2V (sub calc) = 57 degrees . The empirical formula derived from electron microprobe analysis, secondary-ion mass spectrometry and single-crystal structure refinement and calculated on the basis of 24 (O+OH) atoms per formula unit (apfu) is (super A) (Na (sub 0.74) K (sub 0.24) (sub 0.02) ) (sub Sigma 1.00) (super B) (Na (sub 1.52) Ca (sub 0.24) Mn (super 2+) (sub 0.24) ) (sub Sigma 2.00) (super C) (M g (sub 2.54) Mn (super 2+) (sub 1.45) Mn (super 3+) (sub 0.71) Fe (super 3+) (sub 0.26) Ti (sub 0.04) ) (sub Sigma 5.00) (super T) (Si (sub 7.97) Al (sub 0.03) ) (sub Sigma 8. 00) O (sub 22) (super W) [(OH) (sub 1.52) O (sub 0.48) ] (sub Sigma 2.00) . Mangani-eckermannite is monoclinic, space group C2/m, a = 9.9533(4), b = 18.1440(7), c = 5.2970(2) Aa, beta = 103.948(4) degrees , V = 928.39(6) Aa (super 3) and Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, Aa (I, %)(hkl)] are: 8.52(100)(110); 4.54(25)(040); 3.41(29)(131); 3.16(23)(310,201); 2.721(37)(151); 2.533(26)(202). The crystal structure was refined to R (sub 1) = 0.0264 for 1001 independent reflections with I > 2sigma (I). The place of mangani-eckermannite in the nomenclature of the amphibole supergroup is discussed and the status of mangano-ferri-eckermannite as a valid mineral species and successor of 'kozulite' is questioned.