Oxo-magnesio-hastingsite, NaCa2(Mg2Fe33+)(Al2Si6)O22O2, a new anhydrous amphibole from the Deeti volcanic cone, Gregory rift, northern Tanzania

Oxo-magnesio-hastingsite, ideally NaCa2(Mg2Fe33+)(Al2Si6)O22O2, is a new anhydrous amphibole from the Deeti volcanic cone in the Gregory rift (northern Tanzania). The mineral occurs as megacrysts up to 12 cm in size in crystal-rich tuff. Oxo-magnesio-hastingsite is brown with a vitreous lustre and has a perfect {110} cleavage. The measured density is 3.19(1) g/cm(3). Ferri-kaersutite is biaxial (-), alpha = 1.706 (2), beta = 1.715(2), gamma = 1.720(2) (Na light, 589 nm). 2V (calc.) = 73 degrees. Dispersion: r > v, weak; orientation: Y = b;Z(boolean AND)c = 8 degrees; pleochroism: strong, Z: dark brown, Y: brown, X: light brown. The average chemical formula of the mineral derived from electron microprobe analyses, Mossbauer spectroscopy and direct water determination is (Na0.67K0.33)(Sigma 1.00)(Ca1 87Na0.14Mn0.01)(Sigma 2.02)(Mg3.27Fe1.253+Al0.08)(Sigma 5.04)(Al1.80Si6.20O22)(O1.40OH0.60)Sigma(2.00). It has monoclinic symmetry, space group C2/m and unit-cell parameters a = 9.8837(3), b = 18.0662(6), c = 5.3107(2) angstrom, beta = 105.278(1)degrees, V = 914.77(5) angstrom(3), Z = 2. The five strongest powder-diffraction lines [d in angstrom, (I/Io), hkl] are: 3.383 (62) (131), 2.708 (97) (151), 2.555 (100) ((2) over bar 02), 2.349 (29) ((3) over bar 51) and 2.162 (36) (261). The isotopic composition of H and O, as well as the concentration of trace elements in oxo-magnesio-hastingsite suggest its formation from a melt originated from a mantle source metasomatized by slab-derived fluids.