Oxo-magnesio-hastingsite, ideally NaCa2(Mg2mula" id="inline-formula-2">mg class="math mml" alt="Formula" src="2773/embed/mml-math-2.gif" /> )(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/cm3. Ferri-kaersutite is biaxial (–), α = 1.706 (2), β = 1.715(2), γ = 1.720(2) (Na light, 589 nm). 2V (calc.) = 73°. Dispersion: r > v, weak; orientation: Y = m>bm>; m>Zm> ^ m>cm> = 8°; pleochroism: strong, Z: dark brown, Y: brown, X: light brown. The average chemical formula of the mineral derived from electron microprobe analyses, Mössbauer spectroscopy and direct water determination is (Na0.67K0.33)Σ1.00(Ca1.87Na0.14Mn0.01)Σ2.02(Mg3.27mula" id="inline-formula-3">mg class="math mml" alt="Formula" src="2773/embed/mml-math-3.gif" /> Ti0.44Al0.08)Σ5.04(Al1.80Si6.20O22)(O1.40OH0.60)Σ2.00. It has monoclinic symmetry, space group m>C2/mm> and unit-cell parameters m>am> = 9.8837(3), m>bm> = 18.0662(6), m>cm> = 5.3107(2) Å, β = 105.278(1)°, m>Vm> = 914.77(5) Å3, m>Zm> = 2. The five strongest powder-diffraction lines [m>dm> in Å, (m>I/Iom>), m>hklm>] are: 3.383 (62) (131), 2.708 (97) (151), 2.555 (100) (2İ02), 2.349 (29) (3İ51) 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.
