• journal_title：The Canadian Mineralogist
• Contributor：Frank C. Hawthorne ; Neil A. Ball ; Gerald K. Czamanske
• Publisher：Mineralogical Association of Canada
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.48.2.301
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：48
• issue：2
• firstpage：301
• section：Articles

Ferro-obertiite, Na Na₂ (Fe²⁺³ Fe³⁺Ti) Si₈ O₂₂O₂, is a new mineral species of the amphibole group from Coyote Peak, Humboldt County, California, U.S.A., associated with inclusions of lithic-wacke sandstone in an alkali-rich ultramafic diatreme. Ferro-obertiite occurs as euhedral to subhedral crystals up to 1.25 mm across in a matrix of aegirine and feldspar. Crystals are black with a grey streak. Ferro-obertiite is brittle, has a Mohs hardness of 6, and a splintery fracture; it is non-fluorescent, with perfect {110} cleavage, no observable parting, and has a calculated density of 3.330 g cm⁻³. In plane-polarized light, it is pleochroic, X dark brown, Y brown, Z dark grey; X ∧ a = 77.3° (in β acute), Y || b, Z ∧ c = 91.2° (in β obtuse). Ferro-obertiite is biaxial negative, α 1.671(1), β 1.674(1), γ 1.675(1); 2V(obs.) = 60(3)°, 2V(calc.) = 59.9°. It is monoclinic, space group C2/m, a 9.845(4), b 18.018(8), c 5.296(3) Å, β 103.86(3)°, V 912.1(4) ų, Z = 2. The strongest ten X-ray-diffraction lines in the powder pattern [d in Å(I)(hkl)] are: 2.722(100)(3̅31,151), 8.448(80)(110), 3.407(60)(131, 041), 3.144(50)(201, 310), 2.596(50)(061), 2.533(40)(2̅02), 2.178(30)(261), 4.514(20)(040, 021), 3.283(20)(240) and 2.332(20)(3̅51). Analysis by a combination of electron microprobe and crystal-structure refinement gave SiO₂ 52.47, Al₂O₃ 0.09, TiO₂ 6.51, Fe₂O₃ 4.54, FeO 18.43, MgO 5.74, MnO 0.15, CaO 0.90, Na₂O 8.70, K₂O 1.51, Li₂O 0.17, F 0.51, H₂O 0.58, sum 100.09 wt%. The formula unit, calculated on the basis of 24 (O, OH, F), is ^A(Na_0.72 K_0.29) (Na_1.85Ca_0.15) (Mg_1.30Fe²⁺_2.35Mn²⁺_0.02Fe³⁺_0.52Al_0.01Ti_0.75Li_0.10) (Si_7.99Al_0.01) O₂₂ (O_1.16F_0.25OH_0.59). The crystal-structure refinement shows Ti to be strongly ordered at the M(1) site. Ferro-obertiite, ideally Na Na₂ (Fe²⁺₃ Fe³⁺Ti) Si₈ O₂₂O₂, is related to obertiite, Na Na₂ (Mg₃ Fe³⁺Ti) Si₈ O₂₂O₂, by the substitution Fe²⁺ → Mg.