FERRO-OBERTIITE, Na Na2 (Fe2+ 3 Fe3+Ti) Si8 O22O2, A NEW MINERAL SPECIES OF THE AMPHIBOLE GROUP FROM COYOTE PEAK, HUMBOLDT COUNTY, CALIFORNIA

Abstract

Ferro-obertiite, Na Na 2 (Fe 2+3 Fe 3+ Ti) Si 8 O 22 O 2 , 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); 2 V (obs.) = 60(3)°, 2 V (calc.) = 59.9°. It is monoclinic, space group C 2/ m , a 9.845(4), b 18.018(8), c 5.296(3) A, β 103.86(3)°, V 912.1(4) A 3 , Z = 2. The strongest ten X-ray-diffraction lines in the powder pattern [ d in A( I )( hkl )] are: 2.722(100)(331,151), 8.448(80)(110), 3.407(60)(131, 041), 3.144(50)(201, 310), 2.596(50)(061), 2.533(40)(202), 2.178(30)(261), 4.514(20)(040, 021), 3.283(20)(240) and 2.332(20)(351). Analysis by a combination of electron microprobe and crystal-structure refinement gave SiO 2 52.47, Al 2 O 3 0.09, TiO 2 6.51, Fe 2 O 3 4.54, FeO 18.43, MgO 5.74, MnO 0.15, CaO 0.90, Na 2 O 8.70, K 2 O 1.51, Li 2 O 0.17, F 0.51, H 2 O 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 1.85 Ca 0.15 ) (Mg 1.30 Fe 2+ 2.35 Mn 2+ 0.02 Fe 3+ 0.52 Al 0.01 Ti 0.75 Li 0.10 ) (Si 7.99 Al 0.01 ) O 22 (O 1.16 F 0.25 OH 0.59 ). The crystal-structure refinement shows Ti to be strongly ordered at the M (1) site. Ferro-obertiite, ideally Na Na 2 (Fe 2+ 3 Fe 3+ Ti) Si 8 O 22 O 2 , is related to obertiite, Na Na 2 (Mg 3 Fe 3+ Ti) Si 8 O 22 O 2 , by the substitution Fe 2+ → Mg.