ALBERT

Description: Green biotite-like material occurring in Haast Schist at Brighton, eastern Otago, consists of biotite and a kaolin-group mineral interlayered on a 1 μm scale. Electron probe analyses of composite grains show K2O contents of 4.4 to 8.5 wt.%, in part reflecting the kaolin content, and in part, leaching of K+ ions. Phengitic muscovite shows similar but less extreme effects. Kaolin, siderite, calcite, and titanium oxides have formed in the rock during hydrothermal alteration of epidote, sphene, and biotite by carbonated waters. Potassium leaching due to surficial weathering processes appears to be widespread in biotites from the schists of eastern Otago, and biotites (hydrobiotites?) with K2O as low as 4 wt. % are reported. Similar material with 0.8 to 2.4% K2O and with Na2O about 1% is thought to be more highly degraded biotite. Microprobe analyses of kaolin and chlorites from associated rocks are given in addition to those of ‘biotite’ and phengitic muscovite. Analyses of sheet biotite and sheet muscovite from the Mataketake Range pegmatites in the highest grade parts of the Haast Schist terrane are presented for comparison. The study shows that biotite is more widespread in eastern Otago than was previously thought, and that almandine-rich garnet is not confirmed down-grade of the first appearance of biotite in eastern Otago.

Description: Manganoan berthierine, sometimes only a few layers thick, is preserved interlayered with manganoan chlorite (chamosite) in small recrystallized patches in cherty metapelagites matamorphosed in the prehnite–pumpellyite facies at Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand. The Mn occupies 11–12% of the octahedral sites, making this the most Mn–rich berthierine reported so far. The ratio (Fe+Mn)(Fe+Mn+Mg), 0.67–0.70, is at the lower end of those of previously reported occurrences, making these amongst the most Mg-rich berthierines known. Manganoan berthierine and interlayered manganoan chlorite also make up ~11–13% of the cryptocrystalline groundmass in which the berthierine is believed to have crystallized under early diagenetic conditions near a deep-ocean–sediment interface. The occurrence of berthierine in the recrystallized patches lends support to the possibility of a stability field at the temperature of prehnite-pumpellyite facies recrystallization in association with chlorite of similar but not identical composition.

Description: Noélbensonite, a new mineral, is the barium manganese analogue of lawsonite. It is described from the Woods ornamental rhodonite mine, 30 km NNE of Tamworth, New South Wales, Australia, where it occurs as aggregates of blocky to sometimes lamellar crystals ranging from a few micrometres to (rarely) 100 µm in length. It replaces NaMn amphibole, namansilite, and pectolite, and also occurs as tiny monomineralic veinlets 0.05–0.25 mm thick. Rare euhedral crystals are dominated by {100} and {011}, with (011) ^ = 68°. The mineral is orthorhombic, space group apparently Cmcm; a = 6.325(1), b = 9.120(1), c = 13.618(1) Å, V = 785.6(1) Å3, with a : b : c = 0.694 : 1 : 1.493. Noélbensonite is brittle, fracture irregular, Mohs hardness about 4, cleavage and twinning not observed, colour dark brown, streak paler yellow-brown, lustre earthy on some veinlet surfaces to brilliantly vitreous, calculated density 3.87 g/cm3, refractive indices α = 1.82(1),β (calculated from 2V) = 1.835(10), γ = 1.85(1), biaxial negative 2Vα = 46°(3°), strong dispersion r 〉 v, straight extinction to plane of flattening, {100}, α ∥ c, β ∥ b, γ ∥ a with pleochroism in very thin sections: α = orange yellow, β = orange, γ = brownish orange, absorption γ 〉 β ⇐p; α. The average of 23 electron microprobe analyses (wt.%) is SiO2 26.02, Al2O3 0.17, TiO2 0.01, Fe2O3 0.19, Mn2O3 34.76, CaO 0.31, Na2O 0.14, BaO 29.08, SrO 1.51, H2Ocalc 7.87, total 100.06, leading to the simplified formula . Up to 15% Sr and 9% Ca substitute for Ba in the large-cation sites. The strongest lines in the X-ray powder diffraction pattern are [(Iobs) dobs/Å hkl] (100) 4.85 111; (50) 4.557 020; (59) 4.322 021; (77) 3.416 113,004; (80) 2.869 202; (47) 2.849 114; (82) 2.729 024; (45) 2.543 132; (48) 2.428 222; (38) 2.255 223,041. The name is for William Noél Benson (1885–1957), in honour of his classic researches in the New England Fold Belt and of his tenure of the Chair of Geology at the University of Otago.

Description: A gonnardite-thomsonite-chabazite-calcite assemblage forms a cement in the Foveaux Formation, a fossiliferous gabbroic boulder bed that accumulated at the base of a sea cliff cut in a Permian igneous complex during late Oligocene±early Miocene time. Gonnardite was the earliest zeolite to form, locally following minor calcite. It was followed epitaxially by thomsonite, co-precipitating with chabazite. Crystal habits indicate a low-temperature origin. The maximum temperature to which the deposit may have been subjected is estimated as not more than ∽30°C. The chabazites are Ca-poor chabazite-K and chabazite-Na. Representative electron microprobe analyses are as follows, all + nH2O:thomsonite: Na3.77Ca7.73(Al19.39Si20.65)O80 and Na3.78K0.04Ca7.25Mg0.05(Al19.13Si21.05)O80;gonnardite: Na6.95K0.03Ca4.73(Al16.99Si23.15)O80 and Na8.56K0.03Ca4.05(Al17.32Si22.84)O80;chabazite-K: Na1.18K1.72Ca0.08Mg0.23(Al3.51Si8.49)O24 and Na1.67K1.92Ca0.18Mg0.17(Al4.11Si7.85)O24;chabazite-Na: Na2.51K1.13Ca0.17Mg0.02(Al4.08Si7.93)O24.Such a Si-poor zeolite assemblage is unusual for marine sediments and is attributed to precipitation from marine water impoverished in silica in the gabbroic boulder bed and interacting with shell material and calcic plagioclase. In contrast, a dioritic clast in the boulder bed provides an example of less silica-poor zeolites originally formed in the parent igneous complex. Veinlets in the clast contain scolecite averaging Na1.19Ca7.36(Al15.84Si24.14)O80.nH2O, and mesolite averaging Na5.13K0.03Ca5.24 (Al15.93Si24.13)O80.nH2O, in part as sub-microscopic intergrowths. The composition of scolecite closely corresponds to the most Na-rich scolecite reported hitherto.