IMA 2016-066 = murakamiite

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Marco E. Ciriotti
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IMA 2016-066 = murakamiite

Messaggio da Marco E. Ciriotti » ven 08 set, 2017 11:11

Prossima pubblicazione.

▪ Imaoka, T., Nagashima, M., Kano, T., Kimura, J.-I., Chang, Q., Fukuda, C. (2017): Murakamiite, LiCa2Si3O8(OH), a Li-analogue of pectolite, from the Iwagi Islet, southwest Japan. European Journal of Mineralogy, 29, (in press).

Murakamiite (IMA2016-066), ideally LiCa2Si3O8(OH), is a new mineral that was recently discovered in an aegirine-augite albitite exposed on the Iwagi Islet, Ehime Prefecture, Japan. It occurs as prismatic crystals and monomineralic aggregates up to 1.7 mm long, is white to colourless with a white streak, and has a vitreous to silky lustre. It has a Mohs hardness of 4½–5 and is brittle with a splintery fracture and perfect {100} and {001} cleavage. Measured and calculated densities are Dmeas = 2.86(1) and Dcalc = 2.85(1) g·cm−3, respectively. Murakamiite is optically biaxial (+) and non-pleochroic, with refractive indices (in white light) of α = 1.602(1), β = 1.611(1), γ = 1.643(1), and with 2Vmeas = 56–59(2)° and 2Vcalc = 57°. Dispersion is weak, with r > v. The optical orientation is X^c 10–11°, Y^a 10–14°, Z^b 0–5°. Murakamiite is triclinic, belonging to space group P-1 with the unit-cell parameters a = 7.9098(2) Å, b = 7.0320(2) Å, c = 6.9863(2) Å, α = 90.596(2)°, β = 95.589(2)°, γ = 102.767(2)°, V = 376.98(2) Å3 (Z = 2). The eight strongest lines in the X-ray powder diffraction pattern are [dobs/Å(I)(h k l)]: 2.897(100)(2Embedded Image 0), 3.055(49)(012, 102, 1Embedded Image Embedded Image ), 3.295(41) (10Embedded Image ), 3.225(33)(201), 3.845(20)(200), 2.284(19)(10Embedded Image ), 2.720(15)(12Embedded Image , 2Embedded Image 1, 20Embedded Image ), and 6.962(15)(001). The chemical composition is (average of sixteen analyses by LA–ICP–MS, H2O by TG–DTA, wt%): SiO2 54.94, Al2O3 0.01, FeO 0.38, MnO 0.80, MgO 0.04, CaO 34.14, Na2O 4.37, Li2O 2.52 and H2O 2.80. The empirical formula of murakamiite, based on 9 O apfu, is (Li0.55Na0.46)Σ1.01(Ca1.98Mn0.04Fe0.02)Σ2.04Si2.98O8(OH)1.01; thus, muakamiite is a H-bearing pyroxenoid with three-periodicity of SiO4 tetrahedra and the Li-analogue of pectolite. The species is named in honour of the late Professor Emeritus Nobuhide Murakami (1923–1994) of Yamaguchi University, Japan. Type specimens are housed in the National Museum of Nature and Science, Tsukuba, Japan, and the Geological and Mineralogical Museum of Faculty of Science, Yamaguchi University, Japan.
Marco E. Ciriotti

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