Murakamiite, LiCa2Si3O8(OH), a Li-analogue of pectolite, from the Iwagi Islet, southwest Japan

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 {1 0 0} and {0 0 1} cleavage. Measured and calculated densities are D_meas = 2.86(1) and D_calc = 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 2V_meas = 56–59(2)° and 2V_calc = 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) ų (Z = 2). The eight strongest lines in the X-ray powder diffraction pattern are [d_obs/Å(I)(h k l)]: 2.897(100)(⁠$22¯0$⁠), 3.055(49)(0 1 2, 1 0 2, $11¯2¯$⁠), 3.295(41)(⁠$102¯$⁠), 3.225(33)(2 0 1), 3.845(20)(2 0 0), 2.284(19)(⁠$103¯$⁠), 2.720(15)(⁠$121¯$⁠, $22¯1$⁠, $202¯$⁠), and 6.962(15)(0 0 1). 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 (Li_0.55Na_0.46)_Σ1.01(Ca_1.98Mn_0.04Fe_0.02)_Σ2.04Si_2.98O₈(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.