Dalnegorskite Ca5Mn(Si3O9)2, a New Pyroxenoid of the Bustamite Structure, a Rock-Forming Mineral of Calcic Skarns in the Dalnegorsk Borosilicate Deposit, Primorsky Krai, Russia

Abstract: Dalnegorskite, a new pyroxenoid with the crystal chemical formula Ca₂Ca₂MnCa(Si₃O₉)₂and simplified formula Ca₅Mn(Si₃O₉)₂, is a rock-forming mineral in the B-bearing calcic skarns in the Dalnegorsk borosilicate deposit (Dalnegorsk, Primorsky Krai, Russia). It belongs to the bustamite structural type and forms a continuous solid-solution series with isostructural ferrobustamite Ca₂Ca₂FeCa[Si₃O₉]₂. These pyroxenoids form beige, pinkish white and milky white radiated aggregates typically consisting of split thin acicular to fiber-like individuals and are associated with hedenbergite, datolite, andradite, galena, sphalerite, and pyrrhotite. D_meas.= 3.02(2), D_calc.= 3.035 g cm^–3. Dalnegorskite is biaxial negative, α = 1.640 (3), β = 1.647 (3), γ = 1.650 (3)°, 2V_meas.= 75(10)°. The average chemical composition of the holotype (electron microprobe data) is: 0.23 MgO, 40.02 CaO, 5.02 MnO, 3.64 FeO, 50.65 SiO₂, total 99.56 wt %. The empirical formula calculated for 18 O atoms is Ca_5.03Mn_0.51Fe_0.36Mg_0.04Si_6.01O₁₈. The crystal structure of the new mineral was obtained from X-ray diffraction data using the Rietveld method, R_p=0.0345, R_wp=0.0444, R₁= 0.0790, and wR₂= 0.0802. Dalnegorskite is triclinic, P-1, a= 7.2588(11), b= 7.8574(15), c= 7.8765(6) Å, α = 88.550(15), β = 62.582(15), γ = 76.621(6)°, V= 386.23(11) ų, and Z= 1. Dalnegorskite is distinct from the related mineral wollastonite in the infrared spectrum. The wave numbers of the maxima of strong bands in the Si–O stretching vibration region in the IR spectrum of dalnegorskite are (cm^–1): 905, 937, 1025, and 1070. The holotypic specimen of dalnegorskite is kept in the collection of the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, Russia (no. 96201).