Shchipalkina N. V., Pekov I. V., Ksenofontov D. A., Chukanov N. V., Belakovskiy D. I., Koshlyakova N. N.
Dalnegorskite, Ca5Mn(Si3O9)2, a new pyroxenoid of the bustamite structure type, a rock-formingmineral of calcic skarns of the Dalnegorskoe boron deposit (Primorskiy Kray, Russia)
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2019. V. 148. N 2. P. 61-75
Dalnegorskite — the new pyroxenoid with the crystal-chemical formula Ca2Ca2MnCa(Si3O9)2, and the simplified formula Ca5Mn(Si3O9)2, is a rock-forming mineral in the boron-bearing calcareous skarns of the Dalnegorskoe boron-silicate deposit (Dalnegorsk, Primorsky Krai, Russia). It belongs to the structural type of bustamite and forms a continuous solid-solution series with the isostructural mineral ferrobustamite Ca2Ca2FeCa[Si3O9]2. These pyroxenoids form thinly-radiated banded beige, pinkish-white and milky-white aggregates typically consisting of split thin acicular to fiber-like individuals and are associated with hedenbergite, datolite, andradite, galena, sphalerite, and pyrrhotite. Dmeas. = 3.02(2), Dcalc. = 3.035 g·cm–3. Dalnegorskite is optically biaxial, negative, α = 1.640 (3), β = 1.647 (3), γ = 1.650 (3)°, 2Vmeas. = 75(10)º. The average chemical composition of the holotype (electron microprobe data) is: MgO 0.23, CaO 40.02, MnO 5.02, FeO 3.64, SiO2 50.65, total 99.56 wt.%. The empirical formula calculated on 18 O atoms is Ca5.03Mn0.51Fe0.36Mg0.04Si6.03O18. The crystal structure of the new mineral was refined by powder X-ray diffraction data using the Rietveld method, Rp = 0.0345, Rwp = 0.0444, R1 = 0.0790, 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)º, V = 386.23(11) Å3, Z = 1. Dalnegorskite is distinctly different from the related mineral wollastonite in the infrared spectrum. The wave-numbers of maxima of strong bands in the characteristic region of Si—O stretching vibrations in the IR spectrum of dalnegorskite are (cm–1): 905, 937, 1025, 1070. The type specimen of dalnegorskite is deposited in the collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia (No. 96201).
Key words: dalnegorskite, ferrobustamite, bustamite, wollastonite, pyroxenoid, new mineral, crystal structure, Rietveld method, IR spectroscopy, calcic skarn, boron deposit, Dalnegorsk, Primorsky Krai