IMA 2017-092 = siudaite

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Marco E. Ciriotti
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IMA 2017-092 = siudaite

Messaggio da Marco E. Ciriotti » ven 09 mar, 2018 10:22

Prossima pubblicazione.

▪ Chukanov, N.V., Rastsvetaeva, R.K., Kruszewski, L., Aksenov, S.M., Rusakov, V.S., Britvin, S.N., Vozchikova, S.A. (2018): Siudaite, Na8(Mn2+2Na)Ca6Fe3+3Zr3NbSi25O74(OH)2Cl·5H2O: a new eudialyte-group mineral from the Khibiny alkaline massif, Kola Peninsula. Physics and Chemistry of Minerals, 45, (in press).

The new eudialyte-group mineral siudaite, ideally Na8(Mn2+2Na)Ca6Fe3+3Zr3NbSi25O74(OH)2Cl·5H2O, was discovered in a peralkaline pegmatite situated at the Eveslogchorr Mt., Khibiny alkaline massif, Kola Peninsula, Russia. The associated minerals are aegirine, albite, microcline, nepheline, astrophyllite, and loparite-(Ce). Siudaite forms yellow to brownish-yellow equant anhedral grains up to 1.5 cm across. Its lustre is vitreous, and the streak is white. Cleavage is none observed. The Mohs’ hardness is 4½. Density measured by hydrostatic weighing is 2.96(1) g/cm3. Density calculated using the empirical formula is equal to 2.973 g/cm3. Siudaite is nonpleochroic, optically uniaxial, negative, with ω = 1.635(1) and ε = 1.626(1) (λ = 589 nm). The IR spectrum is given. The chemical composition of siudaite is (wt%; electron microprobe, H2O determined by HCN analysis): Na2O 8.40, K2O 0.62, CaO 9.81, La2O3 1.03, Ce2O3 1.62, Pr2O3 0.21, Nd2O3 0.29, MnO 6.45, Fe2O3 4.51. TiO2 0.54, ZrO2 11.67, HfO2 0.29, Nb2O5 2.76, SiO2 47.20, Cl 0.54, H2O 3.5, –O = Cl − 0.12, total 99.32. According to Mössbauer spectroscopy data, all iron is trivalent. The empirical formula (based on 24.5 Si atoms pfu, in accordance with structural data) is [Na7.57(H2O)1.43]Σ9(Mn1.11Na0.88Ce0.31La0.20Nd0.05Pr0.04K0.41)Σ3(H2O)1.8(Ca5.46Mn0.54)Σ6(Fe3+1.76Mn2+1.19)Σ2.95Nb0.65(Ti0.20Si0.50)Σ0.71(Zr2.95Hf0.04Ti0.01)Σ3Si24.00Cl0.47O70(OH)2Cl0.47·1.82H2O. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is trigonal, space group R3m, with a = 14.1885(26) Å, c = 29.831(7) Å, V = 5200.8(23) Å3 and Z = 3. Siudaite is chemically related to georgbarsanovite and is its analogue with Fe3+-dominant M2 site. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.38 (60) (–114), 4.29 (55) (–225), 3.389 (47) (131), 3.191 (63) (–228). 2.963 (100) (4–15), 2.843 (99) (–444), 2.577 (49) (3–39). Siudaite is named after the Polish mineralogist and geochemist Rafał Siuda (b. 1975).
Marco E. Ciriotti

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