Siudaite, Na8(Mn2+2Na)Ca6Fe3+3Zr3NbSi25O74(OH)2Cl·5H2O: a new eudialyte-group mineral from the Khibiny alkaline massif, Kola Peninsula

• Published in: Physics and chemistry of minerals 2018-09, Vol.45 (8), p.745-758
• Main Authors: Chukanov, Nikita V., Rastsvetaeva, Ramiza K., Kruszewski, Łukasz, Aksenov, Sergey M., Rusakov, Vyacheslav S., Britvin, Sergey N., Vozchikova, Svetlana A.
• Format: Article
• Language: English
• Subjects: Chemical composition; Crystal structure; Crystallography and Scattering Methods; Crystals; Density; Diffraction patterns; Earth and Environmental Science; Earth Sciences; Electron probes; Empirical analysis; Geochemistry; Hafnium oxide; Lanthanum oxides; Luster; Massifs; Mineral Resources; Mineralogy; Mossbauer spectroscopy; Nepheline; Niobium oxides; Organic chemistry; Original Paper; Pegmatite; Praseodymium oxide; Silicon dioxide; Single crystals; Titanium dioxide; X ray powder diffraction; X-ray diffraction; Zirconium dioxide
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-018-0959-9

The new eudialyte-group mineral siudaite, ideally Na 8 (Mn 2+ 2 Na)Ca 6 Fe 3+ 3 Zr 3 NbSi 25 O 74 (OH) 2 Cl·5H 2 O, 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/cm 3 . Density calculated using the empirical formula is equal to 2.973 g/cm 3 . 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, H 2 O determined by HCN analysis): Na 2 O 8.40, K 2 O 0.62, CaO 9.81, La 2 O 3 1.03, Ce 2 O 3 1.62, Pr 2 O 3 0.21, Nd 2 O 3 0.29, MnO 6.45, Fe 2 O 3 4.51. TiO 2 0.54, ZrO 2 11.67, HfO 2 0.29, Nb 2 O 5 2.76, SiO 2 47.20, Cl 0.54, H 2 O 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 [Na 7.57 (H 2 O) 1.43 ] Σ9 (Mn 1.11 Na 0.88 Ce 0.31 La 0.20 Nd 0.05 Pr 0.04 K 0.41 ) Σ3 (H 2 O) 1.8 (Ca 5.46 Mn 0.54 ) Σ6 (Fe 3+ 1.76 Mn 2+ 1.19 ) Σ2.95 Nb 0.65 (Ti 0.20 Si 0.50 ) Σ0.71 (Zr 2.95 Hf 0.04 Ti 0.01 ) Σ3 Si 24.00 Cl 0.47 O 70 (OH) 2 Cl 0.47 ·1.82H 2 O. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is trigonal, space group R 3 m , with a  = 14.1885(26) Å, c  = 29.831(7) Å, V  = 5200.8(23) Å 3 and Z  = 3. Siudaite is chemically related to georgbarsanovite and is its analogue with Fe 3+ -dominant M 2 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).