Mendigite, Mn₂Mn₂MnCa(Si₃O₉)₂, a new mineral species from the Eifel volcanic region, Germany

Chukanov N. V., Aksenov S. M., Rastsvetaeva R. K., Van K. V., Belakovskiy D. I., Pekov I. V., Gurzhiy V. V., Schüller W., Ternes B.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2015. V. 144. N 2. P. 48-60

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Language: Russian

Abstract

A new mineral mendigite (IMA No 2014-007) isostructural with bustamite was found in the In den Dellen pumice quarry near Mendig, Laacher Lake area, Eifel Mountains, Rhineland-Palatinate (Rheinland-Pfalz), Germany. Associated minerals are sanidine, nosean, rhodonite, tephroite, magnetite and a pyrochlore-group mineral. Mendigite forms clusters of long-prismatic crystals (up to 0.10.22.5 mm in size) in cavities within sanidinite. Color is dark brown, streak brown. Perfect cleavage is observed on (001). D_calc = 3.56 g/cm³. IR spectrum shows absence of H₂O and OH groups. Mendigite is biaxial (–), α = 1.722 (calc), β = 1.782(5), γ = 1.796(5), 2V_meas = 50(10)°. Chemical composition (electron microprobe, mean of 4 point analyses, the Mn²⁺:Mn³⁺ ratio determined from structural data and charge-balance condition) is, wt %: MgO 0.36, CaO 10.78, MnO 37.47, Mn₂O₃ 2.91, Fe₂O₃ 4.42, Al₂O₃ 1.08, SiO₂ 43.80, total 100.82. The empirical formula is: Mn_2.00(Mn_1.33Ca_0.67) (Mg_0.07)(Ca_0.80)(Si_5.57Al_0.16O₁₈). The idealized formula: Mn₂Mn₂MnCa(Si₃O₉)₂. The crystal structure was refined on a single crystal. Mendigite is triclinic, space group P1; unit-cell parameters: à = 7.0993(4), b = 7.6370(5), c = 7.7037(4) Å, α = 79.58(1)°, β = 62.62(1)°, γ = 76.47(1)°; V = 359.29(4) ų, Z = 1. Strongest lines of the X-ray powder diffraction pattern [d, Å (I, %) (hkl)] are: 3.72 (32) (020), 3.40 (20) (002. 021), 3.199 (25) (012), 3.000 (26), (012, 120), 2.885 (100) (221, 211, 121), 2.691 (21) (222, 210), 2.397 (21) (022, 211, 203, 031), 1.774 (37) (412, 321). The type specimen is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, with registration number 4420/1.

Key words: mendigite, bustamite group, pyroxenoid, new mineral species, crystal structure, Laacher Lake, Eifel, sanidinite, alkaline volcanic rock.