Isomorphism in pyroxmangite-type pyroxenoids: Mg-rich pyroxmangite from sanidinite of the Eifel paleovolcanic region, Rhineland-Palatinate, Germany

The crystal structure of Mg-rich pyroxmangite from sanidinite of the Laach Lake (Laacher See) Volcano, Eifel paleovolcanic region, Rhineland-Palatinate, Germany has been solved based on single-crystal X-ray diffraction data, the final R indices for all data are: R 1 = 0.0302, w R 2 = 0.0617. Parameters of the triclinic unit cell are: a = 9.6410(4), b = 10.4328(6), c = 17.3419(9) Å, α = 112.256(5), β = 102.806(4), γ = 82.935(4)°, V = 1572.72(15) Å 3 . Space group is C -1. The empirical formula of the studied sample is Mn 3.48 Mg 1.78 Fe 1.61 Ca 0.13 (Si 7.00 O 21 ), and the refined crystal-chemical formula is M 1 [(Mn,Fe) 0.90 Mg 0.10 ] M 2 [(Mn,Fe) 0.93 Mg 0.07 ] M 3 [(Mn,Fe) 0.84 Mg 0.16 ] M 4 [(Mn,Fe) 0.63 Mg 0.37 ] M 5 [(Mn,Fe) 0.74 Mg 0.26 ] M 1 [(Mn,Fe) 0.69 Mg 0.31 ] M 1 [(Mn,Fe) 0.54 Mg 0.46 ] [Si 7 O 21 ]. This sample is characterized by a stronger disordering of Mg among the M sites as compared to Mg-rich members of the pyroxmangite-pyroxferroite solid-solution series from xenoliths hosted by basalt of the Bellerberg volcano, Eifel. The genetic factors which could influence to the cation ordering in pyroxmangite-type minerals are discussed.