New data on ferriakasakaite-(La) and related minerals extending the compositional field of the epidote supergroup

Detailed studies of ferriakasakaite-(La) including determination of chemical composition and crystal structure, infrared spectroscopy, optical characteristics in reflected light and micro-indentation hardness have been carried out on a non-metamict sample from young nosean-bearing sanidinite from the Laach Lake volcanic complex, Eifel, Germany. The chemical composition is (electron microprobe, Fe²⁺: Fe³⁺ determined from structural data, wt.%): CaO 6.74, La₂O₃ 13.35, Ce₂O₃ 10.58, Pr₂O₃ 0.42, Nd₂O₃ 0.49, Sm₂O₃, 0.34, Eu₂O₃ 0.18, Gd₂O₃ 0.20, ThO₂ 0.43, UO₂ 0.10, MgO 0.89, MnO 9.98, Al₂O₃ 11.47, Fe₂O₃ 7.39, FeO 4.04, TiO₂ 1.32, SiO₂ 29.80, H₂O (calc.) 1.49, total 99.22. The empirical formula is $(Ca0.68Mn0.322+)Σ1.00(La0.49Ce0.39Pr0.02Nd0.02Sm0.01Eu0.01Gd0.01Th0.01Ca0.04)Σ1.00(Fe0.523+Fe0.042+Al0.34Ti0.104+)Σ1.00Al1.00(Mn0.532+Fe0.342+Mg0.13)Σ1.00(Si2.98Al0.02)Σ3.00O12.00(OH)$⁠. The crystal structure was solved by direct methods and refined to R = 0.018 based on 1259 unique reflections with I > 2σ(I). The H atom was located. The mineral is monoclinic, space group P2₁/m, a = 8.9054(1), b = 5.7545(1), c = 10.1037(2)  Å, β = 114.103(2)°, V = 472.63(1) ų. The IR spectrum confirms the presence of OH groups. Reflectance spectra of ferriakasakaite-(La) obtained in the visible range show reflectance minima at the wavelength of ∼590 nm. Various epidote-supergroup minerals including ferriakasakaite-(La), allanite-(Ce), Al-dominant (at the M1 site) analogue of ferriakasakaite-(Ce), piemontite, piemontite-(Pb), as well as Pb-, Zn- and Cu-bearing varieties of Fe³⁺-dominant (at the M1 site) analogues of piemontite and piemontite-(Pb), have been discovered in sulfide-free metasomatic rocks containing oxide/oxysalt compounds of chalcophile elements (Zn, Cu, Sb, and Pb), within the Pelagonian massif, Republic of Macedonia. Crystal chemical regularities of these minerals are discussed.