The new mineral ferriallanite-(La) (IMA 2010-066), ideally CaLaFe3+AlFe2+(SiO4)(Si2O7)O(OH), is a member of the epidote supergroup and the La analogue of ferriallanite-(Ce). It occurs as a single, thick, tabular {100} crystal (2 mm in its longest dimension) in a void of a sanidinite xenolith found in the In den Dellen pumice quarries, Niedermendig, Mendig, Laach Lake volcanic complex, Eifel Mountains, Rhineland-Palatinate, Germany. Ferriallanite-(La) is associated with sanidine, minor “biotite”, magnetite, nosean (grey, rounded grains) and trace amounts of dark green clinopyroxene. It is black, translucent in very thin splinters, with a brown streak, vitreous lustre, no cleavage, irregular to conchoidal fracture, Mohs hardness of ca. 6, Dcalc. = 4.208 g cm−3 (for empirical formula) and Dx = 4.099 g cm−3. Optically, it is biaxial negative, with α = 1.791(5), β = 1.827(6), γ = 1.845(5) (white light), 2Vα (calc.) = 69°. The mineral is non-fluorescent and shows strong pleochroism, X = pale brownish, Y = greyish brown, Z = dark red-brown; absorption Z > Y >> X, orientation unknown. Dispersion is weak and the sign could not be determined. Electron microprobe analysis yielded the empirical formula (based on 12 O atoms and 1 OH group pfu): Ca0.98(La0.47Ce0.41Th0.08Nd0.02Pr0.02)∑1.00(Fe3+0.81Al0.14)∑0.95Al1.00(Fe2+0.47Mn0.28Ti0.16Mg0.05)∑0.96[(Si0.93Al0.05)∑0.98O4](Si2O7)O(OH). Ferriallanite-(La) is monoclinic, space group P21/m, with a = 8.938(2), b = 5.789(1), c = 10.153(2) Å, β = 114.54(3)°, V = 477.88(6) Å3 (single-crystal data) and Z = 2. Strongest eight lines in the X-ray powder diffraction pattern are [d in Å(I) hkl]: 9.22 (19) 001; 7.96 (34) 1̄01; 3.53 (38) 2̄11; 2.92 (100) 3̄02, 1̄13; 2.72 (50) 120, 013; 2.63 (36) 3̄11; 2.16 (17) 221 and 1.639 (34) 1̄06, 4̄24. A single-crystal X-ray structure refinement [R1(F) = 0.0150] and a derivation of the M3 site population from the chemical-analytical data yielded the formula A1Ca1.00A2(La0.49Ce0.42Ca0.09)M1(Fe0.58Al0.42)M2(Al0.94Fe0.06)M3(Fe0.49Mn0.29Ti0.17Mg0.05)∑1.00(SiO4)(Si2O7)O(OH), in reasonably good agreement with the electron microprobe data. The site refinements clearly show that there is some minor Fe at the M2 site, in contrast to the formula calculated using currently recommended methods for members of the epidote supergroup.

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