Ferriallanite-(La), a new member of the epidote supergroup from the Eifel, Germany

Abstract

The new mineral ferriallanite-(La) (IMA 2010-066), ideally CaLaFe 3+ AlFe 2+ (SiO 4 )(Si 2 O 7 )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, D calc. = 4.208 g cm −3 (for empirical formula) and D x = 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), 2 V α (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): Ca 0.98 (La 0.47 Ce 0.41 Th 0.08 Nd 0.02 Pr 0.02 ) ∑1.00 (Fe 3+ 0.81 Al 0.14 ) ∑0.95 Al 1.00 (Fe 2+ 0.47 Mn 0.28 Ti 0.16 Mg 0.05 ) ∑0.96 [(Si 0.93 Al 0.05 ) ∑0.98 O 4 ](Si 2 O 7 )O(OH). Ferriallanite-(La) is monoclinic, space group P 2 1 / m , with a = 8.938(2), b = 5.789(1), c = 10.153(2) A, β = 114.54(3)°, V = 477.88(6) A 3 (single-crystal data) and Z = 2. Strongest eight lines in the X-ray powder diffraction pattern are [ d in A( I ) hkl ]: 9.22 (19) 001; 7.96 (34) 101; 3.53 (38) 211; 2.92 (100) 302, 113; 2.72 (50) 120, 013; 2.63 (36) 311; 2.16 (17) 221 and 1.639 (34) 106, 424. A single-crystal X-ray structure refinement [ R 1( F ) = 0.0150] and a derivation of the M 3 site population from the chemical-analytical data yielded the formula A 1 Ca 1.00 A 2 (La 0.49 Ce 0.42 Ca 0.09 ) M 1 (Fe 0.58 Al 0.42 ) M 2 (Al 0.94 Fe 0.06 ) M 3 (Fe 0.49 Mn 0.29 Ti 0.17 Mg 0.05 ) ∑1.00 (SiO 4 )(Si 2 O 7 )O(OH), in reasonably good agreement with the electron microprobe data. The site refinements clearly show that there is some minor Fe at the M 2 site, in contrast to the formula calculated using currently recommended methods for members of the epidote supergroup.