Laachite, (Ca,Mn)2Zr2Nb2TiFeO14, a new zirconolite-related mineral from the Eifel volcanic region, Germany

Abstract

The new mineral laachite was discovered in a sanidinite specimen from the Laach Lake (Laacher See) volcano, Eifel region, Rheinland-Pfalz, Germany. Associated minerals are sanidine, allanite-(Ce), baddeleyite, hauyne, hedenbergite, intermediate members of the jacobsite-magnetite series, phlogopite, rhodonite, spessartine, tephroite, thorite, zircon, and a pyrochlore-group mineral. Laachite is deep brownish-red, has an adamantine lustre, and is translucent; the streak is brownish red. It forms long-prismatic crystals up to 0.02 × 0.04 × 0.5 mm, which are present as random intergrowths and twins in cavities within sanidinite. The density calculated from the empirical formula is 5.417 g/cm 3 . The mean refractive index calculated from the Gladstone-Dale relationship is 2.26. The Raman spectrum shows the absence of hydrogen-bearing groups. The chemical composition is (electron microprobe, mean of 5 analyses, wt. %): CaO 4.29, MnO 9.42, FeO 5.73, Y 2 O 3 2.56, La 2 O 3 2.00, Ce 2 O 3 6.37, Nd 2 O 3 2.22, Al 2 O 3 0.99, ThO 2 7.75, TiO 2 10.98, ZrO 2 19.39, Nb 2 O 5 27.82, total 99.52. The empirical formula based on 14 O atoms is: (Ca 0.66 Mn 0.37 Th 0.25 Y 0.20 La 0.11 Ce 0.34 Nd 0.11 )(Zr 1.36 Mn 0.64 )(Nb 1.81 Ti 1.19 )(Fe 0.69 Al 0.17 Mn 0.14 )O 14.00 . The simplified formula, taking into account the structural data, is: (Ca,Mn) 2 (Zr,Mn) 2 Nb 2 TiFeO 14 . Laachite is monoclinic, space group C 2/ c , a = 7.3119(5), b = 14.1790(10), c = 10.1700(7) A, β = 90.072(2)°, V = 1054.38(1) A 3 , Z = 4. The crystal structure was solved using single-crystal X-ray diffraction data. Laachite is a monoclinic analogue of zirconolite-3 O , CaZrTi 2 O 7 , with Nb dominant over Ti in the octahedral sites Nb 1 and Nb 2 and Fe dominant in a site with four-fold coordination. The strongest lines of the powder X-ray diffraction pattern [ d , A ( I , %) ( hkl )] are: 4.298 (22) (022), 2.967 (100) (20–2, 202), 2.901 (59) (042), 2.551 (32) (15–1, 151, 240, 004), 1.800 (34) (24–4, 244), 1.541 (24) (37–1, 371), 1.535 (23) (20–6, 206), 1.529 (23) (046).