Laachite, (Ca,Mn)_2Zr_2Nb_2TiFeO_(14), a new zirconolite-related mineral from the Eifel volcanic region, Germany

摘要

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, haüyne, 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 longprismatic 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_2O_3 2.56, La_2O_3 2.00, Ce_2O_3 6.37, Nd_2O_3 2.22, Al_2O_3 0.99, ThO_2 7.75, TiO_2 10.98, ZrO_2 19.39, Nb_2O_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)_2Nb_2TiFeO_(14). Laachite is monoclinic, space group C2/c, a = 7.3119(5), b = 14.1790(10), c = 10.1700(7)?, b = 90.072(2)°, V = 1054.38(1)? 3, Z = 4. The crystal structure was solved using single-crystal X-ray diffraction data. Laachite is a monoclinic analogue of zirconolite-3O, CaZrTi_2O_7, with Nb dominant over Ti in the octahedral sites Nb1 and Nb_2 and Fe dominant in a site with four-fold coordination. The strongest lines of the powder Xray diffraction pattern [d, ? (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).