Batiferrite, Ba(Ti_2Fe_(10))O_(19), a new ferrimagnetic magnetoplumbite-type mineral from the Quaternary volcanic rocks of the western Eifel area, Germany

摘要

Batiferrite, ideally Ba(Ti_2Fe_(10))O_(19), was found in the Quaternary volcanic rocks near Udersdorf, Graulai, and Altburg, western Eifel area, Germany. The new mineral typically occurs as euhedral platy grains in cavities of melilite- and leucite-nephelinite basalts. Associated minerals are hematite, magnetite, titanite, gotzenite, clinopyroxene, nepheline, and biotite. It exhibits a hexagonal tabular habit flattened on (0001), diameter 0.5-1 mm, thickness 20-125 #mu#m, and (10(1-bar)3), (10(1-bar)0) as observable forms. The mineral is opaque, of black color with submetallic lustre, and shows a ferrimagnetic behavior. VHN_(50) is 793 with a range of 710-841 from ten indentations. The quantitative reflectance measurements of R_o/R_e on oriented grains in air and oil immersion, respectively, are (percent): for 470nm 22.1/20.1 and 8.4/7.1, for 546 nm 21.0/ 19.4 and 7.8/6.6, for 589 nm 20.2/18.8 and 7.4/6.3, and for 650 nm l9.3/18.3 and 6.8/5.9. The bireflectance is distinct (air) to weak (oil), and parallel (0001) a moderate anisotropy with straight extinction can be observed. Typical microprobe analyses give (wt percent] K_2O 0.28-0.33, Na_2O 0.17-0.20, SrO 0.46-0.55, BaO 11.80-12.17, MgO 1.27-1.47, A1_2O_3 0.31-0.33, TiO_2 13.11-l3.63, MnO 2.38-2.57, Fe_2O_3 61.36-63.12, FeO 5.49-5.86 (Fe~(3+)/Fe~(2+) calculated for charge compensation), which is equivalent to (Ba_(0.84)Na_(0.06) K_(0.06)Sr_(0.05))_(1.01) (Fe_(8.48)~(3+)Fe_(0.86)~(2+)Ti_(1.82)Mg_(0.37)Mn_(0.37)A1_(0.06))_(11.96)O_(19) as the average composition based on 19 oxygen atoms. Batiferrite is a magnetoplumbite-type mineral with hexagonal symmetry, space group P6_3 / mmc (no. 194), a = 5.909(1) A, c = 23.369(4) A, V = 706.6(2) A~3, Z = 2, and a calculated density of 5.016 gcm~(-3). The structure was refined to R1= 0.031 for 278 unique reflections with F_o~2 > 4#sigma# (F_o~2) and R1= 0.079 for all 452 unique observations using single crystal X-ray data. The strongest reflections of the X-ray powder diffraction pattern are (d_(obs), I/I_o, (hkl)): 2.631, 100, (114); 2.799, 80,(107); 1.478, 70, (220); 2.429, 60, (203); 1.672, 50, (217). The new mineral is comparable to the other Ba containing magnetoplumbite-type minerals haggertyite and hawthorneite, the iron content, however, is much higher and in the range of magnetoplumbite. The large cation site (A) is dominated by Ba, and four of the five remaining crystallographic cation sites in the structure are dominated by Fe (M1, 2, 3, 5), the octahedrally coordinated M4-site is dominated by Ti. No oxygen vacancy on the O3-site like in plumboferrite can be observed. Batiferrite is named for its main chemical composition and the relationship to the M-type hexaferrites (polytype 5H).