Two new minerals, rondorfite, Ca_8Mg[SiO_4]_4Cl_2, and almarudite, K (square, Na)_2 (Mn, Fe, Mg)_2 (Be, Al)_3 [Si_(12)O_(30)], and a study of iron-rich wadalite, Ca_(12)[(Al_8Si_4Fe_2)O_(32)]Cl_6, from the Bellerberg (Bellberg) volcano, Eifel, German

摘要

Rondorfite, almarudite and iron-rich wadalite have been found in xenoliths in leucite-tephrite lava from a quarry at the Bellerberg (Bellberg) volcano lava field (near Ettringen), 2 km north of Mayen, Eastern Eifel volcanic area, Germany. Rondorfite, not uncommon in Ca-rich xenoliths at this locality, forms anhedral grains (< 0.3 mm) intergrown with ternesite. Both are embedded in a carbonate-quartz matrix and are associated with ettringite/thaumasite, mayenite, cuspidine, larnite, "calcio -olivine", tobermorite, portlandite, hydrocalumite, a member of the ellestadite series, magnetite and hematite. Rondorfite is orange brown to amber, with a light amber streak and vitreous lustre. It is brittle, with no cleavage, has conchoidal fracture and D_(calc) is 3.034g/cm~3. Optically, it is isotropic, with n = 1.676(1). Electron microprobe analysis yielded (wt. percent): Na_2O 0.07, MgO 4.52, CaO 57.05, FeO 0.54, Al_2O_3 0.40, SiO_2 30.51, TiO_2 0.13, Cl 6.71 (-O ident to Cl), sum 99.93, giving the empirical formula (Ca_(7.98)Na_(0.02)) (Mg_(0.87)Fe_(0.06)Al_(0.06))~([4]) [(Si_(3.98)Ti_(0.01))O_(16)] (Cl_(1.92)OH_(0.08)), based on O = 16 + 1. Rondorfite is cubic, space group Fd 3-bar (no. 203), contrasting with Fd 3-bar m (no. 227) reported for a synthetic crystal. It has a = 15.0850(3) A, V = 3432.7(1)A~3 and Z = 8. Strongest lines in the X-ray powder diffraction pattern are: 2.666 (100) 440, 1.540 (50) 844, 2.901 (40) 511, 1.964 (16) 553, 2.549 (30) 531, 1.885 (30) 800 [d in A (I) hkl]. A single-crystal structure refinement (R1 = 2.31 percent) showed that the structure consists of four isolated SiO_4 tetrahedra linked via a central MgO_4 tetrahedron, thus forming a [MgSi_4O_(16)]~(14) pentamer. These pentamers are connected through O-Ca-O bonds. Almarudite has been found in a single silicate-rich xenolith. It forms euhedral, platy crystals flattened on {0001}, with a maximum diameter of about 1.5 mm and a maximum tickness of 0.2 mm. It is associated with tridymite, sanidine, "clinopyroxene", "amphibole", quartz, hematite, sillimanite and rare braunite. Almarudite is yellow to orange, with light orange streak and vitreous lustre. It is brittle with irregular fracture and no cleavage, and D_(calc) is 2.714 g/cm~3. Optically, almarudite is uniaxial negative, with n_(omega) = 1.560(1) and n_(epsilon) = 1.559(1) and a strong pleochroism from orange (/_(omega)) to colourless (/_(epsilon)). Electron microprobe analyses yielded (wt. percent): Na_2O 0.66, K_2O 4.05, BeO 5.18, MgO 1.51, CaO 0.12, MnO 7.31, FeO 4.48, ZnO 0.24, Al_2O_3 4.09, SiO_2 72.31, sum 99.95 (BeO determined by LAM-ICP-MS), giving the empirical formula K_(0.86)~([12])Na_(0.21)~([9]) (Mn_(1.03)Fe_(0.62)Mg_(0.37)Zn_(0.03)Ca_(0.02)~([6]) (Be_(2.09)Al_(0.79))~([4]) [Si_(12)O_(30)], based on Si = 12. Almarudite is hexagonal, space group P6/mcc (no. 192), with a = 9.997(1), c = 14.090(1) A, V = 1219.5(2) A~3, and Z = 2. Strongest lines in the X-ray powder diffraction pattern are: 2.882 (100)114, 3.187 (90) 211, 4.076 (80) 112, 2.732 (50) 204, 7.047 (40) 002, 5.000 (40) 110 [d in A (I) hkl]. A single-crystal structure refinement (R1 = 1.85 percent) confirmed that the structure is isotypic to milarite and related A~([12])B_2~([9])M_2~([6]) T2_3~([4]) [T1_(12)O_(30)] compounds. The A site is dominated by K, the B site is partially occupied by Na, and the M site is clearly dominated by Mn +Fe over Mg. The chemistry at the T2 site can be refined to a Be/(Be+ Al) value close to 0.75; the T1 site is occupied by Si. Type material of rondorfite (M8874) and almarudite (N 1190) have been deposited at the mineral collection of the Naturhistorisches Museum Wien (NHMW), Austria. The Fe-rich wadalite, found in a Ca-rich xenolith from the Bellerberg, is characterised by electron microprobe analyses and X-ray methods. Optically, the lemon-yellow, tristetrahedral crystals are isotropic, with n = 1.700(1), D_(calc) = 3.105 g/cm~3. Electron microprobe analysis yielded (wt. percent): MgO 1.68, C