Two new minerals rondorfite, Ca8Mg[SiO4]4Cl2, and almarudite, K(ٱ,Na)2(Mn,Fe,Mg)2(Be,Al)3[Si12O30], and a study of iron-rich wadalite, Ca12[(Al8Si4Fe2)O32]Cl6, from the Bellerberg (Bellberg) volcano, Eifel, Germany.

Autor(en): Tamara Mihajlovic, Christian Lengauer, Theodoros Ntaflos, Uwe Kolitsch, Ekkehart Tillmanns
Abstrakt: 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), 2km north of Mayen, Eastern Eifel volcanic area, Germany. Rondorfite, not uncommon in Ca-rich xenoliths at this locality, forms anhedral grains (<0.3mm) 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 3.034 g/cm3. Optically, it is isotropic, with n = 1.676(1). Electron microprobe analysis yielded (wt.%): Na2O 0.07, MgO 4.52, CaO 57.05, FeO 0.54, Al2O3 0.40, SiO2 30.51, TiO2 0.13, Cl 6.71 (-O ≡ Cl), sum 99.93, giving the empirical formula (Ca7.98Na0.02)(Mg0.87 Fe0.06Al0.06) [4][(Si3.98Ti0.01)O16](Cl1.92OH0.08), based on O = 16+1. Rondorfite is cubic, space group Fd 3 (no. 203), contrasting with Fd 3m (no. 227) reported for a synthetic crystal. It has a = 15.0850(3) Å, V = 3432.7(1)Å3 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 Å (I) hkl]. A single-crystal structure refinement (R1 = 2.31%) showed that the structure consists of four isolated SiO4 tetrahedra linked via a central MgO4 tetrahedron, thus forming a [MgSi4O16]-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/cm3. Optically, almarudite is uniaxial negative, with nω = 1.560(1) and nε = 1.559(1) and a strong pleochroism from orange (//nω) to colourless (//nε). Electron microprobe analyses yielded (wt.%): Na2O 0.66, K2O 4.05, BeO 5.18, MgO 1.51, CaO 0.12, MnO 7.31, FeO 4.48, ZnO 0.24 Al2O3 72.31, sum 99.95 (BeO determined by LAM-ICP-MS), giving the empirical formula K0.86 [12]Na0.21 [9](Mn1.03Fe0.62Mg0.37Zn0.03Ca0.02)[6](Be2.09Al0.79)[4][Si12O30], based on Si = 12. Almarudite is hexagonal, space group P6/mcc (no. 192), with a = 9.997(1), c = 14.090(1)Å, V = 1219.5(2)Å3, 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 Å (I) hkl]. A single-crystal structure refinement (R1 = 1.85 %) confirmed that the structure is isotypic to milarite and related A [12] B 2 [9] M 2 [6] T2 3 [4][T1 12O30] 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 (N1190) 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/cm3. Electron microprobe analysis yielded (wt.%): MgO 1.68, CaO 41.55, Al2O3 23.06, Fe2O3 8.06, SiO2 15.66, TiO2 0.72, Cl 9.79 (-O ≡ Cl), sum 100.52, giving the empirical formula Ca12.00[(Al7.10Si4.54Fe1.34Mg0.87Ti0.15)O31.99]Cl5.84 assuming a sum of cations of 26. In the rim of the crystals, the coupled substitution Mg2++Si4+↔Al3++Fe3+ is observed. The cubic mineral crystallises in space group I 43d (no. 220), with a = 12.0343(2) Å, V = 1742.9(1)Å3 and Z = 2. Strongest lines in the X-ray powder diffraction pattern are: 2.691 (100) 420, 3.008 (48) 400, 2.456 (46) 422, 1.669 (34) 640, 1.608 (31) 642, 2.360 (21) 510 [d in Å (I) hkl]. The structure refinement (R1 = 2.38 %) confirms isotypy with the mayenite group compounds, M 12[T2 8 T1 6O32]X (X = O2-, S2-, 2F-, 2OH-, 2Cl-), which can be classified as a 3,4-connected, interrupted framework structure. Based on the single-crystal structure refinement and bond-valence calculations the structural formula can be calculated as Ca12.00[(Si3.88Al2.65Fe1.32Ti0.15)(Al4.73Mg0.68Si0.30Fe0.29)O32] Cl5.69.
Organisation(en): Institut für Mineralogie und Kristallographie
Journal: Neues Jahrbuch für Mineralogie. Abhandlungen
Band: 179
Seiten: 265-294
Anzahl der Seiten: 30
ISSN: 0077-7757
DOI: https://doi.org/10.1127/0077-7757/2004/0179-0265
Publikationsdatum: 2004
Peer-reviewed: Ja
ÖFOS 2012: 1051 Geologie, Mineralogie
Link zum Portal: https://ucrisportal.univie.ac.at/de/publications/66feab66-d4de-41ab-8472-2e9f3df5e22c

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