Friedrichbeckeite, K (□_0.5Na_0.5)₂ (Mg_0.8Mn_0.1Fe_0.1)₂ (Be_0.6 Mg_0.4)₃ [Si₁₂ O₃₀], a new milarite-type mineral from the Bellerberg volcano, Eifel area, Germany

Autor(en)

C. L. Lengauer, N. Hrauda, U. Kolitsch, R. Krickl, E. Tillmanns

Abstrakt

Friedrichbeckeite is a new milarite-type mineral. It was found in a single silicate-rich xenolith from a quarry at the Bellerberg volcano near Ettringen, eastern Eifel volcanic area, Germany. It forms thin tabular crystals flattened on {0001}, with a maximum diameter of 0.6 mm and a maximum thickness of 0.1 mm. It is associated with quartz, tridymite, augite, sanidine, magnesiohornblende, enstatite, pyrope, fluorapatite, hematite, braunite and roedderite. Friedrichbeckeite is light yellow, with white to light cream streak and vitreous lustre. It is brittle with irregular fracture and no cleavage, Mohs hardness of 6, calculated density is 2.686 gcm^-3. Optically, it is uniaxial positive with n_ω; = 1.552(2) and n_ε = 1.561(2) at 589.3 nm and a distinct pleochroism from yellow (//ω) to light blue (//ε). Electron microprobe analyses yielded (wt.%): Na₂O 2.73, K₂O 4.16, BeO 4.67, MgO 11.24, MnO 2.05, FeO 1.76, Al₂O₃ 0.15, SiO₂ 73.51, (Σ CaO, TiO₂ = 0.06) sum 100.33 (BeO determined by LA-ICP-MS). The empirical formula based on Si = 12 is K_0.87 Na_0.86 (Mg_1.57Mn_0.28Fe_0.24) _Σ2.09 (Be_1.83ΣMg_1.17) _Σ3.00 [Si₁₂O₃₀], and the simplified formula can be given as K (□_0.5 Na_0.5)₂ (Mg_0.8Mn_0.1 Fe_0.1)₂ (Be_0.6Mg_0.4) ₃[Si₁₂O₃₀]. Friedrichbeckeite is hexagonal, space-group P6/mcc, with a = 9.970(1), c = 14.130(3) Å, V = 1216.4(3) ų, and Z = 2. The strongest lines in the X-ray powder diffraction pattern are (d in Å / I_obs / hkl): 3.180 / 100 / 121, 2.885 / 70 / 114, 4.993 / 30 / 110, 4.081 / 30 / 112, 3.690 / 30 / 022. A single-crystal structure refinement (R 1 = 3.62 %) confirmed that the structure is isotypic with milarite and related ^[12] C^[9] B ₂^[6] A₂^[4] T2₃ [^[4] T1₁₂O₃₀] compounds. The C-site is dominated by potassium, the B-site is almost half occupied by sodium, and the A-site is dominated by Mg. The site-scattering at the T2-site can be refined to a Be/(Be + Mg) value close to 0.61; the T 1-site is occupied by Si. Micro-Raman spectroscopy reveals an increasing splitting of scattering bands around 550 cm^-1 for friedrichbeckeite. The mineral can be classified as an unbranched ring silicate or as a beryllo-magnesiosilicate. With respect to the end-member formula K (□0.5Na0.5)₂ Mg₂ Be₃ [Si₁₂O₃₀] friedrichbeckeite represents the Mg-dominant analogue of almarudite, milarite or oftedalite. The mineral and its paragenesis were formed during pyrometamorphic modifications of the silicate-rich xenoliths enclosed in Quaternary leucite-tephritic lava of the Bellerberg volcano. Holotype material of friedrichbeckeite has been deposited at the mineral collection of the Naturhistorisches Museum Wien, Austria. The mineral is named friedrichbeckeite in honour of the Austrian mineralogist and petrographer Friedrich Johann Karl Becke (1855-1931).

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Johannes Kepler Universität Linz, Naturhistorisches Museum Wien (NHM)

Journal

Mineralogy and Petrology

Band

96

Seiten

221-232

Anzahl der Seiten

12

ISSN

0930-0708

Publikationsdatum

05-2009

Peer-reviewed

Ja

ÖFOS 2012

105116 Mineralogie, 105113 Kristallographie

ASJC Scopus Sachgebiete

Geophysics, Geochemistry and Petrology

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/b9eda6e4-0b9f-46b8-91b7-9f5fd058dcf2