Friedrichbeckeite, K (□0.5Na0.5)2 (Mg0.8Mn0.1Fe0.1)2 (Be0.6 Mg0.4)3 [Si12O30], a new milarite-type mineral from the Bellerberg volcano, Eifel area, Germany (original) (raw)

Abstract

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 nω = 1.552(2) and nε = 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 K0.87 Na0.86 (Mg1.57Mn0.28Fe0.24)Σ2.09 (Be1.83 Mg1.17)Σ3.00 [Si12O30], and the simplified formula can be given as K (□0.5Na0.5)2 (Mg0.8Mn0.1Fe0.1)2 (Be0.6 Mg0.4)3 [Si12O30]. Friedrichbeckeite is hexagonal, space-group _P_6/mcc, with a = 9.970(1), c = 14.130(3) Å, V = 1216.4(3) Å3, 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] A 2 [4] _T_23 [[4] _T_112O30] 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 _T_2-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).

Access this article

Log in via an institution

Subscribe and save

Buy Now

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Similar content being viewed by others

References

Download references

Acknowledgements

The authors thank J. Jahn (Neuwied, Germany) for providing the material studied in the present paper, M. Wagner for sample preparations as well as T. Ntaflos, U. Klötzli, M. A. Götzinger and G. Giester (all Wien, Austria) for their support with the chemical microanalyses and single-crystal X-ray diffraction. The useful comments of CNMNC members and the detailed suggestions of M. Cooper (Winnipeg, Canada) and an anonymous reviewer as well as the considerable improvements of the Associate Editor A. Chakhmouradian (Winnipeg, Canada) are gratefully acknowledged. The work was supported by the International Centre for Diffraction Data through Grant 90–03.

Author information

Authors and Affiliations

  1. Institut für Mineralogie und Kristallographie, Universität Wien-Geozentrum, Althanstrasse 14, A-1090, Wien, Austria
    C. L. Lengauer, R. Krickl & E. Tillmanns
  2. Institut für Halbleiter- und Festkörperphysik, Johannes Keppler Universität Linz, Linz, Austria
    N. Hrauda
  3. Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Wien, Austria
    U. Kolitsch

Authors

  1. C. L. Lengauer
    You can also search for this author inPubMed Google Scholar
  2. N. Hrauda
    You can also search for this author inPubMed Google Scholar
  3. U. Kolitsch
    You can also search for this author inPubMed Google Scholar
  4. R. Krickl
    You can also search for this author inPubMed Google Scholar
  5. E. Tillmanns
    You can also search for this author inPubMed Google Scholar

Corresponding author

Correspondence toC. L. Lengauer.

Additional information

Editorial handling: A. R. Chakhmouradian

Rights and permissions

About this article

Cite this article

Lengauer, C.L., Hrauda, N., Kolitsch, U. et al. Friedrichbeckeite, K (□0.5Na0.5)2 (Mg0.8Mn0.1Fe0.1)2 (Be0.6 Mg0.4)3 [Si12O30], a new milarite-type mineral from the Bellerberg volcano, Eifel area, Germany.Miner Petrol 96, 221–232 (2009). https://doi.org/10.1007/s00710-009-0050-9

Download citation

Keywords