On the mineralogy and paragenesis of tugtupite Na8Al2Be2Si8O24(Cl,S)2 from the Ilímaussaq alkaline intrusion, South Greenland

Abstract

Tugtupite, Na8Al2Be2Si8O24(Cl,S) 2, is colourless to red, tetragonal, pseudocubic and forms triplet twins on {101}. c: a = 1.0269 ± 0.0003, d = 2.33 g/cm3, nε = 1.499 ± 0.001, nα = 1.495 ± 0.001. a0 = 8.637-8.643, c0 = 8.867 - 8.870, V0 = 662 Å3. The description of the crystal structure is taken from Danø (1966). The mineral occurs in hydrothermal veins in the Ilímaussaq alkaline intrusion, South Greenland, and in the Lovozero alkaline intrusion, Kola Peninsula (beryllosodalite). A number of occurrences of tugtupite in Ilimaussaq are described. Tugtupite is most commonly associated with albite, analcime and Li-mica and is generally secondary after chkalovite. The paragenesis and origin of the mineral and a number of associated beryllium silicates are discussed.

Changes in Halogen (F, Cl, Br, and I) and S Ratios in Rock-Forming Minerals as Monitors for Magmatic Differentiation, Volatile-Loss, and Hydrothermal Overprint: The Case for Peralkaline Systems

“…Due to low sodalite contents in intrusions other than Ilímaussaq it was only possible to measure a single sodalite sample from Tamazeght and none from the other intrusions. In Ilímaussaq, sodalite in roof and floor cumulates is known to be close to the Cl-endmember composition [33,54,88], containing around 7 wt.% Cl. Most of the bulk sodalite CIC data scatter around this value but some samples showed, however, much lower Cl contents.…”

“…However, the samples are fresh and do not contain any obvious inclusions, as was the case for some of the sodalite samples. Rather, the presence of Be in tugtupite at the wt.% level [88] may explain this, as many Be compounds show covalent properties. If parts of the Cl are present after combustion as BeCl 2 or Be-Cl ionic complexes, this would result in an overall under determination of the actual Cl concentrations, as only chloride is detected during CIC.…”

“…Literature data for F, Br, and I are absent for tugtupite. For S, rather old publications indicate S at the thousands of µg/g level [88,89], while modern studies [90,91] showed S to be below their respective detection limits of <50 and <5 µg/g, respectively. Moreover, a Wavelength-Dispersive Spectroscopy (WDS) scan for S in the tugtupite used as Cl standard at the electron microprobe facility at University of Tübingen revealed no detectable S, indicating that the apparently low S contents for the investigated samples are trustworthy.…”