Fluor-elbaite, lepidolite and Ta–Nb oxides from a pegmatite of the 3000 Ma Sinceni Pluton, Swaziland: evidence for lithium–cesium–tantalum (LCT) pegmatites in the Mesoarchean

Abstract

Mineral evolution is concerned with the timing of mineral occurrences, such as the earliest reported occurrences in the geologic record. Minerals containing essential Li have not been reported from rocks older than ca. 3000 Ma, thus the lithian tourmaline (fluor-elbaite) and mica (lepidolite) assemblage from a pegmatite near Zishineni associated with the ca. 3000 Ma Sinceni Pluton presents unusual interest. Fluor-elbaite (0.75-0.98 F per formula unit) forms green crystals up to 50 mm long. Spindle stage measurements give v = 1.652(1), e = 1.627(1) (589.3 nm). Optical absorption spectroscopy shows Fe and Mn are divalent; infra-red spectroscopy demonstrates the presence of Li and indicates the presence of (OH) at both the (OH) sites. Electron microprobe analysis of 330 points on several prisms, the largest of which is zoned in Fe and Ca, gives the following average and standard deviations in wt%: SiO 2 37.29 (0.26), TiO 2 0.05 (0.05), Al 2 O 3 38.14 (0.35), Cr 2 O 3 0 (0.02), MgO 0.02 (0.01), MnO 3.57 (0.25), FeO 2.48 (0.60), Na 2 O 2.48 (0.09), K 2 O 0.03 (0.12), CaO 0.77 (0.21), F 1.80 (0.11), Cl 0 (0.01) wt%. Nuclear reaction analyses gave Li 2 O 0.91 (0.04) and B 2 O 3 10.55 (0.45). The empirical formula of fluor-elbaite was determined by integrating crystal-chemical data from electron microprobe analysis, nuclear reaction analysis, crystal structure refinement using X-ray diffraction, infra-red and optical absorption spectroscopy:

Are the [6]-coordinated sites in tourmaline in certain cases partially vacant?

“…Similar Y-site vacancies (up to 0.25 pfu) have been reported from B-rich olenite, also an Al-rich tourmaline (Ertl and Brandstätter 1998, Ertl et al 2007. The samples with the highest reported Y-site vacancies are also Al-rich tourmalines: a Mn-and Fe-bearing fluor-elbaite with 0.35 pfu vacancies (Grew et al 2018) and a Fe-, Mn-, and Li-bearing olenite with 0.36 pfu vacancies (Ertl et al 2012b). A natural Al-rich foititic tourmaline with ~0.35 pfu Y-site vacancies was also described (Wodara and Schreyer 2001).…”

“…Later, for the elbaite-schorl series short-range orders with (OH/F) at the W site were assumed (Ertl et al 2010a). Recently, a tourmaline was described that consists of 39% fluor-elbaite, 34% of the component Na Y (Al 2 ) Z Al 6 (BO 3 ) 3 [Si 6 O 18 ] V (OH) 3 W F, 16% fluor-tsilaisite and 11% fluorschorl (Grew et al 2018).…”

Mn-bearing purplish-red tourmaline from the Anjanabonoina pegmatite, Madagascar

“…Moreover, the low variability of counts, verified by measurements on uniform matrix materials such as that of NIST glass samples, allowed us to perform measurements on tourmaline samples which normally have chemical zoning. The calibration, obtained using spectra recorded on two NIST standard glasses (SRM 610 and 612) and four Li-bearing tourmalines from Bosi et al (2005), Filip et al (2012), Grew et al (2018) and Bosi et al (2019d), resulted in two calibration curves: one below and the other above 0.1 Li 2 O wt.% (Fig. 3).…”

“…The calibration resulted in two branches (one below and the other above 0.1 Li 2 O wt.%) described by two regression lines. Black squares represent samples from NIST standard glasses (SRM 610 and 612) and Filip et al (2012), black circles are from Bosi et al (2005, 2019d) and Grew et al (2018); and a red filled circle represents the present sample.…”

Celleriite, ☐(Mn22+Al)Al6(Si6O18)(BO3)3(OH)3(OH), a new mineral species of the tourmaline supergroup

“…with a delay of 2 μs after the second pulse and were integrated for 1 ms. Quantitative data were obtained by generating a linear regression using the main Li emission line intensity (670.706 nm corresponding to resonance transition 1s 2 2s > 1s 2 2p), which is particularly sensitive to Li amounts. The linear fit was made using spectra recorded on two NIST standard glasses (SRM 610 and 612) and three Li-bearing tourmaline samples from Filip et al (2012), Grew et al (2018) and Bosi et al (2019d). The result corresponds to 0.42 wt% of Li 2 O (Table 4).…”