Cu- and Mn-bearing tourmalines from Brazil and Mozambique: crystal structures, chemistry and correlations.

Andreas Ertl,Martin Okrusch,Ekkehart Tillmanns,Ulrich Schüssler,Hermann Bank,Gerald Giester,Helene Brätz

doi:10.1007/s00710-012-0234-6

Abstract

Cu- and Mn-bearing tourmalines from Brazil and Mozambique were characterised chemically (EMPA and LA-ICP-MS) and by X-ray single-crystal structure refinement. All these samples are rich in Al, Li and F (fluor-elbaite) and contain significant amounts of CuO (up to ~1.8 wt%) and MnO (up to ~3.5 wt%). Structurally investigated samples show a pronounced positive correlation between the <Y-O> distances and the (Li + Mn2+ + Cu + Fe2+) content (apfu) at this site with R 2 = 0.90. An excellent negative correlation exists between the <Y-O> distances and the Al2O3 content (R 2 = 0.94). The samples at each locality generally show a strong negative correlation between the X-site vacancies and the (MnO + FeO) content. The Mn content in these tourmalines depends on the availability of Mn, on the formation temperature, as well as on stereochemical constraints. Because of a very weak correlation between MnO and CuO we believe that the Cu content in tourmaline is essentially dependent on the availability of Cu and on stereochemical constraints.

Highlights

The investigated tourmalines from Brazil and Mozambique are all enriched in Al, contain relatively high amounts of Li and have a pronounced content of MnO, CuO and F (Table 1)
We conclude that the excellent negative correlation, which exists between the distances and the Al2O3 content, can be used to predict the bond-length, when no crystal structure analysis was performed
We conclude that the Mn content in these tourmalines depends on the availability of Mn, on the formation temperature, as well as on stereochemical constraints

Summary

Introduction

In tourmaline, which has the generalised structural formula XY3Z6(BO3)3T6O18V3W, the individual structural sites can be occupied by the following cations: site X 0 Na, Ca, K, □ (vacancy); Y 0 Li, Mg, Fe2+, Mn2+, Al, Cr3+, V3+, Fe3+, Ti4+, Zn, Cu; Z 0 Al, Mg, (Fe2+), Fe3+, Mn3+, V3+, Cr3+, (Ti4+); T 0 predominantly occupied by Si, but sometimes by Al and B; V 0 OH-, O2-; W 0 OH-, F-, Cl-, O2- (e.g., Povondra and Čech 1976; Foit and Rosenberg 1979; Deer et al 1986; Foit 1989; Grice and Ercit 1993; Hawthorne et al 1993; Lussier et al 2009, 2011; MacDonald and Hawthorne 1995a, b; Hawthorne 1996; Henry and Dutrow 1996; Ertl et al 1997, 2003, 2005, 2007, 2008, 2009, 2010a, 2012; Dyar et al 1998; Hawthorne and Henry 1999; Hughes et al 2001, 2004; Marler and Ertl 2002; Henry et al 2011; Ertl and Tillmanns 2012). The crystal structure of Cu-bearing tourmalines from Paraiba, Brazil, was described for the first time by MacDonald and Hawthorne (1995b). They refined two Cubearing elbaites, which contain 0.38 and 0.81 wt% CuO and 0.69 and 0.30 wt% Mn2O3, respectively. Burns et al (1994) refined the crystal structures of eight Mn-bearing tourmaline samples (from Nepal, Zambia and the San Diego Mine, California, USA). These samples contained up to 6.23 wt% MnO. The crystal structure of Mn2+rich tourmalines (with up to 8.66 wt% MnO) from Eibenstein an der Thaya, Lower Austria, was described by Ertl et al (2003). Bosi et al (2005a, 2012) published refinements of the crystal structures of Mn2+-rich tourmalines (with up to 9.6 wt% MnO) from the island of Elba, Italy

Results

Discussion

Conclusion