First high-pressure synthesis of rossmanitic tourmaline and evidence for the incorporation of Li at the X site

Author(s)
Martin Kutzschbach, Bernd Wunder, Marija Krstulovic, Andreas Ertl, Robert Trumbull, Alexander Rocholl, Gerald Giester
Abstract

Lithium is an important component of some tourmalines, especially in chemically evolved granites and pegmatites. All attempts at synthesizing Li-rich tourmaline have so far been unsuccessful. Here we describe the first synthesis of rossmanitic tourmaline at 4 GPa and 700 °C in the system Li

2O–Al

2O

3–SiO

2–B

2O

3–H

2O (LASBH) from seed-free solid starting materials consisting of a homogenous mixture of Li

2O, γ-Al

2O

3, quartz and H

3BO

3. The solid run products after 12-day run duration comprise rossmanitic tourmaline (68 wt%), dumortierite (28 wt%) and traces of spodumene (3 wt%) and coesite (1 wt%). Tourmaline forms idiomorphic, large prismatic crystals (30 × 100 μm), which are inclusion free and chemically unzoned. The refined cell dimensions of the tourmaline are: a = 15.7396(9) Å, c = 7.0575(5) Å, V = 1514.1(2) Å

3. Conventionally, the Li

+ ion is assumed to exclusively occupy the octahedral Y site in the tourmaline structure to a maximum of 2 Li per formula unit (pfu). However, the chemical composition of our synthetic tourmaline determined by electron microprobe and secondary ion mass spectroscopy results in the formula:

X(☐

0.67(11)Li

0.33(11))

Y(Al

2.53(10)Li

0.47(10))

Z(Al

6)

T(Si

5.42(15)B

0.58(15))O

18

B(BO

3)

3

V+W[(OH)

2.40(3)O

1.60(3)], wherein a significant amount of Li occupies the X site for charge balance requirements. Reliable assignment of the OH-stretching vibrations in a polarized single-crystal Raman spectrum such as a single-crystal XRD structure refinement, confirms the incorporation of Li at the X site [0.24(9) and 0.15(5)

XLi pfu, respectively]. The SREF data show that the Li–O1 distances are shortened significantly in order to compensate for the smaller ionic radius of Li

+ compared to Na

+, K

+ or Ca

2+ at the X site, i.e., Li is closer to the Si

6O

18 ring and to a sevenfold coordination with oxygen.

Organisation(s)
Department of Mineralogy and Crystallography
External organisation(s)
Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Naturhistorisches Museum Wien (NHM), Technische Universität Berlin, Universität Potsdam
Journal
Physics and Chemistry of Minerals
Volume
44
Pages
353-363
No. of pages
11
ISSN
0342-1791
DOI
https://doi.org/10.1007/s00269-016-0863-0
Publication date
12-2016
Peer reviewed
Yes
Austrian Fields of Science 2012
105116 Mineralogy, 105113 Crystallography
Keywords
ASJC Scopus subject areas
Geochemistry and Petrology, General Materials Science
Portal url
https://ucrisportal.univie.ac.at/en/publications/5beebf9f-a1be-4a31-86a5-d135356d02ba