TY  - JOUR
A1  - Kutzschbach, Martin
A1  - Wunder, Bernd
A1  - Krstulovic, Marija
A1  - Ertl, Andreas
A1  - Trumbull, Robert B.
A1  - Rocholl, Alexander
A1  - Giester, Gerald
T1  - First high-pressure synthesis of rossmanitic tourmaline and evidence for the incorporation of Li at the X site
JF  - Physics and chemistry of minerals / in cooperation with the International Mineralogical Association (IMA)
N2  - 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 degrees C in the system Li2OAl2O3SiO2B2O3H2O (LASBH) from seed-free solid starting materials consisting of a homogenous mixture of Li2O, gamma-Al2O3, quartz and H3BO3. 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 X 100 mu m), which are inclusion free and chemically unzoned. The refined cell dimensions of the tourmaline are: a = 15.7396(9) angstrom, c = 7.0575(5) angstrom, V = 1514.1(2) angstrom 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)(square Li-0.67(11)(0.33(11)))(Y)(Al2.53(10)Li0.47(10))(Z)(Al-6)T(Si5.42(15)B0.58(15))O-18(B)(BO3)(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) Li-X pfu, respectively]. The SREF data show that the LiO1 distances are shortened significantly in order to compensate for the smaller ionic radius of Li+ compared to Na+, K+ or Ca2+ at the X site, i.e., Li is closer to the Si6O18 ring and to a sevenfold coordination with oxygen.
KW  - High-pressure synthesis
KW  - Tourmaline
KW  - Rossmanite
KW  - Crystal chemistry
KW  - X site occupancy
KW  - SIMS
KW  - SREF
KW  - Li isotope fractionation
Y1  - 2016
U6  - https://doi.org/10.1007/s00269-016-0863-0
SN  - 0342-1791
SN  - 1432-2021
VL  - 44
SP  - 353
EP  - 363
PB  - Springer
CY  - New York
ER  -