ACICULAR ZONED TOURMALINE (MAGNESIO-FOITITE TO FOITITE) FROM A QUARTZ VEIN NEAR TISOVEC, SLOVAKIA: THE RELATIONSHIP BETWEEN CRYSTAL CHEMISTRY AND ACICULAR HABIT

摘要

Distinctly chemically zoned tourmaline was found in a quartz vein near Tisovec, Slovak Ore Mountains, Slovakia. The tourmaline forms radial aggregates of light grey to green thin prismatic to acicular crystals growing in cracks in the host rock. The root zone of the aggregates has dravitic to oxy-dravitic compositions, shifting to magnesio-foitite in the middle parts of the crystals and to foititic compositions in the outer parts of the aggregates. The optimized formula, considering the single-crystal structure refinement (SREF) and the chemistry of the magnesio-foititic zone, is similar to(X)(square 0.5Na0.4Ca0.1) (Y)(Mg1.1Al1.0Fe0.82+Fe0.13+) Z(Al5.7Fe0.33+) (Si5.9Al0.1O18) (BO3)(3) (OH)(3) [O-0.6(OH)(0.3)F-0.1], with lattice parameters (SREF) a 15.929(2) angstrom, c 7.163(1) angstrom, and V 1574.0(7) angstrom(3). SREF data as well as a detailed study of bond lengths indicate a dominancy of Al3+ at the Z site substituted by a small proportion of Fe3+, while Fe2+ and Mg2+ essentially occupy the Y site along with a significant amount of Al. The chemical composition is controlled by the substitutions FeMg-1, Al square(Fe, Mg)(-1)Na-1, and AlO(Fe, Mg)(-1)(OH)(-1). The AlO(Fe, Mg)(-1)(OH)(-1) substitution is dominant in the transition from oxy-dravite to magnesio-foitite, and the Al square(Fe, Mg)(-1)Na-1 substitution prevails in the magnesio-foitite and foitite compositions. Both proton-and alkali-deficient substitutions produce the enrichment in Al which can be linked to the forming of the acicular habit. Aluminum strongly prefers the Z-site, whose octahedra form a network of chains parallel to c. Consequently, polymerization of the ZO(6) octahedral network will dominate growth in the a direction. Therefore, the chemical composition, mainly Al enrichment, which is common for most of the acicular to fibrous tourmalines, can be an important factor promoting preferential growth in the c direction. The rate and time for crystallization, which depend mostly on the temperature and cooling rate, are also factors which should be considered.