Tourmaline structure

Nuber, B. Schmetzer, K. (1979) The lattice position of Cr3 in tourmaline structural refinement of a chromium-rich Mg-Al tourmaline. Neues Jahrb. Mineral., Abh., 137, 184—97. 

A variety of minerals are prized for their exquisite beauty, rarity, and exceptional durability. These extraordinary materials are classified as gemstones. One such mineral, silica, with a chemical composition of SK>2 (silicon dioxide), exhibits several crystal structures. Several gemstones are crystalline forms of silica, including amethyst, aquamarine, emerald, garnet, peridot, topaz, tourmaline, and zircon J l... 

The mineral beryls, cordierite, and tourmaline, collectively designated as cyclosilicates, have large open channels in their crystal structures. These channels are large enough to accommodate atoms and molecules extraneous to the structure, and these minerals are well known as hosts of such extraneous elements as noble gases. Beryl and tourmaline are characteristic minerals of pegmatites, and cordierite is a common metamorphic mineral. 

Foit, F. F. Rosenberg, P. E. (1979) The structure of vanadium-bearing tourmaline and its implications regarding tourmaline solid solutions. Amer. Mineral., 64, 788-98. 

In addition to the chemical groups of the Dana system, there are also structural groups of minerals. These describe two or more mineral species that have the same or similar crystalline structure, while differing chemically. Not all species are placed within structural groups. Well-known groups include garnets, tourmalines, feldspars, clays, and micas. 

Cyclosilicates are those compounds with three or more tetrahedra linked in closed, ring-like structures. The ratio between Si and O is 1 3. This group includes the important gem species beryl and the tourmaline group. The 11 members of the tourmaline group display an unusual property known as piezoelectricity. When pressure is applied to a crystal, an electrical charge builds up at either end of it. This makes these minerals useful in pressure gauges and other electrical equipment. Quartz and a few other, less common minerals, also display this property. 

FIGURE 7.8 The crystal structure of tourmaline. The 3-fold axis can be seen. The lattice parameters are a = 1.58 and c = 0.71 nm. Most of the nominal 140 atoms in the oell sit in tetrahedral or octahedral sites, but the important boron ion sits at the center of three planar anions. 

Preparation of cellulose/tourmaline nanocomposite was by dispersing tourmaline nanociystalsNaOH/thiourea aqueous solutions and then blended with cellulose aqueous solutions. These aqueous solutions are coagulated with CaCl. Characterization of the fihnrevealed tourmaline nanociystals structure is remain unaltered in the cellulose matrix and tourmaline contentof 4-8 wt% composite films possessed good tensile strength (92-107 MPa) and exhibited antibacterial properties against Staphylococcus aureus (Ruan et al., 2003). 

Ruan, D., Zhang, L., Zhang, Z.,and Xia, X. Structure and properties of regenerated cellulose/ tourmaline nanocrystal composite films. J. Polym. Sci. Part B Polym. Phys., 42, 367-373 (2003). 

---