[Chemical composition] Tourmaline is a borosilicate mineral, that is, in addition to the silicon oxide backbone, there is also a [BO 3 ] complex anion group. Wherein Na + may be partially replaced by K + and Ca 2+ , (OH) - may be replaced by F - but there is no Al 3+ instead of Si 4+ . The R position is similar in kind, with four main components, namely:
Magnesium tourmaline (Dravite) R=Mg
Black tourmaline (Schorl): R = Fe;
Lithium tourmaline (Elbaite): R = Li + Al;
Sodium manganite (Tsilaisit) R = Mn.
Between the tourmaline-black tourmaline and the black tourmaline-lithium tourmaline, two complete isomorphic series are formed, and between magnesium tourmaline and lithium tourmaline is an incomplete analog image. Fe 3+ or Cr 3+ can also enter the position of R, and Cr 2 O 3 in the chromium tourmaline can reach 10.86%.
[Crystal Structure] Trigonal System; 
a 0 = 1.584 to 1.603 nm, c 0 = 0.709 to 0.722 nm; Z = 3. The basic feature of the tourmaline crystal structure is that the [SiO 4 ] tetrahedron constitutes a complex trigonal ring. The B coordination number is 3, which constitutes a planar triangle; the Mg coordination number is 6 (two of which are OH - ), which constitute an octahedron and is connected to [BO 3 ]. There is a monovalent cation Na + distribution with a coordination number of 9 in the void above the complex trigonal ring of the [SiO 4 ] tetrahedron. The [AlO 5 (OH)] octahedron is connected between them (Fig. G-24).
Figure G-24 tourmaline crystal structure
(quoted from Pan Zhaoyu et al., 1993)
[Form] The crystal is columnar, and the crystal faces at both ends of the crystal are different because the crystal has no center of symmetry. Longitudinal lines often appear on the cylinder surface, and the cross section is a spherical triangle (Fig. G-25, G-26). This is due to the development of a series of high-index crystal faces. As for why a series of high-index crystal faces are developed, possibly with the surface. It can be related because, from the geometric point of view, the surface energy of the three-column column is relatively large, and the development of a spherical three-sided column will reduce the surface energy, but the spherical three-sided column will lead to the development of some high-index crystal faces. Bismuth (1011) or (4041) develops, but is less common. The aggregate is in the form of a rod, a radial, a needle, or a dense block or a cryptocrystalline block.
Figure G-25 Crystals of tourmaline
(quoted from Pan Zhaoyu et al., 1993)
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Figure G-26 Tourmaline crystals are columnar and longitudinally appear on the cylinder
[Physical properties] Color varies with composition: Fe-rich tourmaline is black, tourmaline rich in Li, Mn and Cs is rosy and light blue, and Mg-rich tourmaline is often brown and Yellow, Cr-rich tourmaline is dark green. In addition, tourmaline often has a ribbon phenomenon, and the vertical c-axis forms a horizontal ribbon from the center to the outside, or the c-axis has different colors at both ends. Glass luster. No cleavage; sometimes there may be a split of vertical L 3 . Hardness 7 to 7.5. The relative density is 3.03~3.25. As the content of Fe and Mn in the composition increases, the relative density also increases. Not only piezoelectric, but also pyroelectric (because its unidirectional axis L 3 is the only polar axis).
[genesis and production] Tourmaline is rich in volatile components B and H 2 O, so it is related to gas formation, and is mostly produced in granitic pegmatite and gas-forming hydrothermal deposits. Generally, black tourmaline is formed at a higher temperature, and green and pink are generally formed at a lower temperature. The early formed tourmaline is long columnar, and the late is short column. In addition, tourmaline production is also found in metamorphic deposits.
[Identification features] columnar crystal form, the column surface has vertical lines, the cross section is spherical triangle, no cleavage, high hardness is characteristic.
[Main use] Its piezoelectricity can be used in the radio industry; its pyroelectricity can be used in the infrared detection and refrigeration industry. Bright color, clear and transparent can be used as a gemstone material (commonly known as tourmaline).
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