Tourmaline (tourmaline)

Tourmaline (tourmaline) has complex chemical composition, and is a silicate mineral characterized by boron, which is characterized by boron, aluminum, sodium, iron, magnesium and lithium. Tourmaline is a mineral of silicic acid. Its process name is tourmaline. It is generally found in granite pegmatite type and high temperature gas forming hydrothermal deposit, and is one of the typical high temperature gas forming minerals. Tourmaline was first discovered in Sri Lanka, when it was regarded as precious as diamond and ruby. It is noticed that this gemstone will bring electric charge when heated. This phenomenon is called pyroelectric effect, so it is called tourmaline. Tourmaline is also called tourmaline, broken gold and electrified stone. The gemstone level is called tourmaline. A list of middle - grade jewels.
1. [chemical composition] that is, besides silica oxide backbone, there are [BO3] complex anion groups. Where Na+ can be locally replaced by K+ and Ca2+, (OH) - F- can be substituted, but no Al3+ is used instead of Si4+. R positional homogeneous image is widely used, mainly with 4 end member components: magnesium tourmaline (Dravite) R=Mg; black tourmaline (Schorl): R=Fe; lithium tourmaline (Elbaite): R=Li+Al; sodium manganese tourmaline (Tsilaisit) R=Mn.
Two. [crystal structure] three square crystal system; -R3m; a0=1.584 ~ 1.603 nm, c0=0.709 ~ 0.722 nm; Z=3. The basic characteristics of tourmaline crystal structure are [SiO4] tetrahedron complex three square ring. The B coordination number is 3, forming a plane triangle, and the Mg coordination number is 6 (two of which are OH-), forming eight faces and connecting with BO3. In the gap above [SiO4] tetrahedron complex three ring, there is a univalent cation Na+ with a coordination number of 9. The connection between the eight sides of [AlO5 (OH)].
Three. [morphology] crystals are columnar, and crystal faces at both ends are different, because the crystal has no symmetry center. The vertical lines appear on the cylinder and the cross section is spherical triangle (Figure G-25, G-26), which is caused by the development of a series of high exponential crystal surfaces. As to why a series of high exponential crystal surfaces can be developed, it may be related to the surface energy, because the surface energy of the three square columns is larger and developed into a spherical three square column from a geometric point of view. The surface energy will be reduced, but the spherical three column will lead to the development of some high index surfaces. Bimorph is developed according to (101) or (401), but is rare. The aggregate is rod-shaped, radial, needle like, and also compact or massive.
Four. [physical properties] the color varies with the composition: the tourmaline rich in Fe is black. The tourmaline rich in Li, Mn and Cs is rosy and light blue. The Mg rich electrics are often brown and yellow, and the Cr rich tourmaline is dark green. In addition, the tourmaline often has a ribbon phenomenon. The vertical c axis forms a horizontal ribbon from the center, or the ends of the c axis are different in color. Glass luster. No cleavage; sometimes there is a split of the vertical L3. The hardness is 7 to 7.5. The relative density is 3.03 to 3.25, and the relative density increases with the increase of Fe and Mn content. It is not only piezoelectric, but also pyroelectric, because its uniaxial L3 is the only polar axis. Tourmaline crystals are columnar, and the aggregates are rod like, radial, and small needle like, and are also dense or hidden crystals.