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Topaz
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula that crystallizes in the orthorhombic system with additional anions (F- and OH-). The name "topaz" of the mineral is usually derived from the Green according to the ancient name of the island of Zabargad in the Red Sea - "Topaz" by Anselm Boethius de Boot of Brugge (circa 1550 - June 21, 1632) in the work "Gemmarum et lapidum historia", published in 1609, with reference on Pliny the Elder.
Chemistry: Al2[SiO4](F,OH)2;
Crystal system: Rhombic, rhombo-dipyramidal crystal class (point symmetry group);
Color: Colorless, less often - brown, pink, reddish-orange, purple, violet, orange, yellow, blue, rarely green.
The violet color of topaz comes from the isomorphous displace of Al3+ ions by Cr3+ ions in octahedral site. An irradiation and irradiation-heat treatment may make topaz being other colors. Smoky color is due to the replacement of fluorine ion O- (leaving the structure of the silicon–oxygen tetrahedron, and forming the vacancy [SiO3]3-). Yellow, wine, blue and green color varieties are due to irradiation defects associated with the anionic complex (F,OH)-.
Identification properties
| Physical properties | |
|---|---|
| Mohs hardness: | 8 |
| Density: | 3.49 – 3.57 g/cm3 |
| Cleavage: | perfect on (001) |
| Fracture: | Conchoidal, uneven |
| Optical properties | |
|---|---|
| Optical character: | anisotropic, biaxial, positive |
| Refractive Index: | np =1.606 – 1.629, nm =1.609 – 1.631, ng =1.616 – 1.638 |
| Birefringence: | 0.008 – 0.011 |
| Pleochroism: | weak to distinct |
| Dispertion: | 0.014 (BG) |
| Luster: | vitreous |
Inclusions and structural inhomogeneities
Gem basic treatments
| Treatment | Goal |
|---|---|
| Heat treatment | Removing the smoky and brownish hue |
| Irradiation treatment with heat treatment followed | Altering the apparent color of gem material |
| Diffusion treatment (ion-implant doping including) | Improving the apparent clarity and color of gem material |
| Surface coating by applying a various coloring agent | Altering the gem visual color |
| Surface coating by applying a various coloring agent (including metallization) | Improving the apparent clarity and color of gem material |
Inclusions and structural inhomogeneities
Coating application
Synthetic or Imitation gem materials
Synthetic topaz for the jewelry industry is not produced. However, there is an information on the hydrothermal synthesis of topaz in laboratory conditions as an experiment in the literature and in patent documents.
The most commonly encountered synthetic topaz is as follows:
- colorless quartz, as well as quartz coating treated;
- synthetic blue quartz;
- danburite;
- doublet: topaz + quartz; topaz + glass;
- glasses
