Gemmological Characteristic of Hydrothermal Synthetic Paraíba-Colour Beryl

The appearance of a kind of synthetic beryl introduced into the market by Tairus Created Gems recently, is very similar to copper and manganese-bearing greenish blue elbaite tourmaline (referred to as “Paraíba” in the trade). Thus, the chemical compositions, spectrum characteristics and colouring mechanism of hydrothermal synthetic Paraíba-colour beryl samples were researched by using the conventional gemmological methods and modern analytical techniques including electron probe micro-analyzer (EPMA), Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible absorption spectrometry (UV-Vis). The results showed that the dominant wavelength, brightness and saturation (CIE 1931) of hydrothermal synthetic Paraíba-colour beryl samples are 487.7-490.2 nm, 17.1%-31.5% and 47.8%-93.7% respectively. The refractive index and birefringence are 1.584-1.589 and 0.005, and density is 2.771-2.789 g/cm³, which are slightly higher than natural beryl. The gemmological microscope investigations revealed that hydrothermal synthetic Paraíba-colour beryl exhibits typical growth patterns such as micro-wave, stair-step, cross, disorder and subparallel ones. EPMA analysis showed that the hydrothermal synthetic Paraíba-colour beryl is characterized by poor alkali (the content of K₂O and Na₂O content is generally below 0.2%), rich copper (the content of CuO is higher than 5.00%) and iron (the content of FeO_T is 1.76%-3.00%), and nickel-bearing (the content of NiO is 0.03%-0.15%). The FT-MIR spectra defined the vibration areas of silica tetrahedron group in six-membered rings. The symmetry and asymmetry stretching modes of O—Si—O and Si—O—Si mainly appear at 970, 1 246 cm^-1 respectively. The FT-NIR analysis indicated that Type-Ⅰ and Type-Ⅱ water molecules are coexisting within the ring channel of hydrothermal synthetic Paraíba-colour beryl, and the absorption bands relating to the combination modes of type-Ⅰ and type-Ⅱ H₂O mainly appear at 5 450, 5 127 and 5 270 cm^-1. Besides, a certain amount of mineralizing agent used to synthetize beryl hydrothermally locates within the ring channel, and absorption bands appearing at 2 736 cm^-1 and 2 450 cm^-1 relating to chloridion are considered to be one of the identification characteristics of hydrothermal synthetic Paraíba-colour beryl. The UV-Vis spectra and EPMA results showed that the Paraíba-colour of hydrothermal synthetic beryl is mainly attributed to Cu²⁺, Fe²⁺, Fe³⁺ and Ni²⁺ in the crystal structure. The Fe³⁺ with ⁶A_1g→⁴E_g(⁴D) and ⁶A_1g→⁴E_g+⁴A_1g(⁴D) d—d electron transition absorption bands appear at 370 nm and 426 nm. The Ni²⁺ with ³A_2g→³T_1g(³P) d—d electron transition absorption band appears at 460 nm. A combination of the Cu²⁺ with ²E→²T₂(²D), the Fe³⁺ with ⁶A_1g→⁴T_2g(⁴G), the Fe²⁺ with ⁵T_2g→⁵E_g(⁵D) d—d electron transition and Fe²⁺—Fe³⁺ charge transfer absorption bands result in the strong absorption between 613-638 nm and 1 500 nm.