Study On The Color-Induced Mechanism Of Colorful Tourmaline

Tourmalines are gem minerals that have rich,bright,and potent colors.The beauty and diversity of tourmaline colors enhance their value as a gem mineral.The chemical composition of tourmaline mineral crystals is highly complex;the chemical formula of tourmalines is XY₃Z₆[Si₆O₁₈][BO₃]₃V₃W,and they usually contain various transition metal ions.Hence,understanding the color-causing mechanisms of tourmalines has always been a difficult problem in the field of mineralogy.Research on the color-causing mechanisms of tourmalines can guide efforts to improve the color of tourmaline gemstones,serve as a diagnostic reference for tourmaline identification tests,and provide a reliable source of information about the minerogenetic mechanisms of tourmaline.In this work,a systematic and in-depth investigation into the color-causing mechanisms of tourmaline was conducted.The CASTEP quantum chemistry package was used to investigate the crystal lattice and transition metal ion doping system of tourmalines,and to analyze their optical properties and state densities.Electron probe microanalysis?EPMA?,X-ray photoelectron spectroscopy?XPS?,X-ray diffractometry?XRD?,UV-visible?UV-vis?spectroscopy,Fourier-transform infrared?FTIR?spectroscopy,and cathodoluminescence?CL?imaging were used to probe typical colored tourmaline crystals to shed light on their color-causing mechanisms.Based on quantum chemistry calculations for the crystal lattice and transition-metal ion doping system of tourmalines,the bandgap of a tourmaline shrinks upon the doping of the transition metal ions in the Y or Z sites of its crystal structure,which enhances the response of a tourmaline to visible light.Transition metal doping at the Y-site results in more bandgap shrinkage than that which would occur at the Z-site.Hence,Y-site doping is the root cause of bandgap shrinkage in tourmalines.The doping of Mn,Ni,or Cu at the Y-site redshifts the primary absorption peak of a tourmaline system,consequently changing its color from pink to yellow,and then blue.The color of tourmaline is attributed to the transition metal ion that occupies the Y-site of its crystal structure.The results of theoretical and experimental analyses strongly support this claim.In addition to the site of transition metal ion occupation,the color of tourmaline is also affected by the valence state of the transition metal ion and the identity of the coordinating anion.Pink is attributable to the occupation of Y-sites by F-coordinated Mn²⁺ions.Yellow is attributable to the occupation of Y-sites by O-coordinated Ni²⁺ions.Green is attributable to the occupation of Y-sites by O,OH,and F-coordinated Fe³⁺ions.Rose red is attributable to the occupation of Y-sites by O and F-coordinated Mn²⁺,and O-coordinated Ni²⁺.Blue is attributable to the occupation of Y-sites by O,OH,and F-coordinated Fe³⁺,and F-coordinated Mn²⁺.Although charge transfers usually occur between ions that occupy different crystal sites,no charge transfer was detected between Y-and Z-site ions in the tourmaline crystal lattice.Instead,the color of tourmaline was found to be directly influenced by the d-d transitions of the Y-site-occupying transition metal ion.The Z-site-occupying transition metal ion did not directly influence the color of tourmalines.Although a few transition metal ions were detected in the colored tourmalines,these ions were always found to occupy the Z-site of the crystal structure.In pink and yellow tourmalines,Fe and Cr were present as Fe³⁺and Cr³⁺,respectively.In rose-red tourmalines,Fe was present as Fe³⁺.In blue tourmalines,Cr was present as Cr³⁺.In green tourmalines,Mn,Ni,and Cu were present as Mn²⁺,Ni²⁺,and Cu²⁺,respectively.The cathodoluminescence of the tourmaline samples was found to be consistent with their respective chemical compositions and chemical states of their characteristic elements.As compared to the colored tourmaline samples,uncolored tourmalines exhibited a higher degree of crystallinity.The decreased crystallinity in the colored samples was due to the doping of the transition metal elements,which reduced tourmaline crystallinity,and also generated color.Hence,decreases in tourmaline crystallinity are conducive to color generation.The results showed that the color of a tourmaline gradually changes from the red series to the green-blue series as its lattice parameter increases in value.The[MO₆],[SiO₄],[BO₃],OH,and H₂O groups in the tourmaline crystal structure were found to be strongly infrared?IR?active.Here,it was found that the shape and position of the IR absorption peaks of[MO₆]and OH significantly differ between differently colored tourmalines.These differences arise from variations in the octahedral distortion of each tourmaline.In addition to the site of occupancy,the valence state and coordinating anion of the transition metal ions,as well as distortions at the Y octahedral site,were found to affect the extranuclear electron configuration of the color-causing transition metal ions,thus influencing the selective absorption of visible light by tourmalines.Changes to the M-O distance due to structural distortions were found to decrease the octahedral field-splitting parameter,causing the tourmaline to become darker in color.Structural distortions at the Z octahedral site did not directly affect tourmaline color;however,they are capable of inducing distortions at the Y octahedral site,which shares an edge with the Z octahedral site via O3.