Maxence Vigier

Iron-titanium (Fe-Ti) charge transfer is mentioned in numerous articles as the source of the coloration of many natural minerals and some manufactured materials, but no global review of this phenomenon has been provided so far. Iron and titanium are ubiquitous in nature and are often found in the same material as Fe2+ and Fe3+, and Ti4+ (more rarely Ti3+). When Fe and Ti ions are in close geometric proximity in an oxide or (alumino)silicate structure, charge transfer can occur between the two ions, even though their concentration might be below 100 ppm. This results in a variety of absorption features that contribute to the color of minerals. A debate remains on the exact nature of Fe/Ti electronic transition, i.e., Fe2+ + Ti4+ → Fe3+ + Ti3+ or the reverse, but solving this issue is not within the scope of the present work. Ascertaining a metal-metal charge transfer is often not straightforward. This review compiles existing knowledge on Fe-Ti charge transfer in both crystalline and amorphous materials and identifies several key characteristics in more than 40 different materials. A charge transfer is associated with broad, intense, optical absorption bands that decrease in intensity at elevated temperatures. It is also strongly pleochroic in non-isotropic materials. Until now, Fe-Ti charge transfer transitions have been primarily described in the 2.25 to 3.1 eV range, corresponding to yellow to orange to brown colors, with notable exceptions such as blue sapphire or kyanite, and green andalusite. This review suggests that Fe-Ti charge transfer can occur across the entire visible spectrum, and the position of the absorption band correlates with the Fe-Ti interatomic distance. This correlation highlights the presence of multiple crystallographic sites for both Fe and Ti in many oxides, leading to multiple Fe-Ti bands within these materials (e.g., sapphire, ilmenite, pseudobrookite). Finally, the use of metal-metal distances is suggested to differentiate this heteronuclear Fe-Ti charge transfer from the common homonuclear charge transfer Fe2+-Fe3+.

The gadolinium garnet Gd ₃ Al ₂ Ga ₃ O ₁₂ co‐doped with Ce ³⁺ and Cr ³⁺ (GAGG‐Ce,Cr) has been widely studied due to its unusual bright yellow long‐lasting persistent luminescence properties. Here, rapid containerless melt‐quenching is used as part of a two‐step glass‐crystallisation synthesis process to obtain a new highly nonstoichiometric form of this garnet, of composition Gd _3+x [Al ₂ Ga ₃ ] _1‐x/5 O ₁₂ with 0 ≤ x ≤ 0.6 (ns‐GAGG). For compositions x > 0, powder X‐ray diffraction analysis confirms that excess Gd ³⁺ is accommodated at the Al ³⁺ /Ga ³⁺ sublattice in octahedral coordination, by substituting up to 30% of these sites. This mode of substitution complexifies the local structure of the garnet host, which is shown to influence certain luminescence properties in the analogous highly nonstoichiometric Y _3+x Al _5‐x O ₁₂ (0 < x < 0.4) and Gd _3+x Al _5‐x O ₁₂ (0 < x < 0.6) systems. Co‐doping ns‐GAGG with Ce ³⁺ and Cr ³⁺ produces green‐yellow persistent luminescence when x = 0, which undergoes a redshift to yellow‐orange as the Gd ³⁺ content increases to x = 0.4. However, this radical modification of the host composition does not strongly affect the afterglow kinetics. These results demonstrate an effective and high‐precision way of decoupling color and kinetics in persistent luminescent garnets, which is usually hard to achieve using the standard stoichiometric material engineering approach.

The blue luminescence commonly observed in natural opal (amorphous, hydrated silica) under 3.40 eV light remains poorly understood. Previous studies, lacking evidence of extrinsic activators, have attributed this emission to intrinsic defects. Raman spectroscopy has enabled the quantification of Q ⁿ units a reliable indicator of silica polymerization degree. Thus, we investigated possible correlation between blue luminescence spectra and opal polymerization degree. We used Raman spectroscopy to measure the proportions of Q ⁿ units on eight natural samples. We then decomposed the blue luminescence spectra of the same samples and observed 3 components at 2.49, 2.73, and 3.00 eV (respectively 498, 454, and 413 nm). The intensity of each contribution exhibits a linear relationship with the proportions of Q ³ , Q ⁴ , and Q ² centers, respectively. As Q ⁴ alone cannot induce luminescence, we propose that the 2.73 eV emission band is linked to an oxygen deficiency center.

Fifteen opal samples of gem quality from the Spencer Opal Mines in Idaho (USA) were examined. These opals display play-of-colour, with three of them having a pink body colour. The analysis was conducted using standard gemmological tools, along with non-destructive spectroscopic, chemical, and imaging methods. All samples were identified as opal-A, with their play-of-colour resulting from a regular stacking of hydrated silica spheres. However, at times, the spheres are large to allow diffraction. Opal-A are only occasionally found in nature as gems. Inclusion observed in the studied samples included alunite (KAl₃(SO₄)₂(OH)₆), anhydrite (CaSO₄), and iron-rich spherules (hematite). The pink body colour in the studied specimens is caused by the scattering of light from reddish dendrites. The samples show significant stratification, with fluorescence ranging from green to blue to orange, the latter of which represents a new phenomenon in opals not associated with quinones. Additionally, the orange fluorescence is accompanied by emissions from Sm²⁺ in anhydrite inclusions.

A batch of 5 samples, sold in 2010 as a set of aquamarines in Madagascar, caught the eye of our team. Although these stones were kept in a safe away from light and humidity/temperature variations for around ten years, a thin fibrous white layer appeared on all of the so-called aquamarines. To date, this type of transformation has not yet been described in the gemological literature. The use of gemological characterization techniques coupled with laboratory characterization techniques (XRD, SEM/EDX, UV-Vis absorption) made it possible to elucidate the mystery of the nature of these stones and the associated deposit. These samples are in reality glasses of soda-silicate composition whose bluish-green color, to imitate aquamarine, is associated with the presence of iron (mainly Fe2+). As for the fibers, these were identified as sodium hydroxycarbonate [Na3H(CO3)2(H2O)2]. The observation of numerous cracks in the sample as well as the chemical composition of the film suggests that the samples are not stable over time, and their surface is modified. The presence of water and CO2 coupled with the sodium of this porous sample is fertile ground for the appearance of these films and hairs on the surface.

This article reviews blue shortwave-excited luminescence (BSL) in natural minerals and synthetic materials. It also describes in detail the emission of seven minerals and gems displaying BSL, as well as three references in which BSL is caused by titanate groups (TiO₆): benitoite, Ti-doped synthetic sapphire and spinel. Emission (under 254 nm shortwave excitation) and excitation spectra are provided, and fluorescence decay times are measured. It is proposed that BSL in beryl (morganite), dumortierite, hydrozincite, pezzotaite, tourmaline (elbaite), some silicates glasses, and synthetic opals is due to titanate groups present at a concentration of 20 ppmw Ti or above. They all share a broad emission with a maximum between 420 and 480 nm (2.95 to 2.58 eV) (thus perceived as blue), and an excitation spectrum peaking in the short-wave range, between 230 and 290 nm (5.39 to 4.27 eV). Furthermore, their luminescence decay time is about 20 microseconds (from 2 to 40). These three parameters are consistent with a titanate emission, and to our knowledge, no other activator.