The Ti-influence on the tourmaline color

Abstract

Titânio em turmalinas pretas foi analisado por espectrofotometria entre 12500 e 27000 cm -1 (800-370 nm) em um microscópio MPV-5P, Leitz (Alemanha), e analisado quimicamente em uma micro-sonda eletrônica Camebax Microbeam, Cameca (France) (WDS-mode). Quatorze turmalinas orientadas com faces paralelas ao eixo trigonal, cortadas e polidas até espessuras finas mostraram, em espectros polarizados, duas bandas largas de absorbância a aproximadamente 14000 cm -1 (715 nm) e 24000 cm -1 (417 nm). Medidas de espessura das amostras foram usadas no cálculo de coeficientes de absorção α (cm -1 ). Para a banda em torno de 24000 cm -1 (417 nm), foi observada uma correlação linear entre α e a concentração em massa de TiO 2 (%). Outra regressão foi observada entre α e o produto da concentração em massa de TiO 2 (%) e FeO (%). Cátions localizados em grupos de octaedros Y e Z conectados através dos seus lados podem contribuir para as transferências de carga Ti 3+ -Ti 4+ , Fe 2+ -Ti 4+ , Ti 3+ -Ti 4+ -Fe 3+ .Titanium was examined in different black tourmalines by spectrophotometric analyses in the region between 12500 and 27000 cm -1 (800-370 nm) using a microscope spectrophotometer MPV-5P, Leitz (Germany), and chemically analyzed on an electron microprobe Camebax Microbeam, Cameca (France) in WDS Mode. Fourteen tourmaline samples with their face oriented parallel to the principal axis, cut and polished down to thin sections, showed polarized spectra of two broad absorption bands at approximately 14000 cm -1 (715 nm) and 24000 cm -1 (417 nm). Precision thickness measurements were used to calculate the absorption coefficients α. For the absorption around 24000 cm -1 , a linear correlation was observed between α and the TiO 2 content (mass %). Another linear regression was observed between α and the product of TiO 2 and FeO (%) contents. Cations located in the Y-and Z-octahedrons of the structure that are connected over edges might contribute to intervalence charge transfer transitions (IVCT) between Ti 3+ -Ti 4+ , Fe 2+ -Ti 4+ and Ti 3+ -Ti 4+ -Fe 3+ .

Dutrowite, Na(Fe2+2.5Ti0.5)Al6(Si6O18)(BO3)3(OH)3O, a new mineral from the Apuan Alps (Tuscany, Italy): the first member of the tourmaline supergroup with Ti as a species-forming chemical constituent

“…All spot analysis data are deposited in the Supplement. Brown domains are usually enriched in Ti, in agreement with previous results obtained, for instance by da Fonseca-Zang et al (2008). The blue domain, whose average chemical composition is given in Table 2, has the following empirical ordered formula: X (Na 0.66 Ca 0.14 K 0.01 ) 0.81 Y (Mg 1.33 Fe 2+…”

Oxy-dravite from Wołowa Góra Mountain, Karkonosze massif, SW Poland: Crystallochemical and structural studies

“…In accordance with the spectroscopic data of Mattson and Rossman (1988) and da Fonseca-Zang et al . (2008), these data suggest that the colouration is mainly a result of the Fe 2+ –Ti 4+ charge-transfer transitions. The transitions are responsible for the absorption at ~24,000 cm –1 and the yellowish-brown to brown colour.…”

“…In the primary tourmaline, the Fe content is tiny and the probability of the Fe 2+ –Ti 4+ charge-transfer transition is very small, which even at a relatively high Ti 4+ (d 0 ) content only results in yellowish colouration. The increase of Fe 2+ in the secondary tourmalines elevates the probability of the Fe 2+ –Ti 4+ transitions and absorption at ~24,000 cm –1 ; however, an absorption at ~14,000 cm –1 appears simultaneously which is responsible for a blue colour, originating mainly from the spin-allowed crystal-field transitions of Fe 2+ (Mattson and Rossman, 1988; da Fonseca-Zang et al ., 2008). The colouration of greenish tints is a superposition of both types of absorptions, and may occur at relatively high contents of Fe 2+ and elevated Ti 4+ .…”

Polymetamorphic evolution of pelites inferred from tourmaline zoning – the Rędziny hornfels case study at the eastern contact of the Karkonosze Granite, Sudetes, Poland

“…t2g ↔ Fe 2+ eg and Fe 2+ ↔ Fe 3+ electron transitions that induce an absorption band at around 14000 cm -1 (714 nm) (da Fonseca-Zang et al 2008) and generate the bluish colour of this tourmaline variety. In the mantle tourmaline, the Ti contents are much higher, which results in high abundances of Fe 2+ ↔Ti 4+ , Ti 3+ ↔Ti 4+ or Ti 3+ ↔Ti 4+ ↔Fe 3+ charge-transfer transitions with absorption around 24000 cm -1 (416 nm) connected with various shades of brown.…”