M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NAS of Ukraine

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Spectroscopic characteristics, crystal chemistry and typomorphism of minerals of the ring silicates group

ABSTRACT

The report on scientific project: 219 p., 90 figs., 30 tables, 214 references.

The object of investigation is natural ring silicates and their synthetic analogues.

The aim of the work is investigation of the peculiarities of isomorphic entering of the transition metal ions into the structure of ring silicates.

The methods of investigations are optical absorption spectroscopy at different temperatures and pressures, photo- and X-ray luminescence, Fourier-transform infrared spectroscopy (FTIR), Mössbauer spectroscopy,X-ray fluorescence spectroscopy andmicroprobe analysis.

By the complex of spectroscopic methods, predominantly by Mössbauer, optical and FTIRspectroscopies there were studiedtourmalines, beryls and cordierites, i.e., the main minerals of the group of ring silicates. From thetransition metal ions, the admixture of Ti⁴⁺, Cr³⁺, Fe²⁺ і Fe³⁺ dominates in their composition. They are the main transition metal ions which dominatein oxygen-based minerals on the whole, the ions, which participate in many processes of mineral and rock formation, cause their colors, influence on thermal and electricconductivities and other properties of minerals, rocks, geological complexes and geological landscapes on the whole.

The investigation of optical spectra of tourmalines, beryls and cordierites at different temperature and pressureshowed that absorption bands of different nature, caused by electronic transitions of different types, i.e.,dd-transitions within the electronic d-shell of transition metal ions, charge-transfer transitions of metal-metal and ligand-metal types and the transitions in exchange-coupled pairs of such ions differently behave under influence of Т and Р.On one hand, it is very important for the interpretation of the optical spectra, on other hand, it gives a unique information about the electronic state and related characteristics of minerals in the conditions of earth depths at high PT-parameters.

Investigation of Fe- and FeTi-bearing tourmalines evidenced that iron ions Fe²⁺ and Fe³⁺ occupy predominately the Y-octahedra of the structure. At increasing degree of oxidation, i.e. of Fe³⁺ content, the intensity of near infrared spin-allowed dd-bands of Fe²⁺ (Y) in E⟂c-polarization strongly increases that evidences that the concentration of the exchange-coupled pairs of Fe²⁺ (Y) - Fe³⁺ (Y) increases. This process is accompanied byintensification of coloration from light green to blue and, in case of schorls, to black. In the E||c-polarization the intensity of these bands and, correspondingly, the intensity of color grows with the increase of iron content much slower that in high-iron tourmalines causes a strong dichroism E⟂c >> E || c.The presence in such crystals of an admixture of Ті⁴⁺ in Y-position of the structure gives rise to appearance of brownish and red-brown components in coloration, caused by broad intense bands of electronic charge-transfer transition Fe²⁺ + Ті⁴⁺ → Fe³⁺ + Ті³⁺ with maximum, depending onthe general composition of tourmaline, between 22000 and 25000 cm^-1.

Investigation of tourmalines with high Mg-content, wherein the excess of these ions occupies the smaller octahedral Z-sites of the structure, evidences that in difference from others, no replacement of Mg by Fe²⁺ in Z-octahedra takes place and all iron concentrates in the Y-site.

Dark-red color and strong dichroism of high-iron oxi-dravites are caused by a high concentration of Fe³⁺ in Y-sites of the structure and an appearance of electronic exchange interaction in Fe³⁺ (Y) - Fe³⁺ (Y) pairs. Investigation of optical absorption spectra of such crystals at different pressures evidences that the absorption bands in them can be interpreted as the spin-forbidden transitions of Fe³⁺ (Y) - Fe³⁺ (Y) pairs, increased by such interaction.

The detailed investigation of the chromium-bearing tourmalines from Tanzania evidences that Usambara-effect (change of color from green to red at increasing thickness of the sample)by the nature is close to alexandrite effect, but in difference from the later, the change of color is not caused by change of the spectral composition of illumination, but the spectral positions of the spin-allowed absorption bands of Cr³⁺, a specific ratio of lighttransmission in two windows of transparency, green and red, and by non-linear, exponential dependence ofthe light transmittance on the thickness of a sample.A threshold chromium content must be exceeded for theUsambara effect to show, that is, sufficient chromium for there to be two deep and well-demarcatedwindows of transparency in the visible range. It was shown that an admixture of other chromophoreions, namely, Fe2+and Fe3+, can suppress the Usambara effect in tourmaline.

On the vast collection of natural and grown beryls of various compositions spectroscopic features of entering of admixtures of Fe²⁺ and Fe³⁺ in the beryl structure and their role in color and dichroism of heliodors, aquamarines and various intermediate variously colored varieties are established. As shown for the first time, a spatial distribution of Fe²⁺ in tetrahedral structural site of Be²⁺ along elongation of the crystals differs in the samples from different deposits and can evidence on physic-chemical peculiarityof their crystallization.

The optical absorption and FTIR spectra, chemical peculiarities of emeralds from the occurrence Kruta Balka in Western Peri-Azovian region, Ukraine were studied in comparison with emeralds from manyother deposits of the world.

The spectroscopic investigation of bazzite (a rear Sc-variety of beryl) from Kent (Kazakhstan) and Tørdal (Norway)deposits has shown that in difference from ordinary beryl the ions Fe²⁺ and Fe³⁺ occupy exceedingly the octahedral sites of the structure. As in beryls, the blue coloration and dichroism (E || c >> E ⟂ c) are caused by electronic charge-transfer transition between Fe²⁺ and Fe³⁺. The infrared spectroscopy evidences that in difference from beryl, where in the structural chanels there are usually H₂O molecules of both types, spectra of bazzite from Kent show the presence of H₂O type ІІ only.

Authors:

Taran Mykhailo Mykolaiovych

Khomenko Volodymyr Mykhailovych

Tarashchan Arkadii Mykolaiovych

Lupashko Tetiana Mykolaivna

Ilchenko Kateryna Oleksandrivna

Hrechanovska Olena Yevhenivna

Other authors:

Keywords:

ring silicates, spectra, isomorphism, transition metals, OH-groups, water molecules

Head of the research:

Taran Mykhailo Mykolaiovych

Registration Card (RC):

0115U003008

Execution period of the research:

2015 - 2019

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