key words [glass iron valence state]

Mossbauer spectroscopy methods have been used to investigate natural and synthetic silicate glasses. The aim of this work was to determine the structural and valence states of iron in natural and synthetic silicate glasses produced at various temperatures of the initial melt. Natural glasses - tengizites (tz glasses ) formed during the petroleum fire (the combustion period was about a year) in one of the boreholes of the Tengiz deposit (Kazakhstan). The temperatures of the melt from which the glasses formed range from 2600 to 3000^oC. The chemical composition of

the glasses complies with that of andesite with high concentration of CaO and a lower concentration of FeO. The synthetic glasses are compositionally close to the mineral kirsteinite CaFeSiO₄ (kr glasses obtained from melts having the temperatures 1200-1400^oC) and to ferrisilicate feldspar KFeSi₃O₈ (fs glasses obtained from melts having the temperatures 1050-1350^oC). The melts were kept at the appropriate temperature for no longer than 2 h. The processing and analysis of the Mossbauer spectra using the program DISTRI, developed by us, were performed through resetting the independent functions of the distribution parameters of the hyperfine interaction of ⁵⁷Fe nuclei, corresponding to Fe²⁺ and Fe³⁺ ions. The obtained distribution functions were analyzed by means of the one-modal distribution characteristics. The value state and the dominant coordination of Fe ions were determined from the mean isomeric shift value, the occurrence of other coordination states was determined from its distribution width. The mean value of the quadrupole shift and its distribution width made it possible to estimate the degree of distortion of the oxygen polyhedrons of Fe atoms and the variations of their junction angles.

The studies performed suggest the following conclusions. In natural tz glasses Fe²⁺ and Fe³⁺ ions are present in approximately the same amounts. Fe³⁺ occupies octahedral positions. Fe²⁺ions occur only in the state of coordination number 5. This fact is associated with the specific features of the chemical composition of tengizites, namely, with the high content of the cation-modifier Ca²⁺ (CaO~ 17¸ 18 wt%) and a rather high content of the cation-net former Al³⁺ (Al₂O₃ ~ 9 wt%). As the temperature of the melt is increased from 2600 to 3000^oC, the share of Fe²⁺ increases by several percents but the structural state remains unvariable. Kr glasses comprise bivalent and trivalent iron ions in the ratio Fe²⁺ :Fe³⁺» 1:6.

The coordination number of Fe³⁺ ions is 6. The oxygen octahedrons are strongly distorted, the degree of distortion changes from site to site. The mean value of the isomeric shift for Fe²⁺ ions is between the values characteristic of tetrahedral and pentahedral coordinations. The greater distribution width of the hyperfine parameters bears witness to the fact that Fe²⁺ ions occupy the positions of both these types. No changes are observed in the coordination state of iron with the growing temperature. In fs glasses iron is present only in the trivalent state in the tetrahedral coordination. However, the broad distribution of the hyperfine parameters indicates that even for the oxygen tetrahedrons of the closest neighborhood of Fe³⁺ ions strong deformations and variations of their junction angles are characteristic. The iron does not alter its valence state with the growing temperature of the melt, but during this event the degree of the distance distortion in the oxygen polyhedrons grows.

So, with virtually invariable Fe²⁺:Fe³⁺ ratio the coordination states of Fe ions are independent of the temperature of the silicate melt. However, an increase of the degree of the distance distortion in the oxygen polyhedrons can be observed, the mean lengths of the Fe-O bonds being invariable. Under the conjecture that during the melt glass transition only small changes occur in the closest neighborhood of Fe ions, the obtained results can be extended for silicate melts as well.