THE STRUCTURE TRANSFORMATIONS IN ANADOL ALANITE DURING ANNEALING PROCESS (BY X-RAY, INFRARED-SPECTROSCOPY AND ELECTRON PROBE MICROANALYSES DATA)

Abstract

Anadol occurrence of cerium group REE ores is located in the eastern part of the Azov re-gion. The main mineral-concentrator of REE in Anadol deposit is allanite − the REE epidote ana-logue. Its content in the rock is about 7090 %, and total content of rare earths oxides (Ce,La,Nd)2O3 in the mineral is 18.82 wt. %. The “anadol-type” allanite formed two morphologic-genetic types: 1) lamellar crystals of the older generation (allanite-1) and 2) elongated prismatic ones of the later generation (allanite-2), which differed by the unit cell parameters (UCP), nm: а = 0. 8942 (1), b = 0. 5779 (6), с = 1.0147 (4), β = 114. 65° (allanite-1) and а = 0. 8907 (5), b = 0. 5763 (7), с = 1. 0177 (2), β = 114. 46° (allanite-2). The X-ray data of more than 20 allanite monofractions show a wide variations in UCP, nm: а = 0.8907–0.8942, b = 0.5747–0.5779, с = 1.0147–1.0177, β = 114. 46–114.72° (al-lanite-1) and а = 0.8974 (1), b = 0.5760 (6), с = 1.0229 (4), β =114.969° (allanite-2). Such UCP variability connects with the different REE content in allanite structure. It varies from 24 % in al-lanite-1 to 15 % in allanite-2. The structure changes of Anadol allanite under the step-by-step annealing were investigated by means of X-ray and infrared spectroscopy (IR). The formula of “anadol-type” allanite: (Ca1.33Ce0.29La0.22Nd0.13)1.97M2(Al1.44Mg0.17Mn0.11Fe3+0.09Na0.02K0.01)1.84M3(Fe2+0.67Fe3+0.29Ti0.04)1.0 × × [SiO4][Si2O7]O(OH), calculated for 12.5 oxygen atoms. By the unit cell parameters data the investigated allanite sample consists mainly of its earlier and more heat-resistant modification allanite-1. Its very small amount is stored in the sample even after high-temperature annealing at 1 050 C. At the temperature 900 С the structure collapse of some allanite grains takes place with formation of three new phases – cerianite, hematite and quartz. The alanite amount in the annealed sample considerably reduces. The further temperature increase (above 950 С) results in formation of another new phase – REE oxysilicate with the apatite structure (oxyapatite) – REE4.67[SiO4]3O□0.9. We investigated the composition of this phase and specified by means of electron probe mi-croanalysis. The crystal chemical formula of REE oxysilicate with the apatite structure, calcu-lated for 13 oxygen atoms, is (Ce1.87La1.74Nd0.62Pr0.31Sm0.14)4.67[SiO4]3O□0.9. The IR-spectra studies have shown that the initial investigated phase is crystalline allanite with some bastnaesite impurity. By IR-data the new annealed phase is indeed REE oxysilicate with the apatite structure. It differs from britholite structure mainly by X-anions composition. X-sites in REE oxysilicate are occupied mainly by oxygen atoms. Since fluorine or chlorine in the initial allanite sample were absent and OH–-groups, PO43–, CO32– or other molecular anions in REE oxysilicate structure were not found, thus some X-sites may be vacant to satisfy the re-quirement of the structure charge balance.