Grechanovskaya and 4 co-authors reported in 2017 that REE-rich epidote group mineral allanite is the main mineral-concentrator of cerium group. REE on Anadol ore occurrence (Eastern Azov Region, Ukrainian Shield). Its crystal chemical formula is CaREEAl2Fe[Si2O7][SiO4]O(OH). The Anadol occurrence of cerium group REE ores is represented by the vein ore body of epidote-allanite and quartz-fluorite-allanite composition ("orthite dyke"). Allanite from "orthite dyke" of Anadol occurrence forms two varieties: allanite-1 and allanite-2, which differ in crystal morphology, the unit cell parameters and degree of iron oxidation. The chemical composition of allanite samples from epidote-amphibole-allanite metasomatites and "allanite concentrate" from "orthite dyke" of anadol occurrence and products of its step-by-step annealing, which are formed as a result of partial or complete destruction of its structure, were
investigated by the methods of X-ray analysis, electron-probe microanalysis and infrared spectroscopy (IR).
Investigation of the chemical composition of allanite samples showed their heterogeneity. They contain two types of the britholite inclusions, the total amount of rare earths and yttrium ΣREEE2O3 + Y2O3 in which varies from 59 to 62%. Allanite from metasomatites is represented by the late generation of allanite-2, but the "allanite concentrate" there are both generations. During the annealing of the allanite sample in the temperature range 400—800°C, one can observe a significant decrease in parameters a, b and the volume of its elementary cell V, with increase of parameter c. This is probably due to the gradual oxidation of iron and leads to the destruction of allanite-2 structure and formation of "oxyallanite".
Further annealing in the temperature range 950—1,050°C leads to a partial amorphization of the allanite structure and formation of a crystalline phase which by the values of the structural parameters is close to the britholite-(la). The anionic part of which is composed only of oxygen — La9.31 [Si1.04O4] +6O2. So the new phase can be called a phase with britholite structure, enriched with oxygen. At the same time, the contents of cerianite and hematite and their crystallinity increases. When the "Anadol" allanite sample is annealed at temperature 1,050°C during the period the amorphous phase disappears. By IR-data the newly formed high-temperature annealing phase is similar to britholite, from which it differs mainly by X-anions composition in the structure channels, as well as to the lanthanum silicate La9.60 (SiO4)6O2.4 with apatite structure, synthesized by mechanical method.
This is probably the case that X structural positions in the newly formed phase with britholite structure are only partly occupied by oxygen atoms, and a certain part of them remains vacant, which is due to the need for a balance of charges. It was established that the formation of new phases that appear during high-temperature annealing of a highly crystalline allanite sample from the "orthite dyke" of Anadol occurrence and metamict samples from the Azov Zr-REE deposit and the Ilmen Mountains (urals) occurs regardless of the degree of their crystallinity. But the insignificant variability of the temperature of their formation and the ratio of appropriate phases are dependent only on the features of the chemical composition of the initial samples.
The authors' studies have shown that the thermal treatment of allanite ore, which resulting in formation of cerium oxide and a phase with britholite structure enriched with oxygen with rare-earth content increased up to 63% and this can be used to separate these phases in the ore dressing process.
Keywords used in the Ukraine article: allanite, "orthite dyke", Anadol occurrence, the unit cell parameters, infrared spectroscopy, britholite, cerianite, hematite, quartz.
Managed & Built by I2M Consulting, LLC | © 2010-
