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AESCHYNITE GROUP MINERALS FROM TUFTANE, NORVAY AND BEAR LAKE, ONTARIO, CANADA - A PROBLEM OF METAMICT MINERALS

The minerals of aeschynite group have orthorhombic symmetry (spance group Pnmb) and general formula AB2O6, with A site usually occupied by Ca, Na, U, Th and REE, and B site by Nb, Ta, Ti. Two mineral samples originating from pegmatites in the area of Tuftane, Norway and Bear Lake, Ontario, Canada were investigated. The mineral from Tuftane is naturally metamict. After heating at 8000C for 21 hour diffraction pattern was obtained and recognized to be similar to aeschynite-(Y) (PDF 20-1401). Indexed powder pattern is used for calculation of unit cell parameters: a=5.196(2), b=10.956(8), c=7.409(3)Ă3. The mineral originating from Bear Lake pegmatite showed also to be naturally metamict and yielded its diffraction pattern after heating at 800°C for 21 hours.Sharper maxima are obtained if the mineral is heated to 1000°C. The diffraction pattern belongs to a Ca, Th and Nb rich variety of aeschynite, formerly called lyndochite (PDF 18-765). It is of orthorhombic symmetry with following unit cell dimensions: a=5.290(3), b=10.993(6), c=7.468(4) Ă, V=434.3(3)Ă3. In the recent editions of PDF data the mineral is called yttropyrochlore-(Y), although concerning the symmetry it does not belong to pyrochlore group. Also, it could be confused with another mineral with the same name but cubic symmetry. Chemical analysis of both minerals, performed using EMPA, gave the results in accordance with the composition of aeschynite. Empirical formulae calculated on the basis of six oxygens are: Aeschynite-(Y), Tuftane (REE0.83Th0.13U0.06Ca0.05Fe0.04)(Ti1.49Nb0.37Ta0.10W0.03)O6 Aeschynite-(Y), Bear Lake (REE0.53Ca0.31Th0.18Fe0.11U0.01)(Nb0.94Ti0.91Ta0.07)O6. In both minerals Y3+ is the most abundant REE and the dominant element in A site. The difference in the contents of REE, Ca, Ti and Nb between two minerals could be interpreted by substitution of Ca+(Nb,Ta) for REE+Ti, common for aeschynite group. Enrichment of HREE in Tuftane aeschynite and that of LREE in Bear Lake aeschynite suggets the different character of the parent pegmatites. Metamictization is very common for Nb-Ta-REE oxides. Destroyed crystal structure is caused by radiation coming from U and Th. Also, the complex chemistry of the minerals could lead to the breakdown of long-range order and formation of microphases with simple chemistry. On heating original mineral recrystallizes, but also some additional phases could appear. That was observed in the case of the investigated aeschynites, where additional diffraction maxima appeared along with those of aeschynite. Unfortunately, the phases have not been determined. TGA curve of Bear Lake aeschynite shows two transitions with losses of mass occurring at about 450°C (2.611%) and 900°C (2.284%).

Aeschynite group minerals; metamict minerals

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