Columbite-tantalite, petscheckite and a columbite-tantalite group related mineral from Tiltvika, Nordland, Norway (CROSBI ID 483941)
Prilog sa skupa u zborniku | sažetak izlaganja sa skupa | međunarodna recenzija
Podaci o odgovornosti
Tomašić, Nenad ; Raade, Gunar ; Bermanec, Vladimir
engleski
Columbite-tantalite, petscheckite and a columbite-tantalite group related mineral from Tiltvika, Nordland, Norway
The minerals of columbite-tantalite group have orthorhombic symmetry and general formula AB2O6, where A = Fe2+, Mn2+, Mg and B = Nb5+, Ta5+. Minor substitutions of Sn4+, Ti4+, Sc, W6+ and Fe3+ are also possible. A mineral sample from Tiltvika pegmatite, Nordland, Norway was investigated. Macroscopically the sample is dark grey to black with glassy lustre. Under ore microscope (12.5×10 magnification) cleavage and weak anisotropy could be observed and the sample seems to be homogenous. X-ray diffraction analysis of the unheated sample yielded the diffraction pattern of columbite-tantalite along with the increase of base line suggesting the presence of amorphous (metamict) phase. After heating the sample at 650°C for 21 hours new diffraction peaks appear along with columbite-tantalite. These peaks could be assigned to petscheckite (PDF 29-1426), a very rare oxide with formula U4+Fe2+(Nb,Ta)2O8 and hexagonal symmetry. On further heating at 800, 1000 and 1100°C for 21 hours the suspected petscheckite diffraction peaks sharpen and intensify as columbite-tantalite diffraction peaks decrease and those of originally lower intensity disappear. At 1000°C diffraction peaks of the third phase appear and further heating to 1100°C intensifies them. Thorough investigation of those peaks does not give any complete solution but they are closest to the diffraction pattern of an unnamed phase without definite formula (PDF 22-1116). This phase has columbite type structure and its composition can be described as Fe-Mn-Nb-Ta-W-O. Unit cell parameters for the phases found: Columbite-tantalite: a = 5.742 (3), b = 14.322 (8), c = 5.063 (2) Å, V = 416.3 (3) Å 3 Petscheckite: a = 6.42 (2), c = 4.02 (1) Å, V = 143.5 (6) Å 3 Third phase: a = 5.005 (5), b = 14.05 (2), c = 5.64 (1) Å, V = 397.3 (8) Å 3. In the late magmatic stages a lot of incompatible elements concentrate in the cooling magma forming a large number of rare-element minerals. Sometimes these minerals are naturally metamict. Metamictization could be caused by the presence of a radioactive element, as in the case of petscheckite. However, there is always a question whether the phases, such as the third phase found in this investigation, have ever been structurally ordered considering their complex composition and the structural limitation.
columbite-tantalite; petscheckite; metamict state
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Podaci o prilogu
55-x.
2001.
objavljeno
Podaci o matičnoj publikaciji
Tenth Croatian-Slovenian Crystallographic Meeting - Book of Abstracts
Zagreb: Hrvatska kristalografska zajednica HAZU
Podaci o skupu
Tenth Croatian-Slovenian crystallographic meeting
predavanje
21.06.2001-24.06.2001
Lovran, Hrvatska