Naslov
Columbite-tantalite, petscheckite and a columbite-tantalite group related mineral from Tiltvika, Nordland, Norway

Autori
Tomašić, Nenad ; Raade, Gunar ; Bermanec, Vladimir

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Tenth Croatian-Slovenian Crystallographic Meeting - Book of Abstracts / - Zagreb : Hrvatska kristalografska zajednica HAZU, 2001

Skup
Tenth Croatian-Slovenian Crystallographic Meeting

Mjesto i datum
Lovran, Hrvatska, 21.06.2001. - 24.06.2001

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
columbite-tantalite; petscheckite; metamict state

Sažetak
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.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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