prvi kongres hrvatskih znanstvenika iz domovine i inozemstva - HATZ

PRVI KONGRES HRVATSKIH ZNANSTVENIKA IZ DOMOVINE I INOZEMSTVAZagreb - Vukovar, 15. - 19. studenog 2004.P - 10PROPERTIES AND CLASSIFICATION PROBLEM OF A Nb- AND Y-RICH MINERAL OF AESCHYNITE MINERAL GROUPBermanec, V.; Tomašić, N.; KniewaldFaculty of Science, University of Zagreb, CroatiaComplex oxides of Nb, Ta, Ti and REE frequently show a tendency for large scale cationsubstitution on certain atomic positions in crystal structure. The aeschynite mineral groupwith general formula AB2O6 is a typical representative of this phenomenon as cations aremutually interchanged on A and B sites of the crystal structure. REE3+, Ca2+ and Th4+predominantly occupy site A while Ti4+, Nb5+ and Ta5+ occupy site B. The dominantsubstitution mechanism is Ca2+ + (Nb5+, Ta5+) REE3+ + Ti4+. This mechanismleads to an occurence of different mineral group members. The diffraction data of theinvestigated mineral sample originating from Bear Lake, Ontario, Canada, are closest tothose of aeschynite mineral group being most similar to the mineral once calledlyndochite. The comparison of its chemical data to those of aeschynite group mineralsalso shows a close similarity to previous findings of lyndochite and to niobo-aeschynitesas well. The chemical composition also partly resembles euxenite. However, diffractiondata with the most prominent peaks (intensities in parentheses) 3.088 (20), 3.018 (100)and 2.939 (69) are more common for aeschynite group minerals. This is also the casewith the unit cell parameters (a = 5.290(4)Å, b = 10.983(5)Å, c = 7.467(4)Å, V =433.8(3)Å3) having the values between those for niobo-aeschynite-(Ce) and aeschynite-(Y).The calculated chemical formula is(REE0.53Ca0.31Fe2+0.11Th0.18U0.009)(Nb0.95Ti0.92Ta0.07)O6. On site B Nb isdominant, thus the mineral is close to niobo-aeschynites. However, all known nioboaeschyniteshave Ce or Nd as the dominant cation on site A while Y is the dominantcation in this case. In aeschynite group Y is dominant cation on position A in the case ofaeschynite-(Y). So, the investigated mineral could be treated as a boundary casebetween aeschynite-(Y) and niobo-aeschynite.Presented on:18th General Meeting of the International Mineralogical AssociationEdinburgh 1. -6. 09.2002.Principal Investigator : Dr.sc. Vladimir BermanecProject No. 011942012