CRYSTAL STRUCTURES OF NEW MINERALS OF THE EUDIALYTE GROUP
Rastsvetaeva R.K., Khomyakov A.P.
Institute of Crystallography RAS, Moscow, Russia, Email: bolotina@rsa.crystal.msk.su
Eudialyte is a very complicated silicate with varying amounts of Na, Ca, Sr, K, REE, Y, Mn, Fe, Zr, Nb, Ti, Si, Cl, F and some other atoms. Due to the ability to concentrate REE among the other useful elements (Zr, Ti, Nb...) eudialyte is one of the most important ore mineral of XXI centure in a number of regions of alkaline deposits . The sum of the REE elements varies from 0 up to 11.78 mas % REE2O3; 0-6.3 for TiO2; 0-4.4 for Nb2O5 and 8.6-16.7 for ZrO2. In order to extract REE and other useful elements from the mineral it is necessary to know whether these elements are concentrated on individual sites or dispersed on several sites in the structure. The basic feature of all eudialytes structure is a stable framework of the three-membered and nine-membered rings of SiO4 tetrahedra linked together by Zr or Ti octahedra. A characteristic property of eudialyte is the presence of cages along the triad axes containing different elements and some of them can be competitors in certain micro regions. We have investigated crystal structure of 8 eudialyte samples from Khibina massif with different chemical compositions [1-5]. Recently we have studied the structure of 3 new members of the eudialyte group from Lovozero massif (Kola Peninsula) and compared them with our earlier published structural data on minerals of this family. First one is a mineral with very low content of FeO and Fe2O3 (0.5 mas % instead of usual 5-7%) ; REE2O3 content 2.2 % (among them Ce2O3 1.1, Nd2O3 0.57 and La2O3 0.55 %). New mineral is trigonal with the cell parameters a=14.170(4), c=30.35(2)A; space group R3m; ENRAF-NONIUS, Rhkl=0.048, 925 reflections, 55 unique positions. In the previous structures with different amount of REE2O3 (1-6 %) we found out that REE are situated in the same Na position into 7-8 fold polyhedra with REE-O distances in the range 2.5-2.9A. As can be seen from crystal-chemical formula (Z=1): Zr9 [Si3O9]6 [Si9O27]6 [Ca12.8Mn3.3Ce1.5Na0.4][Na36(Na6.3 Sr1.8K0.9)][Na7(Nb0.9Ti0.6Fe3+0.5)][Si4.2Al0.3Mn1.5(OH)10]Cl2(H2O)3.2 atoms REE occupy the other position - in Ca octahedra with REE-O=2.3-2.4A.
Thus we have found two positions with qualitatively and quantitatively different occupation by REE depending on their ionic radii. The second new mineral is characterized by very high (6.3%) content of TiO2. The cell parameters are: a=14.153(9), c=60.72(5)A; space group R3m; ENRAF-NONIUS, Rhkl=0.068, 3402 reflections, 96 unique positions. REE atoms occupy both of the positions simultaneously in the same structure. Ti atoms with Nb occupy half of Zr octahedra causing to the doubl c-parameter. The rest of Ti atoms are distributed on two other positions - octahedron in the middle of 9-membered Si-ring and 3 semioctahedra. These 4 polyhedra are connected each other through the common vertices forming cluster. The third representative of the group is hyperzirconium mineral with ZrO2 content 16.7% instead of usual 12%. Cell parameters are: a=14.222(3), c=30.165(5)A; space group R3m; Syntex P1, R=0.039, 2292 reflections, 52 unique positions. REE atoms occupy only one position with Ca, Mn, Fe, Sr and Na. Atoms of Zr are distributed on two positions - in usual octahedral and in semioctahedra instead of Fe, wich as a rule occupy this polyhedra together with the flat square.
More details and numerical results will be presented in the
full-length paper.