The crystal chemistry of the eudialyte group

Seventeen crystals of eudialyte (sensu late) differing in provenance and sh owing a wide range of chemical compositions were chosen for crystal-structu re analysis and electron-microprobe analysis, supplemented with thermogravi metric analysis, infrared analysis, Mossbauer and optical absorption spectr oscopy on selected samples. The structure consists of layers of six-membere d rings of [M(1)O-6] octahedra linked together by [M(2)O-n] polyhedra sandw iched between two pseudocentrosymmetrically related layers of three-membere d and nine-membered silicate rings forming a 2:1 composite layer. The 2:1 c omposite layers are crosslinked by Zr in octahedral coordination and are re lated to one another in accordance with the rhombohedral symmetry. This ope n structure is filled with [Na phi(n)] polyhedra in which Na may have vario us coordinations. The silicate network is relatively uniform in composition . The Zr site usually has a small amount of Ti. M(1) is normally occupied m ostly by Ca; the main substitutions are Mn, REE and Y. In one crystal, more than 50% Ca is replaced by Mn and REE, resulting in an ordering in two dis tinct sites, M(la) (Mn) and M(lb) (Ca + REE), and the symmetry reduced to R 3. M(2) is either a four-fold Fe- dominated site, or a five-fold site, usua lly Mn-dominated. M(3) and Si(7) are two central sites of the two crystallo graphically distinct nine-membered silicate rings, and related by the subst itution M-[6](3) + Si-[4](7) double left right arrow 2([4])Si(7). M(3) is d ominated by Nb (or W), and Si(7), by (Si,AI). Na is present in five distinc t sites (acentric eudialyte) of which the Na(4) site is particularly prone to incorporation of heavy elements such as REE and Sr. The O(19) site plays a key role in the structure, being the ligand shared by the M(3), M(2,5) a nd Na(4) sites. The degree of acentricity is strongly correlated with the c hemical elements in specific atomic sites. Eudialyte sensu stricto represen ts the most centrosymmetric structure, R (3) over bar m. As substitutions s uch as Si double right arrow (Nb,W), Fe double right arrow Mn, (Ca,Na) doub le right arrow (REE,Sr) take place, the structures become more and more non centrosymmetric, R3m. The R3 eudialyte has an incongruously low \ E-2 - 1 \ value as a result of merohedral twinning. A complete formula for eudialyte requires structural data. An acceptable empirical formula, Na-15[M(1)](6)[ M(2)](3)Zr-3 [M(3)](Si25O73) (O,OH,H2O)(3) X-2, can be calculated on the ba sis of 29 (Si,Al,Zr,Ti,Hf,Nb,W,Ta) apfu (Z = 3).