CHEMICAL VARIATION IN EUDIALYTE

A large suite of eudialyte samples has been investigated by electron m icroprobe analysis, out of which 36 are presented. All analyses are tr ansformed into formula units (pfu) based on 78 anions (Z = 3), what is proposed to be the best procedure at present. Regression analyses of the elemental variation are performed and correlations between element s and groups of elements are discussed in the light of present structu ral knowledge from single crystal data. Selected eudialytes have been further studied by high resolution X-ray diffractometry and examples o f the variation of X-ray data with chemistry are presented. In additio n, a few eudialyte samples have been investigated by TEM in order to l ook for crystal defects causing non-stoichiometry of which none were f ound. An overview of the eudialyte structure is given using the site n otation from JOHNSEN et al. (in press) comprising ([6])M1, ([4-5])M2, ([6])M3, ([4])M4 and Na1 through. Na5 in 6-9 coordination in addition to the framework, composed of three-membered and nine-membered rings o f SiO4 tetrahedra linked together by Zr octahedra. A special property of eudialyte is the existence of oblong cages along the triad axes car rying additional bonded and possibly also non-bonded components. Our d ata show that the framework is stable and uniform in composition, whil e considerable variation takes place in other parts of the structure; Na, though, is always present in large amounts (13.95-17.44 pfu). The Na <-> Ca substitution often proposed for eudialyte is not supported b y our data. Ca, generally assigned to the M1 site, varies from 1.92 to 7.82 pfu with an average of 5.45 pfu; Mn and REE are the main substit utional elements. One eudialyte from Mt. St.Hilaire has 9.78 wt% REE2O 3 (Y2O3 not included), the highest content recorded so far. Fe, assign ed to M2 in four-fold coordination in an almost planar arrangement, va ries from 0.68 to 3.33 pfu, average only 1.73 pfu, which is just sligh tly more than Mn (av. 1.39 pfu). Mn also substitutes for Fe, but is fi ve-fold coordinated because of the incorporation of an extra oxygen (O 19) close to the triad axes. The average content of Si is 25.89 pfu, higher than the 24 pfu necessary for the formation of the framework. T his confirms the presence of one or two Si positions on the triad axes (M4) thus transforming the nine-membered rings into [Si10O28](16-) pl atforms. The amount of Si is negatively correlated to that of Nb, not because these elements substitute in a classical sense, but because th ey are competitors in certain micro regions in the cages. Nb is accomm odated in M3 sharing O 19 with M2. Combining significant correlations leads to a eudialyte series between two end-members, one relatively ri ch in Si, Ca, Fe and Cl and another relatively rich in Nb, REE, Mn and F. Plots of X-ray data vs. chemistry show that the Sigma Nb,REE,Mn,F component is negatively correlated to the a parameter, positively to D r calc., and the intensity of some reflections, especially reflection (003) proves to be a good indicator of the Sigma Nb,REE,Mn,F/Sigma Si, Ca,Fe,Cl-ratio. Finally, a procedure for calculating the formula of an analysed eudialyte is proposed.