The systematic investigation of the x-ray diffraction symmetry of scapolites covering the whole range of solid solution show that all scapolites possess space group P4(2)/n, except the pure end-members, marialite and meionite, which will possess space group I4/m, if they exist. The intensity of the weak superlattice reflections violating the body-centred symmetry, continuously increases from zero at the marialite end-member as Me% increases, reaches a maximum around 37%+/-2% Me then approximately symmetrically decreases to 75% Me followed by a slower decrease to zero at the meionite end-member. Hence, all the intermediate scapolites of the marialite-meionite series actually have a pseudosymmetric structure, i. e., a pseudobody-centred structure, and as the chemical composition approaches both end-members, the scapolite structure approaches the structure with a body-centred lattice. The weak super-lattice reflections are directly related to the following two correlated factors: (1) the relative order-disorder of A1 and Si distribution in the tetrahedral framework, and (2) the structural distortion from the body-centred symmetry. Accompanied with the above facts is the systematic deviation of cell dimensions from the linear regression in such a way that the further the structure deviates from the body-centred symmetry, the smaller the cell volume and the a(o) become relative to the evaluated one from the linear regression. Scapolites are regarded as a continuous, perfect solid solution with a long-range disordering, and unlikely to be composed of submicroscopic twins or different domains. The structural analyses and refinements of three scapolites (ON8, 20% Me; XL, 52% Me; ON45, 93% Me) have been carried out by using 3-dimensional intensity data collected by integrated precession film techniques (ON8, ON45) and an automated single-crystal diffractometer (XL), and using the full-matrix least0squares method. The result has clarified the ambiguities arising from the previous studies and showed that the crystal structure is essentially continuous along the marialite-meionite series, with a slight structural variation as a function of chemical composition and ordering of Al, Si. Several important quantitative relationships regarding the crystal structure and crystal chemistry of the scapolite solid solution series have been established, namely, (1) the relationship between the superlattice intensity r(ΣI(h+k+l=2n+1)/ΣI(h+k+l=2n)) and the chemical index % Me. (2) the exponential relationship between the superlattice intensity ration r and the atomic displacement from the mirror plane consistent with the space group I4/m. (3) The linear relationship between the superlattice intensity ratio r and the difference of Al occupancy between T2 and T3 sites. (4) The Al occupancy (%) of tetrahedral sites as a function of the chemical composition of scapolites. The ordering of Al-Si, the c-axis displacement of (Ca, Na), the tilting of CO3 and the relative amount of CO3 and Cl in the same scapolite are all inter-related. The mechanism to cause all such related structural phenomena and even the abnormal stoichiometry of scapolite can all be interpreted in terms of the internal strain created by the two greatly different anions, Cl- and CO3-,sharing the same set of equivalent sites. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/18599 |
| Date | 09 1900 |
| Creators | Lin, Szu-bin |
| Contributors | Burley, B. J., Geology |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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