Theoretical and experimental study of Si,Al distribution rule in tectoaluminosilicate minerals

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1. Characterization of Si,Al distributionSi,Al distribution on the tectoaluminosilicate frameworks was characterized by introducing a new concept of substituted concentric cluster(SCCL). It was disclosed that various kinds of Si,Al distribution on the framework are fairly reflected on the types and the frequencies of the SCCL , and the connective relations between Si and Al atoms are clearly visualized on the SCCL.2. Prediction of Si,Al distributionA graph theoretical derivation of the possible SCCLs was tried under the restriction of Loewenstein (avoidance rule of Al-Al pair in the 1st neighbor) as well as Dempsey rule (minimization rule of Al-Al pairs in the 2nd neighbor). The 1st SCCLs were obtained by an application of simplex method, while the 2nd SCCLs by using a mathematical vector relation . Si and Al distribution on a given framework could be predicted by assigning the above SCCLs on the framework. In order to avoid the production of infinite combinations of them. special attention was paied on the translational symmetry of the framework and the compatibility relationship of the SCCLs to be combined. 3-dimensional computer graphics method was newly developed for the procedure.3. Determination of Si,Al distribution by X-ray diffraction methodIn order to confirm the predicted Si,Al distribution , crystal structure of scapolite mineral, which forms a complete solid solution between Al/Si = 3 and 1 , was determined by X-ray diffraction method. Data were collected using a four circle automatic X-ray diffratometer, Rigaku AFC-M. Bond distances between T(Si,Al) and 0 atoms were closely examined and found that the T - 0 distance was significantly longer than the other T , T - 0 distaces. This indicated that the probability of Al occupancy at site 2 was very high. The distribution was completely consistent with one of the predicted ones.