Summary
X-ray diffraction studies reveal that most scapolites belong to space groupP4 2 /n. The mean intensity of weak reflections violating body-centered symmetry, reaches a maximum around 37 per cent meionite and extrapolates to zero at the pure end-members. All intermediate scapolites of the marialitemeionite series are pseudobody-centered, and as the composition approaches that of the end-members the structure becomes truly body-centered (I4/m).
The weakh+k+l odd reflections are directly related to (1) orderdisorder of Al and Si in the tetrahedral framework and (2) structural distortion from body-centered symmetry.
Scapolites may be regarded as a continuous, homogeneous solid solution. The cell dimensions are only approximately linearly related to composition and there is a maximum deviation from linearity, for those scapolites (25% Me ≈ 45% Me) whose symmetry departs the most from body-centered symmetry. These phenomena are interpreted in terms of the structural characteristic of the scapolites.
Zusammenfassung
Röntgenbeugungs-Untersuchungen zeigen, daß die meisten Skapolithe der RaumgruppeP4 2 /n angehören. Die durchschnittliche Intensität der schwachen Reflexe, welche der Raumzentrierung widersprechen, erreicht bei 37% Meionit-Momponente ein Maximum — für die reinen Endglieder nähert sie sich dem Wert Null. Alle intermediären Skapolithe der Marialith-Meionit-Reihe sind pseudo-raumzentriert und mit Annäherung der Zusammensetzung an jene der Endglieder wird die Struktur echt raumzentriert (I4/m)
Die schwachenh+k+l ungerade Reflexe stehen (1) mit Ordnung-Unordnung von Al und Si im Tetradergerüst und (2) mit strukturellen Abweichungen von der Raumzentrierung in Beziehung.
Die Skapolithe können als kontinuierliche homogene Mischkristallreihe betrachtet werden. Die Zelldimensionen ändern sich nur ungefähr linear mit der Zusammensetzung; die stärkste Abweichung von der Linearität tritt bei jenen Skapolithen (25% Me bis ≈ 45% Me) auf, deren Symmetrie am stärksten von der Raumzentrierung abweicht. Diese Phänomene werden aus den strukturellen Eigenheiten der Skapolithe interpretiert.
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Lin, S.B., Burley, B.J. On the weak reflections violating body-centered symmetry in scapolites. TMPM Tschermaks Petr. Mitt. 20, 28–44 (1973). https://doi.org/10.1007/BF01082100
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DOI: https://doi.org/10.1007/BF01082100