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The crystal structure of Ca7Mg9(Ca,Mg)2(PO4)12

Die Kristallstruktur von Ca7Mg9(Ca,Mg)2(PO4)12

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The unit cell of Ca7Mg9(Ca,Mg)2(PO4)12 isa=22.841(3) Å,b=9.994(1) Å,c=17.088(5) Å and β=99.63(3)° at 24° C. The space-group is C2/c with four formula weights per cell. The crystal structure has been determined from 6330 X-ray reflections measured from a single crystal by a counter method and has been refined toR w =0.044,R=0.046 (based on 4227 observed reflections and 322 of the unobserved reflections). One cation site may be occupied by Ca or Mg and gives rise to variability in composition as is reflected in the formula give above. In the sample studied, Ca and Mg occupy the site approximately equally. The\(\left[ {10\bar 2} \right]\) direction in the unit cell is pseudo-hexagonal. The structure of Ca7Mg9(Ca,Mg)2(PO4)12 is related to that of K3Na(SO4)2 in that along\(\left[ {10\bar 2} \right]\) it has columns of cations and columns of cations and anions. These columns are arranged in a K3Na(SO4)2-type pseudo-cell. In the cation-anion columns, every other cation site in K3Na(SO4)2 is vacant in Ca7Mg9(Ca,Mg)2(PO4)12.


Die Gitterkonstanten von Ca7Mg9(Ca,Mg)2(PO4)12 sind (bei 24° C)a=22,841(3) Å,b=9,994(1) Å,c=17,088(5) Å und β=99,63(3)°; Raumgruppe: C2/c;Z=4. Die Kristallstruktur wurde aus 6330 Röntgendiffraktometer-Einkristalldaten bestimmt und (auf der Basis von 4227 beobachteten und 322 nicht-beobachteten Reflexen) aufR w =0,044 undR=0,046 verfeinert. Eine Kationenlage kann von Ca oder Mg besetzt werden, was eine Variabilität der Zusammensetzung ergibt, wie sie obige Formel ausdrückt. In der untersuchten Probe besetzen Ca und Mg diese Punktlage etwa zu gleichen Teilen. Die\(\left[ {10\bar 2} \right]\)-Richtung der Elementarzelle ist pseudo-hexagonal. Die Struktur von Ca7Mg9(Ca,Mg)2(PO4)12 ist zu der von K3Na(SO4)2 darin verwandt, daß sie längs\(\left[ {10\bar 2} \right]\) Säulen von Kationen und Säulen von Anionen hat. Diese Säulen sind in einer Pseudozelle vom K3Na(SO4)2-Typ angeordnet. In den Kation-Anion-Säulen ist jede zweite Kationen-Lage des K3Na(SO4)2 in Ca7Mg9(Ca,Mg)2(PO4)12 unbesetzt.

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Dickens, B., Brown, W.E. The crystal structure of Ca7Mg9(Ca,Mg)2(PO4)12 . TMPM Tschermaks Petr. Mitt. 16, 79–104 (1971).

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  • Reflection
  • Crystal Structure
  • Geochemistry
  • Counter Method
  • Cation Site