Crystal structure parameters have been determined for a compositionally intermediate scapolite (Na, nrCa,.rrK' ro) (Si8.0sAl3 e5)Or4Cl' u"(COr)0.(SOr)o on in space group P4r/nbefore and after a heating cycle. In addition, unit-cell parameters have been determined with roomtemperature data col lected before and after heating, and with data col lected at 400",600o, 700', 800',900o and 1000"C. Both a and volume increase with temperature while c remains constant. Thermal expansion of a results from rotat ion of four-membered r ings of tetrahedra in the (001) plane. Tetrahedral bond distances suggest that this sample has a highly ordered Si-Al distribution with Tl and 73 occupied by Sio+ and T2 occupied by Al3+ . The irreversible change in tetrahedral bond distances after the sample was heated to 1000'C suggests a slight amount of disordering has taken place. This change also affected the unit-cell parameters. The carbonate group was refined as a rigid body with realistic bond distances and angles using the least-squares computer program RrtNE4. The refinement confirms the model of Papike and Stephenson (1966) and suggests that the group t i l ts only sl ightly out of the (001) plane. Normalized primitive reflections which violate 14/m symmetry decrease in intensity with temperature up to 1000'C but do not dissappear.
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