Scapolite : alkali atom configurations , antiphase domains , and compositional Yariations
Electrostatic energy calculations on an intermediate scapolite indicate that Na+ should prefer to be adjacent to Clanions and Ca2* to be adjacent to CO3radicals. The calculations also indicate that short-range ordering of NarCl and CaaCO3 clusters is energetically favored and might give rise to antiphase domains. Electrical neutrality considerations on the anion/radical site in scapolite suggest that the unusual compositional variations observed in the marialite-meionite series are the result of crystallographic constraints on the two possible independent exchange reactions in scapolite: NaCICaCO-3 and NaSie6-Ai. The compositions of naturally occurring scapolites may be explained in terms of the scapolite composition that represents local charge balance between Ca2*, Na* cations and Cl-, COIanions.
Scapolite crystal chemistry; aluminum-silicon distributions, carbonate group disorder, and thermal expansion
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.
Unit-Cell Dimensions of Natural and Synthetic Scapolites
In natural scapolites the cell dimension a shows a regular increase from marialite to meionite composition, while c remains constant. Both a and c of synthetic meionite are larger than the corresponding dimensions of synthetic marialite. The cell volume of both natural and synthetic scapolites is a nearly linear function of composition. Variations in cell dimensions of scapolites may be caused by differences in structural state similar to those in plagioclase feldspars.