THE CRYSTAL CHEMISTRY OF THE SCAPOLITE-GROUP MINERALS. I. CRYSTAL STRUCTURE AND LONG-RANGE ORDER

Scapolite-group minerals are a group of rock-forming framework aluminosilicates common in a wide variety of igneous and metamorphic rocks. They have the general formula M(4) [T(12) O(24)] A, where M represents Na, K, Ca, Sr, Ba and Fe(2+), T stands for Si, Al, (Fe(3+)), and A represents Cl, (CO(3)), (SO(4)). They constitute a non-binary solid-solution series involving the end members marialite (Na(4) [Al(3) Si(9) O(24)] Cl, Ma) and meionite (Ca(4) [Al(6) Si(6) O(24)] CO(3), Me). The crystal structures of eighteen scapolite-group minerals from Me(4.9) to Me(92.8) [Me = 100 Sigma M(2+) / Sigma(M(2+) + M(+))] have been refined with MoK alpha X-radiation. The crystals, a 12.045-12.208, c 7.564-7.586 similar to, V 1099.7-1130.1 similar to(3), I4/m or P4(2)/n, Z = 2, were analyzed with an electron microprobe subsequently to collection of the X-ray intensity data. There are subtle differences between the behavior of the distinct T tetrahedra in the I4/m and P4(2)/n structures. For Si and Al, determinative curves for the assignment of site populations from < T-O > distances were developed for each T site in each structure type, and T-site populations were assigned from these curves. In I4/m marialite, there is no Al at the T(1) site. In P4(2)/n scapolite, Al is strongly ordered at T(2), and Si is strongly ordered at T(3), but this order is never complete. The A-site cation (Na, Ca, K) is [8]-coordinated in end-member marialite and is (on average) [8.5]-coordinated in end-member meionite. Each (M phi(n)) polyhedron shares two edges with adjacent (Mfn) polyhedra, forming a three-dimensional framework that interpenetrates the [T(12)O(24)] silicate framework. In (CO(3))-bearing structures, the model proposed by previous workers, four (CO(3)) groups arranged about the (4) over bar axis (I4/m) or 4 axis (P4(2)/m), is consistent across the series from marialite to meionite. Any (SO4) present is disordered by rotation of 90 about the central 4-fold axis; in I4/m structures, each arrangement is 50% occupied, whereas in P4(2)/n structures, the occupancies of the two arrangements are not required to be equal.