The crystal structure of nordite and its relationship to melilite and datolite-gadolinite

The crystal structure of nordite, a = 14.27 ± 0.03, b = 5.16±0.01, c= 19.45 ± 0.15; Pcca, Z = 4 X (La_2.1Ce_1.7Ca_0.2) (Sr_2.1Ca_1.9) Na₂ (Na_0.8Mn_0.2) (Zn_1.5Mg_1.3Fe²⁺_0.7Mn_0.5) (Si_5.85 Fe³⁺_0.15)O₁₇ has been solved on the basis of a three-dimensional Patterson function. The refinement with 1335 independent photographic reflections gave R_hkl = 0.122.

The structure consists of compact layers of RE, (Sr, Ca) and Na square antiprisms and Na, Mn octahedra which are connected along the b-axis by a new kind of T₇O_17.∞∞ flat net of tetrahedra formed by four-, five- and eight-rings. The structure of nordite is closely related to the structures of melilite and datolite-gadolinite, and can be considered as an unusual combination of both.