Structure variation in low cordierites

Crystal structures of six metamorphic low cordierites with Fel(Fe + Mg) ranging from 0.07 to 0.42 (atomic) were refined from X-ray data. No evidence of a superstructure was found, confirming that, in chemically intermediate cordierites, Fe and Mg are located o1 a single site. Although T-O distances remain nearly constant, increased Fe content causes (l) the M site to enlarge, (2) the channels to move apart, and (3) a and D to increase. In addition, chan-nel rings rotate, change their shape, and move closer together within each channel, causing a decrease in c. Diference Fourier maps confirm that channel water is located in a split loca-tion near (0,0,y4).Its orientation is interpreted as being largely controlled by an excess nega-tive charge on O(4) and O(5). The H-O-H plane is oriented nearly parallel to (100) and the H-H vector may either be close to [001] or [010]. Channel cations may be located at several locations at the channel walls in addition to the well-defined site at (0,0,0). Low framework cation-site occupancies are explained by 3-4 percent vacancies that balance for cations in the channel. Vacancies on T(2) and T(5) sites are inversely correlated, and on these sites an in-creasing number of vacancies produces an enlargement of tetrahedra. Minor differences in mean T-O distances, usually ascribed to Al-Si substitution, may be caused by tetrahedral distortions and vacancies.