Statistical interpretation of structural parameters of tourmalines: the ordering of ions in the octahedral sites
European Journal of Mineralogy Volume 11 Number 2 (1999), p. 243 - 252
published: Apr 19, 1999
manuscript accepted: Nov 25, 1998
manuscript received: Jan 3, 1998
ArtNo. ESP147051102002, Price: 29.00 €
Abstract Basing on the results of structural analyses (bond lengths) published so far and on chemical compositions of 50 tourmalines of different varieties (schorl, dravite, elbaite, uvite, feruvite, liddicoatite, tsilaisite, olenite, foitite, povondraite, chromdravite, V-tourmaline), the relationships between the size and the cation population of X, Y, Z, B and T polyhedra and also between the observed a and c unit-cell parameters and mean bond lengths of X, Y, Z, B and T polyhedra have been statistically analysed. The influence of cations of the second coordination sphere was considered only when adjacent polyhedra share common edges (Y and Z sites). The analysis of Y and Z octahedra has been limited to the tourmalines with Al > 6 apfu, i.e. in 29 samples. Linear equations of multiple regression (at the 95 % confidence level) have been characterized in each case by the values of the correlation coefficient (R), standard error of estimate (SE) and mean absolute error (MAE) as well as by plots of the observed vs. predicted values of the mean bond lengths and the a and c unit-cell parameters, the plots of residuals vs. predicted values of dependent variables, and the normal probability plots of residuals. High correlation coefficients R of multiple linear regression models, describing the influence of the substitutions in adjacent cation sites on the dimensions of Y and Z polyhedra in tourmalines with Al > 6 apfu, prove the lack of a significant disordering of octahedral ions. They also point to a considerable influence of the substitutions in adjacent cation sites on the size of the octahedral sites. This factor has been omitted so far in the analyses of the tourmaline structure. Very high, close to unity, correlation coefficients of multiple linear regression models, describing relationships between the observed a and c unit-cell parameters and mean bond lengths of X, Y, Z, B and T polyhedra (when all Z octahedra are occupied by Al) also exclude the possibility of a significant disordering of octahedral divalent ions and Al in Z site.