Titre du document / Document title

The geometric effects of ^vFe²⁺ for ^vMg substitution on the crystal structures of the grandidierite-ominelite series

Auteur(s) / Author(s)

DZIKOWSKI Tashia J. ; GROAT Lee A. ; GREW Edward S. ;

Résumé / Abstract

The electron microprobe compositions and crystal structure of seven members of the grandidierite-ominelite (MgAl₃BSiO₉ -Fe²⁺Al₃BSiO₉) series with X = (Fe²⁺ + Mn + Zn)/(Fe²⁺ + Mn + Zn + Mg) ranging from 0.00 to 0.52 were studied to determine the geometric effects of Fe substitution for Mg on the crystal structures. Calculating Fe³⁺ from the electron microprobe analyses gave 0.04-0.06 Fe³⁺ apfu, but such small amounts at the Al sites could not be detected in the refinements. Regression equations derived from single-crystal X-ray diffraction data show that b increases by 0.18 Å from X= 0-1. The crystal structure refinements show that the most significant changes involve the (Mg,Fe²⁺)O₅ polyhedron, which increases in volume by 0.36 ų (5.0%), largely as a result of expansion of the MgFe-05, -02, and -06 (x2) bond distances, which increase by 0.09 (4.4%), 0.06, and 0.04 Å, respectively. Other significant changes include increasing Ol-MgFe-02 (3.44°) and -A13-05a angles (1.9°) and a decreasing 06-MgFe-06b (-2.20°) angle. Significant increases are also seen in the lengths of the 01-02 (0.13 Å) and 06-05a (x2) (0.09 Å) edges. The SiO₄ tetrahedra appear to respond to changes in the surrounding polyhedra by changing O-Si-O angles such that the tetrahedral angle variance and mean tetrahedral quadratic elongation increase withX. The BO₃ triangles appear to behave as relatively invariant units in the crystal structure. Regression equations obtained from the MgFe-O bond distances were used to determine a radius for ^VFe²⁺ of 0.70 Å. Although our samples show little Mn, the presence of Mn²⁺ at the MgFe site would be expected to cause more distortion than an equivalent amount of Fe²⁺. Substitution of Zn likely would have little effect. The presence of Cr³⁺ at any of the Al sites would be expected to increase the size of the coordination sphere, but the substitution of P⁵⁺ for Si at the Si sites would most likely decrease the Si-O bond distances.

Revue / Journal Title

The American mineralogist   ISSN 0003-004X   CODEN AMMIAY 

Source / Source

2007, vol. 92, n^o5-6, pp. 863-872 [10 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Mineralogical Society of America, Washington, DC, ETATS-UNIS  (1916) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 503, 35400016224876.0140