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The crystal chemistry of gorceixite, grandidierite, and traskite

University of British Columbia

This thesis reinvestigates the crystal structures of gorceixite and traskite, and the geometric effects of vFe²+ for vMg substitution on the crystal structures of the grandidieriteominelite series. All data was measured using using MoKα radiation on an automated Bruker X8 single-crystal diffractometer with a SMART APEX CCD (charge coupled device) detector. The crystal structure of gorcexite (BaA1₃(P0₃0,0H) ₂(OH₆),a, 7.0538(3), c l7.2746(6) , Å V 744.4(2) ų, space group R 3̄m, Z = 3, has been refined to an R₁ index of 2.3% based on 253 unique reflections. The results indicate that this specimen has rhombohedral rather than monoclinic Cm symmetry as was previously reported for the species. The crystal-structure refinement shows that the atomic arrangement of gorceixite is similar to that of other members of the plumbogummite group. The chemical compositions and crystal structures of seven members of the grandidieriteominelite (MgA1₃BSiO₉--Fe²⁺ A1₃BSiO₉) 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. Regression equations derived from single-crystal X-ray diffraction data show that b increases by 0.18 for the range 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. Numerous space groups were tried in an attempt to solve and refine the crystal structure of the traskite (Ba₉Fe²⁺₂Ti₂(SiO₃)₁₂(F,C1,OH)₆ 6H₂O.The most successful was P3̄1̕n with a 17.863(3), c 12.298(3) Å, and Z = 3. The Rint = 5.3% and R₁ = 5.3% values indicate that the data are good and that the model is close to being correct; however, split Ba, O, and C1 sites indicate that there are missing symmetry elements within the structure. Attempts to refine the structure in PG/mmm, which contains the supposed missing symmetry elements, were unsuccessful.

Text

2010-01-08

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http://hdl.handle.net/2429/17928

Geological Sciences

University of British Columbia

UBCV

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