REFINEMENT OF THE COLUMN ANION STRUCTURE AND CHEMISTRY OF SYNTHETIC APATITES IN THE F-OH AND OH-CL SOLUTION SERIES

Apatite sensu lato [Ca₁₀(PO₄)₆(OH,F,Cl)₂; OH = hydroxylapatite, F = fluorapatite, Cl = chlorapatite], as one of the most abundant minerals and the most abundant phosphate on Earth, is of great importance to many aspects of the geological sciences. Apatite is also of great importance in biology and material science, thus making its significance highly diverse and interdisciplinary. Of the many important crystal chemical properties of this mineral, the substitution of F, OH and Cl has recently gained increased attention due to the newly discovered uncertainty of the OH component in bone and the role of apatite in the water budget of the moon and Mars. Due to varying ionic radii these three substituents (F, OH, Cl) occupy slightly different positions within the anion column of the atomic structure. The difference in position enables the anion chemistry to be refined using structural analysis. This study applies and tests the precision of this technique using a suite of synthetic apatite samples. The crystal structures of twelve samples, falling along two binary solid solutions, F-OH and OH-Cl, were solved and refined using single crystal X-ray diffraction data, with R values between 1.3 and 1.5%. Anisotropic thermal parameters were modeled for all atoms except those in the column anion positions.

In the F-OH samples all of the F occupies the standard position at z = 0.25, and the OH at z = 0.197-0.206, with the total refined column anion site occupancies very close to 100%. The refined F/OH ratios are also very close to that predicted from the starting chemistry of the experimental charges. In the OH-Cl samples the OH position varies from z=0.173-0.201 and the Cl resides in a bimodal distribution of positions. The first is on the special position of Z=0.0 and the second, with much higher occupancy, has a range of z = 0.058-0.145. Half of the OH-Cl samples have total refined column anion site occupancies very close to 100% while the other half refined to values as high as 117%, indicating a limitation in accuracy. Also, on average, the refined column anion chemistry of the OH-Cl samples showed 13% more Cl than predicted from the starting chemistry of the experimental charges. Very precise site occupancies can be determined with such superlative refinements.