THE ATOMIC ARRANGEMENT OF NATURALLY OCCURRING FLUOR-CHLORAPATITE

Apatite [Ca₁₀P₆O₂₄(F, Cl, OH)₂] is the tenth most abundant mineral on earth, but there are large gaps in our knowledge of its atomic arrangement. Apatite is one of the rare minerals in which the atomic arrangements of the binary and ternary (F, OH, Cl) solid solutions are not predictable from those of the end-members in the ternary system. For decades, scientists have sought natural samples along the F-Cl apatite binary to determine the method of accommodation of F and Cl in the anion columns. However, no such natural apatites have been analyzed from Earth or extraterrestrial samples. Recently, samples along the F-Cl binary have been successfully synthesized for the first time, and the atomic arrangement of these samples has been unraveled. Subsequently, the first naturally occurring samples of OH-free, F-Cl apatites have been found, and their atomic arrangement has been solved. These samples of naturally occurring F-Cl apatite were found in the Three Peaks area, Utah, and their atomic arrangement solved using single crystal x-ray diffraction. It was found that the atomic arrangement of this naturally occurring F-Cl apatite matched that of the synthesized F-Cl apatite. Solid solution of the Three Peaks apatite is attained by creation of a new, off-mirror F site, allowing accommodation of the seemingly incompatible F and Cl ions in the apatite anion column. The results of the crystal structure solution of this binary member in the natural apatite ternary system will be presented, the first natural F-Cl apatite structure that has been elucidated.