NEW METHODS FOR INFRARED SPECTROSCOPY OF OH IN APATITE
Hydroxyl groups occur in end member hydroxylapatites as well as in mixed fluorine- and chlorine- bearing apatites. A crucial observation is that the OH stretching vibration in the range 3500-3600 cm-1 is strongly anisotropic, polarized parallel to the c-axis, and absorption parallel to the a-axis is zero. The sole exception is a very small absorption at 3555 cm-1, probably related to a CaII vacancy that charge-balances a carbonate substitution.
The equations of K&S 2008 relate unpolarized absorbance to the minimum and maximum absorbance of the OH group within the mineral. In the case of the apatites, these equations can be simplified further. Previous work (Tacker et al. 2010, Goldschmidt Conference) allows determination of the effective sample thickness directly from the phosphate combination absorbance bands in each unpolarized spectrum. These two elements combine to eliminate much of the polishing involved in sample preparation. Simple modifications of the microscope X-Y mapping stage allow for rapid analysis of single grains of apatite, with polarized or unpolarized radiation. We have applied these methods to the determination of OH absorbance in end member hydroxylapatite and to analysis of apatites separated from pumice from the Soufriere Hills volcano. In addition, we are exploring the feasibility of analyzing synthetic apatites in KBr pellets using unpolarized radiation.
This work was supported by NSF grants EAR-0929898 and EAR- 0910202 to RCT.