EFFECTS OF CRYSTAL CHEMISTRY, HABITUS, AND EXPERIMENT APPARATUS ON APATITE DISSOLUTION ACROSS SCALES

The weathering of apatite is the foundation of the phosphorus cycle and essential to life, yet our understanding of the dissolution of phosphate minerals remains limited. To gain a better understanding of the effects of the crystal chemistry of phosphate minerals on apatite dissolution mechanisms across scales and determine the occurrence of experimentally-induced artifacts on apatite weathering rates, we designed two sets of flow-through weathering experiments on apatite minerals in mixed flow reactors and columns. We hypothesize that experiment apparatus will have little to no effect on the dissolution rates and response of the apatite samples. To test this hypothesis, we compared the macro- to nanoscale dissolution response of prismatic and crushed apatite samples.

Prismatic and crushed apatite (Ca₅(PO₄)₃(F,Cl)) anionic endmembers were hydrolyzed in a HNO₃-H₂O solution at a pH of 3, ambient temperature, and 1 bar for 6-10 days in a 3D-printed mixed flow reactor and flow-through column. At the macroscale, the steady-state dissolution rates were similar to published values. At the microscale, SEM analysis revealed a striking anion-dependency of surface weathering response, with etch pits formation on F-apatite but not on Cl-apatite. This discrepancy was confirmed at the nanoscale via HRTEM imaging which revealed that the Cl-apatite surface transitioned to an amorphous layer more sharply than the Cl-apatite. In an effort to attribute this discrepancy to crystallographic parameters we are monitoring their alteration pre- and post-weathering using single-crystal XRD.

Our experiments indicated that the strongest control on apatite weathering mechanisms resides in its chemical composition rather than its habitus and that both flow-through columns and mixed-flow reactors are appropriate to determine its macroscale weathering.