The Structures of Two Hole Centers in the Mineral Brasilianite NaAl₃(PO₄)₂(OH)₄

Abstract

Two hole centers were detected in natural brazilianite by EPR at X-band and temperatures between 20 and 300 K. For the more intense center a hyperfine splitting (hfs) due to two nonequivalent Al nuclei could be completely analyzed by comparison of the hfs patterns with simulations for 1 ^ A\lA~i ^ 1.4. The orientation of the principle axes of these hfs tensors allowed to assign this hole to a specific oxygen in the lattice. The phosphorus bound to it is evidently exchanged against a lower valency cation, most likely Si . In the second center a hfs splitting due to one Al and one P nucleus was identified. Although a complete analysis of their hfs tensors was not possible, this center could also be assigned to a particular oxygen for which evidently one adjacent Al is exchanged against a divalent ion.

An Electrolytically Generated, Localized Hole Center in Quartz

“…It may indicate a relatively short A l... O" distance, but cal culations based on a purely electrostatic model [18] do generally not lead to very consistent results. A twice as large value was recently observed for one of two Al adjacent to an (also localized) hole in the mineral brazilianite [19], where due to sixfold coordination of the Al these distances should be significantly larger than in quartz.…”