A trapped-hole center causing rose coloration of natural quartz

In rose-colored natural quartz crystals two new hole centers were detected and analyzed by electron paramagnetic resonance (EPR). A hole center with characteristic doublet and sextet hyperfine structures was identified as an O− ion bridging between a substitutional aluminum and phosphorus atom each. This center slowly bleaches in UV light and can be enhanced by X-ray irradiation. It is most likely responsible for the rose coloration and also explains the repeatedly reported bleaching of “rose” quartz in daylight. Light-induced transfer of the hole in the PO4, not the AlO4 unit is assumed to cause this strong visible absorption. Several features of this center are quite unexpected: The localization of the hole between Al and P contradicts electrostatic considerations. It is evidently caused by the longer Al … O distance resulting from substitution of Si by the smaller P. This longer distance also explains why this center is still observable by EPR at room temperature in contrast to the unperturbed Al … O− (smoky quartz) center. It also has an unexpectedly high thermal stability compared to aluminum-related hole centers with nearby positive ions. All the phosphorus present is obviously associated with aluminum, in agreement with estimates of the equilibrium distribution based on a purely Coulomb attraction between Al3+ and P5+ on sites of Si4 +. The isotropic 27Al superhyperfine structure is four times smaller than in the unperturbed smoky quartz center, evidently as a result of the strong electron attraction by the pentavalent phosphorus opposite to Al which also gives rise to a large and practically isotropic 31P doublet superhyperfine splitting.