| Popis: |
The biaxial absorption bands in amethyst quartz, with peaks at 2.28 eV and 3.54 eV related to Fe4+ and a peak at 3.02 eV—which is the A3 band related to the [AlO4]° trapped hole center, have orientations of maximum light absorption in the basal plane of Brazil-twinnedr-growth sectors paralleling the planes of Brazil optical twinning. Absorption minima are at 90° to the maxima in all cases. The Brazil twinning planes always parallel thea-axes(1210,etc) of quartz and in many cases also parallel planes perpendicular to ther-faces(1011,etc.). These are directions of channels in the quartz structure. The anisotropy ratio,σmax/σmin, of the Fe4+ band is that of the A2 absorption band in smoky quartz as would be expected if Fe3+ furnishes electrons to quench the trapped holes causing this absorption band. In the absence of the A1 and A2 absorption bands, the A3 absorption band width at half-maximum decreases from 1.43 to 0.36 eV indicating decreased charge-transfer character of the [AlO4]° center in the absence of the other types of Al trapped-hole centers in quartz. The key to the Brazil twinning in α-quartz are the channels which fill with large Fe3+ ions that force twinning to relieve strain in the structure. Amethyst color results only if aluminum is present substitutionally in the quartz as well as the interstitial iron plus ionizing radiation. |
