Colourless aegirine in metamorphic rocks from Bayan Obo (Inner Mongolia): lack of charge transfer transitions as possible explanation

摘要

Aegirine is a Na-pyroxene well known for exhibiting bright green colour under the microscope, or else green to light brown pleochroism within the aegirine-augite solid-solution. We report the occurrence of aegirine from metamorphic rocks associated to the Bayan Obo REE-Fe-Nb ore deposits (Inner Mongolia), which displays a peculiar optical characteristic, being colourless in thin section. Detailed compositional data for the Bayan Obo aegirine and for aegirine and aegirine-augite from nepheline syenites from the Boavista Island (Cape Verde Archipelago), and from other geological settings, reveal that the Bayan Obo aegirine is characterised by significantly lower Ti contents (＜0.5 wt.% TiO_2) with respect to the aegirine from other occurrences. Besides the colour effects, normally ascribed to Fe~(2+)-related crystal field transitions and to Fe~(2+)–Fe~(3+) charge transfer transitions, our chemical and spectrophotometric data suggest that Fe~(2+)–Ti~(4+) charge transfer transitions may be the main colour-enhancer mechanism for the typical deep colour of aegirine.