UHP-metamorphic rocks from Dora Maira, Western Alps: Cathodoluminescence of silica and twinning of coesite

Hot cathode cathodoluminescence (CL) microscopy is introduced as a powerful tool to distinguish between different silica phases and between different generations of the same breakdown product of coesite. The rock type investigated is a fine grained pyrope-quartzite from Pangi, Dora Maira Massif, Italy. While coesite inclusions within pyrope display bright bluish-green luminescence colours, their palisade-like breakdown products of quartz are characterized by heterogeneously brownish-violet colours. Cracks within coesite are filled by quartz which shows brown luminescence colours indicating a different generation. Quartz of the matrix exhibits a homogeneously dark blue luminescence. Fine-grained, partly also fibrous chalcedony which is difficult to distinguish from quartz breakdown products of coesite under the polarizing microscope shows bright brownish-yellow luminescence colours. Consequently, even tiny crystals of coesite can easily be separated from different quartz generations or chalcedony due to their characteristic CL emission. Thus, CL-microscopy is an elegant and inexpensive method to discover chronological breakdown sequences of silica and to reveal specific features and to obtain information of minerals which may have become otherwise overlooked. Twinning of metamorphic coesite resembles polysynthetic plagioclase exhibiting albite- and pericline-law individuals. Two respective types of lamellae were identified which intersect each other at an angle of ca. 90; they are twinned after (021) and (101).