Rocks / Tonalite, from Coed-y-Brenin.

The name tonalite is derived from a type locality rock, adjacent to the Tonale line, which is a major structural lineament in the Alps. This specimen was collected on a field trip to Coed-y-Brenin in the Snowdonia national park, North wales. Tonalite has the same petrological significance as andesite, but has remained at depth, crystallising under plutonic conditions, allowing crystals to grow to a reasonable size, and evolving chemically in a felsic sense. Tonalite is composed of essential quartz, Plagioclase feldspar, mafic minerals ( biotite, pyroxene, hornblende ), and minor alkali feldspar. The inserts show crystals of pargasite, which have been exposed on a fractured surface.  A full account of the field trip to Coed-y-Brenin  in November 2020 can be found on the MWGC website

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