Gneiss is a metamorphic rock, with a banded or foliated structure, formed at great depths from igneous or sedimentary rocks.
Photo courtesy Brent Eades.
Display rock 5 is a metamorphic rock. It’s a Precambrian age sample of intensely-folded mafic gneiss. Mafic rocks contain an abundance of dark-coloured iron and magnesium-rich minerals.
This rock was found in a local farm field. It was brought to our area from an unknown northern location by the last glacier. Its dark blue to black colouration is due to an abundance of iron and magnesium-rich minerals. Look closely for some light-coloured minerals – quartz and feldspar – that have been stretched and rotated along the foliated layering. Having been metamorphosed so strongly that the minerals and structure of the original igneous or sedimentary rock are obliterated, it’s a challenge to determine what the parent rock might have been.
Photo by Roy Bassoo.
Folding details can be seen too if you take a close look.