Anglesey rocks under the microscope
Dr Rob Crossley has created a presentation showing Anglesey rocks under the microscope.
Dolerite is a basic intrusive igneous rock, usually found in dykes [or sills] – the same chemistry and mineralogy as basalt [or gabbro]. Looking at the plane polarised imaeg [left] those grains with bold black lines [‘high relief’] are olivine. The highly coloured crystals are pyroxene or olivine, the black and white striped crystals are plagioclase feldspar. See these rocks in situ here
Sandstones are sedimentary rocks with a grain size of 0.05-2mm [though they grade imperceptibly into fine gritstones of up to 5mm] and vary enormously in composition. This one is principally quartz, and comes from within the Carboniferous limestone sequence from Penmon. See these rocks in situ here
This rock started life as a sedimentary rock, a muddy sandstone but has been metamorphosed to a schist – meaning that the clay minerals have been recrystallised to mica type minerals.
This rock also staarted life as a sedimentary rock but was principally a quartz sandstone. Quartz cannot change composition whether heated or pressurised, but it can change structure, giving it an unduloose extinction pattern. this means that instead of a single grain changing into extinction as one unit, the dark extinction colour seems to ‘flow’ across the grain. See this rock in situ here on Holyhead Mountain.
