ROCKS under the microscope

The ROCKS you need to know granite diorite gabbro dolerite basalt shelly limestone sandstone arkose greywacke marble schist

biotite biotite muscovite plagioclase plagioclase muscovite quartz quartz PPL XPL Granite Note how light and clear it is under PPL and grey/white/black under XPL; main minerals – feldspars (often microcline), quartz, biotite and muscovite

amphibole amphibole biotite biotite plagioclase plagioclase PPL XPL Diorite Darker under PPL than granite with more coloured minerals; much plagioclase and amphibole/biotite, only small amount of quartz

pyroxene pyroxene plagioclase plagioclase olivine olivine olivine PPL XPL Gabbro Coarse-grained; darker under PPL and brightly coloured under XPL; main minerals – plagioclase, pyroxene and olivine

plagioclase plagioclase pyroxene pyroxene PPL XPL Dolerite Medium-grained; darker under PPL and brightly coloured under XPL; main minerals – plagioclase (often in form of laths), pyroxene (intergrown through other crystals) and olivine

plagioclase & iron minerals fine groundmass of plagioclase & iron minerals pyroxene pyroxene PPL XPL Basalt Fine-grained; darker under PPL and brightly coloured under XPL; main minerals – plagioclase, pyroxene and olivine; often porphyritic (large well-formed crystals in fine groundmass)

calcite calcite PPL XPL Shelly limestone Dark under PPL and dark pearly shades under XPL; main mineral – calcite; note the fossils.

quartz quartz PPL XPL Sandstone Rounded quartz fragments; light under PPL and grey/white/black under XPL; main mineral – quartz

microcline microcline quartz quartz PPL XPL Arkose More angular quartz and feldspar fragments; light under PPL and grey/white/black under XPL; main minerals – quartz and microcline

muddy matrix muddy matrix quartz quartz rock fragment rock fragment PPL XPL Greywacke Fragments variable in shape, size and type; light under PPL and grey/white/black under XPL; main mineral – quartz but with rock fragments, feldspar and minor micas; look out for the muddy matrix!

calcite calcite PPL XPL Marble Crystalline with perfect cleavages evident; light under PPL and pearly shades under XPL; main mineral – calcite.

muscovite muscovite garnet garnet quartz quartz PPL XPL Schist Minerals showing foliation; light under PPL and brightly coloured under XPL; main minerals – muscovite, biotite, garnet, quartz and feldspar