ROCKS

under the microscope

The ROCKS you need to know

• granite• diorite• gabbro• dolerite• basalt• shelly limestone• sandstone• arkose• greywacke• marble• schist

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

quartzquartz

muscovite

muscovite

biotitebiotite

plagioclaseplagioclase

plagioclase

amphibole

amphibole

plagioclase

biotite biotite

PPL XPL

Diorite

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

PPL XPL

Gabbro Coarse-grained; darker under PPL and brightly coloured under XPL; main minerals –

plagioclase, pyroxene and olivine

olivine

olivine

olivine

pyroxenepyroxene

plagioclaseplagioclase

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 plagioclase

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)

pyroxenepyroxene

fine groundmass of

plagioclase & iron minerals

calcite calcite

PPL XPL

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

fossils.

quartzquartz

PPL XPL

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

mineral – quartz

microcline

quartz quartz

microcline

PPL XPL

Arkose More angular quartz and feldspar fragments; light under PPL and grey/white/black under

XPL; main minerals – quartz and microcline

quartzquartz

muddy

matrixmuddy

matrix

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!

calcitecalcite

PPL XPL

Marble Crystalline with perfect cleavages evident; light under PPL and pearly shades under XPL;

main mineral – calcite.

garnet

muscovite

quartz

PPL XPL

Schist Minerals showing foliation; light under PPL and brightly coloured under XPL; main

minerals – muscovite, biotite, garnet, quartz and feldspar

garnet

muscovite

quartz