Minerals under the microscope

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Presentation transcript:

Minerals under the microscope

The MINERALS you need to know quartz microcline plagioclase muscovite biotite amphibole pyroxene olivine garnet calcite

Quartz grains in sandstone PPL XPL Quartz grains in sandstone

Plagioclase with repeated twinning PPL XPL Plagioclase with repeated twinning

microcline microcline PPL XPL Microcline grain in centre showing its distinctive cross-hatched twinning

muscovite muscovite PPL XPL PPL XPL Muscovite clear under PPL but with perfect cleavage and bright interference colours

feldspar feldspar biotite biotite PPL : lower view rotated to show pleochroism Biotite pleochroic with one perfect cleavage; strong natural colour masks polarisation colours making look greenish under XPL XPL biotite

amphibole amphibole PPL : lower view showing pleochroism Amphibole pleochroic with 2 cleavages at 60o or 120o distinguishing it from biotite or pyroxene XPL

pyroxene pyroxene enlarged PPL XPL Pyroxene well-formed crystal seen here with 2 cleavages at 90o: brightly coloured under XPL

olivine olivine plagioclase plagioclase PPL Olivine looks like a stained-glass window; bright interference colours and thick black fractures XPL

garnet garnet Garnet rounded crystal, transparent under PPL but goes extinct under XPL PPL XPL

calcite calcite Calcite may show both high and low relief, transparent under PPL with 2 perfect cleavages at 120o. Under XPL it shows pearly shades PPL XPL