Minerals under the microscope The MINERALS you need

Minerals under the microscope

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

quartz PPL quartz Quartz grains in sandstone XPL

plagioclase PPL Plagioclase with repeated twinning XPL

microcline PPL microcline Microcline grain in centre showing its distinctive crosshatched twinning XPL

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

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

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

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

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

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

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