1. Minerals under the microscope

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

3. Quartz grains in sandstone
PPL
XPL
Quartz grains in sandstone

4. Plagioclase with repeated twinning
PPL
XPL
Plagioclase with repeated twinning

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

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

7. 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

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

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

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

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

12. 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