ROCKS under the microscope

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

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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
biotite
biotite
muscovite
plagioclase
plagioclase
muscovite
quartz
quartz
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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
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PPL XPLGabbro Coarse-grained darker
under PPL and brightly coloured under XPL main
minerals plagioclase pyroxene and olivine
pyroxene
pyroxene
plagioclase
plagioclase
olivine
olivine
olivine
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PPL XPLDolerite 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
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PPL XPLBasalt 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)
fine groundmass of plagioclase iron minerals
pyroxene
pyroxene
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PPL XPLShelly limestone Dark under PPL
and dark pearly shades under XPL main mineral
calcite note the fossils.
calcite
calcite
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PPL XPLSandstone Rounded quartz
fragments light under PPL and grey/white/black
under XPL main mineral quartz
quartz
quartz
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PPL XPLArkose More angular quartz and
feldspar fragments light under PPL and
grey/white/black under XPL main minerals
quartz and microcline
microcline
microcline
quartz
quartz
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PPL XPLGreywacke 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!
muddy matrix
muddy matrix
quartz
quartz
rock fragment
rock fragment
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PPL XPLMarble Crystalline with perfect
cleavages evident light under PPL and pearly
shades under XPL main mineral calcite.
calcite
calcite
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PPL XPLSchist Minerals showing
foliation light under PPL and brightly coloured
under XPL mainminerals muscovite biotite
garnet quartz and feldspar
muscovite
muscovite
garnet
garnet
quartz
quartz