Post on 28-Mar-2015
Minerals
under the microscope
The MINERALS you need to know
• quartz• microcline• plagioclase• muscovite• biotite• amphibole• pyroxene• olivine• garnet• calcite
Quartzgrains in sandstone
XPLPPL
quartz
quartz
quartz
quartz
Plagioclasewith repeated twinning
XPLPPL
plagioclase
plagioclase
Microclinegrain in centre showing its distinctive cross-
hatched twinning
XPLPPL
microcline microcline
Muscovite clear under PPL but with perfect cleavage and
bright interference colours
XPLPPL
muscovite
muscovite
PPL XPL
Biotitepleochroic with one perfect cleavage; strong
natural colour masks polarisation colours making look greenish under XPL
XPL
PPL : lower view rotated to show pleochroism
biotite
biotite
biotite
feldspar
feldspar
Amphibolepleochroic with 2 cleavages at 60o or 120o distinguishing it from biotite or pyroxene
XPL
PPL : lower view showing pleochroism
amphibole
amphibole
Pyroxenewell-formed crystal seen here with 2 cleavages at 90o:
brightly coloured under XPL
XPLPPL
pyroxenepyroxene enlarged
Olivinelooks like a stained-glass window; bright interference
colours and thick black fractures
PPL XPL
olivineolivine
plagioclase plagioclase
XPLPPL Garnetrounded crystal, transparent under PPL but goes
extinct under XPL
garnetgarnet
XPLPPL
Calcitemay show both high and low relief, transparent under PPL with 2 perfect cleavages at 120o. Under XPL it shows pearly shades
calcite calcite