Earth Science - Volcanic Hazards
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15 Terms
PPL vs XPL
Relief – how much the mineral stands out relative to other minerals(high relief = clear, low relief = does not stand out)
Colour – most minerals are colourless, some retain some colour
Pleochroism – some minerals change colour when stage is rotated in PPL
Cleavage – planes of weakness in structure, lines can be seen in thin section
Isotropic minerals – turn black under XPL as they do not refract light
Birefringence – when light refracts, colour changes, can be classed as 1st, 2nd, 3rd order based on colours presented
Extinction – anisotropic minerals(ones that show birefringence colours) when rotated align every 90 degrees with polarizer, turn black
Twinning – some mineral crystals grow in alternating orientations, reflect like a mirror, two types; simple and multiple
features to look for under microscope
o High relief
o No cleavage but irregular fracture
o Colourless-olive green
o 3rd order birefringence
o No twins
identifying features of Olivine
o Two cleavage at 90 degrees to eachother
o Moderate-high relief
o Colourless-pale brown green
o Simple or multiple
o Clino-pyroxene – 2nd order birefringence
o Orthopyroxene – 1st order birefringence
identifying features of Pyroxene
o Two cleavage at 120/60 degrees to each other
o Moderate relief
o Yellow-green
o Pleochroic white-green
o Simple twins
o 2nd-3rd order birefringence
identifying features of amphibole
o One perfect cleavage
o Pleochroic brown-yellow
o Pleochroic halos
o Moderate relief
o 3rd – 4th order birefringence
o No twins
identifying features of biotite mica
o One perfect cleavage
o Colourless
o 3rd order birefringence
o Low relief
o No twins
identifying features of muscovite mica
o Multiple twins
o First order birefringence
o Colourless
o Low relief
o 2 cleavages – hard to see
identifying features of plagioclase feldspar
o Tartan or simple twinning
o First order birefringence
o Low relief
o Colourless
o 2 cleavages – not seen in thin section
identifying features of potassium feldspar
o Low first order birefringence
o No twinning
o No cleavage
o Low relief
o Boring
identifying features of quartz
- Coarser grain size = intrusive(cooled slowly)
- Finer grain size = extrusive(cooled quickly)
what does grain size tell you about the rocks formation
o Dark – basalt
o Grey – andesite
o Light – rhyolite
types of extrusive igneous rocks
o Dark – gabbro
o Grey – diorite
o Light - granite
types of intrusive igneous rocks
- Euhedral - hexagonal
- Subhederal – semi hexagonal but more wobbly?
- Anhedral - wobbly, not uniform
types of shapes of crystals?
porphyritic - large crystals(phenocrysts)
equigranular - all crystals roughly equal in size
vesicular - bubbles of gas trapped in rock making rock porous
Porphyritic vs equigranular vs vesicular
Note 
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