Volcano Labs Flashcards
(27 cards)
Checklist of describing a rock
- Colour
- Crystalline or fragmented
- Size of crystals and if there is variation in size
- Identify minerals or describe no. of different colour minerals and their quantified %
- Draw rock with scale
- Does the rock have a texture or special shape
Porphyritic texture
2 crystal sizes
Large crystals = phenocrysts
Small crystals = matrix
Tells you igneous rock had 2 stages of cooling
Vesicular texture
Bubbles caused by gas escaping and expanding during eruption but rock freezes before gas escapes.
More vesicles = more gas rich magma and therefore more explosive eruption.
Amygdaloidal texture
Vesicles get filled up with new crystals formed by ppt. of minerals.
Indicates rock has been affected by fluid after emplacement.
Crystal sizes (numerical and names)
Coarse = 6-22mm Medium = 2-6mm Fine = 0.06-2mm Glassy = No crystals
Crystal sizes and relative cooling
Larger the crystal, slower the cooling
How to describe colour of rock?
Shade - Dark or light
The simply the colour
Relief on an optical microscope
- Observation of how much the mineral visually stands out
- High relief is bold, low relief is faint
Pleochroism (minerals)
When minerals change colour when rotated in plane polarised light
Cleavage (minerals)
- Some minerals have planes of weakness along their structural plane
Cross polarised light
- Putting a 2nd filter (the analyser) in the way of the 1st
Cross polarised light: Isotropic minerals
Minerals do not refract light when it passes through, and so appear black in x polarised light
Cross polarised light: Anisotropic minerals
Minerals refract light and make the thin section appear multiple colours due to birefringence
Birefringence and 1st, 2nd and 3rd order
Birefringence = refraction of light in x polarised light resulting in appearance of multiple different colours
1st: dull, yellow or white
2nd: Yellow, white and extended into rainbow
3rd: Colours become pastel shades
Extinction in x polarised light
When anisotropic minerals rotate and align their refraction direction w/ the polarised light & turn BLACK.
Happens every 90 degrees
Twinning (optical microscope)
- Mineral crystals grow in alternating orientations
- Usually reflection or rotation
- 2 main types: Simple and multiple
Optical microscope images: OVALINE
Key indicators:
- High relief
- No cleavage but irregular fracture
Other features:
- Colourless-olive green
- 3rd order
- No twins
Optical microscope images: PYROXENE
Key indicators:
- 2 cleavage plates @ 90 degrees to each other
Other features:
- Moderate-high relief
- Colourless pale brown-green
- Simple or multiple twins
Clin-pyroxene is 2nd order, Orthopyroxene is 1st order
Optical microscope images: Amphibole
Key features:
- 2 Cleavage @ 120-160 degrees to each other
Other features:
- Moderate relief
- Yellow-green
Optical microscope images: Biotite mica
Key indicators:
- 1 perfect cleavage
- Pleochroic brown-yellow
- Pleochroic halo
Other features:
- Moderate relief
- No twins
Optical microscope images: Muscovite mica
Key indicators:
- 1 perfect cleavage
- Colourless
- 3rd order birefringence colours
Other indicators:
- Low relief
- No twins
Optical microscope images: Plagioclase feldspar
Key indicators:
- Multiple twins
- 1st order birefringence
- Colourless
- Low relief
Optical microscope images: Potassium feldspar
Key indicators:
- Tartan or simple twins
- 1st order birefringence
- Low relief
- Colourless
Optical microscope images: Quartz
Key indicators
- Low 1st order birefringence
- No twinning
- No cleavage
- No twinning
