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Earth Science > Volcano Labs > Flashcards

Volcano Labs Flashcards

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1
Q

Checklist of describing a rock

A
  1. Colour
  2. Crystalline or fragmented
  3. Size of crystals and if there is variation in size
  4. Identify minerals or describe no. of different colour minerals and their quantified %
  5. Draw rock with scale
  6. Does the rock have a texture or special shape
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2
Q

Porphyritic texture

A

2 crystal sizes
Large crystals = phenocrysts
Small crystals = matrix
Tells you igneous rock had 2 stages of cooling

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3
Q

Vesicular texture

A

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.

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4
Q

Amygdaloidal texture

A

Vesicles get filled up with new crystals formed by ppt. of minerals.
Indicates rock has been affected by fluid after emplacement.

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5
Q

Crystal sizes (numerical and names)

A 
Coarse = 6-22mm
Medium = 2-6mm
Fine = 0.06-2mm
Glassy = No crystals
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6
Q

Crystal sizes and relative cooling

A

Larger the crystal, slower the cooling

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7
Q

How to describe colour of rock?

A

Shade - Dark or light

The simply the colour

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8
Q

Relief on an optical microscope

A
  • Observation of how much the mineral visually stands out

- High relief is bold, low relief is faint

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9
Q

Pleochroism (minerals)

A

When minerals change colour when rotated in plane polarised light

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10
Q

Cleavage (minerals)

A
  • Some minerals have planes of weakness along their structural plane
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11
Q

Cross polarised light

A
  • Putting a 2nd filter (the analyser) in the way of the 1st
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12
Q

Cross polarised light: Isotropic minerals

A

Minerals do not refract light when it passes through, and so appear black in x polarised light

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13
Q

Cross polarised light: Anisotropic minerals

A

Minerals refract light and make the thin section appear multiple colours due to birefringence

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14
Q

Birefringence and 1st, 2nd and 3rd order

A

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

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15
Q

Extinction in x polarised light

A

When anisotropic minerals rotate and align their refraction direction w/ the polarised light & turn BLACK.

Happens every 90 degrees

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16
Q

Twinning (optical microscope)

A
  • Mineral crystals grow in alternating orientations
  • Usually reflection or rotation
  • 2 main types: Simple and multiple
17
Q

Optical microscope images: OVALINE

A

Key indicators:

  • High relief
  • No cleavage but irregular fracture

Other features:

  • Colourless-olive green
  • 3rd order
  • No twins
18
Q

Optical microscope images: PYROXENE

A

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

19
Q

Optical microscope images: Amphibole

A

Key features:
- 2 Cleavage @ 120-160 degrees to each other

Other features:

  • Moderate relief
  • Yellow-green
20
Q

Optical microscope images: Biotite mica

A

Key indicators:

  • 1 perfect cleavage
  • Pleochroic brown-yellow
  • Pleochroic halo

Other features:

  • Moderate relief
  • No twins
21
Q

Optical microscope images: Muscovite mica

A

Key indicators:

  • 1 perfect cleavage
  • Colourless
  • 3rd order birefringence colours

Other indicators:

  • Low relief
  • No twins
22
Q

Optical microscope images: Plagioclase feldspar

A

Key indicators:

  • Multiple twins
  • 1st order birefringence
  • Colourless
  • Low relief
23
Q

Optical microscope images: Potassium feldspar

A

Key indicators:

  • Tartan or simple twins
  • 1st order birefringence
  • Low relief
  • Colourless
24
Q

Optical microscope images: Quartz

A

Key indicators

  • Low 1st order birefringence
  • No twinning
  • No cleavage
  • No twinning
25
Q

Shape of mineral: Euhedral

A

All sharp edges

26
Q

Shape of mineral: Subhedral

A

Partially sharp edges

27
Q

Shape of mineral: Anhedral

A

No sharp edges

Earth Science (3 decks)

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