GEO 2163 Lecture Notes - Lecture 2: Alkali Feldspar, Albite, Perthite
13 May 2018
School
Department
Course
Professor
GEO2164: Lab 5
L4-A: Strained quartz in metamorphic rocks
- Undulatory extinction (extinct in one place and not in another)
- Mostly anhedral grains
- Low interference, first order grey to white in XPL
- Not pleochloric
- Colorless in PPL
L4-B: beta-quartz in rhyolitic rocks
- Dipyramidal hexagonal crystals
- Low interference, 1st order grey to white in XPL
- Colorless in PPL
- Undulatory extinction
L4-C: Optical determination of the composition of plagioclase
- Albite
- Albite twinning, parallel (all the way through grain, pericline perpendicular twinning, not parallel)
- Colorless in PPL
- Low interference colors, first order grey to white in XPL
L4-D: Tartan twinning in microcline
- Microcline has tartan twinning
- Low interference 1st order grey to white
- Colorless in PPL
- No pleochroism
- Large crystal ~2mm
L4-E: Exsolution features in alkali feldspar: perthite
- Albite twinning observed in perthite “patches”
- Colorless in PPL
- low interference in XPL, 1st order
- inclined extinction: anywhere from 0-90 degrees
- parallel extinction: N-S
- symmetrical extinction: 45 degrees
L5-A: Sample 2162-11
- This pyroxene is clinopyroxene
- Anisotropic
- Biaxial +
- transparent, translucent
- The mineral is pale brown in PPL and has high interference colors (second order) in XPL
- No pleochlorism in this thin section
- High birefringence in XPL
- High positive relief
- One cleavage plane is found intersecting at 88-90 degrees
- Lamellar twinning
- Upon lining the cleavage plane N-S, I rotated the stage until extinction and noticed it to be inclined
(calculated roughly from N-S aligned cleavage to extinction = 46 degrees).
- Grain size: ~0.1mm
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GEO2164: Lab 5
L5-B: Sample 2162-4
- This pyroxene is orthopyroxene
- Transparent, translucent
- This mineral in XPL has low birefringence colors and 1st order grey to yellow
- Pleochlorism is observed in PPL, very faint and hard to notice
- Parallel extinction is observed N-S and E-W
- In PPL the color is pale brown, slightly green tint
- Two cleavage planes that intersect at around 88 degrees
L5-C: Sample 2162-18
- This amphibole is either anthophyllite or gedrite
- Anisotropic
- It has parallel extinction which limits it to either anthophyllite or gedrite
- Low birefringence high 1st order orange-yellow in XPL
- In PPL color is very light-pale brown
- I estimate this mineral to be gedrite because of its lower birefringence, but can be confused with anthophyllite
- Simple and lamellar twins observed
- Cleavage also observed in 2 directions at 55 degrees and 120 degrees
- Pleochrosim is very weak in PPL
- High relief
- Crystal habit, appears fibrous
L5-D: Sample 2162-33
- The amphibole appears to have inclined
extinction
- It shows only one cleavage plane intersecting
at 90 degrees
- In XPL it appears orange to magenta/purple,
making it high 1st order/low 2nd order with
relatively low birefringence
- Not pleochroic
- In PPL it is very weakly colored
- The amphibole has lower relief than the
pyroxene
- This amphibole may be either cummingtonite-
grunerite or tremolite according to the
amphibole flow chart
- The pyroxene has more square shaped
cleavage with two intersecting at 90 degrees
- Low birefringence, in XPL low first order grey
to white
- The pyroxene has a higher relief than
amphibole
- Also shows inclined extinction
- This pyroxene may be clinopyroxene
(diopside)
L5-E: Sample 2162-5
Orthopyroxene:
- Two cleavage planes intersecting at 90 degrees
- Low interference colors
- Parallel extinction
- Lamellar twinning
Clinopyroxene:
- One cleavage plane at 90 degrees
- High interference colors, orange-yellow
- Inclined extinction
Olivine:
- Fractures
- High interference colors, 2nd order green
- Parallel extinction (along fractures)
- Colorless in PPL
Spinel:
- Black in XPL
- Brown in PPL
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Undulatory extinction (extinct in one place and not in another) Low interference, first order grey to white in xpl. Low interference, 1st order grey to white in xpl. L4-c: optical determination of the composition of plagioclase. Albite twinning, parallel (all the way through grain, pericline perpendicular twinning, not parallel) Low interference colors, first order grey to white in xpl. Low interference 1st order grey to white. Colorless in ppl low interference in xpl, 1st order inclined extinction: anywhere from 0-90 degrees parallel extinction: n-s symmetrical extinction: 45 degrees. The mineral is pale brown in ppl and has high interference colors (second order) in xpl. One cleavage plane is found intersecting at 88-90 degrees. Upon lining the cleavage plane n-s, i rotated the stage until extinction and noticed it to be inclined (calculated roughly from n-s aligned cleavage to extinction = 46 degrees). This mineral in xpl has low birefringence colors and 1st order grey to yellow.
