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Igneous Petrology (WWU) Cheat Sheet by

A quick guide to ID igneous minerals and rocks
geology     igneous     petrology     igneouspetrology     mineralogy     westernwashingtonuniversity     wwu

Common Igneous Minerals (Thin Section)

Amphibole (Hornblende)
Irregular fractures, pale yellow pleoch­roism, 1st order yellow to 2nd order pink birefr­ingence
Biotite
Light brown to yellow pleoch­roism, 2nd order yellow birefr­ingence
Clinop­yroxene (Augite)
Weakly pleoch­roic, pale green or brown pleoch­roism, 2nd order green to yellow birefr­ing­ence, inclined extinction
Olivine
High relief, high birefr­ing­ence, conchoidal fracture
Orthoclase (K-Fel­dspar)
No pleoch­roi­sm/­tra­nsl­ucent in plane light, 1st order gray to yellow birefr­ing­ence, can display twinning
Orthopyroxene (Enstatite)
Pink to green pleoch­roism, 1st order gray to purple birefr­ing­ence, parallel extinction
Plagio­clase
No pleoch­roism, 1st order gray to yellow birefr­ing­ence, twinning common and simple
Quartz
Transl­ucent in plane light, 1st order gray to yellow birefr­ing­ence, conchoidal fracture

Common Igneous Minerals (Hand sample)

Amphibole (Hornblende)
Black to brown, vitreous, elongate columnar, cleavage at 60 and 120
Biotite
Black to brown, vitreous, flaky sheets,
Clinopyroxene (Augite)
Green to black or pale green, stubby columnar, vitreous, stair-step cleavage
Olivine
Green, vitreous, granular texture, conchoidal fracture
Orthoclase (K-Feldspar)
Pink or white, tabular, vitreous, lamellae
Orthop­yroxene
Brown, shreddy texture, elongate columnar, resinous
Plagio­clase
White, vitreous, massive
Quartz
White to gray or transl­ucent, vitreous, massive granular texture, conchoidal fracture

QAPF Minerals

Alkali-feldspars
Feld­spa­tho­ids
K-Feldspar
Nepheline
Albite
Leucite
 

Rock Textures & Structures

Phaneritic
Grains are visible without assistance
Aphanetic
Grains seen with a micros­cope, but not glassy
Porphy­ritic
Large crystals surrounded by smaller ones
Phenocryst
A large crystal in comparison to others
Microc­rys­talline
A hand lens is needed to discern grains
Groundmass/Matrix
Surrou­nding crystals of similar size
Crypto­cry­sta­lline
Grains unable to be seen with a micros­cope, but not glassy
Holocr­yst­alline
Entirely made of formed crystals
Hypocr­yst­alline
Some crystals and some glass
Holohy­aline
Entirely glass
Cumulus
Euhedral (perfect) crystals surrounded by other material
Interc­umulus
Subhedral to anhedral crystals grown secondary to the crystals enclosed in its matrix
Vesicular
Rounded, hollow cavities in the rock (caused by gas bubbles typically)
Pandio­morphic granular
Most crystals formed have similar size and are euhedral in perfection
Hypidi­omo­rphic granular
Most crystals formed have similar size and are subhedral or anhedreal in perfection
Poikilitic
Phenoc­rysts surround other crystals entirely
Rapakivi
A corona effect of specif­ically plagio­clase surrou­nding K-spar
Pyrocl­astic
Fragments of crystals formed together
Tuffaceous
Mostly ash instead of crystals
Trachytic
Crystals are aligned parallel or sub-pa­rallel to one another
Welded
Any vesicles in the rock have been deformed to look cigar-­shaped
Amygdule
Caviti­es/­ves­icles in a rock that have been filled with minerals
Scoria­ceous
A highly vesicular rock, typically sharp where vesicles meet
Pumaceous
A highly vesicular rock with a lighter color
Devitr­ifi­cation
The process of delayed crysta­lli­zation of a glass
Flow banding
Wavy layers in a glassy rock; gives evidence of variable devitr­ifi­cation

How to Write Rock Descri­ptions

1. Rock name + color
2. Textures in the rock
Degree of crysta­llinity
Grain size
3. Mineral modes present
4. Mineral descri­ptions
Groundmass
Vesicles
Crystal perfection
Noteable mineral textures

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