A quick guide to ID igneous minerals and rocks
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Amphibole (Hornblende)
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Irregular fractures, pale yellow pleochroism, 1st order yellow to 2nd order pink birefringence
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Biotite
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Light brown to yellow pleochroism, 2nd order yellow birefringence
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Clinopyroxene (Augite)
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Weakly pleochroic, pale green or brown pleochroism, 2nd order green to yellow birefringence, inclined extinction
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Olivine
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High relief, high birefringence, conchoidal fracture
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Orthoclase (K-Feldspar)
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No pleochroism/translucent in plane light, 1st order gray to yellow birefringence, can display twinning
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Orthopyroxene (Enstatite)
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Pink to green pleochroism, 1st order gray to purple birefringence, parallel extinction
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Plagioclase
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No pleochroism, 1st order gray to yellow birefringence, twinning common and simple
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Quartz
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Translucent in plane light, 1st order gray to yellow birefringence, conchoidal fracture
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Amphibole (Hornblende)
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Black to brown, vitreous, elongate columnar, cleavage at 60 and 120
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Biotite
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Black to brown, vitreous, flaky sheets,
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Clinopyroxene (Augite)
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Green to black or pale green, stubby columnar, vitreous, stair-step cleavage
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Olivine
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Green, vitreous, granular texture, conchoidal fracture
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Orthoclase (K-Feldspar)
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Pink or white, tabular, vitreous, lamellae
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Orthopyroxene
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Brown, shreddy texture, elongate columnar, resinous
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Plagioclase
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White, vitreous, massive
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Quartz
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White to gray or translucent, vitreous, massive granular texture, conchoidal fracture
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Alkali-feldspars
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Feldspathoids
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K-Feldspar
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Nepheline
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Albite
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Leucite
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Phaneritic
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Grains are visible without assistance
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Aphanetic
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Grains seen with a microscope, but not glassy
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Porphyritic
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Large crystals surrounded by smaller ones
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Phenocryst
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A large crystal in comparison to others
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Microcrystalline
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A hand lens is needed to discern grains
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Groundmass/Matrix
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Surrounding crystals of similar size
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Cryptocrystalline
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Grains unable to be seen with a microscope, but not glassy
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Holocrystalline
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Entirely made of formed crystals
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Hypocrystalline
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Some crystals and some glass
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Holohyaline
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Entirely glass
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Cumulus
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Euhedral (perfect) crystals surrounded by other material
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Intercumulus
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Subhedral to anhedral crystals grown secondary to the crystals enclosed in its matrix
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Vesicular
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Rounded, hollow cavities in the rock (caused by gas bubbles typically)
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Pandiomorphic granular
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Most crystals formed have similar size and are euhedral in perfection
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Hypidiomorphic granular
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Most crystals formed have similar size and are subhedral or anhedreal in perfection
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Poikilitic
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Phenocrysts surround other crystals entirely
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Rapakivi
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A corona effect of specifically plagioclase surrounding K-spar
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Pyroclastic
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Fragments of crystals formed together
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Tuffaceous
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Mostly ash instead of crystals
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Trachytic
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Crystals are aligned parallel or sub-parallel to one another
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Welded
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Any vesicles in the rock have been deformed to look cigar-shaped
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Amygdule
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Cavities/vesicles in a rock that have been filled with minerals
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Scoriaceous
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A highly vesicular rock, typically sharp where vesicles meet
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Pumaceous
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A highly vesicular rock with a lighter color
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Devitrification
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The process of delayed crystallization of a glass
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Flow banding
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Wavy layers in a glassy rock; gives evidence of variable devitrification
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1. Rock name + color
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2. Textures in the rock
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Degree of crystallinity
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Grain size
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3. Mineral modes present
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4. Mineral descriptions
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Groundmass
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Vesicles
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Crystal perfection
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Noteable mineral textures
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