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 MineralsAlkali-feldspars | Feldspathoids | K-Feldspar | Nepheline | Albite | Leucite |
| | Rock Textures & StructuresPhaneritic | 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 Descriptions1. 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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