Igneous Rock Identification

Granite

Composition: Felsic
Texture: Phaneritic
Formation Process: Intrusive
Mineralogy: Quartz, Plagioclase, Orthoclase, Hornblende, Biotite, Muscovite
Other: Major igneous, continental crust rock, has black shiny specks

Diorite

Composition: Intermediate
Texture: Phaneritic
Formation Process: Intrusive
Mineralogy: Plagioclase, Hornblende, Biotite, Minor Quartz
Other: Has small sprinkle-like black lines

Gabbro

Composition: Mafic
Texture: Phaneritic
Formation Process: Intrusive
Mineralogy: Plagioclase, Pyroxene, Olivine, Hornblende
Other: Reflective

Peridotite

Composition: Ultramafic
Texture: Phaneritic
Formation Process: Intrusive
Mineralogy: Olivine, Pyroxene
Other: Intrusive but may exhibit relatively fine grains, green hue

Pumice

Composition: Felsic - Intermediate
Texture: Vesicular
Formation Process: Extrusive
Mineralogy: Quartz, Feldspars
Other: Lower specific gravity than water, floats

Rhyolite

Composition: Felsic
Texture: Aphanitic
Formation Process: Extrusive
Mineralogy: Quartz, Plagioclase, Orthoclase, (Hornblende, Biotite)
Other: A little light, not too heavy

Obsidian

Composition: Felsic
Texture: Glassy
Formation Process: Extrusive
Mineralogy: Mostly SiO2 (glass)
Other: Fe and Mg impurities give the dark color

Tuff

Composition: Felsic - Intermediate
Texture: Pyroclastic
Formation Process: Extrusive
Mineralogy: Varies; Orthoclase, Quartz, Plagioclase
Other: Dull, can see different minerals, jumbled texture associated with explosive volcanism

Andesite

Composition: Intermediate
Texture: Aphanitic
Formation Process: Extrusive
Mineralogy: Plagioclase, Hornblende, Biotite, Pyroxene
Other: Porphyritic

Basalt

Composition: Mafic
Texture: Aphanitic
Formation Process: Extrusive
Mineralogy: Plagioclase, Pyroxene, Olivine, Hornblende
Other: Porphyritic Basalt

Scoria

Composition: Intermediate - Mafic
Texture: Vesicular
Formation Process: Extrusive
Mineralogy: Pyroxene, Olivine, Plagioclase, Hornblende
Other: Low specific gravity