Igneous Rock Identification Practice

Texture of Granite

phaneritic

Composition of granite

felsic

Minerals in granite

quartz, potassium feldspar

Distinguishing properties of granite

All grains relatively the same size; slightly pink;

Texture of diorite

phaneritic

Composition of diorite

intermediate

Minerals in diorite

amphibole and plagioclase feldspar

distinguishing properties of diorite

Darker version of granite; whiter appearance; large grains

Texture of peridotite

phaneritic

composition of peridotite

ultramafic

minerals of peridotite

olivine and pyroxene

Distinguishing properties of peridotite

Has a green hue in light; extremely dark; large grains

Texture of gabbro

phaneritic

Composition of gabbro

mafic

Minerals of gabbro

Pyroxene and plagioclase feldspar

Distinguishing properties of gabbro

Darker; single large grains

Texture of rhyolite

aphanitic

composition of rhyolite

felsic

Minerals of rhyolite

Quartz and potassium feldspar

Distinguishing properties of rhyolite

Only straight pink mineral; smooth; fine-grained

Texture of andesite

aphanitic

Composition of andesite

intermediate

Minerals of andesite

Amphibole and plagioclase feldspar

Distinguishing properties of andesite

grey and/or brown, smooth; fine grained

Texture of basalt

aphanitic

Composition of basalt

mafic

minerals of basalt

pyroxene and plagioclase feldspar

Distinguishing properties of basalt

Almost black(dark grey); smooth; fine-grained

Texture of granite porphyry

porphyritic

Composition of granite porphyry

felsic

Minerals of granite porphyry

Quartz and potassium feldspar

Distinguishing properties of granite porphyry

two very distinct grain sizes; pinkish coloring

Texture of andesite porphyry

Porphyritic

Composition of andesite porphyry

Intermediate

Minerals of andesite porphyry

amphibole and plagioclase feldspar

Distinguishing properties of andesite porphyry

No pink; two distinct grain sizes

Texture of basalt porphyry

Porphyritic

Composition of basalt porphyry

mafic

Minerals of basalt porphyry

Pyroxene and plagioclase feldspar

Distinguishing properties of basalt porphyry

Dark with some lighter minerals; two distinct grain sizes

Texture of obsidian

glassy

Composition of obsidian

felsic

Minerals of obsidian

quartz and potassium feldspar

Distinguishing properties of obsidian

Black; looks like glass

Texture of scoria

vesicular

Composition of scoria

Intermediate

Minerals of scoria

Pyroxene and plagioclase feldspar

Distinguishing properties of scoria

Rough like pumice; rusty to grey looking; light weight

Texture of pumice

vesicular

Composition of pumice

felsic

Minerals of pumice

quartz and potassium feldspar

Distinguishing properties of pumice

Very rough; light grey; light weight

Texture of tuff

pyroclastic

Composition of tuff

Felsic

Minerals of tuff

Quartz and potassium feldspar

Distinguishing properties of tuff

light to heavy; light color; like pumice but with no pores

Texture of volcanic breccia

Pyroclastic

Composition of volcanic breccia

Mafic

Minerals of volcanic breccia

Pyroxene and plagioclase feldspar

Distinguishing properties of volcanic beccia

Looks like chunks of rocks; color varies

Granite

Diorite

peridotite

Gabbro

rhyolite

Andesite

Basalt

Granite porphyry

Andesite porphyry

Basalt porphyry

obsidian

scoria

pumice

Tuff

volcanic breccia