Igneous Rock Classification Lab What is a rock? rocks are composed of minerals, mineraloid or organic materials Granite (composed of minerals) Opal (composed of mineraloids) Coal (composed of organics) Elements Minerals Rocks

Igneous Rock Classification Lab Let’s form an igneous rock Two major types of igneous rocks Intrusive – magma solidifies below the Earth’s surface magma cools very slow crystals form interlocking “mosaic” textures very coarse-grained visible minerals referred to as Plutonic rocks Extrusive – magma solidifies above the Earth’s surface magma cools very fast minerals can not be seen with un-aided eye very fine-grained texture (no visible minerals referred to as Volcanic rocks Extrusive rocks Intrusive rocks

Igneous Rock Classification Lab Identification of Igneous rocks is based on two main characteristics Texture – the appearance of the rock due to the rate of magma cooling Composition – the type of minerals found in the rock (mineral composition) Textures of igneous rocks Intrusive rocks (Textural terms) phaneritic texture – crystals are visible and form a mosaic of interlocking mineral aggregates (less than 1 cm) Interlocking crystal grains phaneritic texture

Igneous Rock Classification Lab pegmatitic texture – crystals are very large (>1cm) Large orthoclase and plagioclase minerals Extrusive Igneous Rocks (textural terms) aphanitic texture – crystals are too small to see See, you really can not see any mineral crystals

Igneous Rock Classification Lab porphyritic texture – crystals can be separated into two distinct visible sizes. There can be small grains or large grains, but crystals appear in 2-distinct sizes Phenocrysts Groundmass (matrix)

Igneous Rock Classification Lab Vesicular texture- sponge like appearance, texture contains numerous cavities or holes Vesicles – gas bubbles cavities Pyroclastic texture – textures created by rapidly cooling lava that is “hurled” through the air picking up fragments (tuffaceous texture) rock fragments – pieces of rock “incorporated” into the rock (tuffaceous)

Intrusive Extrusive Igneous Rock Textures Rock cooled slowly Rock cooled rapidly Texture types: phanartitic pegmatitic Texture types: aphanitic porphyritic vesicular pyroclastic tuffaceous

Igneous Rock Classification Lab Igneous Rock Composition mineral composition = mineral assemblages= chemistry The mineral is either ferromagnesian (dark colored) or felsic (light colored ferromagnesian (mafic) minerals rich in Fe, Mg – creates a dark colored rocks Pyroxene (Augite) Amphibole (hornblende) Mica - Biotite

Igneous Rock Classification Lab Igneous Rock Composition Mineral composition = Mineral assemblages = Chemistry The mineral is either ferromagnesian (dark colored) or felsic (light colored) Felsic – mineral composition is light colored minerals Ca-plagioclase K-feldspar (orthoclase) Mica- muscovite Quartz

Texture Origin Intrusive Extrusive X Igneous Rock Classification Felsic Mafic 80% Origin Texture 40% 20% Rock Descriptions Very coarse crystal grains, usually all grains are approximately the same size w/ respect to each other. Grains> one inch Coarse Very Granite Pegmatite Mafic Pegmatite Intrusive A mosaic of coarse-grains that are easily visible to the unaided eye – grains greater than 1 mm Phane- ritic Grano- diorite Diorite Gabbro Peridotite Granite Aphan- itic Fine to very fine grains, usually to small to be distinguished with the unaided eye or even with a hand lens Rhyolite Dacite X Andesite Basalt Glassy Glassy – non-crystalline, non-granular Color varies between black and brown Obsidian Extrusive Por- ous Highly vesicular, finely crystalline to glassy Pumice (light), Scoria (dark) Pumice Scoria Composed of fragments of all sizes Produced in volcanic environments, Ash Partially re-fused or cemented Rhyolitic Tuff Basaltic Tuff mental Frag-

Igneous Rock Classification Lab Let’s use Texture and Composition to identify igneous rocks Steps for identifying igneous rocks Identify the texture (phaeritic/aphanitic) If the rock is phaneritic, estimate the % of felsic and mafic minerals High mafic = dark colored rock High felsic = light colored rock 50% felsic, 50% mafic = intermediate color Use your igneous rock schematic and cross reference the texture and composition (light, intermediate, dark) and name the rock. Add modifying terms (porphyritic, vesicular ect…….)