Intermediate contain mostly feldspar and some dark

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Intermediate Contain mostly feldspar and some dark minerals (usually amphibole), usually no quartz. Mafic Contain feldspar and abundant dark minerals, no quartz (therefore dark color). Ultra Mafic Contain almost entirely dark minerals such as pyroxene and olivine. No extrusive equivalents. Felsic Mafic Intermediate SiO 2 FeO+Fe 2 O 3 MgO+CaO Al 2 O 3 Na 2 O+K 2 O Silica Iron Magnesium+calcium Alumina Sodium+potassium Composition of Igneous Rocks Important Igneous Minerals Mineral Diagnostic Mineral Properties Composition Quartz Many colors (often dull, sometimes translucent), hard (scratches glass), conchoidal fracture (can break in smooth curves), no cleavage Felsic Mica Brown/translucent, soft (scratches with fingernail), 1 good cleavage (flakes off into thin sheets) Feldspar White/pink/tan, hard (scratches glass), good cleavage in 2 directions Amphibole/ Pyroxene Dark (black or greenish gray), hard (scratches glass), good cleavage Olivine Green (weathers to orange), hard (scratches glass), no cleavage Mafic Figure 6. Igneous rock-forming minerals and their properties. Figure 5. Average chemical compositions of mafic, intermediate, and felsic igneous rocks. Note that silica (SiO 2 ) is the dominant oxide in all three types of igneous rocks. 0% 100%
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Lab #2: Igneous Rocks 35 Figure 8. Igneous rocks identification chart, including important minerals. Light color (white/pink) Intermediate color (gray) Dark color (black) Green (from olivine) (can have black flecks) OBSIDIAN SCORIA increasing viscosity increasing silica content Special Textures PUMICE Vesicles (bubble holes) may or may not occur in any composition of volcanic (extrusive) rock. Typically vesicles are smaller in more felsic magma and larger in more mafic magma. Rhyolite Andesite Basalt No common extrusive equivalent Granite Diorite Gabbro Peridotite Dunite quartz feldspar mica amphibole feldspar amphibole pyroxene feldspar- pyroxene amphibole olivine olivine pyroxene olivine 75 wt% SiO 2 45 wt% SiO 2 Extrusive (fine grained or porphyritic) Intrusive (coarse grained) Minerals
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Lab #2: Igneous Rocks 36 Magma Viscosity As defined previously, viscosity is resistance to flow. A fluid with high viscosity resists flow and is sticky (flows like cold honey; Fig. 9). A fluid with low viscosity is runny (flows like water; Fig. 9). Viscosity determines: The shape of volcanoes Whether a volcanic eruption will be explosive or relatively quiet How easily a magma will rise through the crust to Earth’s surface. This can control where a magma might solidify (whether deep underground or on Earth’s surface). Gas bubbles form in magma as it rises. The gas bubbles are less dense (buoyant) than the magma, so they cluster at the top of the magma body. As the gas bubbles cluster, they enduce pressure build-up within the magma body, which pushes the magma upward. Therefore gases that can’t escape prior to an eruption increase the explosive potential of a volcano.
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