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(Solved): 3(a) Identify the rock's mafic colour index (MCI) as shown in the top of figure 5.5.3(b) The sample ...
3(b) The sample has pink coloured minerals, which are most likely….
3(c) The dark minerals are most likely….
3(d) Describe the texture of the rock by using figure 5.4.
3(e) Determine the name of the rock, using the flowchart in figure 5.4 and the expanded classification chart in figure 5.5.
\r\nIGNEOUS ROCK ANALYSIS AND CLASSIFICATION STEP 1 \\& 2: \\( \\mathrm{MCl} \\) and Mineral Composition Quartz hard, transparent, gray, crystals with no cleavage Plagioclase Feldspar hard, opaque, usually pale gray to white crystals with cleavage, often striated Potassium Feldspar hard, opaque, usually pastel orange, pink, or white crystals with exsolution lamellae Muscovite Mica flat, pale brown, yellow, or colorless, crystals that scratch easily and split into sheets Biotite Mica flat, glossy black crystals that scratch easily and split into sheets Amphibole hard, dark gray to black, brittle crystals with two cleavages that intersect at 56 and 124 degrees Pyroxene (augite) hard, dark green to green-gray crystals with two cleavages that intersect at nearly right angles Olivine (gemstone peridot) hard, transparent to opaque, pale yellow-green to dark green crystals with no cleavage STEP 3: Texture Pegmatitic mostly crystals larger than \\( 1 \\mathrm{~mm} \\) : very slow cooling of magma Phaneritic crystals about 1-10 mm, can be identified with a hand lens: slow cooling of magma Porphyritic large and small crystals: slow, then rapid cooling and/or change in magma viscosity or composition Aphanitic crystals too small to identify with the naked eye or a hand lens; rapid cooling of lava Glassy rapid cooling and/or very poor nucleation Vesicular like meringue: rapid cooling of gas-charged lava Vesicular some bubbles: gas bubbles in lava Pyroclastic or Fragmental: particles emitted from volcanoes STEP 4: Igneous Rock Classification Flowchart Texture is pegmatitic or phaneritic Feldspar \\( > \\) mafic minerals K-spar \\( > \\) Plagioclase quartz present. K-spar < Plagioclase no quartz GRANITE \\( ^{1,2} \\) SYENITE \\( ^{1,2} \\) DIORITE \\( ^{1,2} \\) Feldspar \\( < \\) mafic minerals \\( \\mathrm{MCl}=45-85 \\) GABBRO \\( { }^{1,2} \\) \ felsic minerals \\( ) \\) PERIDOTITE felsic \\( (\\mathrm{MCl}=0-15) \\) and/or pink, white, or pale brown. RHYOLITE 2,3 Also refer to Figure 5.2 Texture is intermediate \\( (\\mathrm{MCl}=15-45) \\) and/or green to gray. ANDESITE \\( ^{2,3} \\) mafic \\( (\\mathrm{MCl} \\geq 45) \\) and/or dark gray to black BASALT \\( ^{2,3} \\) vesicular mafic with abundant vesicles (resembles a sponge) SCORIA intermediate or felsic with abundant tiny vesicles - like meringue, floats in water... PUMICE OBSIDIAN fragments \\( \\leq 2 \\mathrm{~mm} \\) TUFF fragments \\( >2 \\mathrm{~mm} \\) VOLCANIC BRECCIA \\( { }^{1} \\) Add pegmatite to end of name if crystals are \\( >1 \\mathrm{~cm} \\) (e.g., granite-pegmatite). \\( { }^{2} \\) Add porphyritic to front of name when present (e.g., porphyritic granite, porphyritic ihyolite). \"Add vesicular to front of name when present (e.g., vesicular basalt). FIGURE 5.4 Igneous rock analysis and classification. Step 1-Estimate the rock's mafic color index (MCl). Step 2-Identify the main rock-forming minerals if the mineral crystals are large enough to do so, and estimate the relative abundance of each mineral (using a Visual Estimation of Percent chart from GeoTools Sheet 1 or 2). Step 3-Identify the texture(s) of the rock. Step 4-Use the Igneous Rock Classification Flowchart to name the rock. Start on the left side of the flowchart, and work toward the right side to the rock name.\r\nrock (Step 4). Also refer to FIGURE 5.4 and the examples of classified igneous rocks in FIGURES 5.8-5.14.