Some rocks are defined by their vesicles scoria

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viscous magma because they can coalesce more easily. Some rocks are defined by their vesicles: Scoria (larger, less abundant bubbles in reddish-black rock, called a scoreacous texture . See Fig. 4) Pumice (a lot of little bubbles, almost foamy and very light weight, called a pumiceous texture ). Pumice often has enough bubble space that it will float in water. II. Igneous Compositions Color is a quick way to estimate the chemical composition of most (but not all) igneous rocks. The color of most igneous rocks is controlled by the types of minerals present. To generalize, a darker color commonly indicates that the rock is composed of minerals with a higher iron/magnesium content. A lighter color indicates the rock contains minerals with high silica (SiO 2 , mineral name quartz ) and low iron/magnesium content. Dark colors (such as black obsidian, or nearly black andesite) may indicate the lack of crystals. In these rocks, the black color is a result of the light being absorbed by glass rather than being reflected back to our eyes form mineral grains. Most magmas can be grouped into three broad categories based on their chemical composition. These categories are mafic ( less silica, more Mg and Fe), intermediate , and felsic ( more silica, less Mg and Fe). In geology, rock chemical composition is presented in oxide form. For instance, Si contents would be listed as SiO 2 . The average chemical compositions of felsic, intermediate, and mafic igneous rocks are shown in Fig. 5 on the next page. This graph reveals that most igneous rocks are composed of similar elements in different amounts. Note that silica is the most abundant in all three compositional groups, with content ranging from approximately 50% to 70%. The silica content of igneous magmas is very important because it influences the viscosity of the magma, which determines the behavior of the magma/lava. Figure 2. Obsidian showing glassy texture and conchoidal fracture. Figure 4. Scoria (scoriaceous texture). Figure 3. Welded tuff.
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Lab #2: Igneous Rocks 34 Igneous Rock-Forming Minerals The minerals present in an igneous rock indicates the rock’s chemical composition. Thankfully, there is a correlation that determines the kinds of minerals that will occur together in a rock. This correlation is determined by the magma’s chemical composition, as well as the pressure and temperature of crystallization (Fig. 6-7). You will need to be able to identify the igneous rock-forming minerals. Luckily, there are only eight common igneous rock-forming minerals, which have been grouped into 4 broad compositional categories listed in Fig. 7 above. For additional information on igneous rock classification refer to the Igneous Rocks and Associated Minerals diagram (Fig. 8). Figure 7. Characteristic minerals for each compositional group. Composition Mineral Content Felsic Must contain feldspar and quartz, very minor dark minerals (therefore light in color).
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