Three major mineralogical criteria are used to classify igneous rocks: 1. Presence or absence of quartz. Quartz is an essential mineral in silicic rocks (rocks high in silica [Si]) and an accessory mineral in intermediate or mafic rocks—rocks high in magnesium (Mg) and iron (Fe). (In the classification of igneous rocks, an “essential mineral” is a mineral on which the rock classification is based. An accessory mineral occurs in minor amounts and is not a factor in classification.) 2. Composition of the feldspars. Potassium feldspars and sodium plagioclase are essential minerals in silicic rocks but are rare or absent in intermediate and mafic rocks. Calcium plagioclase is characteristic of mafic rocks. 3. Proportion and kinds of ferromagnesian minerals. As a general rule, mafic rocks are rich in ferromagnesian minerals, whereas silicic rocks are rich in quartz. Olivine is generally restricted to mafic rocks. Pyroxenes and amphiboles are present in mafic to intermediate rocks. Biotite is common in intermediate and silicic rocks. The graph at the top of Figure 3.8 is not as complex as it may first appear. It shows the range in composition of the major igneous rock types. Each color zone represents a rock family (yellow—the granite-rhyolite family; yellow green—the diorite-andesite family; green—the gabbro-basalt family; and dark green—the peridotite family). The area on the graph allotted to each mineral represents the percentage of the rock composed of that mineral. For example, peridotite may be composed entirely of olivine, but pyroxene can occur in amounts up to 30% and Ca-plagioclase in amounts up to 15%. In the granite-rhyolite family, K-feldspar may form over 25% of the rock (the extreme left side of the graph). Note how the percentage of K-feldspar decreases as quartz increases; at the right margin of the granite-rhyolite zone, both quartz and K-feldspar decrease and plagioclase content increases. OTHER IGNEOUS ROCKS Note that some types of igneous rock do not fit well in the classification system used in Figure 3.8. The most important exceptions are described here. Obsidian. A massive volcanic glass, usually jet-black due to the presence of dust-like particles of magnetite and ferromagnesian minerals. Most obsidian is rich in silica and has a chemical composition similar to that of granite and rhyolite. Pumice. A porous volcanic glass with a texture consisting of subparallel glass fibers tangled together. Tuff and Volcanic Breccia. A volcanic rock of pyroclastic texture, formed from consolidated volcanic ash. Tuff is fine grained, similar to fine sand or mud. Breccia is coarse grained, like a gravel. The composition of pyroclastic rocks is variable. Ash-Flow Tuff. If the grains of ash, pumice, and crystal fragments in a pyroclastic rock are fused together, the term ash-flow tuff is used. 7 From Figure 3.8 we can make the following summary of the composition of the igneous rock families: The Granite-Rhyolite Family. The granite-rhyolite family (Figure 3.9) is characterized by the following mineral composition:  Quartz 10-40%  Potassium feldspar 15-30%  Plagioclase (high Na) 0-33%  Biotite and amphibole 8—15% These rocks are commonly referred to as silicic. The magmas that produce them are high in potassium, silicon, and sodium and are low in iron, magnesium, and calcium. Granites and rhyolites are therefore characteristically light colored. The Diorite-Andesite Family. The diorite-andesite family (Figure 3.10) is intermediate in composition between the granite-rhyolite and gabbro-basalt families. It is characterized by the following composition:  Plagioclase 55-70%  Amphibole and biotite 15-40% Plagioclase is approximately 50% albite and 50% anorthite. Potassium feldspar and quartz are present in minor amounts only. The diorite-andesite family is therefore characteristically gray in color. The Gabbro-Basalt Family. The gabbro-basalt family (Figure 3.11) has the following composition:  Plagioclase (high Ca) 25-70%  Ferromagnesian minerals (olivine, pyroxene, and amphibole) 25-75% These rocks crystallize from magmas that are relatively high in iron, magnesium, and calcium, but deficient in silica. Rock coloration is characteristically black or dark green. 8 The Peridotite Family. The peridotite family (Figure 3.12) is characterized by the following mineral composition:  Olivine 65-100%  Pyroxene 0-25%  Ca-plagioclase 0-5%  Ore minerals (i.e., magnetite, ilmenite, chromite) 0-10% Hints on Mineral Identification in Hand Specimens Identifying the major rock types in Earth’s crust may be difficult for you at first because the most obvious properties we use to distinguish most things in the physical world (color, size, shape, and form) are not important in identifying rock type. Indeed, the specific color, size, and shape of a given rock have little significance. The rock properties that permit us to discriminate between the various igneous rock types are mineral composition and texture. In a way, identifying rocks is like identifying woven fabric of cloth. The color and shape of a piece of fabric are insignificant. The composition of its threads—their size and the way they are woven—determines the nature of the fabric. It is much the same with rocks. A specimen’s color, size, and shape are insignificant properties. Mineral composition and texture are the properties that distinguish one rock type from another and provide information about their genesis. A student of geology, then, must learn to ignore a rock specimen’s more obvious features—specific color, shape, and size— and to concentrate instead on composition and texture. The following is a brief summary of the characteristics of the important minerals in igneous rocks. Quartz. Occurs as irregular, glassy grains, commonly clear to smoky appearance; it has no cleavage. (Figure 3.9C) Muscovite. Brass-colored flakes associated with quartz or K-feldspar. Perfect cleavage in one direction. K-Feldspar. Porcelain luster. Commonly colored pink, white, or gray. Cleavage in two directions at right angles may be detected by a reflection of light when specimen is rotated. (Figure 3.9C) Plagioclase. Usually gray or white in granite, dark-bluish color in gabbro. Striations common. Two cleavage directions at right angles may be detected. (Figure 3.9B) Biotite. Small black flakes. Perfect cleavage in one direction. Reflects light. (Figure 3-9C) Amphibole. Long, black crystals in a light-colored matrix. Cleavage at 60 and 120 degrees. (Figures 3.10C and 3.11B) Pyroxene. Short, dull, greenish-black minerals in darker rocks. Cleavage in two directions at 90 degrees. (Figure 3.11C) Olivine. Glassy, light-green grains. (Figure 3.12A) PHOTOGRAPHS OF ROCK SPECIMENS The photographs on the following pages illustrate the characteristics of the major igneous rock types as seen in hand specimens and under the microscope. The hand specimens are actual size. Some show a polished surface and others a natural fractured surface. All microscopic views show a magnification of approximately 20 times actual size. Remember, the photographs are not a key to rock identification. Use them as a visual reference only for examples. Note that the specimens in Figures 3.9-3.12 are arranged in families corresponding to the classification chart in Figure 3.8. Note that in the microscopic view, the colors of minerals are variable. Pyroxene may be yellow, green, or brown. Quartz may appear colorless, blue, or yellow. This is because the photomicrograph was taken under polarized light—a technique used extensively in advanced mineralogical studies. 9 GRANITE-RHYOLITE FAMILY 10 DIORITE-ANDESITE FAMILY 11 BASALT-GABBRO FAMILY 12 ULTRAMAFIC ROCKS 13 OTHER IGNEOUS ROCKS 14

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