Granite is an igneous rock typically composed of the

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Granite is an igneous rock typically composed of the minerals quartz, feldspar, and mica. Schist is a metamorphic rock typically composed of the minerals quartz, feldspar, and mica. Shale is a sedimentary rock typically composed of clay minerals. Coal is another sedimentary rock composed entirely of carbon that was derived from plant material. Identifying Rocks I. Texture Hint : For most of us, the texture means the feel or appearance of the surface of an object; whether it has a rough or smooth surface. In geology, the texture of a rock refers to the characteristics of the material that makes up the rock, as opposed to the feel of the outer surface. Rock texture is the size of the minerals (or fragments), their shape, and how they are stuck together. The texture helps determine the origin of the rock (Fig. 2). The texture of the rock is perhaps the most important tool used to determine whether the origin of a rock is igneous, sedimentary or metamorphic. The texture reflects the geologic processes involved in the formation of the rock. Key rock-identifying observations: 1. Size of the mineral (or fragment) constituents that make up the rock Can you see the minerals/grains with your naked eye, or do you need a microscope? Are particles all the same size or a mixture of different sizes? 2. Arrangement of mineral grains is a property that you will eventually use to tell if the rock is igneous, sedimentary, or metamorphic in origin. Are the minerals intergrown together? Are individual particles cemented together? Are there holes in the rock from the escape of gas bubbles? Does the rock have a squashed look? Do the minerals in the rock appear to have a preferred alignment? 3. Shape of the particles that make up the rock not the shape of the hand sample Are the particles that make up the rock angular or rounded? Are they well-formed crystals or rounded fragments? *A mineral can have both cleavage and fracture , but some only have fracture.
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Rock Cycle 29 Observation Texture Interpretation Particles cemented together Clastic Particles transported, deposited, and stuck together (Fig. 3) Parallel alignment of minerals Foliated Growth of minerals in preferred orientation due to pressure conditions (Fig. 4) Interlocking crystals Crystalline Minerals grown together from magma crystallization (Fig. 5) Different sized crystals in a Porphyritic Different cooling rates (associated with extrusive igneous groundmass rocks; Fig. 6) Igneous, Sedimentary, and Metamorphic Textures Careful examination of rock texture places most (not all) rocks into one of three categories that depend on how they formed Particles cemented together Common texture: clastic Interlocking crystals Common texture: crystalline Banded minerals Common texture: foliated Lithification of sediment produced rock Rock crystallized from magma Rock subjected to increased pressure & temperature causing parallel alignment of minerals Igneous Sedimentary Metamorphic Figure 2. Rock identification based on texture.
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Lab #2: Igneous Rocks 30 II. Composition Rock c omposition
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