A rock is a solid mass of geological materials. Geological materials include individual mineral crystals, inorganic non-mineral solids like glass, pieces broken from other rocks, and even fossils. The geological materials in rocks may be inorganic, but they can also include organic materials such as the partially decomposed plant matter preserved in coal. A rock can be composed of only one type of geological material or mineral, but many are composed of several types (Figure 6.2).
Concept Check: Rock or Mineral?
Whether we refer to something as a rock or mineral may depend on context. Here is a story to illustrate. Fill in each blank space with either rock or mineral.
Emily went for a walk and found two interesting geological specimens. One was white and the other was multi-coloured. After investigating, she learned that the white was quartzite, and made entirely of crystals of the quartz. Even though it was quartzite, she added it to her collection as an example of quartz because it had the same physical properties.
Emily learned that the multi-coloured was granite. Granite contains quartz crystals, but it also contains crystals of the potassium feldspar and the biotite.
One day Emily’s friend Liz came for a visit because she heard that Emily could show her what quartz looked like. Unfortunately, Emily had put her specimen collection with quartz samples in a safe place, so try as she might, she couldn’t find it. Instead, she showed Liz the granite and pointed out which crystals within the granite were the quartz.
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Three Main Types of Rock
Rocks are grouped into three main categories based on how they form. Igneous rocks form when melted rock cools and solidifies. Sedimentary rocks form when fragments of other rocks are buried, compressed, and cemented together; or when minerals precipitate from solution, either directly or with the help of an organism. Metamorphic rocks form when heat and pressure alter a pre-existing rock. Although temperatures can be very high, metamorphism does not involve melting of the rock.
Do You Know Your Rock Types?