Discover Rocks and Minerals: Properties and Classification

When you look at the ground beneath your feet, you are looking at Earth materials and two of the most important are rocks and minerals. Understanding these materials connects to what you already know about Natural Resources and how Earth's surface is made up of different substances.

A mineral is a naturally occurring solid found inside the Earth that has never been alive. A rock is a solid made of one or more minerals packed tightly together. For example, granite is a rock made of the minerals quartz, feldspar, and mica all combined.

Scientists use several special properties to identify minerals. You can use these same properties to tell minerals apart, even when two minerals look similar.

• Luster how light reflects off a mineral's surface (shiny, glassy, or dull)
• Streak the true color of a mineral's powder, found by rubbing it on a streak plate
• Hardness how difficult the mineral is to scratch, measured on the Mohs hardness scale
• Cleavage how a mineral breaks along smooth, flat surfaces
• Texture how the surface of a rock or mineral feels (smooth or grainy)
• Color the outer color, though this alone is not always reliable for identification

Because many minerals share the same color, scientists always use more than one property together to make an accurate identification. For example, if two minerals look the same color, you can test their hardness to tell them apart.

Scientists sort rocks into three main groups based on how they form. You can connect this to what you know about Formation Processes like erosion, deposition, and weathering.

Igneous Rock

Igneous rock forms when melted rock called magma underground or lava at the surface cools down and becomes solid. Granite and basalt are common examples. If you see a rock with large visible mineral grains, it likely cooled slowly underground, like granite.

Sedimentary Rock

Sedimentary rock forms when small pieces of material called sediment like sand, mud, or shells settle in layers and get pressed together over millions of years. Sandstone and limestone are examples. You can often spot sedimentary rock by its visible layers or fossils inside it. Rocks with fossils are always sedimentary because fossils are preserved in layered sediment.

Metamorphic Rock

Metamorphic rock forms when an existing rock is changed by great heat and pressure deep underground. Marble, for example, was once limestone before it was transformed. Mica is a mineral found in metamorphic rock that splits into thin, flat sheets a perfect example of cleavage.

Mineral: A mineral is a naturally occurring solid found in the Earth that was never alive and has a specific chemical makeup. Quartz is an example of a mineral you can find inside many types of rocks.

Rock: A rock is a solid made of one or more minerals packed tightly together. Granite is a rock made of quartz, feldspar, and mica.

Luster: Luster describes how light reflects off a mineral's surface. A mineral can look shiny, glassy, metallic, or dull depending on its luster.

Streak: Streak is the true color of a mineral's powder. You find it by rubbing the mineral across a white streak plate. The streak color can be very different from the mineral's outer color.

Hardness: Hardness tells you how resistant a mineral is to being scratched. Scientists measure hardness using the Mohs hardness scale, which goes from 1 (softest) to 10 (hardest). Talc is the softest mineral (1) and diamond is the hardest (10).

Mohs Hardness Scale: The Mohs hardness scale ranks minerals from softest to hardest on a scale of 1 to 10. If a fingernail can scratch a mineral, that mineral has a very low hardness rating.

Cleavage: Cleavage means a mineral breaks along smooth, flat surfaces in a predictable pattern because of its crystal structure. Mica is a great example it splits into thin, flat sheets cleanly.

Texture: Texture describes how the surface of a rock or mineral feels some are smooth, while others feel rough or grainy. Sandstone feels gritty because it is made of sand-sized particles.

Igneous Rock: Igneous rock forms when melted rock (magma or lava) cools and hardens into solid rock. Granite and basalt are common igneous rocks.

Sedimentary Rock: Sedimentary rock forms from layers of sediment that are pressed together over time. Sandstone and limestone are examples, and they often contain fossils.

Metamorphic Rock: Metamorphic rock forms when an existing rock is changed by heat and pressure deep underground. Marble (changed from limestone) and slate are examples.

Magma: Magma is melted rock found underground. When it reaches the surface, it is called lava.

Sediment: Sediment is tiny pieces of rock, shells, or plant material that settle in layers. Over millions of years, sediment can harden into sedimentary rock.

Fossil: A fossil is the preserved remains or traces of an ancient living thing found inside sedimentary rock. Fossils form when organisms are buried in sediment that slowly hardens.

Density: Density, also called specific gravity, describes how heavy a mineral feels for its size. It is a property scientists use to help identify minerals.

You can practice identifying minerals by testing their properties. Try scratching a mineral with your fingernail if it leaves a mark, the mineral is softer than your nail. You can also observe how a mineral's surface reflects light to determine its luster.

Sorting a rock collection by how each rock formed is a great way to practice classification. This skill connects to Measurement and Data Analysis, where you record observations and look for patterns in your results.

You have already learned about Properties of Solids including shape, volume, and structure which gives you a strong foundation for understanding mineral properties like hardness and cleavage.

Your knowledge of Weather Impact and Effects on Earth's Surface helps you understand how rocks can be broken down over time. You also know about Natural Resources, which shows you why rocks and minerals are so valuable to people and the environment.

Rocks and minerals connect to many other important topics in Earth science. Here is how they all fit together in your learning journey:

• Soil Composition You will discover that soil is partly made from broken-down rocks and minerals, so understanding rocks helps you understand soil.
• Formation Processes: Erosion, Deposition, and Weathering You will learn how wind, water, and ice break rocks apart and move sediment, which is how sedimentary rock begins to form.
• Types of Landforms: Mountains, Valleys, and Plains You will see how different rock types shape the landforms you can observe on Earth's surface.
• Changes Over Time: Rapid and Slow Changes You will explore how rocks change slowly over millions of years through the rock cycle.
• Resource Use and Effects on the Environment You will think about how humans use rocks and minerals as resources and what impact that has on Earth.
• Internal Structure: Layers of the Earth Next, you will go deeper and learn about the layers inside Earth where magma forms and metamorphic rocks are created.
• Surface Features: Mountains, Valleys, and Oceans You will connect rock types to the major features you see on Earth's surface.
• Physical Properties: Mass, Volume, and Density You will build on what you know about mineral density and explore these properties in more depth.
• Chemical Properties: Reactivity, pH, and Combustibility You will learn how minerals can also be identified and studied through their chemical properties.
• Materials Science: Properties and Applications You will explore how the properties of rocks and minerals make them useful in everyday materials and technology.