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Rocks and Mineral Facts Kids


The Merriam-Webster online dictionary defines a mineral as a solid homogeneous crystalline chemical element or compound that results from the inorganic processes of nature. Rocks are more complex. Rocks are solid, inorganic materials that can be made up of one or more minerals. For example: quartz, feldspar and biotite are all minerals. Combined together, these individual minerals form a rock named granite. There are 3 categories scientists use to define rocks: igneous, sedimentary and metamorphic. Igneous rocks are formed by heat or fire. Rocks formed as lava from a volcano cools are igneous rocks. Sedimentary rocks were formed over centuries of time. As the igneous rock eroded over eons of time, these little particles, called sediment settled in the bottom of oceans and lakes. The pressure from the upper layers of sediment caused the lower layers to form rocks. Metamorphic rocks are rocks that have changed over time. They were once igneous or sedimentary but pressure and heat applied over an extensive period of time have caused a chemical reaction within the igneous or sedimentary rocks forming a new type of rock.

Common Minerals

Quartz is the most common element in the earth’s surface. It has a Mohs hardness rating of 7, and usually colorless and shiny.

Potassium is the 7th most abundant mineral in the earth’s crust. It is a highly reactive metal. In nature it is always found in combination with other elements.

Feldspar is a group of rock forming minerals. If all the different feldspars are counted together they make up 60% of the earth’s crust. The feldspars are hard minerals, most frequently clear, light orange or cloudy in color. Plagioclase is an commonly occurring feldspar.

Olivine while a very common mineral but usually in quantities too small to be detected without a microscope. Olivine comes in various shades of green, brow, yellow and white. It has been found in meteorites.

Pyroxene is a hard shiny black mineral frequently found in meteorites.

Amphibole is a hard black, brown or green mineral often shaped like a blade.

Mica is a soft shiny flat shaped mineral that is easily peeled into layer.

Calcite is a very soft, highly reactive mineral. Stalagmites that are seen hanging from cave walls are composed in part of calcite.

Common Igneous Rocks

Rhyolite is a buff to pink hued, fine textured silicate rock.

Granite is a rock that comes in many colors. It is a very hard rock, 7-8 on the Mohs scale. Most mountains are made out of granite.

Andesite is a common volcanic rock found in mountains along the Pacific Rim. Andesite is coarse in texture, pink and gray in color and has 7.4 on the Mohs hardness scale.

Dioriteis a coarse, dark colored rock that makes up much of the Sierra Nevada mountain range.

Basalt is a fine grained dark colored, hard rock. An excellent example of basalt is the Palisades of New Jersey.

Gabbro is a coarse, dark colored rock formed deep within the earth’s crust.

Peridotite is similar in structure to granite. It makes up by volume the majority of the earth’s mantle layer.

Obsidian is different than other igneous rock in that is it a type of volcanic glass. It is shiny with very shape edges.

Pumice is another form of volcanic glass formed as cooling lava is filled with gas bubbles.

Scoria is basalt that is filled with gas as it cools. It can be red, black or gray in color. Scoria is filled with holes of the same size and shape.

Common Sedimentary Rocks

Shale is formed as clay is compressed over a long period of time.

Sandstone is formed from compressed grains of various types of igneous rock.

Conglomerate is formed from large grains of sand and pebbles being compressed by water.

Limestone is formed over time from the compression of shells and skeletons of sea animals. It is found in the bottom of the ocean.

Tilliteis formed from unsorted sediment deposits left as the glaciers melted.

Common Metamorphic Rocks

Slate is formed of shale compressed deep underneath the earth’s surface.

Phyllite is formed by the combination of mica, quartz, and plagioclase igneous rocks.

Schist is form from both or either igneous and sedimentary rocks compressed in layers. Because of its layered structure it flakes easily.

Gneiss formed of large layers of rocks; granite quartz and pegmatite are among the most common rocks to make up gneiss.

Marble is formed from limestone compressed under high pressure.

Quartzite is metamorphosed sandstone.