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Earth Science Concepts

It's All About: Minerals

Azurite Gypsum Sphalerite Gypsum Flower Fluorite
Learning about rocks and minerals gives students a deeper appreciation of the story behind the scenery in our national parks.

Many minerals are coveted around the world for their striking beauty, rarity, and gem quality. But what is a mineral?

A mineral is a solid, naturally occurring, inorganic substance that generally forms crystals. They are the building blocks of rocks and are described based on their physical properties which include: luster, cleavage, streak, hardness, color, and specific gravity.

To many, the National Park System is one of America's favorite mineral collections which can be viewed in the various rock formations and features around the country.

Natural objects, such as rocks and minerals, contribute to the beauty and wonderment of the National Parks and should be left, as they were found, so that others can experience a sense of discovery.

Fundamental Concepts

  • Cleavage: The tendency of a mineral to break along weak planes.
  • Color: Most minerals have a distinctive color while others are variable in color.
  • Hardness: A measure of a mineral's resistance to scratching. This is measured by scratching it against another substance of known hardness on the Mohs Hardness Scale.
  • Luster: The reflection of light from the surface of a mineral, described by its quality and intensity. Luster is described as metallic, glassy, dull, earthy, etc.
  • Nonsilicate minerals: A mineral without silicon (Si).
  • Silicate: Refers to the chemical unit silicon tetroxide, SiO4, the fundamental building block of silicate minerals. Silicate minerals make up most rocks we see at the Earth's surface.
  • Streak: Streak refers to the color of the residue left by scratching a mineral on a tile of unglazed porcelain.

Common Minerals

  • Quartz
  • Potassium Feldspar
  • Plagioclase Feldspar
  • Mica
  • Amphibole
  • Olivine
  • Calcite
  • Talc
  • Fluorite
  • Quartz
    Quartz crystal. Photo courtesy of Tina Kuhn.

    Quartz is one of the most common minerals in the Earth's crust. It is made of silicon dioxide (SiO2), otherwise known as silica. Varieties of quartz based on color include: amethyst, smoky quartz, rose quartz, and citrine. Quartz has a glassy luster and a hardness of 7.

    Quartz occurs in all three rock types and and can be seen in parks such as Glacier National Park, Black Canyon of the Gunnison National Park, and Great Smoky Mountains National Park.

    Potassium feldspar
    Potassium feldspar with perthitic texture. Photo courtesy of Tina Kuhn.

    Potassium feldspar (or alkali feldspar or K-spar) is a member of the feldspar mineral family and is a silicate mineral. It contains a considerable amount of potassium and is typically a pink salmon to white in color.
    Potassium feldspar has a hardness of 6. The crystals are stubby prisms and have a streaky appearance called perthitic texture (as seen in image on the right).

    A significant amount of potassium feldspar is found in the slope sediments and granite at Acadia National Park.
    Plagioclase Feldspar
    Plagioclase feldspar. Photo courtesy of Tina Kuhn.

    Plagioclase is a member of the feldspar mineral family. Plagioclase feldspars are yet another silicate that contains considerable sodium or calcium. Feldspar crystals are stubby prisms, generally white to gray and have a glassy luster.

    Plagioclase feldspar can be found in the igneous and metamorphic rocks at Grand Teton National Park and City of Rocks National Reserve.

    Mica- Biotite
    Biotite mica. Photo courtesy of Tina Kuhn.

    Micas are another group of silicate minerals composed of varying amounts of potassium, magnesium, iron, aluminum, silicon and water. All micas form flat, book like crystals that peel apart along one cleavage plane into individual sheets of smooth flakes. Biotite (pictured to the right) is dark, black, or brown mica; muscovite is light-colored or clear mica. Mica is so soft it can be scratched with a fingernail.

    Mica typically occurs in metamorphic and igneous rocks. Biotite and muscovite are two of the primary minerals in the metamorphic rocks at Mount Rushmore National Monument.

    Hornblende, a dark green to black amphibole. Photo courtesy of Tina Kuhn.

    The amphiboles are a family of silicate minerals that form crystals which are prism or needle-like. Amphiboles are generally dark colored and contain iron, calcium and aluminum. Hornblende is the most common amphibole and is dark green to black in color. Amphiboles are a component in igneous and metamorphic rocks.

    Amphiboles can be found in the intrusive igneous bodies at Denali National Park and Preserve and in the metamophosed gneiss at Weir Farm National Historic Site.

    Olivine. Photo courtesy of Tina Kuhn.

    Olivine [(Fe, Mg)2SiO4] is a silicate mineral containing iron and magnesium. It is a green, glassy mineral that forms at high temperatures. It is common in basalt and ultramafic rocks. Gem-quality olivine is called peridote. A rock made up entirely of olivine is called dunite.

    Olivine most commonly occurs in igneous rocks and can be found in andesite at Mount Rainier National Park and Devils Postpile National Monument, as well as in basalt at Yosemite National Park.

    Calcite. Photo courtesy of Tina Kuhn.

    Calcite is made of calcium carbonate (CaCO3). Generally white to clear, calcite is easily scratched with a knife. Due to the presence of carbonates (CO3), calcite reacts to most acids (such as hydrochloric acid, HCl) and effervesce on contact. Most seashells are made of calcite or related minerals.

    Calcite can be found in many cave and karst formations such as the calcite features at Jewel Cave National Monument and Wind Cave National Park.

    Talc. Photo courtesy of Tina Kuhn.

    Talc (Mg3Si4O10(OH)2) is the softest known mineral and can be scratched with a fingernail. Upon contact, talc has a distinctive greasy feel and a waxy/pearly luster.

    Talc is a foliated mineral and associated with metamorphic rocks. It is an alteration product from the metamorphism of minerals such as serpentine, pyroxene and amphibole.
    Talc can be found in talc schist at the George Washington Memorial Parkway and the National Capital Parks - East.

    Fluorite. Photo courtesy of Tina Kuhn

    Fluorite (CaF2) is considered to be one of the most colorful minerals in the world. Common colors of fluorite include purple, green, yellow, and blue. Fluorite is often mistaken for quartz but it has a lower hardness of 4.
    Fluorite is the state mineral of Illinois which was once the largest fluorite producer in the United States.

    Mohs Hardness Scale

    Mohs hardness scale


    The Mohs Hardness Scale is used to help identify minerals. A mineral's hardness is a measure of a mineral's resistance to scratching. This is measured by scratching it against another substance of known hardness on the Mohs Hardness Scale. This simple graphic outlines the index minerals and common objects used to determine a mineral's hardness.

    Color Graphic [1.09 MB JPEG]
    Graphic for Printing [890 KB JPEG]

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    Last Updated: January 03, 2017