Minerals

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 identified and described according to their physical properties of:
  • Cleavage: The tendency of a mineral to break (cleave) along weak planes.
  • Color: Most minerals have a distinct 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.
  • Streak: refers to the color of the residue left by scratching a mineral on a tile of unglazed porcelain, like a piece of chalk.
  • Specific gravity: the ratio of the density of a mineral to an equal volume of water
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
  • 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.
Common Minerals
Quartz
Quartz crystal.

Photo courtesy of Tina Kuhn

Quartz

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 (purple), smoky quartz (grey), rose quartz (pink), and citrine (yellow-green). 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
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 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 left).
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 feldspar
Plagioclase is a member of the feldspar mineral family. Plagioclase feldspars are yet another silicate that contains considerable sodium or calcium. Plagioclase 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

Micas are another group of silicate minerals composed of varying amounts of potassium, magnesium, iron, aluminum, silicon and water. All micas form flat, sheet like crystals that peel apart along one cleavage plane into individual smooth flakes. Biotite (pictured to the left) is a black or brown mica; muscovite is light-colored or clear mica. Mica is so soft (2.5 on Mohs scale) that 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
Hornblende, a dark green to black amphibole.

Photo courtesy of Tina Kuhn

Amphiboles
The amphiboles are a family of silicate minerals that form prism or needle-like crystals. 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 common 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
Olivine

Photo courtesy of Tina Kuhn

Olivine

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
Calcite

Photo courtesy of Tina Kuhn

Calcite

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 effervesces 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
Talc.

Photo courtesy of Tina Kuhn

Talc

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
Fluorite

Photo courtesy of Tina Kuhn

Fluorite
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. Flourite is also visible under ultraviolet light, a property that gets its name, flourescence, from the mineral. 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]