What Materials Compose Landscapes?
WHAT MATERIALS MAKE UP THE WORLD around us? What do we see if we look closely at an exposure of rock? How does the rock look when viewed with a magnifying glass? Such properties of these rocks control landscape formation. We investigate these questions using the beautiful scenery of Yosemite National Park in California.
What Is the Difference Between a Rock and a Mineral?
Observe this photograph of Yosemite Valley, the heart of Yosemite National Park. What do you notice about the landscape? This landscape is dominated by dramatic cliffs and steep slopes of massive gray rock perched above a green, forested valley. The valley is famous for waterfalls and for huge rock exposures. The appropriately named Half Dome is on the right side of the photograph. What would we see if we got closer to this landscape?
Closer examination reveals several different-colored crystals in the rock: whitish, pinkish, clear gray, and black. So what looks like a homogeneous gray rock from a distance is actually composed of different types of crystals. To better observe a rock at this scale, a geoscientist may collect a hand-sized piece, called a hand specimen. When examined more closely, the rock contains four different types of crystals with distinct appearances. The clear gray crystals all have similar chemical composition and physical properties, and so they represent one kind of solid substance called a mineral. The whitish crystals are a different kind of mineral, as are the black and pinkish crystals.
Rocks are composed of minerals, but what characteristics define a mineral? To be considered a mineral, a substance must fulfill all of the criteria listed below. A mineral is a naturally occurring, inorganic solid with an ordered internal (crystalline) structure and a relatively consistent chemical composition.
A mineral must be natural. These crystals of fluorite grew naturally from hot water flowing through a rock. If a mineral must form naturally, then natural diamonds are minerals, but synthetic diamonds grown in the lab are not.
This crystal is inorganic and a mineral. Many shells have the same composition as the crystal, but they were made by clams and other creatures; they are not considered to be a mineral by most geoscientists.
All minerals are solid, not liquid or gaseous. Ice, a solid, is a mineral, but liquid water is not, even though it has the same composition.
Ordered Internal Structure
A mineral has an ordered internal structure, which means that atoms are arranged in a regular, repeating way. Such substances are considered to be crystalline, and they can form well-defined geometric crystals. This mineral is crystalline, and the shape of the crystals shows the internal arrangement of its atoms. If atoms are arranged in a random way, such as a volcanic glass, it is not crystalline and not a mineral.
Minerals are homogeneous and so have specific chemical compositions that do not depend on the size of the sample that is analyzed. Table salt, which is the mineral halite, contains atoms of the chemical elements sodium (Na) and chlorine (Cl) in equal proportions, no matter how big or small the specimen. Most minerals have a specific chemical formula, like NaCl for halite.
What Are Some Common Minerals?
Quartz is a common mineral, with a formula of SiO2. It is generally transparent to nearly white, but it can be many colors. It is a relatively hard mineral that commonly occurs as well-formed crystals, but it fractures irregularly, along smoothly curving surfaces.
Feldspar is a very common mineral, coming in two varieties: potassium feldspar (shown here) and plagioclase. Feldspars contain varying amounts of aluminum, potassium, sodium, and calcium. Feldspars are pink to cream-colored to gray.
Mica refers to a family of minerals that breaks into flakes and sheets that are typically partially transparent and shiny because the flat surfaces reflect light. Micas contain potassium, aluminum, silicon, and oxygen and can be clear, silvery-gray, green, brown, or black.
Mafic minerals are silicate minerals that, in addition to silicon and oxygen, contain magnesium and iron, such as in the mineral olivine. These minerals are generally dark colored and include the common mineral groups amphiboles, pyroxenes, and dark micas.
Carbonate minerals contain carbon and oxygen bonded with other elements. The most common carbonate mineral is calcite, a common calcium-carbonate mineral (CaCO3). It may be almost clear but commonly has a cream to light-gray color. Other important carbonates include dolomite, which also contains magnesium.
Oxide minerals contain oxygen bonded with a metallic element, like iron, copper, or titanium. Hematite is a common iron oxide (Fe2O3) that can be black, brown, silvery gray, or earthy red; it is the red color in rust and many red-rock landscapes. Another iron-oxide mineral is magnetite (Fe3O4), which is strongly magnetic.
Salt minerals include several families of minerals, including halite (NaCl), which is the mineral in table salt. Halite grows and breaks into cubic shapes and generally forms from the evaporation of water. Other salt-like minerals include gypsum, a calcium-sulfate mineral that forms in environments similar to those in which halite forms.
Sulfide minerals all contain sulfur bonded with a metallic element, like iron or copper. The most common sulfide mineral is pyrite, an iron-sulfide mineral (FeS2). Pyrite has a pale bronze to brass-yellow color (it is “fool’s gold”). It commonly forms cube-shaped crystals. Other sulfide minerals contain copper, zinc, lead, and other metals.
The term clay can refer either to a family of silicate minerals or to any very fine particles that are less than 0.002 millimeters in diameter. Clay minerals consist of sheets, similar to mica minerals, but the sheets are weakly held together, so they easily slip past one another, giving clays their slippery feel. When some clay minerals get wet, water pushes apart the weakly bonded sheets, causing the clay to expand.
Most clay minerals have light colors but may appear dark if mixed with other material (dark minerals or organic debris). Most clay minerals form by breaking down rocks (weathering) at Earth’s surface, and they are an important component of soil. The fine grain size and low density mean that clay particles are easily picked up and transported long distances by wind and water. Clay can be deposited on land, but some makes it to the open ocean, where it finally settles to the ocean floor, forming submarine mud.