Badlands National Monument - South Dakota
July 1999
LAB #1:
Rocks and Minerals
Badlands National Monument - South Dakota
July 1999
All rocks can be found in one of three major groups -- igneous, sedimentary and metamorphic:
These groups are defined and differentiated by how they are formed. Their formation is cyclical, that is, one type can become another based on physical and chemical reactions. The dependency of this cycle can be illustrated as follows:
(From: "Rock Cycles" by Monica Whitney. http://science.coe.uwf.edu/SH/Curr/rockcyc/rockcyc.htm)
You can start anywhere in the cycle to understand these changes. Let’s start with the sedimentary rocks, which are nothing more than sediments cemented together. An example of this type of rock is a rock that you find at the beach, where bits of seashells have been stuck together to form a rock. Sedimentary rocks can then become metamorphic rocks when they are exposed to extreme heat and pressure. (This process can also happen to igneous rocks.) The last link in the cycle is the change of metamorphic rock into igneous rock. This occurs when metamorphic rock is exposed to the extreme heat of Earth’s interior and it melts to make magma. As the magma cools either inside or outside of the Earth or when it comes to the surface it forms igneous rocks. These igneous rocks then erode and deposit sediment which later become sedimentary rock.
And so the cycle goes on and on…
Whether you are atop a mountain, walking through a meadow or forest, or strolling on a beach, you are standing on the thin outermost layer of Earth, the crust. Within this crust are the rocks and mineral that geologist study to reveal the history of our planet. Anywhere this fragile crust has been disrupted or "broken", i.e., near quarries, on newly-cut roads, along beaches or riverbanks, one can find examples of these treasures. You don’t even have to step outside to find samples of minerals. They’re in aluminum foil; copper plumbing in the walls; and the quartz in your wristwatch. They’re in the stone, steel, and brick buildings, cement sidewalks and gravel roads. Pencils are made of soft graphite, and black-and-white photographs are pictures painted in mineral silver. Minerals are even inside you --- from the calcium in your bones and teeth to the iron in your blood. Just as all living organisms are composed of functional units called cells, so are rocks --- the building blocks of the Earth --- composed of the elementary functional units of minerals. The structures of minerals are based upon the defined bonds of chemical elements (a substance that cannot be broken into simpler substances by ordinary chemical or physical means) or compounds (a substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements). These types of bonds can be:
1. ionic (very strong bonds) – bonding that occurs when two opposite charged ions attract each other (EX --- sodium chloride, NaCl)
2. covalent (weaker bonds than ionic) – bonding in which two electrons are shared by two atoms (EX --- gaseous form of oxygen, O2
3. metallic (shared bonds) – bonding in which electrons are shared in some metals, as copper, silver, aluminum and gold. This type of bonding allows for the high conductivity of these metals.
Because of these defined structures, each mineral takes on a definite structure – one that is its own and one that helps to identify it. This orderly arrangement seen in minerals is called a crystal, and each of the over 4000 minerals known on Earth has its own unique pattern, much like individual snowflakes or human fingerprints. Some of these crystalline patterns are composed of only one element, as gold, sulfur or graphite (carbon). These minerals are called native elements. Other minerals are formed by combinations of elements or compounds.
All of the minerals and rocks that make up the Earth’s crust --- the 5-70 km thick outermost layer of the Earth --- are composed mainly of eight elements. The following chart lists these elements of the crust, as well as the eight elements that are the most prevalent in ALL of the layers of the Earth. (Note that six elements are common to both lists.)
Identification of the various minerals is made by a combination of physical and chemical methods. Most of these methods are based on examination of the physical structure or by the results of various chemical tests or physical stresses applied to the mineral in question. The most commonly used techniques are discussed here.
1. Examination of crystalline structure --- This procedure identifies the unique structure of each mineral. Sometimes these structures can be seen with the naked eyes, like the linear layer crystal of mica or the hexagonal crystals with pyramid ends of quartz. The use of a magnifier aids in this examination. But the best studies are performed using sophisticated polarization microscopes of very thin layers of the mineral in question.
2. Luster --- This characteristic describes the appearance or quality of light reflected from the surface of the mineral. Luster is further defined as metallic (as seen in gold or pyrite) or non-metallic (pearl-like) or none.
3. Color --- This observation is an obvious characteristic, but is somewhat unreliable, as impurities can change the color of a mineral, as in the various colors of quartz (milky, pink, purple, yellow, etc.)
4. Streak – This characteristic is observed in the color a mineral leaves when streaked on the surface of a piece of unglazed porcelain. The color is due to the powdered form of the mineral that is left behind with the process. Streaking is a reliable testing method because the color of a streak does not vary from sample to sample. The streak test is also used to determine luster: metallic materials usually produce a dark streak; non-metallic materials do not.
5. Hardness ---This test is probably the best known of identification methods. Hardness is the measure of resistance of a material to abrasion or scratching. A mineral of unknown hardness is rubbed against an item of known hardness or vice versa. A numerical value is given to this hardness by using the Mohs Scale:
6. Cleavage --- This property is the tendency of a mineral to break along planes of weak (chemical) bonding. Not all minerals have these planes, but those that do can be identified by these distinctive surfaces. (EX - Mica and fluorite) Cleavage can be smooth, as seen in mica and fluorite; poor, as in magnetite; or virtually non-existent as in kaolinite. When minerals break evenly in more than one direction, cleavage is described by the number of planes seen and the angles by which they meet.
7. Fracture --- If a mineral does not have cleavage, it fractures. These lines of fracture can be:
However, most fractures are quite irregular.
8. Specific gravity --- This characteristic is a number defined by the ratio of the weight of the mineral to that of an equal volume of water. Thus, if the weight of 1 cc of quartz is compared to that of 1 cc of water (= 1 gram), the quartz will probably weigh between 2.5 - 3.0 gms. Therefore, the ratio is stated as 2.5-3.0 : 1.0, and quartz is described of having a specific gravity of 2.5-3.0. (The s.g. of galena is 7.5; that of gold is 20)
9. Other Properties --- Certain minerals can be identified by distinctive characteristics. Some of these characteristics are:
The following chart lists the most common minerals found in each of the rock groups. The minerals are listed in decreasing order of frequency.
1. TALC --- Talc is a secondary mineral which forms when magnesium silicate is altered by the process of metamorphic rock building. Its structure is generally massive (compact without any crystal structure) or foliated (easily separated into plates). Talc can be easily recognized by its softness (Mohs scale = 1.0) and greasy feel. Compact, massive talc is more commonly known as soapstone. The Chinese, Eskimos and Egyptians used it for everything from insecticides to cosmetics. We know it best for its use in talcum powder.
TALC
2. PYRITE --- Perhaps more widely recognized by the general public as "fool’s gold". Pyrite is the most common of the sulfide minerals. It forms at both high and low temperatures produced when magma cools and segregates. It may be found in igneous, metamorphic or sedimentary rocks. Pyrite can be recognized by its pale brassy-yellow color, metallic luster and crystal shape which is generally like a cube. It can be easily distinguished from real gold by its brittleness and hardness (Mohs scale = 6.0-6.5). Pyrite is used to make sulfuric acid.
PYRITE
3. FLUORITE --- Fluorite is a common, widely distributed mineral. It is commonly found in hydrothermal veins; areas where minerals have been dissolved and redeposited by very hot water generally associated with metamorphic rocks. Fluorite can generally be identified by its cubic crystalline form and by the way it breaks into eight equilateral triangles (octahedral cleavage) when struck. (Mohs scale = 4.0) Fluorite comes in many colors including blue and green, and has the appearance of melted or impure glass. Fluorite sometimes glows in UV light – its name gives us the term fluorescence. This mineral is used in dental products and is an ingredient of hydrofluoric acid, an acid so potent that it etches glass.
FLUORITE
4. GYPSUM --- Gypsum is a sulfate mineral commonly found in sedimentary rocks. It is often found in naturally occurring thick layers and is characterized by its softness (Mohs scale = 2.0) and its perfect cleavage in one direction --- it breaks along a single, definite plane. Gypsum is general colorless to a white or gray and usually has a pearly, silky luster. It is mined for making paints, plaster, tile, drywall and cement. A fine-grained form, alabaster, is used for carving. The sand of White Sands Desert, in southern New Mexico, is formed by wind erosion of gypsum beds.
GYPSUM
5. HALITE --- Commonly recognized as rock salt. Halite is a common mineral that was precipitated by evaporation and the drying up of landlocked bodies of salt water. As a result, it is usually found in extensive deposits. Halite is generally colorless to white and can be identified by its salty taste and cubic crystalline structure. (Mohs scale = 2.0-2.5) In addition to using it as a seasoning for our food, halite is used as a preservative, and the chlorine from it is used in bleach and soap production.
HALITE
6. MAGNETITE --- Magnetite is a mineral sometimes found in large, concentrated bodies within igneous rock formations. However, it is more often associated with metamorphic rocks. Magnetite is characterized chiefly by its strong magnetism, its black color and its hardness (Mohs scale = 5.5-6.5). It is a source of iron ore. Some specimens, called lodestones, were used in early compasses.
MAGNETITE
7. MICA --- The micas are a group of silicate minerals which generally develop into schists (a metamorphic rock form) and are commonly found in igneous rocks. Mica develops by stacking layers of silicates on top of each other. This arrangement gives micas their characteristic thin sheets which are generally flexible and have a pearly luster. It is relatively soft (Mohs scale = 2.0-2.5). Mica is valuable as insulation, in electronics, and in gardening to condition the soil.
MICA
8. CALCITE --- Calcite is one of the most common rock minerals. It occurs in sedimentary rock masses such as limestone. As a mineral, calcite is easily recognizable by its rhombohedral cleavage which makes crystal look like a "kicked over" cube. It is soft (Mohs scale = 3.0) and can be scratched by a fingernail. It has a pearly luster and is transparent to opaque. Deposited in caves through the process of evaporation, calcite forms stalactites and stalagmites. (Two easy ways to remember which grows from the ceiling and which one from the floor: The "C" in stalactite stands for "ceiling". Since it develops on the floor, and the "G" in stalagmite stands for ground.) Calcite is used in making precise ocular instruments like microscopes, and also used to manufacture cement, fertilizers, and other chemicals.
CALCITE
9. FELDSPAR --- The feldspars are a mineral group made up of aluminum silicates and is our most common mineral. The presence of one of three associated minerals determines the specific form of feldspar that is present. These three are potassium, sodium and calcium. All feldspars have a good cleavage in two directions. On the Mohs scale, it has a hardness of 6.0. Feldspar occurs in nearly all rocks, including those found on the moon. It is used in the ceramics industry, including the making of fine porcelain.
FELDSPAR
10. QUARTZ --- Quartz is a common rock-forming mineral associated with igneous and metamorphic rocks. As a mineral, quartz may be found in the form of a crystal that is generally prismatic. Quartz crystals can be clear, purple (amethyst), smoky or rose. Massive quartz (having no crystalline form) is generally milky or white and can be distinguished by its greasy feel and conchoidal fracturing. (Mohs scale = 7.0) Quartz is used in making radios, televisions, radar devices and timepieces. Glass is made by melting quartz sand.
TALC
11. GALENA --- Galena is a very common metallic sulfide, found in veins formed when pre-existing fractures or fissures in the Earth’s surface are filled with the mineral. It is the ore for lead. Galena is easily recognized by its weight (about 400 pounds per cubic foot), cubic crystalline structure and its bright silvery color. It is fairly soft (Mohs scale = 2.5). It is used in the manufacture of batteries. (Note: it is generally recognized that lead does not pose a health problem in its sulfide form.)
GALENA
12. HEMATITE --- Hematite is widely distributed in rocks of all ages and forms. It is the most abundant and important ore of iron. Hematite is heavy, quite brittle and steel gray to black in color in either crystalline or massive form. It is also found as a red, earthy mineral. (Mohs scale = 5.5-6.5) The mineral gets its name from hemo, the Greek word for blood. It is used in making steel. In ancient times, it was used as a paint pigment and also used as a cosmetic, rouge.
HEMATITE
13. GARNET --- Garnets as a group are relatively common in highly metamorphosed rocks and in some igneous formations. They form under the high temperatures and/or pressures that those types of rocks must endure. As a gemstone, garnets have had a mixed reputation. Garnets are greatly variable in colors and varieties, though, and many of these are both rare and beautiful, producing genuinely precious gemstones. The general formula for garnets is A3B2(SiO4)3. The A represents divalent metals such as calcium, iron, magnesium and manganese. The B represents a trivalent metal such as aluminum, chromium, iron, and other elements found in rarer members of the group. Most garnets are red in color. They are also considered as gems.
GARNET
14. OLIVINE --- Olivine is found in igneous rock, but not too frequently on the Earth’s surface. As a result, it must be mined. It is a hard mineral (Mohs scale 6.5-7.0). When it weathers, it forms serpentine. It is also found in a gem form, peridot. Olivine is used in sandblasting (cleaning surfaces of buildings; cleaning statues; and in taking rust off of bridges). It is also used in casting and other foundry processes.
OLIVINE
REFERENCES
1. "Common Uses of Minerals" http://home.navisoft.com/alliance/afaweb/minerals.htm
2. "Nature Finder - A Guide for Minerals". Hubbard Scientific. 1996. Chippewa Falls, WI.
3. "Mineral Finder Specimens". Rockman McLean Trading Company. 1998. Loveland, CO.
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