Mineral Resources of the Midwestern US
What is a mineral?
A mineral is a naturally occurring solid with a definite chemical composition and crystalline structure. Minerals provide the foundation of our everyday world. Not only do minerals make up the rocks we see around us in the Midwest, they are used in nearly every aspect of our lives. The minerals found in the rocks of the Midwest are used in industry, construction, machinery, technology, food, makeup, jewelry, and even the paper on which these words are printed.
Elements: The Building Blocks of Minerals
Elements are the building blocks of minerals. The mineral quartz, for example, is made of the elements silicon and oxygen. Most minerals present in nature are not composed of a single element, though there are exceptions such as gold (Au). Eight elements make up (by weight) 99% of the Earth&ramp;rsquo;s crust, with oxygen being by far the most abundant (46.4%). The remaining elements in the Earth&ramp;rsquo;s crust occur in very small amounts, some in concentrations of only a fraction of one percent (Figure 5.1). Since silicon (Si) and oxygen (O) are the most abundant elements in the crust by mass, it makes sense that quartz (SiO2, silicon dioxide or silica) is one of the most common minerals in the Earth&ramp;rsquo;s crust and is found all over the Midwest.
Minerals provide the building blocks for rocks. For example, granite, an igneous rock, is typically made up of crystals of the minerals feldspar, quartz, mica, and amphibole. Sandstone may be made of cemented grains of feldspar, quartz, and mica. The minerals and the bonds between the crystals define a rock&ramp;rsquo;s color and resistance to weathering.
Metallic minerals are vital to the machinery and technology of modern civilization. However, metallic minerals occur in extremely small amounts in the crust. In addition, it is almost always necessary to process ore minerals in order to isolate the useful element. A mineral is called an ore when one or more of its elements can be profitably removed. For example, chalcopyrite (CuFeS2), which contains copper, iron, and sulfur, is referred to as a copper ore when the copper can be profitably extracted from the iron and sulfur.
Non-metallic minerals do not have the flash of a metal, though they may have the brilliance of a diamond or the silky appearance of gypsum (CaSO4·2H2O). Generally much lighter in color than metallic minerals, non-metallic minerals can transmit light, at least along their edges or through small fragments.
Although defined by their chemical composition and crystal structure, minerals are identified based on their physical properties. A variety of properties must usually be employed in identifying a mineral, each eliminating possible alternatives.
Hardness is a very useful property for identification, as a given mineral can only exhibit a narrow range of hardnesses, and it is easily testable, which quickly and simply minimizes the number of possibilities. Hardness is important because it helps us understand why some rocks are more or less resistant to weathering and erosion. Quartz, with a rating of 7 on the Mohs scale, is a relatively hard mineral, but calcite (CaCO3), rating 3 on the Mohs scale, is significantly softer. Therefore, it should be no surprise that quartz sandstone is much more resistant to erosion and weathering than is limestone, which is primarily made of the mineral calcite. Quartz is a very common mineral in the Earth&ramp;rsquo;s crust and is quite resistant due to its hardness and relative insolubility. Thus, quartz grains are the dominant mineral type in nearly all types of sand.
Mohs Scale of Hardness
In 1824, the Austrian mineralogist Friedrich Mohs selected ten minerals to which all other minerals could be compared to determine their relative hardness. The scale became known as Mohs scale of hardness, and it remains very useful as a means for identifying minerals or for quickly determining their hardness. A fingernail has a hardness of around 2, a penny 3, window glass 5.5, and a knife blade 6.5.
Color is helpful in identifying some minerals like sulfur, but it is uninformative or even misleading in others like garnet. Luster describes how light is reflected from a mineral&ramp;rsquo;s surface and can range from adamantine, seen in diamonds, to dull or earthy (effectively no luster), like kaolinite. Crystal form, if visible, can be diagnostic. For example, fluorite and calcite may appear superficially similar, but fluorite forms cubic crystals while calcite forms trigonal-rhombohedral crystals. Relatedly, crystals may have planes of weakness that cause them to break in characteristic ways, called cleavage. Or they may not, and instead display fracture when broken. Mica and graphite have very strong cleavage, allowing them to easily be broken into thin sheets, while quartz and glass (the latter not being a mineral) have no cleavage, instead displaying a distinctive curved fracture form known as conchoidal. The density of a mineral may also aid in identifying it (e.g., metals tend to be very dense). Finding the exact density is straightforward, but it does require measuring the volume of the sample. Placing an unknown mineral in water (or other liquid) to find its volume by displacement can be a risky undertaking since several minerals react violently with water, and many more break down with exposure. A mineral&ramp;rsquo;s streak is obtained by dragging it across a porcelain plate, effectively powdering it. The color of the powder eliminates conflating variables of external weathering, crystal habit, impurities, etc. Some minerals are magnetic (affected by magnetic fields), while a few are natural magnets (capable of producing a magnetic field).
There are many more interesting and distinguishing properties that minerals may possess, and there are many more elaborate and precise means for identifying them. The branch of geology that studies the chemical and physical properties and formation of minerals is called mineralogy.
Most minerals can be identified by process of elimination after examining a few of these properties and consulting a mineral identification guide. Mineral testing kits often include several common objects used to test hardness: a porcelain streak plate, a magnet, and a magnifying glass. Some minerals have rare properties, which may be more difficult to test. For example, there are minerals that exhibit luminescence of all types, giving off light due to a particular stimulus. Some minerals are radioactive, usually due to the inclusion of significant amounts of uranium, thorium, or potassium in their structure. Carbonate minerals will fervesce when exposed to hydrochloric acid. Double refraction describes the result of light passing through a material that splits it into two polarized sets of rays, doubling images viewed through that material. For example, a single line on a sheet of paper will appear as two parallel lines when viewed through a clear calcite crystal.
What distinguishes a regular mineral from a gem?
Beauty, durability, and rarity of a mineral qualify it as a gemstone. Beauty refers to the luster, color, transparency, and brilliance of the mineral, though to some degree it is dependent on the skillfulness of the cut. Most gems, including tourmaline, topaz, and corundum, are durable because they are hard, making them scratch-resistant. On the Mohs scale of hardness, the majority of gemstones have values greater than 7. Isolated deposits of semi-precious gemstones can be found in each state in a variety of places, but they are not mined commercially (except as noted below), and precious gemstones are extremely rare in the Midwest.
Geologists looking for particular minerals do not make haphazard guesses as to the location of ore bodies. The occurrence of minerals in the Earth&ramp;rsquo;s crust is due to the geologic processes that formed certain rock types in a given area. An understanding of the environments in which minerals form, the minerals that make up different rocks, and the geologic history of an area, all help geologists ascertain where minerals of interest are concentrated. Metallic minerals are often associated with igneous and metamorphic rocks, which typically occur in either very ancient rocks (Precambrian) or in areas of severe deformation of the crust, such as where continents have collided.
Mineral deposits may be formed in several ways: precipitation out of water, crystallization of magma or lava, recrystallization after exposure to heat and pressure, or the dissolution and later precipitation of minerals by hot water moving through cracks and openings in rock located well below the surface. A mineral is not necessarily restricted, however, to one method of concentration or environment of formation. For example, gypsum may form as a precipitate from evaporating water, but it is also associated with volcanic regions where limestone and sulfur gases from the volcano have interacted.
What are hydrothermal solutions?
Hot water enriched in salts such as sodium chloride (NaCl), potassium chloride (KCl), and calcium chloride (CaCl2) is called a hydrothermal solution, or simply “brine.” The brine is as salty or even saltier than seawater and, surprisingly, may contain minute bits of dissolved minerals such as gold, lead, copper, and zinc. The presence of salt in the water stops the metallic minerals from precipitating out of the brine because the chlorides in the salt preferentially bond with the metals. Additionally, because the brine is hot, the minerals are more easily dissolved, just as hot tea dissolves sugar more easily than cold tea does. These hot water brines can have varying origins. As magma cools, it releases its mineral-enriched, super-heated water into surrounding rock. Rainwater becomes a hydrothermal solution by picking up salt as it filters through rocks. Seawater, which is already enriched in salt, often becomes a hydrothermal solution in the vicinity of volcanic activity on the ocean floor where tectonic plates are pulling apart. Rapid cooling of the hydrothermal solution over short distances allows concentrations of minerals to be deposited. Water moving quickly through fractures and openings in the rock, experiencing changes in pressure or composition and being diluted with groundwater, can rapidly cool a hydrothermal solution.