Only $2.99/month

Terms in this set (97)

In the crystal structure of many minerals, some atomic bonds are weaker than others. It is along these weak bonds that minerals tend to break when they are stressed. Cleavage (kleiben=carve) is the tendency of a mineral to break (cleave) along the planes of weak bonding. Not all minerals have cleavage, but those that do can be identified by the relatively smooth, flat surfaces that are produced when the mineral is broken. The simplest type of cleavage is exhibited by the micas. Because these minerals have very weak bonds in one direction, they cleave to form thin, flat sheets. Some minerals have excellent cleavage in one, two, three, or more directions, whereas others exhibit fair or poor cleavage, and still others have no cleavage at all. When minerals break evenly in more than one direction, cleavage is described by the number of cleavage directions and the angle(s) at which they meet. Each cleavage surface that has a different orientation is counted as a different direction of cleavage. For example, some minerals cleave to form six-sided cubes. Because cubes are defined by three different sets of parallel planes that intersect at 90 degree angles, cleavage is described as three directions of cleavage that meet at 90 degrees. Do not confuse cleavage with crystal shape. When a mineral exhibits cleavage it will break into pieces that all have the same geometry. By contrast, the smooth sided quartz crystals in figure 2.1 do not have cleavage. If broken, they fracture into shapes that do not resemble one another or the original crystals.
Except for quartz (SiO2) the basic structure (chains, sheets, or three-dimensional frame-works) of most silicate minerals has a net negative charge. Therefore, metal ions are required to bring the overall charge into balance and to serve as the "mortar" that holds these structures together. The positive ions that most often link silicate structures are iron (Fe2+), magnesium (Mg2+), potassium (K1+), sodium (Na1+), aluminum (Al3+), and calcium (Ca2+). These positively charged ions bond with the unshared oxygen ions that occupy the corners of the silicate tetrahedra. As a general rule, the hybrid covalent bonds between silicon and oxygen are stronger than the ionic bonds that hold one silicate structure to the next. Consequently, properties such as cleavage, and to some extent hardness, are controlled by the nature of the silicate framework. Quartz (SiO2), which has only silicon-oxygen bonds, has great hardness and lacks cleavage, mainly because of equally strong bonds in all directions. By contrast, the mineral talc (the source of talcum powder), has a sheet structure. Magnesium ions occur between the sheets and weakly join them together. The slippery feel of talcum powder is due to the silicate sheets sliding relative to one another, in much the same way sheets of carbon atoms slide in graphite, giving it its lubricating properties. Recall that atoms of similar size can substitute freely for one another without altering a mineral's structure. For example, in the mineral olivine, iron (Fe2+) and magnesium (Mg2+) substitute for each other. This also holds true for the third most common element in earth's crust, aluminum (Al3+), which often substitutes for silicon (Si) in the center of silicon-oxygen tetrahedra. Because most silicates will readily accommodate two or more different positive ions at a given bonding site, individual specimens of a particular mineral may contain varying amounts of certain elements. As a result, many silicate minerals form a mineral group that exhibits a range of compositions between tow end members. Examples include the olivine's, pyroxenes, amphiboles, micas, and feldspars.
Nonsilicate minerals are typically divided into groups, based on the negatively charged ion or complex ion that the members have in common. For example, the oxides contain the negative oxygen ion (O2-), which is bonded to one or more kinds of positive ions. Thus, within each mineral group, the basic structure and tyupe of bonding is similar. as a result, the minerals in each group have similar physical properties that are useful in mineral identification. Although the nonsilicates make up only about 8 percent of earth's crust, some minerals, such as gypsum, calcite, and halite, occur as constituents in sedimentary rocks in significant amounts. Furthermore, many others are important economically. Some of the most common nonsilicate minerals belong to one of three classes of minerals--the carbonates (CO32-), the sulfates (SO42-), and the halides (Cl1-,F1-,Br1-). The carbonate minerals are much simpler structurally than the silicates. This mineral group is composed of the carbonate ion (CO32-) and one or more kinds of positive ions. The two most common carbonate minerals are calcite, CaCO3 (Calcium carbonate), and dolomite, CaMg (CO3)2 (calcium/magnesium carbonate). Because these minerals are similar both physically and chemically, they are difficult to distinguish from each other. Both have a vitreous luster, a hardness between 3 and 4, and nearly perfect rhombic cleavage. They can, however, be distinguished by using dilute hydrochloric acid. Calcite reacts vigorously with this acid, whereas dolomite reacts much more slowly. Calcite and dolomite are usually found together as the primary constituents in the sedimentary rocks limestone and dolostone. When calcite is the dominant mineral, the rock is called limestone, whereas dolostone results from a predominance of dolomite. Limestone has many uses, including as road aggregate, as building stone, and as the man ingredient in Portland cement. Two other nonsilicate minerals frequently found in sedimentary rocks are halite and gypsum. Both minerals are commonly found in thick layers that are the last vestiges of ancient seas that have long since evaporated. Like limestone, both are important nonmetallic resources. Halite is the mineral name for common table salt (NaCl). Gypsum *CaSO4.2H2O), which is calcium sulfate with water bound into the structure, is the mineral of which plaster and other similar building materials are composed. Most nonsilicate mineral classes contain members that are prized for their economic value. This includes the oxides whose members hematite and magnetite are important ores of iron. Also significant are the sulfides, which are basically compounds of sulfur (S) and one or more metals. Examples of important sulfide minerals include galena (lead), sphalerite (zinc), and chalcopyrite (copper). In addition, native elements, including gold, silver, and carbon (diamonds), plus a host of other nonsilicate minerals--fluorite (flux in making steel), corundum (gemstone, abrasive), and uraninite (a uranium source)---are important economically.
Useful metallic minerals that can be mined at a profit. In common usage, the term ore is also applied to some non-metallic minerals such as fluorite and sulfur. However, materials used for such purposes as building stone road aggregate, abrasives, ceramics, and fertilizers are not usually called ores; rather, they are classified as industrial rocks and minerals. Recall that more than 98 percent of earth's crust is compsed of only eight elements, and except for oxygen and silicon, all other elements make up a relatively small fraction of common crustal rocks. Indeed, the natural concentrations of many elements are exceedingly small. A deposit containing the average percentage of a valuable element such as gold has no economic value, because the cost of extracting it greatly exceeds the value of the gold that could be recovered. To have economic value, an element must be concentrated above the level of its average crustal abundance. For example, copper makes up about 0.0135 percent of the crust. For a deposit to be considered as copper ore, it must contain a concentration that is about 100 times this amount. Aluminum, on the other hand, represents 8.13 percent of the crust and can be extracted profitably when it is found in concentrations only about four times its average crustal percentage. It is important to realize that a deposit may become profitable to extract or lose its profitability because of economic changes. If demand for a metal increases and prices rise sufficiently, the status of a previously unprofitable deposit changes, and it becomes an ore. The status of unprofitable deposits may also change if a technological advance allows the ore to be extracted at a lower cost than before. Conversely, changing economic factors can turn a once profitable ore deposit into an unprofitable deposit that can no longer be called an ore. This situation was illustrated at the copper mining operation located at Bingham Canyon, Utah, one of the largest open-pit mines on earth. Mining was halted there in 1985 because outmoded equipment had driven the cost of extracting the copper beyond the current selling price. The owners responded by replacing an antiquated 1000-car railroad with conveyor belts and pipelines for transporting the ore and waste. These devices achieved a cost reduction of nearly 30 percent and returned this mining operation to profitability. Over the years, geologists have been keenly interested in learning how natural processes produce localized concentrations of essential minerals. One well-established fact is that occurrences of valuable mineral resources are closely related to the rock cycle. That is, the mechanisms that generate igneous, sedimentary, and metamorphic rocks, including the processes of weathering and erosion, play a major role in producing concentrated accumulations of useful elements. Moreover, with the development of the theory of plate tectonics, geologists have added another tool for understanding the processes by which one rock is transformed into another. As these rock-forming processes are examined in the following chapters, we will consider their role in producing some of our important mineral resources.