Geology ch 3

most common mineral group

Feldspar

Hybrid Bonds

Many chemical bonds are actually hybrids that exhibit some degree of electron sharing and some degree of electron transfer

_________ the second-most-abundant mineral in the continental crust, is the only common mineral made completely of silicon and oxygen.

Quartz

What differences in their silicate structures account for the slipperiness of talc and the hardness of quartz?

Quartz has only silicon-oxygen bonds and thus has great hardness and lacks cleavage, mainly because it has equally strong bonds in all directions. Talc, on the other hand, has a sheet structure. Magnesium ions occur between the sheets and weakly join them together. The slippery feel is due to the silicate sheets sliding relative to one another.

ratio of silicon and oxygen for every structure

The ratio of oxygen ions to silicon ions differs in each type of silicate structure. In independent tetrahedra (SiO4) there are four oxygen ions for every silicon ion. In single chains, the oxygen-to-silicon ratio is 3:1 (SiO3), and in three-dimensional frameworks, as found in quartz, the ratio is 2:1 (SiO2). As more oxygen ions are shared, the percentage of silicon in the structure increases. Silicate minerals are therefore described as having a low or high silicon content, based on their ratio of oxygen to silicon.

silicon-oxygen tetrahedron

These tetrahedra are not chemical compounds but rather complex ions (SiO44—) having a net charge of -4. To become electrically balanced, these complex ions bond to positively charged metal ions. Specifically, each O2— has one of its valence electrons bonding with the Si4+ located at the center of the tetrahedron. The remaining 1— charge on each oxygen is available to bond with another positive ion, or with the silicon ion in an adjacent tetrahedron. It is important to note that the silicon-oxygen tetrahedron is the fundamental building block that all silicate minerals have in common.

Based on the chart in Figure 3.34, what do muscovite and biotite have in common? How do they differ?

They are both micas and have one plane, but Biotite contains Iron and Magnesium whereas Muscovite does not.

Garnet

another common silicate, is also composed of independent tetrahedra that are ionically bonded by positive ions. Both olivine and garnet form dense, hard, equidimensional crystals that lack cleavage.

The only way to distinguish the feldspars physically is to look for a multitude of fine parallel lines

called striations Striations are found on some cleavage planes of plagioclase feldspar but are not present on potassium feldspar

Light: Feldspar Group

can form under a wide range of temperatures and pressures, which partially accounts for their abundance

___________originate as products of the chemical breakdown (chemical weathering) of other silicate minerals

clay minerals; clay minerals make up a large percentage of the surface material we call soil.

Muscovite

common member of the mica family. has excellent cleavage in one direction. In thin sheets, muscovite is clear, a property that accounts for its use as window "glass" during the Middle Ages. Because muscovite is very shiny, it can often be identified by the sparkle it gives a rock. If you have ever looked closely at beach sand, you may have seen the glimmering brilliance of the mica flakes scattered among the other sand grains.

positive ion

created when an atom loses an electron

Biotite

dark, iron-rich member of the mica family (see Figure 3.34). Like other micas, biotite possesses a sheet structure that gives it excellent cleavage in one direction. Biotite also has a shiny black appearance that helps distinguish it from the other dark ferromagnesian minerals. Like hornblende, biotite is a common constituent of igneous rocks, including the rock granite.

Some atoms bond to form ionic compounds, some form molecules, and still others form metallic substances. how?

electrical forces hold atoms together and bond them to each other. These electrical attractions lower the total energy of the bonded atoms, which, in turn, generally makes them more stable. Consequently, atoms that are bonded in compounds tend to be more stable than atoms that are free (not bonded)

It is worth noting that only eight elements make up the vast majority of the rock-forming minerals and represent more than 98 percent (by weight) of the continental crust ...

elements, in order of most to least abundant, are oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg).

_______ are by far the most plentiful silicate group, comprising more than 50 percent of Earth's crust

feldspar

Olivine Group

high-temperature silicate minerals, is black to olive green in color and has a glassy luster and a conchoidal fracture

The most common dark silicate minerals are

olivine, the pyroxenes, the amphiboles, dark mica (biotite), and garnet.

The Light Silicates nonferromagnesian

re generally light in color and have a specific gravity of about 2.7, less than that of the dark (ferromagnesian) silicates. These differences are mainly attributable to the presence or absence of iron and magnesium, which are "heavy." The light silicates contain varying amounts of aluminum, potassium, calcium, and sodium rather than iron and magnesium.

minerals are the building blocks of

rocks

As a general rule, the hybrid covalent bonds between silicon and oxygen are_________ than the ionic bonds that hold one silicate structure to the next.

stronger properties such as cleavage and, to some extent, hardness are controlled by the nature of the silicate framework

negative ion

when an atom gains an electron

Most silicate minerals form when

when molten rock cools and crystallizes. Cooling can occur at or near Earth's surface (low temperature and pressure) or at great depths (high temperature and pressure). The environment during crystallization and the chemical composition of the molten rock determine, to a large degree, which minerals are produced. For example, the silicate mineral olivine crystallizes early, whereas quartz forms much later in the crystallization process.

Earth materials that are classified as minerals exhibit the following characteristics:

1.Naturally occurring 2. Solid Substance 3. Orderly Crystalline Structure (which means that their atomic ions are arranged in repetitive manner, if not they are not considered a mineral. 4. inorganic 5. Definite chemical composition that allows for sine variation.

granite

A usually light-colored rock that is found in continental crust quartz, hornblende, feldspar

Five basic silicate structures

A. Independent tetrahedra. B. Single chains. C. Double chains. D. Sheet structures. E. Three-dimensional framework

Silicate Structures

All silicate minerals have the same fundamental building block, the silicon-oxygen tetrahedron (SiO44—). This structure consists of four oxygen ions that are covalently bonded to one comparatively small silicon ion, forming a tetrahedron—a pyramid shape with four identical faces

quartz

As such, the term silica is applied to quartz. Because quartz contains a ratio of two oxygen ions (O2—) for every silicon ion (Si41), no other positive ions are needed to attain neutrality.

octet rule

Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons. Although there are exceptions to the octet rule, it is a useful rule of thumb for understanding chemical bonding.

halite and gypsum

Both minerals are commonly found in thick layers that are the last vestiges of ancient seas that have long since evaporated

What is the most common carbonate mineral?

Calcite (and dolomite)

List six common nonsilicate mineral groups. What key ions or elements define each group?

Carbonates - CaCO3, CaMg(Co3)2 Halides - NaCl, CaF2, KCl Oxides - Fe2O3, Fe3O4, Al2O3, H2O Sulfides - PbS, ZnS, FeS2, CuFeS2, HgS Sulfates - CaSO4 (dot) 2H2O, CaSO4, BaSO4 Native Elements - Au, Cu, C, C, S, Ag

List eight common nonsilicate minerals and their economic uses.

Carbonates - Portland cement Halides - Common salt, Used in steelmaking, Used as fertilizer Oxides - Ore of iron, gemstone, solid form of water Sulfides - Ore of lead, ore of zinc, sulfuric acid production, ore of copper, ore of mercury Sulfates - Plaster, drilling mud Gold - Trade, Jewelry Diamond - Gemstone, abrasive

Sketch the silicon-oxygen tetrahedron and label its parts.

Center = Si4+ Surrounding bulbs = O2-

Recall that atoms of similar size can substitute freely for one another without altering a mineral's structure.

For example, in 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 (Si4+) in the center of silicon-oxygen tetrahedra.

What is the ratio of oxygen to silicon found in single tetrahedrons? How about framework structures? Which has the highest silicon content?

Four oxygen ions covalently bonded to one comparatively small silicon ion (4:1) In framework structures the Oxygen to Silicon ratio is 2:1 (2 oxygen per one Silicon) Silicate minerals with three-dimensional structures have the highest silicon content, while those composed independent tetrahedrons have the lowest.

Garnet

Garnet is similar to olivine in that its structure is composed of individual tetrahedra linked by metallic ions. Also like olivine, garnet has a glassy luster, lacks cleavage, and exhibits conchoidal fracture. Although the colors of garnet are varied, this mineral is most often brown to deep red. Well-developed garnet crystals have 12 diamond- shaped faces and are most commonly found in metamorphic rocks

Amphibole Group

Hornblende is usually dark green to black in color, and except for its cleavage angles, which are about 60 degrees and 120 degrees, it is very similar in appearance to augite. In a rock, hornblende often forms elongated crystals. This helps distinguish it from pyroxene, which forms rather blocky crystals. Hornblende is found in igneous rocks, where it often makes up the dark portion of an otherwise light-colored rock

rocks with nonminerals

In addition, some rocks are composed of nonmineral matter. These include the volcanic rocks obsidian and pumice, which are noncrystalline glassy substances, and coal, which consists of solid organic debris.

What Is a Rock?

In contrast to minerals, rocks are more loosely defined. Simply, a rock is any solid mass of mineral, or mineral-like, matter that occurs naturally as part of our planet.

Generally inorganic.

Inorganic crystalline solids, such as ordinary table salt (halite), that are found naturally in the ground are considered minerals. (Organic compounds, on the other hand, are generally not. Sugar, a crystalline solid like salt but extracted from sugarcane or sugar beets, is a common example of such an organic compound.) Many marine animals secrete inorganic compounds, such as calcium carbonate (calcite), in the form of shells and coral reefs. If these materials are buried and become part of the rock record, geologists consider them minerals.

Apart from their difference in color, what is one main distinction between light and dark silicates? What accounts for this difference?

Light silicates have a specific gravity of about 2.7, whereas dark silicates have a specific gravity of about 3.2-3.6. These differences are mainly attributable to the presence or absence of iron and magnesium, which are "heavy" elements. The light silicates contain varying amounts of aluminum, potassium, calcium, and sodium rather than iron and magnesium (which the dark silicates possess).

unique properties of metals

Metals are good conductors of electricity because the valence electrons are free to move from one atom to another. Metals are also malleable, which means they can be hammered into thin sheets, and ductile, which means they can be drawn into thin wires. By contrast, ionic and covalent solids tend to be brittle, and fracture when stress is applied. Consider the difference between dropping a metal frying pan and a ceramic plate onto a concrete floor.

Definite chemical composition that allows for some variation

Minerals are chemical compounds having compositions that can be expressed by a chemical formula. For example, the common mineral quartz has the formula SiO2, which indicates that quartz consists of silicon (Si) and oxygen (O) atoms in a 1:2 ratio. This proportion of silicon to oxygen is true for any sample of pure quartz, regardless of its origin. However, the compositions of some minerals vary within specific, well-defined limits. This occurs because certain elements can substitute for others of similar size without changing the mineral's internal structure.

geode

Minerals can also precipitate from slowly moving groundwater filling in fractures and voids in rocks and sediments. One interesting example, called a geode, is a somewhat spherically shaped object with inward-projecting crystals that were gradually deposited by groundwater (Figure 3.12). Geodes sometimes contain spectacular crystals of quartz, calcite, or other less common minerals.

Naturally occurring

Minerals form by natural geologic processes. Synthetic materials, meaning those produced in a laboratory or by human intervention, are not considered minerals.

Is color a good way to distinguish between orthoclase and plagioclase feldspar? If not, what is a more effective means of distinguishing them?

No. The only way to distinguish between these groups is to note whether striations (a multitude of fine parallel lines) are present or not. They can be found on plagioclase feldspar but NOT on orthoclase feldspar.

kaolinite

One of the most common clay minerals

Minerals with Independent Tetrahedra

One of the simplest silicate structures consists of independent tetrahedra that have their four oxygen ions bonded to positive ions, such as Mg2+, Fe2+, and Ca2+.

Minerals with Chain or Sheet Structures

One reason for the great variety of silicate minerals is the ability of SiO4 tetrahedra to link to one another in a variety of configurations. This important phenomenon, called polymerization,

Solid substance.

Only solid crystalline substances are considered minerals. Ice (frozen water) fits this criterion and is considered a mineral, whereas liquid water and water vapor do not. The exception is mercury, which is found in its liquid form in nature.

Minerals with Independent Tetrahedra : olivine

The mineral olivine, with the formula (Mg,Fe)2SiO4, is a good example. In olivine, magnesium (Mg2+) and/or iron (Fe2+) ions pack between comparatively large independent SiO4 tetrahedra, forming a dense three-dimensional structure

Deposition as a Result of Biological Processes

Water-dwelling organisms are responsible for transforming substantial quantities of dissolved material into mineral matter. For example, corals are organisms capable of creating large quantities of marine limestones, rocks composed of the mineral calcite. These relatively simple invertebrate animals use calcium (Ca) ions from seawater and in turn secrete external skeletons composed of calcium carbonate (CaCO3). Over time, these small organisms are capable of creating massive limestone structures called reefs. Mollusks (such as clams) and other marine invertebrates also secrete shells composed of the carbonate minerals calcite and aragonite. When the remains of these shells are buried, they become the major component of the sedimentary rock limestone. Limestone is a very common rock at Earth's surface and is visible across the entire United States. Although most water-dwelling organisms produce hard parts made of calcium carbonate, some, such as diatoms and radiolarians, produce glasslike silica skeletons. During burial, this material forms microscopic silicon dioxide (quartz) crystals that are the main constituents of rocks such as chert and flint.

mineral species

a collection of specimens that exhibit similar internal structures and chemical compositions. Some common mineral species are quartz, calcite, galena, and pyrite. However, just as individual plants and animals within a species differ somewhat from one another, so do most specimens of the same mineral.

hen the electrons are shared between the atoms, the bond is

a covalent bond

Pyroxene Group

a group of diverse minerals that are important components in dark-colored igneous rocks. The most common member, augite, is a black, opaque mineral with two directions of cleavage that meet at nearly a 90-degree angle. Augite is one of the dominant minerals in basalt

Except for quartz (SiO2), the basic structure (chains, sheets, or three-dimensional frameworks) of most silicate minerals has ___________

a net negative charge. 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.

Geologists define mineral as

any naturally occurring inorganic solid that possesses an orderly crystalline structure and a definite chemical composition that allows for some variation.

Olivine and related forms are typically found in ______________a common igneous rock of the oceanic crust and volcanic areas on the continents, and are thought to constitute up to 50 percent of Earth's upper man tle.

basalt

oxides

contain the negative oxygen ions (O2—), which bond to one or more kinds of positive ions

plagioclase feldspar

contains both sodium and calcium ions that freely substitute for one another, depending on the environment during crystallization

The Dark Silicates

ferromagnesian; are minerals containing ions of iron (ferro 5 iron) and/or magnesium in their structure. Because of their iron content, ferromagnesian silicates are dark in color and have a greater specific gravity,

Quartz (SiO2), which has only silicon-oxygen bonds, has great hardness and lacks cleavage, mainly because i________

has 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 that sheets of carbon atoms in graphite slide, giving this mineral its lubricating properties.

Silicate minerals with three-dimensional structures have the________ silicon content, while those composed of independent tetrahedra have the _________.

highest; lowest

When the valence electrons are transferred between the elements to form ions, the bond is an

ionic bond

polymerization

is achieved by the sharing of one, two, three, or all four of the oxygen atoms with adjacent tetrahedra. Vast numbers of tetrahedra join together to form single chains, double chains, sheet structures, or three-dimensional frameworks,

When calcite is the dominant mineral, the rock is called ______, whereas dolostone results from a predominance of dolomite.

limestone

When the valence electrons are shared among all the atoms in a substance, the bonding is

metallic

Silicates

minerals that contain silicon and oxygen and usually one or more other elements More than 800 silicate minerals are known, and they account for more than 90 percent of Earth's crust.

Minerals with Three-Dimensional Frameworks

n the most common silicate structure, all four oxygen ions are shared, producing a complex three-dimensional framework

Mineral species are assigned to mineral classes.

ome important mineral classes are the silicates (SiO44—), carbonates (CO32—), halides (Cl1—, F1—, Br1—), and sulfates (SO42—). Minerals within each class tend to have similar internal structures and, hence, similar properties. For example, minerals belonging to the carbonate class react chemically with acid—albeit to varying degrees—and many exhibit similar cleavage. Furthermore, minerals within the same class are often found together in the same rock. For example, halite (NaCl) and silvite (KCl) belong to the halide class and commonly occur together in evaporite deposits.

Every silicate mineral contains the two most abundant elements in Earth's crust,

oxygen and silicon

ions

positively and negatively charged atoms

One group of feldspar minerals contains potassium ions in its structure and is therefore referred to as

potassium feldspar.

three ways minerals can form

precipitation of mineral matter from a solution, crystallization of molten rock by cooling, and mineral matter deposition as a result of biological processes

mineral varieties

pure quartz (SiO2) is colorless and transparent. However, when small amounts of aluminum are incorporated into its atomic structure, quartz appears quite dark, in a variety called smoky quartz. Amethyst, another variety of quartz, owes its violet color to the presence of trace amounts of iron.

other ways silicate minerals form

some silicate minerals form at Earth's surface from the weathered products of other silicate minerals Still others are formed under the extreme pressures associated with mountain building. Each silicate mineral, therefore, has a structure and a chemical composition that *indicate the conditions under which it formed. *

Clay Minerals

term used to describe a category of complex minerals that, like the micas, have a sheet structure.

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—).

Crystallization of Molten Rock

the crystallization of minerals from molten rock, although more complicated, is a process similar to water freezing (Figure 3.13). When magma is hot, the atoms are very mobile, but as the molten material cools, the atoms slow and begin to chemically combine. Crystallization of a molten mass generates igneous rocks that consist of a mosaic of intergrown crystals that tend to lack well-developed planar surfaces, or faces

Because most silicate structures 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 two end members. Examples?

the olivines, pyroxenes, amphiboles, micas, and feldspars.

Precipitation of Mineral Matter

Perhaps the most familiar way in which minerals grow is from an aqueous (water) solution containing dissolved material matter (ions). As long as the solution is not saturated, the motion of the dissolved ions keeps them from joining together. Two factors—a drop in temperature and water lost through evaporation—cause the solution to become closer to reaching saturation. Once saturation is reached, the ions begin to bond, forming crystalline solids (called salts) that precipitate from (settle out of) the solution.

Salt being mined in Bolivia's Salar de Uyuni

The Salar de Uyuni is a salt flat that overlies a lake of brine up to 20 meters (66 feet) deep. It is the world's largest salt flat—25 times larger than the Bonneville Salt Flats in Utah. This region is completely surrounded by mountains and is quite arid, so water that enters the valley leaves via evaporation—which results in deposition of salts, including sodium chloride, magnesium chloride, and lithium chloride.

few minerals, such as native gold, silver, and copper, are made entirely of metal atoms packed tightly together in an orderly way

The bonding that holds these atoms together results from each atom contributing its valence electrons to a common pool of electrons, which freely move throughout the entire metallic structure. The contribution of one or more valence electrons leaves an array of positive ions immersed in a "sea" of valence electrons

quartz bonds since hard, resists weathering, and does not have cleavage.

three-dimensional framework is developed through the complete sharing of oxygen by adjacent silicon atoms. Thus, all the bonds in quartz are of the strong silicon-oxygen type