KEY POINTS: ATOMS & ELEMENTS (AtomsElementsMineralsRocks)• Rocks are made up of one or more minerals.• Minerals are composed of one or more elements.• Elements such as oxygen or iron are made up of groups of like atoms.• Atoms are composed of combinations of protons, neutrons, and electrons.• Atoms of elements are joined together by bonds.
Minerals•Rocks are made up ofone or more minerals.•Geologists haveidentified more than4,000 different minerals.•But only about 20minerals are commonlyfound in rocks.•Typically, individualrocks are composed ofjust a handful ofminerals.•Granite (R) is composedof 4:quartz, feldspar, hornblende and biotite (a typeof mica).
Most Common • Granite is an igneous rock that isElements in composed primarily of fourGranite Minerals minerals. Magma cooling very•Quartz (oxygen & slowly far under the earthssilicon). surface, allows crystals of the four•Feldspar minerals to grow large enough to be(oxygen, silicon, aluminu easily seen by the naked eye. Thesem, calcium, sodium & minerals arepotassium) quartz, feldspar, mica, and usually•Mica hornblende.(oxygen, silicon, aluminum, iron, calcium, magnesium & hydrogen).•Hornblende [a.k.a.amphibole](oxygen, silicon, aluminum, iron, calcium, magnesium & hydrogen).
Quartz vs.Hornblende (a.k.a.amphibole)The mineral quartz(top R) is composed ofjust two elements:silicon and oxygen.Other minerals maycontain manyelements.Hornblende oramphibole (bottom R)is made up ofcalcium, magnesium, iron, aluminum, silicon, and oxygen.
Four of the Most Common Minerals• Quartz quartz is a common mineral that is found in many different types of rocks. The chemical formula is Silicon oxide (SiO2). Impurities in the rock at the time of formation causes the quartz crystal to produce different colors. Quartz can be colored yellow, milky white, rose, brown to black, blue and the best known color purple amethyst. Feldspar Feldspar is the most abundant mineral in rocks that are located at or near the earths surface. Feldspar can have a glassy white, blue, green, pink or red crystals. When feldspars are exposed to the atmosphere they weather easily. Clays are formed by weathered feldspar. Kaolinite is the highest quality of the feldspar clays used by potters.• Hornblende: Hornblende is a mineral that contains magnesium, iron, silica and aluminum. Hornblende is black, brown and green in color. It occurs in crystals of many igneous rocks.• Mica: Mica can be clear, black, green, red, yellow, brown, purple lapidolite and other colors too. Clear mica was named Muscovite because it was found near Moscow, Russia. It was used as window glass in the Muscovites homes. Muscovite contains water which helps to make it clear. Biotite mica is dark green to black in color because it contains metals such as iron and magnesium.
What are MineralsMade of? Elements!•Minerals can becategorized by thechemical elements theycontain.•There are 92 naturallyoccurring elements onEarth.•98% of the rocks in thecrust below our feet aremainly composed of just8 elements.•These eight elements arecommon ingredients ineveryday objects or in thefoods we eat.
Eight Most Common Elements in Continental Crust (The most common minerals in Earth’s crust are composed of the most common elements.)
Certain elements arevital for humanhealth.•Human food containsmany of the commonelements that are naturallyextracted from rocks asminerals break down atEarth’s surface to form soil.•These elements areabsorbed by crops growingin soils and find their wayto our bodies via the foodwe eat.•Many of the elements areessential for good humanhealth.
Elements and Atoms• Elements can’t be subdivided into other materials.• Elements can be separated into individual atoms.• An atom is the smallest particle that retains the characteristics of an element.• Atoms are composed of one or more of the same three basic components: electrons, neutrons and protons.• The number of protons in an atom is unique for each element and is called the element’s atomic number, e.g. oxygen = 8; silicon = 14 (protons).
Helium Atom (not to scale). Note: charges of electrons, protons, & neutrons.
Helium and Neon: In all atoms electrons lie in energy levels around the nucleus. There are only 2 electrons in the firstlevel, and maximum of 8 electrons in the second level. In thesetwo elements the number of electrons = protons, so they don’t form ions and don’t bond with other elements.
Ions and BondsThe outer energy levels ofmany elements areincomplete.These elements may gainelectrons from otherelements or may loseelectrons.Atoms that lose or gainelectrons are known asions and have a positive ornegative electrical charge.Positive and negative Ionsof individual elements areattracted to each otherand bond together to formminerals (note sodiumchloride, R).
How salt is formed from two ions(negatively and positively charged) seeking stability via an ionic bond.
CovalentBondingWater (H20), one of themost common substanceson the planet is composedof two atoms of hydrogen(H) and one of oxygen (O).Each hydrogen atom hasan electron that is sharedwith oxygen to give it a fullenergy level.Likewise, oxygen shares anelectron with each of thehydrogen atoms.This type of sharing iscalled covalent bondingand typically occurs whereatoms share electrons toachieve a stable structure.
Together oxygen and silicon account for 70% of theatoms in the Earth’s continental crust. Many crustal minerals are mainly combinations of these two elements.
Common Silicate Structures (tetrahedron combinationsthat form 1. olivine, 2. pyroxene group, 3. amphibole or hornblende group, 4. mica group, and 5. quartz/feldspar group).
The SilicatesThe bonding of siliconand oxygen atomsforms a pyramid-likeshape known as thesilica tetrahedron.Minerals made ofsilicon and oxygen areknown as silicates.Silicates featureatomic structures thatare characterized bydifferentarrangements of thesilica tetrahedron.
Tetrahedra = Basic Building Blocks• These basic building blocks, the tetrahedra, can be joined together in combination with other elements (using ionic or covalent bonds or both) to form different silicate minerals such as those found in granite.• The amount of silica present in magma is especially significant in controlling magma viscosity.• Silica combines with other elements in magma (e.g. iron, magnesium, potassium).
Silica content determines magma viscosity.• In magmas with low silica content atoms combine to form minerals such as olivine and pyroxene with simple forms like pairs and single chains.• With more silica, the tetrahedra form complex double chairs, sheets, and three-dimensional frameworks.• These larger, more complex forms get tangled together in the magma, making flow difficult and resulting in higher viscosity
Bond Strength/Weakness• Ionic bonds formed by electrical attraction are typically weaker than covalent bonds where electrons are shared.• Minerals composed of ions that form covalent bonds are typically stronger, and therefore more likely to be preserved on the Earth’s surface, than those with only ionic bonds.• The diamond, the hardest mineral, is formed of carbon atoms that share covalent bonds among four neighboring atoms.
Character of Rocks• Rocks’ character depends to a large degree on the atomic structures of their minerals and the way the atoms are bonded together.• Some minerals are strong, like quartz, and appear in a variety of roles in different rock types.• Other minerals have weak bonds and will dissolve in water.• The combination of elements combining to form minerals will determine the melting temperature of rocks and behavior of the magma produced.
MINERALS: KEY POINTS• A mineral is a naturally occurring, inorganic solid with a definite chemical composition and uniform atomic structural elements.• Minerals can be identified on the basis of characteristic physical properties such as:(The four features used most often to characterize minerals)1. Crystal form2. Cleavage3. Hardness4. Color(The two less common features used)5. Luster, and6. Streak
Crystal FormsThe shape of a mineralcrystal is related to theway the chemicalbonds form.Atoms or molecules ofa mineral align in auniform manner toform a crystalstructure.Common shapes areprisms, pyramids, needles, cubes andsheets.
CleavageDepending on howtheir constituentatoms arearranged, mineralsmay brake alongspecific surfaces ofweakness calledcleavage planes.Cleavage planes markthose parts of themineral where ionsare connected byrelatively weak ionicbonds.
Cleavage PlanesCovalent bonds arestrong and less likelyto form cleavageplanes.Mica separates intosheets (R).Other minerals mayhave multiplecleavage planes:sheets, cubes orother shapes.Quartz has none.
HardnessTen minerals make upthe Mohs hardnessscale (R).Minerals not in thetable are rankedrelative to these.For example, a mineralthat could scratchfeldspar by not quartzwould have ahardness of about 6.5.A copper penny ranksaround 3; glass 6.
ColorUsed imperfectly todescribe minerals.Minerals come in a varietyof colors.However, some mineralsexist in a wide range ofcolors.Quartz, forinstance, occurs in over adozen different colors.And, minerals can changecolor when they areexposed to changingnatural conditions(rain, heat) on or near theEarth’s surface.
Luster andStreakLuster = how light isreflected from a mineral.Streak = the markformed when a mineralis scratched across anunglazed piece ofporcelain.Gold creates a yellowstreak; iron sulfidemakes a black streak.Other properties: calcitecreates bubbles of CO2when exposed to acids;and sulfur smells bad.
Three Classifications of Rocks:Igneous, Sedimentary and Metamorphic (differentiated on the basis of their chemical and physical properties)
Igneous Rocks: KEY POINTS• Igneous rocks form from the cooling of melted rock (magma).• When magma reaches the surface it cools quickly and solidifies into small mineral crystals in volcanic igneous rock.• If magma solidifies below the surface, it cools slowly to form large mineral crystals in plutonic igneous rock.• Igneous rocks can be classified by texture and color (indicating mineral composition)• Types of igneous rocks vary with plate tectonic settings.
Plutonic vs.Volcanic IgneousRocksIgneous rock is formedthrough the coolingand solidification ofmagma or lava.Igneous rock may formeither below thesurface from magmaas intrusive (plutonic)rocks or on the surfaceas extrusive (volcanic)rocks from lava.
Igneous landforms: Volcanic process form volcanoes, lava flows and ash falls. Plutonicprocess lead to batholiths, laccoliths, dikes, sillsand crystallized magma chambers called “stock.” Another Version of Figure 7.18
Batholith:a great mass of intruded igneous rock that forthe most part stopped in its rise a considerable distance below the surface (Stone Mt. Georgia & its confederate memorial).
Half Dome, a granite monolith inYosemite National Park is part of the Sierra Nevada batholith.
Laccolith = a mass of igneous rock that is intrudedbetween sedimentary beds and produces a domical bulging of the overlying strata
Sedimentary Rocks: KEY POINTS• Clastic sedimentary rocks: composed of sediments – rock and mineral fragments that form when rocks break apart at or near Earth’s surface. Clastic sedimentary rocks make up the majority of all sedimentary rocks.• Chemical sedimentary rocks: crystallized from a solution, e.g. seawater, as a result of changing conditions.• Biochemical sedimentary rocks: form by the actions of living organisms or are composed of the remains of dead organisms.• Often form in a series of layers called beds or strata.
Clastic Sedimentary Rocks• Clastic sedimentary rocks are formed from rock and mineral fragments (clasts).• This process occurs in a series of steps:1. Generation of clasts due to the breakdown of an original rock by weathering;2. Transportation of the eroded material from the source area; and3. Lithification—the deposition and subsequent conversion of the material to rock.
Sedimentary Rocks: Typically are composed of beds or strata.
Limestone (a sedimentary rock deposit) permits water to seep through it and form caverns.
Metamorphic Rocks: KEY POINTS• Metamorphism relates to changes in mineral composition or texture that occur in solid rocks as a result of increasing pressure or temperature.• The temperature range for rock metamorphism is approx. 200 to 1,100 degrees C (390-2,010 degrees F).• Contact metamorphism: occur when rocks come in contact with a heat source (usually magma).• Regional metamorphism occurs when rocks undergo increased temperatures and pressure typically associated with the plate tectonic processes that form mountains.
Igneous Rocks Relative abundance ofin the earths crust (basalt and gabbro dominate the igneous category).
MetamorphicRocksRelativeabundance ofmetamorphicrocks in theearths crust(gneissdominates).
The Rock Cycle and Minerals: KEY POINTS• The rock cycle links the principal igneous, sedimentary, and metamorphic rocks together in an idealized view of the formation of rocks in the Earth’s crust.• The rock cycle serves as an example the Earth operates as a system and constantly changes.• Minerals form in a range of geologic settings associated with the formation of the most common rock types.
SedimentaryRocksRelativeabundance ofsedimentaryrocks in theearths crust(shaledominates).