Matter and MineralsSummary of Important Concepts to Be Covered•The Earth consists of four interconnected systems: the solid earth or lithosphere (rock and soil); theatmosphere; the hydrosphere; the biosphere; These systems interact and influence each other.•The earth’s crust consists of rocks made of various types of minerals. Minerals are classified into differentgroups according to their chemistry. The silicate mineral group makes up most rock of the crust.•Rocks are composed on one or more minerals. There are three main classes of rocks: –Igneous rocks form from solidification of magma (molten rock) –Sedimentary rocks form from solidification of sediment (particles formed from the erosion of older rocks, or by chemical or biological processes at the earth’s surface). –Metamorphic rocks form from other rocks changed by heat and pressure•One type of rock can be transformed into another type over geologic time; a process known as the rockcycle.Importance of Minerals•The earth’s crust is made of various kinds of rock, and rock is composed of one or more minerals. Forexample, as this picture shows, the rock granite is composed mostly of the minerals quartz, biotite, andfeldspar.•Knowing the chemistry is essential to knowing how they combine to form rocks and the stability of therocks.•Uses: natural resources, nutrients, metals, building materials, jewelry, computer chips.•~4000 species of mineral but only about 20 or so are actually common.Definition of a Mineral•Naturally occurring (not synthesized by man).•Inorganic (not made from living organisms).•Solid that is homogenous (the same throughout).•Ordered crystalline structure (atoms are arranged in a definite pattern that repeats throughout the entiremineral).•Definite chemical composition expressed with a formula.Atoms, atoms, atoms! • An element is electrically neutral. • ~92 naturally occurring, ~17 man made • Smallest unit of matter that cannot be broken down into other substances (i.e. it’s the smallest part of the element that retains the properties of that element).Minerals are made of various chemical elements, or atoms. • Atoms, which form all matter, consist of three kinds of particles. protons with a positive charge and neutrons with a neutral charge -- occupy the center, or nucleus, of the atom. The nucleus contains nearly all the mass of the atom. -- electrons with a negative charge -- orbit around the nucleus. Electrons have almost zero mass. All atoms have this same general structure. By changing the numbers of protons we make different chemical elements!Periodic Table of the Elements http://www.chemicalelements.com
Anatomy of an Atom•Atomic number = # of protons (+). Determines the element.•Atomic mass= # protons and neutrons in nucleus.•Neutrons, no charge, contributes to atomic mass.•Electrons (-), doesn’t contribute to mass.•All atoms are electrically neutral. That means the number of protons (+) in the nucleus eguals the numberof electrons orbiting the nucleus.Electrons•Negatively charged sub-atomic particles that move very quickly around the nucleus.•Electrons are arranged in ‘shells’ or ‘orbitals’ around the nucleus. Electron shells are merely SPACEsurrounding the nucleus at different levels. Electrons within a shell can move anywhere within this space.•The shells are layered where the first shell can only hold up to two electrons. Each successive shell canhold up to 8 (as you get up in atomic number, the number or electrons/shell varies…we will not worry aboutthat).•For an atom to be STABLE, that is, not ‘bond’ or chemically react with another atom, its VALENCEshell (outermost shell) must be full.•Electrons and valence shells are involved in chemical BONDING.•Chemical bonding between elements forms minerals. Covalent and Ionic BondsTO BOND OR NOT TO BOND•If the number of protons equals the number of electrons (+ = -) and the ‘checkbook’ balancesthe atom hasno charge.•Having no charge does not necessarily mean the atom is stable.•Take Na (sodium). Check the periodic table to get its atomic number (the number of protons, that is). Naatomic number is 11. How many electrons in the valence shell? One valence electron. Well, to be stable andnon reactive, we want to have a full valence shell. Na can accept 7 electrons or give up 1. Which seemseasier? Sure! It is much easier to donate that one electron so that the shell is full. What is the charge onsodium if it gives up that electron? +1 It has a plus one charge and is now called an ION, specifically acation (positively charged ion). If an element gives away two electrons it will have a +2 charge and so forthand so on.•Now go look up the atomic number for Chlorine Cl. How about the valence shell? You should come up with 7valence electrons. What a dilemma. Cl only needs ONE more electron to have a full valence shell. If Clobtains one electron it will have 17 protons, 18 electrons and a charge of -1. An anion is a negatively chargedion.•Now we have an electric glue. Opposites attract right? Well, Na+1 and Cl-1 will have to form a bond.Ionic Bonds•An ionic bond is the chemical bond that results from the electrostatic attraction between positive ions(cations) and negative ions (anions). It can form when two atoms meet and an electron is permanentlytransferred from one to the other. These two ionized atoms then stick very tightly together to make sodiumchloride -- also known as table salt (or the mineral halite).•So basically electric glue holds NaCl together. This bond is not that strong (as we can dissolve NaCl easilyin water).Covalent Bond•Okay so what if you have 4 valence electrons. Would you give up or obtain 4 electrons. Probably neither.
Your best bet in this situation is to just SHARE the electrons.•Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms. A Covalent bond isformed when the valence shells of two atoms overlap and the electrons are free to roam between bothshells.•The number of covalent bonds an atom can have depends on the number of valence electrons.•Note: the more covalent bonding a mineral has in its structure, the more stable (stronger) that mineral is,and the more difficult to weather or break it down.Common Elements of the Earth’s Crust•Okay so if there are thousands of minerals, why only a few common ones? The answer is that there are onlya few elements that make up MAJORITY of the earths minerals (and minerals make up rocks). •Si (silicon)S •O (oxygen)OSi and O form the silica tetrahedron •Na (sodium)N+1 •K (potassium)K +1 •Ca (calcium)C +2 •Mg (magnesium)M+2 •Al (aluminum)A +3 •Fe (iron)F +2•98% of the earth’s crust are made of combinations of these 8 elements.•92% of the earth’s crust are made of ‘silicate’ minerals, those containing mostly Si and O.Geologists classify minerals into groups that share the same negatively charged atoms or molecules.For example:The oxide group consists of all minerals formed from various elements bonded to negatively charged oxygen(O -2) atoms. The carbonate group consists of all minerals formed from elements bonded to negativelycharged carbonate ((CaCO3 ) -2 ) molecules. The silicate minerals are the MOST COMMON minerals in theearth’s crust.Silicate MineralsThe silicates are the most abundant and important mineral group. Silicates are minerals composed ofsilicate tetrahedra (SiO4)- 4 bonded to each other and/or to other elements. A single silicate tetrahedronconsists of 1 silicon (Si) atom surrounded by 4 oxygen (O) atoms, forming a tetrahedron shape. Thesetetrahedra bond readily to each other, and to other elements, forming several kinds of abundant minerals,including quartz and feldspar.The Silica Tetrahedron The fundamental building block for the silicate minerals is called the silica tetrahedron. Stats on Si and O are as follows: Si atomic number is 14 How many valence e-? (4) O atomic number is 8 How many valence e-? (6)• Silicon therefore can form four covalent bonds with oxygen atoms. If each oxygen (four total) share one electron with silicon, then silicon’s valence shell is full, but what about oxygen? No, each oxygen has two ‘vacancies’ in its valence shell and will require two covalent bonds. So now what? The chemical formula Si04 will then have a net negative 4 charge. This is because, each of four oxygens have one vacancy in their valence shells. Each Oxygen therefore needs to obtain one more electron to fill its outer shell. If
the MOLECULE accepts four electrons to become STABLE (and it will), the tetrahedron itself will have obtained four electrons (have full valence shells) and will have an overall charge of -4. So where does it get these other 4 electrons? From atoms that want to get rid of their valence electrons (to become cations).• Of the remaining abundant elements of the earth’s crust, you can see that there are many combinations of cations that will form ionic bonds to the tetrahedron and thus form a variety of different minerals. So, eight elements can make many different SILICATE minerals that make up the earth’s crust.• The silica tetrahedron itself is covalently (strong) bound, but each tetrahedra is electrically glued to adjacent tetrahedra via the cations. If the ‘silica tetrahedra backbone’ is arranged in this way, we call this the isolated silicate structure (see next slide).• The OTHER way the silica tetrahedra can ‘fill’ its valence shells is by covalently bonding to adjacent tetrahedra. If it forms two, three, or four covalent bonds (oxygens sharing electrons with oxgyens of adjacent tetrahedra) and subsequent demand for electrons drops, the net negative charge on the tetrahedron becomes less and less.• If all four oxygens share electrons, all covalent bonds are formed. There will be no charge (and thus no bonding to cations).Isolated Silicate Minerals•Olivine is an example.•Consists of one tetrahedron bound by 4+ ions.•It is ‘isolated’ because it shares no oxygen atoms with other tetrahedra.•Si:O = 1:4 = +4:-8, net charge = -4•-4 binds to +4 cations.•Because this silicate structure has the highest negative charge, it will form the MOST amount of ionicbonds with adjacent tetrahedra rendering this the WEAKEST silicate structure.•Olivine contains a lot of Fe and Mg (recall layers of the earth and distribution of Fe, Mg, Si and O).Single Chain Silicates•Augite is an example.•Look at the diagram to the left A-single chains.•View a single blue dot representing the silicon atom—each is surrounded by four beige oxygen atoms.•Pick out any tetrahedra and see how TWO oxygens are being shared with adjacent tetrahedra.•Because of some covalent bonding in this ‘backbone’ the net charge is -2 for this structure.•Per individual tetrahedra, only two electrons are accepted for the two NON covalently bound oxygens.•The demand for + ions is less. These minerals have more covalent bonds than the isolated structure and aresubsequently stronger.Double Chain Silicates• Hornblende is an example.• Examine the B-Double silicate in the figure. Every other tetrahedron shares 3 OR 2 oxygens.These silicates have MORE covalent bonds and LESS ionic bonds than the single chains (and so are stronger yet).• Si:O = 1:2.75• +4:-5.5, net charge –1.5• Double chains of silicates are linked together by +1.5 ‘worth’ of cations.Sheet Silicates• Muscovite and Biotite as examples.
• Suppose we covalently bind three oxygens with adjacent tetrahedra. The only oxygen that had accepted an electron is the top oxgyen (see diagram—it is the one pointed at you).The silica is arranged in sheets, and the sheets are electrically glued together by cations. These minerals break easily between the ionic bonds between the sheets.• Si:O = 1-2.5• +4:-5, net charge –1• Positive ions IONICALLY bond sheets together.•Framework Silicates• Each tetrahedron shares all four oxygens with adjacent tetrahedra.• Si:O = 1:2.• All four oxygens shared with adjacent tetrahedron forms a complex 3-D network, all covalently bound.• Quartz as an example.• Quartz therefore is a pretty stable mineral as it is only SiO2 all covalently bound. •The reason that the formula is SiO2 is because each silicon ‘owns’ half of four oxygens (sharing with other silicon atoms).Quartz (and glass) will not ‘break’ in a straight line or preferential direction (minerals tend to break along planes of weakness resulting from ionic bonds). Four covalent bonds means strength between the atoms is equal in all directions. Hence, glass (and quartz) breaks in curved patterns (wherever the force was the strongest). Silicate Structure Isolated Single Chain Double Chain Sheet Framework Example Olivine Augite Hornblende Mica Quartz Percent SiO2 Lowest Lower Low Medium High Highest Higher High Medium Low Percent Fe, Mg Ultramafic Mafic Mafic Felsic Felsic Si:O 1:4 1:3 1:2.75 1:2.5 1:2 Net Charge +4-8=-4 +4-6=-2 +4-5.5=-1.5 +4-5=-1 +4-4=0 Covalent Bonds Lowest Lower Low Medium High Ionic Bonds Highest Higher High Medium Low Density Highest Higher High Medium Low Stability Lowest Lower Low Medium HighOf the Silicates we can divide them up into smaller groups.Felsic Silicate Minerals FELSIC silicate minerals contain high percentages of Si and O and very little Fe and Mg (sometimes none at all). Examples include Quartz, Orthoclase Feldspar, Plagioclase Feldspar. Felsic minerals ‘tend’ to be light colored. What layer of the earth do you think would be made up of rock composed of felsic minerals? Answer: CONTINENTAL CRUST
Mafic Silicate Minerals Of the silicates, those that have higher percentages of Fe and Mg are called MAFIC minerals. They tend to be darker in color. Examples include Olivine, Hornblende, Augite, Biotite Mica. Mafic minerals, due to their higher percentages of iron and magnesium means they are ‘heavier’ or denser. Which layers of the earth should have the denser stuff? Answer: OCEANIC CRUSTNon-Silicate Minerals do not have the silica tetrahedron.•Any minerals that do NOT contain silica in their chemical formula.•Examples, Halite (NaCl), Fluorite (CaF2), Calcite (CaCO3), Magnetite (Fe3O4).•Metallic minerals include: Al (3rd most common element), Cu copper electrical, Pb lead batteries, Zn zinccoats ‘tin’ cans, Au, Ag, PtMineral IdentificationCrystals Minerals take the form of crystals of different colors and shapes. The different colors result from the ways that different atoms absorb and reflect light. The different shapes result from the different ways that bonded atoms pack together in particular geometric arrangements.Fracture and Cleavage These terms describe the way a mineral breaks. If a mineral fractures, it breaks along rough edges. If it cleaves, then it breaks along smooth, flat surfaces or definite planes as determined by their crystal structure.Hardness This is how resistant a mineral is to being scratched. We use the Mohs scale to classify a given minerals hardness. The technique is rather simple. All you do is try to scratch the unknown mineral with minerals of the mohs hardness scale or various items of known hardness, such as a fingernail (hardness of about 2.5), a penny (3), and a steel nail (5.5). The item that first scratches the unknown is harder than the unknown so then you must estimate.Luster The way a mineral reflects light. Luster can be metallic - looks like a metal; or nonmetallic. Nonmetallic minerals can be described with other terms such as glassy, oily, greasy, and earthy.Streak The color of a mineral when it is powdered is called the streak of the mineral. Streak can be determined for any mineral by rubbing the mineral across the surface of a hard, unglazed porcelain material called a streak plate. The color of the powder left behind on the streak plate is the minerals streak.Mineral Chemistry
•So the take home message is that mineral chemistry determines the ‘strength of the mineral’ andcontributes to the mineral’s properties.•The more covalent bonding a mineral has, the less the ionic bonding and the stronger it is.•Minerals with higher percentages of iron and magnesium (and therefore a lot of ionic bonds) have weakersilicate structures (isolated, single, double) and are denser. These minerals make up denser layers of theearth.•Minerals with lower percentages of iron and magnesium (due to more covalent bonding between adjacenttetrahedra) have stronger silicate structures (sheet, framework) and are lower density.•We will DEFINITELY revisit this stuff during igneous rocks, metamorphic rocks, weathering, etc.