States of Matter
I would imagine that there are students in the class who did not take chemistry in high school or transfer it in from another college. Some fundamental knowledge of chemistry will be required and the lectures and text should provide you with sufficient background. Don't panic if this is new material. Read the texts, read this summary and ask questions! Normally these summaries will not be as much "text" as this one contains. This is because past experience suggests that additional material is needed in this section to bring everyone up to about the same level of understanding. One goal is to make you familiar with some of the important Earth Materials; the amethyst crystals shown above have been highly prized for their esthetic quality. [Illustration from - http://www.nosc. mil/planet_earth/ photos.html ]
States of Matter
Anything that has mass and occupies space is matter. The following classification will help define the states in which matter can occur.
MineralsRead through this material until you come to the section dealing with rocks.
A mineral:
All matter is made up of chemical elements, each of which is made up of particles called atoms. A rather crude, but useful, picture of an atom is that of a rigid sphere. The radii of these atoms are a few angstroms (where an angstrom is 10-10cm. There are 92 so-called naturally occurring elements of which 89 can be detected on Earth at this time. What happened to the "missing 3" will be left for you to think about.
These spheres contain a central nucleus which contains two types of particles:
The atomic number is the number of protons in the nucleus of the atom. The atomic mass number is the number of protons plus neutrons in the nucleus. All atoms of an element have the same number of protons - hence the same atomic number; for example all atoms of hydrogen have one proton in the nucleus. Each element is assigned a symbol - H for hydrogen. You should learn the symbols for the eight most abundant elements in the Earth's crust (Oxygen (O), Silicon (Si), Aluminum (Al), Calcium (Ca), Iron (Fe), Magnesium (Mg), Sodium (Na), and Potassium (K) .
If we could weigh individual atoms of hydrogen we would find some variation in their mass; this must be due to variations in the number of neutrons in their nucleus as all must have the same number of protons. Atoms which have the same atomic number but different mass numbers are isotopes. For example, all carbon atoms have an atomic number of 6 but there are three isotopes of carbon -12, 13, and 14. (How many neutrons does each isotope possess?)
In addition to the particles in the nucleus, there are negatively charged particles - electrons which "orbit" the central nucleus. Recognition that the distribution of electrons around the nucleus is not random was one of the great accomplishments of physicists in the 1920s - quantum mechanics. Electrons occupy "levels" which are separated from each by some volume of space =which electrons can pass through but not remain. In fact, much of the volume of an atom consists of empty space. [I remember that this is how Superman was able to pass through solid objects!] Electrons are "attracted" to the nucleus (positive and negative charges attract). The primary division of these levels is called the principal quantum number. The first level (K) is closest to the nucleus. As the principal number increases 1...2....3....4 the number of electrons that can reside in the level increases.
Level Electrons 1 = K 2 2 = L 8 3 = M 18 4 = N 32The maximum occupancy = 2n2 (two times n squared )
In a neutral atom the number of negative particles equals the number of positive particles (electrons = protons).
A useful exercise is to start with the first element (Hydrogen) with its one proton and one electron and to add one proton and one electron to build up the remaining chemical elements. The build-up process works as follows. Start with a single electron - it occupies the empty level closest to the nucleus The second electron goes into the first level (atomic Helium) and the shell K) is filled. Actually, there are sub levels within the main level (except for the K-shell).
Sub Shell Max Electrons s 2 p 6 d 10 f 14For element 11 (Sodium - Na) there are 11 electrons and 11 protons. Two electrons are in the K shell, 8 in the L shell and one in the M shell. The outer most electron(s) are termed the valence electrons. If one electron were removed what is the balance between positive and negative charges? An ion is a charged atom; that is, there is an excess of positive (cation) or negative charge (anion). If an electron is removed a cation is formed. If an electron is added an anion is formed. Of the eight most abundant elements in the Earth's crust only Oxygen forms anions; the rest form cations by loosing one or more electrons. When there are 8 outer most electrons in the s and p sub shells the atom has a tendency to resist change and ions are formed with great difficulty if at all.
Main Level Orbitals K 1s (max = 2) Total = 2 L 2s (max = 2), 2p (max = 6) Total = 8 M 3s (max = 2), 3p (max = 6), 3d (max = 10) Total = 18
Chemical Bonding - Most elements in the Earth react to form compounds although there are a few which are stable as elements (gold, for example). There are several "bonding models" which need to be summarized.
The Periodic Table provides a framework in which to place the
chemical elements so that their similarities are recognized. Look at the properties of Oxygen and Silicon - the two most abundant elements in the Earth's crust - by clicking on their symbols on the Periodic Table. The recognition that the elements could be arranged in a systematic way so as to emphasize relationships between atoms, was a major break through in the history of chemistry. For example, all of the elements in the first column (the alkali metals) have a single outermost electron in its outermost sub shell (an s sub shell). All of these elements can lose a single electron forming a cation with a +1 charge. All of the elements in the column on the far right (inert or Nobel gases) have two s electrons and eight p electrons in their outermost level (called the valence level). Note that the Periodic Table has the shape of a distorted "H". The vertical bars (sides of the H) contain the A group elements. The central bar contains the "transition" elements. Down at the bottom of the page are two rows - the "Lanthanides" and the "Actinides". Look to see where these rows fit into the "H". If these rows which shown in proper position the Periodic Table would be less compact.
Questions
Think about the statement that oxygen occupies 95% of the volume of the Earth's crust. If oxygens were cubes they could be packed together to fill up space. However, the oxygens are presumed to be spheres and you cannot pack equal sized spheres to fill up all space; some open spaces will remain inside of the framework produced by the oxygens. Other ions fit into these open spaces. In general, these spaces are "regular". One common type of space is that created when there are three oxygens on the bottom and one on the top. This is called a "tetrahedral void". (A tetrahedron is a regular solid consisting of four faces, each of which is an equilateral triangle.) The larger the ion the larger the preferred site. The coordination number of a cation is the number of nearest neighbor anions. Silicon, with one exception, prefers to "sit" in a tetrahedral void formed by packing Oxygen anions together. Thus, silicon usually has a coordination number of 4. Coordination numbers are used to produce a structural formula.
Remember that the subscripts give the number of ions in one formula unit and the numbers above the chemical symbols given the coordination numbers.
Quartz 4 SiO2 Stishovite 6 SiO2
Quartz and Stishovite are polymorphs (many forms). They have identical chemical compositions but differ in structure and hence in physical properties. An increase in pressure favors a larger coordination number. Temperature has the reverse effect. If a large meteorite were to impact quartz then stishovite might form (if the pressure were sufficiently high).Other polymorphs are
In the definition of a mineral it was noted that a mineral has a fixed composition (like quartz) or a composition that is variable within a limit. Consider the two minerals Forsterite and Fayalite.
Forsterite 6 4 Mg2SiO4 Fayalite 6 4 Fe2SiO4
Note that the two minerals differ chemically in that one has Mg (magnesium) and the other Fe (iron). The rest of the chemical formulas are identical. Note that both Mg and Fe sit in 6 fold sites (octahedral). The sizes of these two ions are nearly identical, the charge on both is +2 and the Mg-O and Fe-O bonds are dominately ionic in nature. When such similarities occur the ions may substitute for each other forming a solid solution series. The series between these two end members is called the olivine series. Compositions range from pure Forsterite to pure Fayalite. Not all solid solution series are complete; some exhibit a limited amount of substitution.
Almost all of the common minerals we will work with are solid solution series. Quartz is the notable exception.
Classification of Minerals
The broadest classification of the nearly 3,500 known minerals is based on chemical composition. Thus, we recognize Native Elements (individual chemical elements), Carbonates (containing the CO3 group, Silicates (containing Silicon and Oxygen) and other broad chemical groups.
Chemically, the silicates are very complicated and not much progress was made in understanding until a structural classification was devised. The most common "structural element" is the silicon/oxygen tetrahedron. The simplest structural class of silicates consists of those compounds (minerals) which consist of isolated single tetrahedra - the Nesosilicates. Tetrahedra can share oxygens between themselves. Two, three and four oxygens per tetrahedron can be shared and in some structures two or more sharing schemes exist. The most common minerals are listed below. (keep in mind that all of these contain Si and O. many also contain Al).
Mineral Structural Type Composition
Olivine Isolated Fe and Mg rich
Pyroxene Single Chains Ca, Fe, and Mg rich
Amphibole Double Chains Ca, Fe, Mg and K rich (with "OH")
Mica Sheet Silicates K, Na rich (with "OH")
Plagioclase Tectosiicates Ca and Na rich
Alkali Feldspar Tectosilicates Na and K rich
Quartz Tectosilicates Si
In addition, the following mineral groups are important - especially in the sedimentary rocks
Carbonates
Sulfates
Salts
Silicate Structural Classification. [then return to this page]
Questions
Use the section on Properties to help answer the following questions.
A good index of minerals and gems is available for students at the University of Wisconsin.
If you like to look at mineral specimens you will probably want to look at some images from the Smithsonian Mineral Collection Smithsonian Minerals