Mineralogy is the branch of geology that studies the composition, structure, occurrence and characteristics of minerals.

1. A mineral is naturally occurring and therefore generally found in nature.

2. A mineral forms solid crystals of definite shape under appropriate conditions.

3. A mineral is generally inorganic even though some may contain carbon.

4. A mineral has a specific chemical composition.

1. The atom is the smallest unit of an element that retains the physical and chemical properties of that element.

2. Figure 2.2: An atom consists of different particles called protons, neutrons and electrons.

(a) Proton: Positively charged with an atomic mass = 1.

(b) Neutron: No charge (neutral) with an atomic mass = 1

(c) Electron: Negatively charged with an atomic mass = 0

3. Protons and neutrons comprise the nucleus of an atom while electrons orbit around the nucleus.

4. In an electrically neutral atom, the number of protons = number of electrons.

5. The number of protons an element contains is known as its atomic number.

6. The sum of the masses of all the protons and neutrons in the nucleus of an atom = atomic weight (electrons have no mass). Note in Figure 2.3 that differences in the atomic weight of an element are due to differences in the number of neutrons in the nucleus. However, the number of protons always stays the same.

7. Atoms of different atomic weight for a given element are referred to as isotopes of that element. Figure 2.3 shows different isotopes of the element carbon.

8. Figure 2.6: The Periodic Table defines different elements by the number of protons they contain. For a given element, the number of neutrons may vary but the number of protons always stays the same.

1. Elements may combine with other elements to form compounds. Compounds may also react to form other compounds.

Ca2+ + 2Cl- = CaCl2

CaCO3 + 2HCl = H2CO3 + CaCl2

H2CO3 = CO2 + H2O

1. For an electrically neutral atom, the number of electrons = number of protons such that each positive charge by a proton is counterbalanced by a negative charge from an electron.

2. Figure 2.7: After gain or loss of an electron, the atom is no longer electrically neutral and is therefore called an ion. When a neutral atom loses an electron, it acquires a positive charge and is called a cation (e.g. Na+_, K+). If a neutral atom gains an electron, it acquires a negative charge and is called an anion (e.g. Cl-, Br-). Many elements can gain or lose more than one electron (e.g. Ca2+, O2-). The most stable configuration for an atom is to have 8 electrons in its outer shell.

3. Figure 2.6 again: The tendency of elements to have 8 electrons in the outer shell can be envisioned as an attempt to achieve the stable state of a Noble gas. Elements in the first two columns of the periodic table therefore have a tendency to lose electrons (acquire a positive charge) while elements in the two columns immediately preceding the noble gases have strong tendencies to gain electrons (acquire negative charges).

1. Figure 2.4: Electrons surround the nucleus as a series of concentric spheres called electron shells. In chemical reactions of most elements, only electrons in the outermost shells interact.

2. Some types of reactions occur where one element loses an electron to become positively charged while a different element gains the electron to become negatively charged. The opposite electrical charges for each element causes them to attract and bond together. This type of attraction is called ionic bonding.

1. Figure 2.5: Other types of reactions involve elements that share electrons. In this case, the attraction is not electrical but rather involves elements having their electron orbits (shells) overlap with those of adjacent elements. The sharing of electrons between two or more elements creates a covalent bond. Note overlapping orbits in figure 2.5

2. Groups of elements can form complex ions such as CO32-, SO42- and NO3- through a combination of ionic bonding and electron sharing.

3. A variation of the covalent bond occurs with metallic elements. Atoms of metallic substances tend to lose electrons. Metallic elements usually pack together as cations. The electrons, however, are not confined to rigid orbits but rather are mobile and move about freely. As a result, the electrons are dispersed and shared among the cations. This type of cohesion is called a metallic bond. This type of bonding is found among a small number of minerals like metallic copper and some sulfides.

1. A mineral that appears to the naked eye as a crystal or grain constitutes the orderly arrangement of certain atoms. Minerals can vary in size from microscopic to large, hand-sized crystals.

2. Minerals can form by one of three ways.

(a) Crystallization occurs when random atoms start to bond together in an orderly arrangement to form a solid crystal or grain. During crystallization, certain atoms bond to one another to form a 3-D network of atoms which constitute the atomic structure of that particular mineral. An example is a hot body of magma that cools slowly to the point that solid olivine crystals begin to form.

(b) Other minerals such as halite and gypsum can precipitate during evaporation of saltwater.

(c) A third process involves recrystallization where one mineral changes into another. This occurs as a result of changes in temperature, pressure and/or chemical environment.

Examples Of The Orderly Arrangement Of Atoms In Crystals.

1. Diamond (Fig. 2.8): In diamond, each carbon atom is bonded to four other carbon atoms. The bonding is covalent involving the sharing of electrons among adjacent carbon atoms.

2. Halite (Fig. 2.9): The atomic structure of the mineral halite involves an orderly atomic arrangement of Na and Cl atoms. Each Na atom is bonded to six Cl atoms, while each Cl atom is bonded to six Na atoms. The result is a 3-D network of NaCl atoms in the shape of a cube.

3. Figures 2.9 and 2.11: The overall arrangement of atoms produces crystals of a mineral having a characteristic shape or form. Different minerals have differently-shaped crystals depending on the arrangement of their particular atoms.

4. Atoms are so small that we cannot see the crystalline arrangement of the mineral directly, but rather we can view the overall shape of the crystal. The crystal faces of a mineral are the external expression of the mineral’s internal atomic structure.

5. In perfect crystals, the angles between crystal faces can be measured and used to characterize a particular mineral.

6. Crystallization has to occur slowly in order to form good crystals so that the atoms have sufficient time to arrange themselves in an orderly manner.

7. One mineral may have the same chemical composition of another mineral but differ in crystal structure. These alternative structures for a single chemical compound are called polymorphs. Examples include graphite-diamond and quartz (low temp.) -cristobalite (higher temp.).

8. Sometimes crystallization is too rapid for the bonding atoms to form an orderly structure, in which case the atoms are frozen in place in a more or less random manner. A solid thus forms where the atoms lack any internal ordering. In this case, the solid is called a glass and it lacks well-defined crystal faces but instead appears as an irregular solid.

Figure 2.14: Within actual crystals, ions are packed together rather than widely separated as depicted in the models. Anions like Oxygen and Chloride are usually much larger than cations. As a result, most of the space of a crystal is occupied by the anions whereas cations fit into the spaces between them.

1. Figure 2.16: The basic building block of all silicate mineral structures is the Silicate ion which consists of a Silicon ion (Si4+) surrounded by four oxygen ions (O2-) and represented by the formula (SiO4)4-. This configuration results in a structure resembling a tetrahedron, sometimes called the Silica Tetrahedron. All silicates therefore include Si and O in their chemical formula, in addition to various cations depending on the mineral.

General Formula for silicates: (SiO4)4- + cations (Mg2+, Fe2+, Ca2+, Na,+ K+)

2. Figure 2.17: Within the silicate structure, tetrahedra can be isolated or may be linked together in rings, chains, sheets or frameworks.

3. Important silicates include olivine, proxene, amphiboles, micas, feldspar and quartz.

1. The carbonates have the (CO3)2- complex as the basic unit. There are different carbonate minerals depending on which cation is attached to the carbonate complex.

(CO3)2- + cations

2. The two most important carbonates are calcite [CaCO3] and dolomite [CaMg(CO3)2].

1. Oxides are minerals where cations are bounded to oxygen

O2- + cations

2. Two important oxides are the minerals hematite (Fe2O3) and magnetite (Fe3O4).

1. Many important ore deposits exist as sulfides.

S2- + cations

2. Two important sufides are galena (PbS) and pyrite (FeS2) also known as fools gold.

1. In sulfates, sulfur is present as the sulfate ion (SO4)2-.

(SO4)2- + cations

2. Two important sulfates are anhydrite (CaSO4) and gypsum (CaSO4.2H2O). Gypsum precipitates from evaporating seawater whereas anhydrite can form by removing water from the gypsum structure.

1. Minerals composed of only one type of element.

2. Some important native elements are graphite (C), diamond (C), copper (Cu) and gold (Au) and sulfur(S).

1. Color: May be affected by impurities. Pure quartz is colorless, but impurities can impart a color such as rose, purple, dark grey or even black depending on the impurities.

2. Streak: color left when scratching a mineral on a tile of unglazed porcelain.

3. Hardness: A measure of the ease with which the surface of a mineral can be scratched. Table 2.2 and Mohs scale of hardness

4. Cleavage: The tendency of a crystal to break along flat planar surfaces. Covalent bonds, which are generally strong, give poor cleavage. Ionic bonds, being relatively weak, give good cleavage. Thus quartz has poor cleavage whereas halite and calcite have good cleavage. Cleavage is generally defined by the number and orientation of cleavage planes along with the quality of cleavage surfaces.

5. Fracture: Tendency of a crystal to break along irregular surfaces other than cleavage planes. Fractures may be conchoidal (smooth, curved surfaces), fibrous (similar to splitting wood) or irregular.

6. Luster: Table 2.3: refers to the way the surface of the mineral reflects light.

7. Specific Gravity: Differences in weight related to the density of minerals.

8. Crystal Habit: The shape in which individual crystals grow. Shapes can include blades, plates, needles, pyramidlike, etc.).