Magnesium ranks close behind silicon in mass percentage, making up an estimated 13 percent of the Earth.A Matter of DefinitionoThe abundance of Earth-forming elements is a geological question and as such is governed by the geological definition of the Earth. This definition includes only the solid geosphere composed of the crust, mantle and core. It
does not take into consideration the composition of the atmosphere, the hydrosphere (the Earth's water systems) or the biosphere (the Earth's living systems). These Earth systems must be considered separately in regard to their elemental composition.Silicates (SiO24-) are called the rock-forming minerals.oConstitute almost the entire crust and mantle of EarthoThey are the most common minerals.oExample: quartz (SiO2)oMade of oxygen and silicon with other atomsoSilicate minerals, or silicates, make up over 95% of the con- tinental crust and almost 100% of the oceanic crust. Fur- ther, nearly all of the Earth’s mantle consists of silicates. Thus, silicates are the most common minerals on Earth. As we’ve noted, silicates in the Earth’s crust and upper mantle contain the SiO44– anionic group. In this group, four oxygen atoms surround a single silicon atom, thereby defining the corners of a tetrahedron, a pyramid-like shape with four tri- angular faces (Fig. 3.9a). We refer to this anionic group as the silicon-oxygen tetrahedron (or, informally, as the sil- ica tetrahedron), and it acts, in effect, as the building block of silicate minerals.oOxides (O2-)oMetal cations (Fe2+, Fe3+, Ti2+) are bonded to oxygen.oExamples:Magnetite (Fe3O4)Hematite (Fe2O3)Rutile (TiO2)Sulfides (S-)oMetal cations are bonded to a sulfide anionoExamples:Pyrite (FeS2)Galena (PbS)Sphalerite (ZnS)Sulfates (SO42-)oMetal cations bonded to a sulfate anionic groupoMany sulfates form by evaporation of seawater oExamples:Gypsum (CaSO42H2O)Anhydrite (CaSO4)Silicates: The fundamental component of most silicates in the Earth’s crust is the SiO44– anionic group. A well-known example, quartz (Fig. 3.7a), has the formula SiO2. We will learn more about silicates in the next section. Sulfides: Sulfides consist of a metal cation bonded to a sulfide anion (S2–). Examples include galena (PbS) and pyrite (FeS2; Fig. 3.7c). Oxides: Oxides consist of metal cations bonded to oxygen anions. Typical oxide minerals include hematite (Fe2O3; Fig. 3.7b) and magnetite (Fe3O4; Fig. 3.7g).
Halides: The anion in a halide is a halogen ion (such as chloride [Cl–] or fluoride [F–]),an element from the second column from the right in the periodic table (a periodic table is provided after Chapter 19). Halite, or rock salt (NaCl; Fig. 3.8d), and fluorite (CaF2 ) are common examples. Carbonates: In carbonate minerals, CO32– serves as the anionic group. Elements such as calcium or magnesium bond to this group. The two most common carbonates are calcite (CaCO3; Fig. 3.8e) and dolomite (CaMg[CO3 ]2 ).