This document last updated on 15-Jun-2012 EENS 2110 Mineralogy Tulane University Prof. Stephen A. Nelson Silicate Structures, Structural Formula, Neso-, Cyclo-, and Soro- Silicates Silicate Structures and Structural Formula As we discussed in a previous lecture, the relative abundance of elements in the Earth's crust determines what minerals will form and what minerals will be common. Because Oxygen and Silicon are the most abundant elements, the silicate minerals are the most common. Thus, we will spend some time here discussing the structure, chemistry, and occurrence of silicate minerals. Our systematic discussion of the common rock forming minerals will follow in the lectures throughout the remainder of the course. Element Wt% Atomic% Volume% O 46.60 62.55 ~94 Si 27.72 21.22 ~6 Al 8.13 6.47 Fe 5.00 1.92 Ca 3.63 1.94 Na 2.83 2.34 K 2.59 1.42 Mg 2.09 1.84 Total 98.59 100.00 100 In order to discuss the silicates and their structures it is first necessary to remember that the way atoms are packed together or coordinated by larger anions, like oxygen depends on the radius ratio of the cation to the anion, Rx/Rz. Rx/Rz C.N. Type 1.0 12 Hexagonal or Cubic Closest Packing 1.0 - 0.732 8 Cubic 0.732 - 0.414 6 Octahedral 0.414 - 0.225 4 Tetrahedral 0.225 - 0.155 3 Triangular <0.155 2 Linear Since oxygen is the most abundant element in the crust, oxygen will be the major anion that coordinates the other other cations. Thus, for the major ions that occur in the crust, we can make the following table showing the coordination and coordination polyhedra that are expected for each of the common cations. Silicate Structures, Neso- Cyclo-, and Soro- Silicates 6/15/2012 Page 1 of 13