The basic chemical unit of the members of Silicates Minerals Class is indicated by the (SiO4) tetrahedron shaped anionic group having negative four charge. The charge of the central silicon ion is positive four. While the rest of the oxygen have negative two charge. Thus each silicon-oxygen bond is actually equal to almost one-half of the total bond energy of oxygen. Through this, oxygens have the option to bond to another silicon ion resulting to the linking of one (SiO4) tetrahedron to another and another, etc.
The largest and the most interesting and most complicated mineral class are the Silicates. Some geologists estimated that almost 90 percent of the Earth’s crust is made up of silicates. Also, almost 30 percent of all minerals known are made up of silicates. This is not really surprising because of the great abundance of the two known elements in the Earth’s crust, namely oxygen and silicon, silicates is also abundant.
The complicated but interesting structures formed by these Silicate minerals are truly wonderful and amazing most especially when viewed under petrographic polarizing light microscopes. Silicates can actually form as single tetrahedron units or nesosilicates, double tetrahedron units or sorosilicates, single and double chains or in silicates, sheets or phyllosilicates, rings or cyclosilicates, and framework structures or tectosilicates. Because of these different ways that the silicate tetrahedrons combine, Silicates Mineral Classis considered the largest and the most interesting and most complicated mineral class.
The Nesosilicates or single tetrahedrons are the simplest among the subclasses of silicates. This subclass includes all silicate minerals with SiO4 tetrahedrons unbonded to other tetrahedrons. These Nesosilicates are sometimes referred to as orthosilicates. Their structure produces stronger bonds with ions that are closely packed resulting to a much higher density compared to other subclasses of silicates. Nesosilicate minerals are more often used for gemstone purposes and are usually found exhibiting wonderful appearance when viewed under gemological microscopes.
The Sorosilicates subclass has two silicate tetrahedrons that are found linked by only one oxygen ion. The structure formed is similar to an unusual hourglass shape. Sorosilicates is the smallest subclass of silicate minerals. But then this subclass also includes the normal silicate tetrahedrons as well as the double tetrahedrons. Most members of this group are rare. Only Epidote is widespread in many metamorphic environments.
The Inosilicate Subclass on the other hand contains two distinct groups, namely the single chain and the double chain silicates. The tetrahedrons in the single chain group are found sharing two oxygens with two other tetrahedrons. The chain that has been formed is seemingly endless. When viewed in cross sections, the chain forms a trapezium. This shape produce the angles found between the cleavage directions and the crystal faces that can be seen when evaluated closely with the aid of the geological polarizing light microscope. While in the double chain, the two single chains lie side by side. The double chain, when closely examined, looks like a chain of six sided rings. The cross section view is similar. The only difference is that the trapezium is a bit longer. This difference results to difference in angles. This group has prismatic cleavage that can be seen much clearly visible under polarizing microscope. The cleavage angle of the minerals in this group is close to 120 to 60 degrees usually found forming rhombic cross sections. A cleavage angle is convenient to use when identifying single chain to double chain silicate minerals.
The Cyclosilicates also form chains but these chains link back around on themselves forming rings. Rings can be triangular, square shaped, or even hexagonal. Some other structures can be complicated but are very interesting most especially when viewed under polarizing microscope for geologists. The symmetry of the rings of these tetrahedrons can directly translate to the symmetry of the minerals. Several members of this group are used for gemstone purposes. That is because of their significant hardness, luster, and most of all durability.
The Phyllosilicates have rings of tetrahedrons that are linked by shared oxygens to other rings in two-dimensional plane that produces a sheet-like structure. The symmetry of the rings chiefly controlled the symmetry of the members of this group. But then because of other layers and ions, the symmetry can be possibly altered into lower symmetry. When evaluated closely with the aid of the petrographic polarizing microscope, the typical habit of the Phyllosilicate minerals includes flat, platy, book-like, and often exhibits good basal cleavage. Members of this group as commonly soft and some sheets are rolled into tubes, which can produce fibers as in asbestos serpentine.
The Tectosilicates are often called the Framework Silicates. This is because of the structure formed by the members of this group. The structure of Tectosilicate minerals is composed of the interconnected tetrahedrons going outward in all directions forming an intricate framework that is analogous to the framework of a large building. Tectosilicates are not simple-structured minerals. The aluminum ion in the structure of Tectosilicate mineral can easily substitute for the silicon in the tetrahedrons for almost up to 50 percent. The great variations of this class are due to the additional cations needed to balance the crystal structure.
