Silicates

We call the uppermost part of our planet the crust. It reaches a depth of about 80 km up to the hot outer mantel. It is composed mainly of silicates, which are the main rock-forming minerals of all igneous rocks and metamorphic rocks. Next to the most common quartz, there are feldspars, micas, pyroxenes and amphiboles. After the disintegration of these rocks, they then become part of the sediments as grains. The whole long cycle accompanied by tectonic movements, denudation, volcanism and again decay, ends with the presence of fine silicate particles in sands and topsoil in the field. Rocks full of silicates serve as a building and decorative stone, silicates can be found in porcelain objects, jewelry, works of art… They are simply omnipresent and are very interesting from a mineralogical point of view. Silicon is able to form a large number of compounds due to its unique atomic structure. It forms the strongest bond with oxygen. The silicates of Earth’s crust most often contain together with an oxygen aluminum, iron, magnesium, calcium, sodium and potassium. If we add to this base other rarely occcuring particles, the number of potential silicates will greatly increase. To bring a system into the amount of silicate minerals was difficult. They were originally divided into meta and ortho silicates, derived from hypothetical acids. But already in the 1930s roentgen rays analyzes revealed that the same formation appeared in all silicates: a tetrahedron composed of four oxygen atoms and one silicon atom. After refining the analytical methods, it turned out to be a pyramidal tetrahedron with an oxygen atom in each corner, and with one silicon atom in the middle of the pyramid, i.e. SiO4. By joining two or more tetrahedrons through common oxygen atoms, a number of spatial configurations are created. Six of them are found in nature and they represent a natural classification system of silicates. Simple Nesosilicates are independent, without any bonds. These include olivine, garnets, staurolite, zircon, titanite, kyanite, andalusite with chiastolite, sillimanite, topaz and other less common minerals. Another group are Sorosilicates, whose tetrahedrons join into pairs using common oxygen. The main but more precious silicates of this group will be: epidote, zoizite, clinozoite, vesuvian,

hemimorphite and axinite with ferroaxinite. Cyclosilicates via common oxygens form circles consisting of four or six tetrahedrons. Cordierite, berylliums and tourmalines are the adornment of this group. There is no doubt about the importance of pyroxenes and amphiboles in the Earth’s crust rocks. They belong to the group of Inosilicates, which is characterized by joining of tetrahedrons into single or double planes. Important pyroxenes: augite, diopsite, hedenbergite, egerin, enstatite and spodumene, are an essential part of gneisses, granulites, syenites, corneas… Only from one or more pyroxenes the whole rocks can be composed. The same apply for amphiboles: common and basalt amphibole. Actinolite with tremolite and in a small quantity occuring rhodonite and prehnite represent other minerals of this group. Flat sheets of joined tetrahedrons support minerals of Phylosilicates, which have mostly husky appearance, like micas and chlorites. They are completed by serpentinites, talc, kaolinite, chrysocolla, allophane, chamosite, palygorskite etc. Tectosilicates deserve attention. Their spatial arrangement of tetrahedrons enabled the formation of really omnipresent silicates. These are feldspars: potassium feldspar orthoclase, adularia, microcline with amazonite and plagioclase feldspars: albite, oligoclase, andesine, labradorite, bytownit, anorthite. To feldspars belong also leucite, nepheline, sodalite and a group of scapolites. The same structure have also zeolites. Though they are not rock-forming minerals, the beauty of the crystals in the cavities of extrusive rocks attracts our attention: natrolite, thomsonite, analcite, phillipsite, mordenite… Along with the sun came into existence planets and other objects of our solar system in the same time and from the same basic matter. Everywhere from this space there are reports on the presence of silicates. Our planet hit meteorites containing silicates. We often find them in rocky meteorites and pallasites, and just as often they are silicates that cannot be found on Earth. They are also carried by cosmic dust. The same laws of nature apply to our entire giant Universe. Planets next to the distant suns, galactic and extragalactic clouds of nebulae and in fact the entire thin cosmic environment constantly sparkles with the omnipresent silicates.