Chemical composition of natural stones

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The composition of natural stones, defined rocks, is determined according to the contents of the priority items such as SiO2, TiO2, Al2O3, Fe2O3, Cr2O3, FeO, MnO, MgO, CaO, Na2O, K2O, P188, and H2O expressed as a percentage or in parts per million (ppm). The silicates are the most numerous and widespread minerals in the rocks of the lithosphere. Most mineral deposits originates from chemicalphysical activity that takes place on the edge of the enormous plates of the Earth's crust. Within the Earth's crust, silicon is never at elementary state, but is always combined with silica and silicates.

Quartz is present in nature in significant amounts, it derives from the crystalline modification of amorphous silicon dioxide SiO2. The crystalline form of silica is quartz at normal atmospheric pressure and at room-temperature, instead over 1025° C the shape, thermodynamically fixed, is the cristobalite (Figure 1). The chemical composition of silicon, consists of a silicon atom surrounded by four oxygen atoms, being each of them tied to two silicon atoms.

The hardness, stiffness and thermal stability of silica are due to the silicon-oxygen bond which is covalent significantly polarized towards oxygen atom. The structure of silicates has as its fundamental unit group SiO4 consisting of a tetrahedron at the center of which lies a Silicon ion and at the top of which there are four oxygen ions. Silicates are classified into 5 different groups depending on their way to join the tetrahedrons:

  1. Isolated groups SiO4 (neosilicates = olivine, garnet ecc.);
  2. Isolated groups Si2O7, Si6O18 ecc. (sorosilicates = thortveitite, beryl ecc.);
  3. Groups with chains developed in only one direction (= antiboli and magnesium minerals pyroxene);
  4. Groupings in piano (phyllosilicates = miche clays, talc, chlorite);
  5. Three-dimensional groupings (Tecto-silicate = feldspars).

In silicates is possible a wide possibility of substitution between atoms that have congruent dimensions and fill equivalent positions. The size of bivalent iron ions, magnesium, trivalent iron and aluminium are roughly equivalent to allow mutual exchange within the structural lattice. They can also occur, although in a limited way, substitution of Silicon by atoms of aluminium, which extends even further the range of composition of silicates. These features and elements are mineralogical compounds possessing different nomenclature:

Silicates:

  • Quartz (SiO2), it has a rhombohedral crystallization and a hexagonal bipyramidal shape. it is a very common component in magmatic rocks. It is usually pure and colorless, but can also take various colours depending on the minerals in the form of inclusions. (Figure 3)
  • Feldspars: are potassium, calcium and sodium aluminosilicates, with the presence of other elements. Are very frequent in different types of rocks. They have a structural tecnosilicata shape with a threedimensional lattice consisting of tetrahedra of [(Si, Al) O4]. The two most important terms are:
    • Orthoclase, which is a type of potassic feldspar [K (AlSi3O8)] with a Tetragonal crystallization. It is a component of the eruptive rocks of acid type and in crystalline schists. It is also found in many rocks of sedimentary formation and it has a grayish-white color. (Figure 4)
    • Pagioclase: feldspars consist of variables mixtures of albite [Na (AlSi3O8)] and Anorthite [Ca (Al2Si2O8)]. They crystallize in the triclinic system and normally occur geminate

Micas:

Are aluminium phyllosilicates with some alkali metals, that may contain amounts of fluoride, hydroxyl groups (OH), calcium, iron, magnesium. They have a monoclinic crystallization. Micas are divided into two main groups: the Muscovite (white micas, potassium and aluminium phyllosilicates with hydroxyl and fluorine) and biotite (dark micas, phyllosilicates of potassium, iron, magnesium, manganese and fluorine with hydroxyl):

  • Pyroxene: inosilicates in tetrahedral chains. They can crystallize both in the monoclinic system and in the rhombic one.
  • Amphiboles: inosilicates like pyroxene that have in the structural crystalline lattice double chains of SiO4 tetrahedra. In the structure there are hydroxyl groups (OH) and fluoride ions.
  • Olivine: is also known as Peridot, it is a magnesium and iron nesosilicate [(Mg, Fe)2(SiO4)], and crystallizes in the orthorhombic system. It has a greenish color and it is a mineral found in rocks with low amounts of silica. (Figure 5)

Carbonates:

  • Calcite: is the alfa phase of calcium carbonate, cristallizzazione trigonale.
  • Dolomite: is calcium magnesium carbonate CaMg(CO3)2, with a trigonal-rhombohedral system of crystallization.
  • Aragonite: is the beta phase of calcium carbonate, it has an orthorhombic type of crystallization. (Figure 6)