Silicate

Silicates (including silicates) are the salts and esters of ortho- silicic acid ( Si (OH) 4) and their condensates. The esters are described under silicic acid esters, for condensates see silicas. All salts are built up by SiO4 tetrahedron compounds, the tetrahedron may be linked to each other but in different ways. Unconnected points of the tetrahedron contribute to the charge balancing metal cations at or are possibly present as hydroxide ion (OH - ). With the exception of alkali metal silicates are silicates insoluble in water or other solvents.

Natural silicates ( silicate minerals ) play a major role in mineralogy, as can a lot of minerals in this group of substances assign. The earth's crust consists of more than 90 percent of the Earth's mantle almost entirely of silicates. The most common silicates in the Earth's crust are the feldspars with 50-60 volume percent. Other important rock-forming minerals are mica, clay minerals, amphiboles, pyroxenes, garnet and olivine. The common mineral quartz ( SiO2) is counted in German literature to the oxides, in the Anglo - American literature, however, to the silicates.

• 5.1 As jewelery and gemstones
• 5.2 industry

Structure

Allen Silicatmineralen is a common design principle of self, so they are relatively easy to bring in a systematic order. The basic building blocks of all silicates are SiO4 tetrahedra. A silicon atom is surrounded by four oxygen atoms. The oxygen atoms touch each other because of their size, in the center is place for the relatively small silicon atom ( the free space is called tetrahedral ).

Another feature of the silicates is the ability of the oxygen atoms to participate simultaneously in several SiO4 complexes. This results in addition to isolated SiO4 tetrahedra more composite components:

• Isolated tetrahedra
• Double tetrahedron
• Ring structures
• Single and double chains
• Layer structures
• Framework structures.

Aluminum can replace the chemically similar behaving silicon and substitutes ( one speaks of " isomorphous replacement " ), silicates, in which this happens is called aluminosilicates. Done When installing aluminum ( Al3 instead of Si4 ) in the mineral lattice must charge compensation by incorporation of other positively charged ions ( cations). The Al: Si ratio can not exceed the value 1, pure aluminates do not occur in nature.

Classification of silicate minerals

The silicates form as mentioned a very extensive mineral family. Occur on large differences in chemical composition, crystal symmetry, bond types and structure of the basic building blocks, which is why different classification schemes for silicate minerals exist. The usual in Germany systematics divides them according to the degree of polymerization of the SiO4 tetrahedra.

Remarks on the spelling of the chemical sum formulas

Is a simplified schematic formula of silicates is:

Hydroxide or fluoride ions can take the place of the oxygen-silicon complexes. The position of "M" is occupied by one or more metal ion to balance the charge. In the particularly wide-meshed grid of silicates, water can also be stored. If some of the Si x O y complexes actually to be replaced by ions such as fluoride (F- ) or hydroxide (OH - ) in a particular mineral, this indicates one of the by vertical dashes in the last term of the formula, for example,

Intercalated water is written as follows:

Classification according to the degree of polymerization of the SiO4 tetrahedra

Island silicates ( Nesosilicate )

The Inselsilicaten are isolated SiO4 tetrahedra. representatives:

• Olivine:
• Zircon.

Gruppensilicate ( sorosilicates )

Each two SiO4 complexes are linked via an oxygen atom to form a double tetrahedron, said bridging oxygen called belongs each SiO4 tetrahedron half. The Si: O ratio in group silicates is thus 2:7. This structure is less common, an example is the mineral Gehlenite ( Ca2Al [ (Si, Al) 2o7 ] ).

Ring silicates ( cyclosilicates )

In Ringsilicaten the SiO4 tetrahedra are grouped to isolated three-, four - and six-membered rings. Each Siliciumion divided per two oxygen ions with two adjacent tetrahedra, this results in a Si: O ratio of 1:3 in Ringsilicaten. This results in the following formulas for the ring structures:

• [ Si3O9 ] 6 -
• [ Si4O12 ] 8 -
• [ Si6O18 ] 12 -.

Beryl ( Al2Be3 [ Si6O18 ] ) and the minerals of the tourmaline are among the Ringsilicaten.

Chain and band silicates ( inosilicates )

The Chain are two important groups of rock-forming minerals: pyroxenes and amphiboles. The pyroxenes form one-dimensional single chains, this includes two each of the oxygen ions simultaneously at two tetrahedral complexes, resulting in a Si: O ratio of 1:3 results, as in diopside ( CaMg [ Si2O6 ] ).

Amphiboles form one-dimensional tapes. Two chains are linked laterally over bridge oxygens. Compared with the chains in addition every second tetrahedron each chain with its respective neighbor oxygen ion in common. The Si: O ratio in Bandsilicaten is 4:11. In such siliceous bands voids are present in the (OH ) - ions can occur - and F. In the chemical molecular formula is expressed by a vertical line. A mineral of the amphibole group is actinolite (Ca2 (Mg, Fe) 5 [( OH) 2 | Si8O22 ] ).

Layered silicates ( phyllosilicates )

With a higher degree of polymerization are formed instead of chain structures layer structures of SiO4 tetrahedra. Within a layer, each silicon atom is divided between three of its oxygen ion with its neighbors. The Si: O ratio of 2:5 phyllosilicates is. The phyllosilicates are divided into two and Dreischichtsilicate. A further subdivision into account the structure and the ions are between two tetrahedral layers. The cavity between the two layers can be occupied by ( -OH) (-O -Me ) and link the layers with dipole -dipole forces or ionic bonds.

The layered silicates include mineral groups such as mica, talc, serpentine, and clay minerals such as vermiculite, examples are muscovite ( a three-layer silicate ) ( KAl2 [( OH) 2 | AlSi3O10 ] ) and kaolinite ( a Zweischichtsilicat ) ( Al4 [( OH) 8 | Si4O10 ] ).

Synthetic layered silicates, such as SAS -6 ® () are used in detergents. SAS phyllosilicates exhibit characteristics such as sodium zeolites. The layer- linking, hydrated sodium ions in suspensions selectively interchangeable, for example, against calcium ions, and thus suitable for water softening as ion exchangers and show good properties as alkali wash.

Scaffold ( tectosilicates )

In Gerüstsilicaten each oxygen ion belongs simultaneously to two adjacent tetrahedra ( each tetrahedron is linked via its corners with neighboring tetrahedra ). This creates three-dimensional network structures. This results in the chemical formula SiO2. Representing silica compounds of quartz shown here be mentioned in the modification of β - quartz. For more tectosilicates silicon must be replaced by aluminum. The charge compensation takes place by incorporation of cations. The Gerüstsilicaten include the feldspars and feldspar, one because of their frequency extremely important group of minerals. Examples are minerals from the mixed batch of plagioclase ( albite - anorthite ): ( NaAlSi3O8 - CaAl2Si2O8 ).

In the wide-meshed grid of some Feldspatvertreter even large molecules such as H2O can be installed. At high temperature, the water escapes, but is incorporated into the crystal lattice again at low temperature in a saturated steam environment. These hydrous minerals belong to the group of zeolites ( natrolite ( Na2 [ Al2Si3O10 ] * nH2O ) ).

Amorphous silicates

Opal is amorphous silica with embedded water ( SiO2 · n H2O). It is provided as quartz by some authors to the Oxidmineralen. The highly structured shells of diatoms ( diatom ) and radiolarians ( Radiolaria ) are made ​​of amorphous silica (SiO2) up.

Classification of Kostov

This division is mainly due to the chemical composition of the silicate, and its crystal morphology.

Technical silicates

• Many silicates are produced industrially. Here are most prominently the glasses and glass-ceramics to call but also zirconium silicates as color pigments. These are usually prepared from sand ( SiO2) and metal oxide in a glass melting furnace.

• Soluble in water glasses, so-called water glasses, are of quartz sand and metal carbonates ( Na2CO3 with soda ) in a glass melting furnace. They are used as an adhesive, a filler in the paper industry, for the sealing of damp walls, additional diffusion-permeable plaster in mixtures. From water glass ( silica ), silica, silicates and zeolites are prepared industrially by reaction with acids gels.

• Talk ( mineral ) is versatile. It is used in the paint and glass industry and as a lubricant. As a ground base ( then called talc ) it is included in many cosmetics.

• Asbestos ( chrysotile ) has been due to its fire resistance and its suitability as an insulating and insulating material especially used in construction, is prohibited because of adverse health side effects in the EU since 2005. For the production of refractory and corrosion-resistant materials, the minerals zircon, muscovite, andalusite, sillimanite and kyanite are suitable.

• Kaolinite is an important raw material for the ceramic industry for the production of refractory crucibles and bricks and roofing tiles.

• Zeolites, in particular, the synthetic zeolite A, are used as ion exchangers ( so-called molecular sieves ) and serve as a phosphate substitute for softening water, in particular in detergents. To prevent eutrophication of ornamental ponds, nutrients ( ammonium compounds) are removed with zeolites and particularly inhibited the growth of algae. In addition to these aluminosilicates synthetic sheet were ( so ) with similar properties found.

• Nano silicates are suitable because of their large surface area and adsorption capacity as a carrier for catalyst materials or medicinal agents. Here the production on an industrial scale is still being researched; first passages in the synthesis on a laboratory scale have been achieved by the Stöber process.

• In drinking water treatment silicates are widely used as a corrosion inhibitor. As potent inhibitors of phosphate -silicate mixtures and phosphate-free, carbonate- silicates are known.

Occurrence

• Silicates come in all water samples before dissolved in low concentration.
• Some groups of organisms form siliceous skeletons, the main production takes place presumably by planktonic organisms such as diatoms ( diatoms ) and radiolarians ( Radiolaria ). Some sponges also rely on siliceous frameworks.
• All Earth-like planets are to a large part of silicates.

Use

As jewelery and gemstones

• The island silicates garnet, olivine, topaz and zircon are sold in pure quality gemstones.
• The quartz varieties amethyst, aventurine quartz, chalcedony, citrine, moss agate gemstones are popular.
• The feldspar amazonite, aventurine feldspar, labradorite and Feldspatvertreter sodalite are used as gemstones.
• Emerald, a gemstone is a variety of beryl.

Industry

• In silicate glass ( building glass ) in borosilicate (laboratory glass) as a material for kitchen equipment made of glass ( Duran, Pyrex, Pyrex ) or as a display cover for technical equipment ( Willow Glass)
• In aluminosilicate glass as a glass with high resistance to fire ( DIN EN 1059 ) and as scratch- resistant display cover for technical equipment (Gorilla Glass)
• In lead glass

Proof

One can show silicate with water drops sample. The substance is placed in a lead pot with calcium fluoride and sulfuric acid. In the presence of silica gaseous silicon tetrafluoride, which condenses on a moistened black filter paper over the crucible opening back to white silica produced