Why is mica insoluble in water?

The earth's crust consists of over 90 percent, the earth's mantle almost entirely of silicates. The most common silicates in the earth's crust are feldspars with 50-60 percent by volume. Other important rock-forming minerals are mica, clay minerals, amphiboles, pyroxenes, garnet and olivine. The common mineral quartz (SiO2) is counted among the oxides in German-language literature and among the silicates in Anglo-American literature.


All silicate minerals have a common construction principle, which is why they can be brought into a systematic order relatively easily. The basic building blocks of all silicates are SiO4-Tetrahedron. One silicon atom is surrounded by four oxygen atoms. The oxygen atoms touch because of their size, in the middle there is space for the relatively small silicon atom (the free space is called the tetrahedral gap).

Another property of silicates is the ability of the oxygen atoms to work on different SiOs at the same time4-Complexes to participate. In addition to isolated SiO4-Tetrahedra further composite components:

  • isolated tetrahedron
  • Double tetrahedron
  • Ring structures
  • Single and double chains
  • Layer structures
  • Scaffolding structures

Aluminum can replace or substitute silicon, which behaves chemically in a similar way (one speaks here of "isomorphic replacement"), silicates in which this happens are called aluminosilicates. When installing aluminum (Al3+ instead of Si4+) In the mineral lattice, charge compensation must take place through the incorporation of further positively charged ions (cations). The Al: Si ratio cannot exceed 1, pure aluminates do not occur in nature.

Systematics of the silicate minerals

As already mentioned, the silicates form an extremely extensive mineral family. There are large differences in chemical composition, crystal symmetry, types of bonds and the structure of the basic building blocks. There are therefore various classification schemes for silicate minerals. The system common in Germany divides the silicates according to the degree of polymerization of the SiO4-Tetrahedron.

Comments on the notation of the chemical molecular formulas

A simplified schematic formula of silicates is:


The oxygen-silicon complexes can be replaced by hydroxide or fluoride ions. The position of "M" is occupied by one or more metal ions until the charge is equalized. Water can also be stored in the grid of particularly wide-meshed silicates. If in a given mineral some of the SixOy-Complexes by ions such as fluoride (F-) or hydroxide (OH-) are replaced, this is indicated by vertical hyphens in the last term of the formula, for example

, Kaolinite.

Stored water is noted as follows:

, Analcime.

Classification according to the degree of polymerization of the SiO4-Tetrahedron

Island silicates (nesosilicates)

The island silicates contain isolated SiO4-Tetrahedron in front. Representative:

Group Silicates (Sorosilicates)

Two SiO each4-Complexes are connected via an oxygen atom to form double tetrahedra, this so-called bridging oxygen to each SiO4- Half of the tetrahedron. The Si: O ratio in group silicates is therefore 2: 7. This structure is less common; an example is the mineral gehlenite (Ca2Al [(Si, Al)2O7]).

Ring silicates (cyclosilicates)

In ring silicates the SiO4-Tetrahedra grouped into isolated rings of three, four and six. Each silicon ion shares two oxygen ions with two neighboring tetrahedra. 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 group belong to the ring silicates.

Single and double chain silicates (inosilicates)

The chain silicates belong to two important groups of rock-forming minerals: pyroxenes and amphiboles. The pyroxenes form one-dimensional single chains; two of the oxygen ions belong to two tetrahedral complexes at the same time, resulting in a Si: O ratio of 1: 3 (e.g. diopside (CaMg [Si2O6]).

Amphiboles form one-dimensional double chains. Two single chains are connected laterally via bridging oxygen. Compared to the single chains, every second tetrahedron of each single chain has an oxygen ion in common with its respective neighbor. The Si: O ratio in double chain silicates is 4:11. In such silicate double chains there are cavities into which (OH)-- and F--Ions can enter. In the chemical formula this is expressed by a vertical line. A mineral from the amphibole group is actinolite (approx2(Mg, Fe)5[(OH)2| Si8O22]).

Layered silicates (phyllosilicates)

With a higher degree of polymerization, chain structures are formed instead of Layer structures made of SiO4-Tetrahedra. Within a layer, each silicon atom shares three of its oxygen ions with its neighbors. The Si: O ratio of the sheet silicates is 2: 5. The layered silicates are divided into two and three layer silicates. Another subdivision takes into account the structure and the ions that are between two tetrahedral layers. The void between two layers can be, for example, (-OH), (-O-Me+) be occupied 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 two-layer silicate) (Al4[(OH)8| Si4O10].

Synthetic sheet silicates such as SAS-6 ® ( Na2S.i2O5) are used in detergents. SAS sheet silicates show properties like sodium zeolites. The layer-connecting, hydrated sodium ions are selectively exchangeable in suspensions, e.g. for calcium ions, and are therefore suitable as ion exchangers for water softening and show good properties as washing alkalis.


In the case of tectosilicates, each oxygen ion belongs to two neighboring tetrahedra at the same time. This creates three-dimensional network structures. The result is the chemical formula SiO2; this is quartz. For further framework silicates, silicon has to be replaced by aluminum. The charge balance takes place through the incorporation of cations. The framework silicates include the feldspars and feldspar representatives, an extremely important group of minerals because of their abundance. Examples are minerals from the mixed series of plagioclase (albite - anorthite): (NaAlSi3O8 - CaAl2Si2O8).

Even large molecules such as H.2O can be installed. At high temperatures the water escapes, but at low temperatures it is re-incorporated into the crystal lattice in an environment saturated with water vapor. These water-containing minerals belong to the group of zeolites (e.g. natrolite (Na2[Al2Si3O10] * nH2O).

Amorphous Silicates

Opal is amorphous silicon dioxide with embedded water (SiO2 * nH2O). Like quartz, some authors place it among the oxide minerals. The highly structured shells of diatoms and of radiolucent animals are made of amorphous silicon dioxide (SiO2) built up.

Classification according to Kostov

This classification is mainly based on the chemical composition of the silicate and its crystal morphology, see the point "Further literature" below. "

Technical silicates

  • Glasses soluble in water, so-called glasses of water, are made from quartz sand and metal carbonates (e.g. with Soda Na2CO3) made in a glass melting furnace. They are used as an adhesive, filler, e.g. in the paper industry, for sealing damp masonry, as an additive in vapor-permeable plaster mixtures. Gels (silica gel), silicic acid, silicates and zeolites, among other things, are produced on an industrial scale from water glass by reacting with acids.
  • Talc (mineral) can be used in many ways. It is used in the paint and glass industry and as a lubricant. As a ground raw material (then Talc called) it is contained in many cosmetics.
  • Asbestos (chrysotile) was used because of its fire resistance and its suitability as an insulating and insulating material, especially in the construction industry, but has been banned in the EU since 2005 because of its harmful side effects. The minerals zircon, muscovite, andalusite, sillimanite and disthene are also suitable for the production of refractory and corrosion-resistant materials.
  • Kaolinite is an important raw material for the ceramic industry e.g. B. for the production of refractory crucibles and wall and roof tiles.
  • Zeolites, especially the synthetic zeolite A, are used as ion exchangers (so-called molecular sieves) and serve as phosphate substitutes for softening water, especially in detergents. To prevent eutrophication of ornamental ponds, nutrients (ammonium compounds) are removed with zeolites and, in particular, the growth of algae is inhibited. In addition to these aluminosilicates, synthetic sheet silicates (see above) with similar properties were found.
  • Because of their large surface area and adsorption capacity, nanosilicates are suitable as carriers for catalyst materials or medicinal agents. Here, large-scale production is still being researched; The first breakthroughs were achieved with sturgeon silicates, which can be produced on a laboratory scale.
  • Silkates are widely used as a corrosion inhibitor in drinking water treatment. Phosphate-silicate mixtures and phosphate-free, carbonate-activated silicates are known to be effective inhibitors.


  • Silicates occur in low concentration in dissolved form in all water.
  • Some groups of organisms form pebbly skeletons, the main production is presumably carried out by planktonic living organisms such as diatoms and radiolarians. Some sponges also build up pebble framework structures.
  • All earth-like planets consist to a large extent of silicates.


As jewelry and precious stones

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


One can detect silicate with the water drop sample. To do this, the appropriate substance is placed in a lead crucible and calcium fluoride (quantity ratio 3: 1) is added. This is doused with concentrated sulfuric acid and heated in a water bath. A moistened black filter paper must be placed on the lead crucible. If silicate is present, a white spot will form on the filter paper.

Note: Do not use an excess of CaF2, otherwise H2SiF6 is formed.

Further literature

  • W. L. Bragg (1930): The structure of silicates. Z. Kristallogr., 74: 237--305.
  • W. A. ​​Deer, W. A. ​​Howie and J. Zussman (1982): Rock-Forming Minerals, Volume 1A: Orthosilicates. Longman, London, 2nd edition.
  • I. Kostov (1975): Crystal chemistry and classification of silicate minerals. Geokhimiya, Mineralogiya i Petrologiya. 1: 5--41.
  • F. Liebau (1962): The systematics of silicates. Natural sciences. 49: 481--491.
  • F. Liebau (1985): Structural Chemistry of Silicates. Springer-Verlag, Berlin.
  • S. Matthes (1993): mineralogy. Springer-Verlag, Berlin, 4th edition. (meanwhile there is a new edition, date unknown)
  • S. Na'ray-Szabo (1930): A silicate system based on the crystal structure. Z. physics. Chem. Dept.. B9: 356--377.
  • H. Pichler and C. Schmitt-Riegraf (1987): Rock-forming minerals in thin section. Enke publishing house.
  • P. H. Ribbe (1982): Reviews in mineralogy Volume 5: Orthosilicates. Mineralogical Society of America, Washington, 2nd edition.
  • J. V. Smith and W. L. Brown (1988): Feldspar Minerals, Volume 1. Springer-Verlag, Berlin, 2nd edition.
  • H. Strunz (1978): Mineralogical tables, 3, 7th edition Akademische Verlagsgesellschaft Gees & Portig, Leipzig.
  • W. E. Tröger (1952): Optical determination of the rock-forming minerals, Part 1: Determination tables. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, 4th edition.
  • W. E. Tröger (1967): Optical determination of the rock-forming minerals, part 2: text volume. Schweizerbart'sche Verlagbuchhandlung, Stuttgart,?. Edition.
  • T. Zoltai (1960): Classification of silicates and other minerals with tetrahedral structures. American mineralogist. 45: 960--973

Categories: Fabric Group | Silicates and Germanates