Minerals in rocks.

    Minerals are the basic molecules of our good old Earth. They are characterized by the atoms composing them but also by the shape they take when they are sufficiently developed (these are the crystals).

1-From atom to the molecule:

2-Minerals:

      1-From atom to the molecule:

The arrangement of atoms in a mineral depends links:

- covalent: there sharing the same orbital by two atoms, the connection is stable.
- Ionic: This is the most common. Some atoms have a "surplus" of electrons (cations), while others are missing (anion) to be stable. They unite, the cation providing sound, or, electron (s) extra (s) in which the anion claimed. The compound thus formed becomes stable.
- Metallic: they are found in the metallic minerals (normal right?). It's kind of ionic bond between identical atoms but.
- Van der Walls: They usually occur in middle by attraction of residual charges.
       The intervening atoms in the mineral composition possess low ionic radii. That is, they occupy little space. The smallest is that of hydrogen (H) which is negligible. Then comes the silicon (Si) and aluminum (Al) and magnesium (Mg) and iron (Mn). Sodium (Na), calcium (Ca) are a little bigger, and despite their different valence ( Na  can provide an electron against two for calcium), they can be easily replaced. Potassium (K) and oxygen (O) still take up more space. larger ionic radius of the atoms are hardly present in the crystals due to space!

In reality a cation is not related to a single anion but several at once (the exchange, constant, the electron is on the first come atom and not on a particular). The number of suceptibles anions that bind to a single cation depends on the ionic radius of one: the larger it is, the more anions can be many. It is the coordination rule. And Si and Al, which are small, can not be related with 4 carbon and being, therefore, that the centrede tetrahedra. Al however, can allow the formation of hexahedrons (6 vertices) as Fe and Mg. Ca and Na give octahedra or dodecahedron.

                                                                                                                                                                                    2- Minerals:

Composition and structure :

More than 90% of minerals are based on silicon. We distinguish among them siliceous minerals and silicate minerals. Silica, SiO2, is pure as silicates contain other atoms.

Siliceous minerals

Silica mineral formed the most common is quartz (colored purple amethyst he carries the name of colored yellow, it is called citrine). There are also chalcedony and opal (including agate, jasper, onyx ...). At high temperature is formed cristobalite (above 1470 ° C) or tridymite (between 870 and 1470 ° C).

                                                                                                                                                                          Silicate minerals:

The silicates are formed by the same basic unit as silica (SiO 4) 4-. This group takes the form of a pyramid (the peaks being formed by the oxygen atoms) due to its ionic radii. The pyramid or tetrahedron of SiO4 will be associated in various ways, forming different structures:

   Tectosilicates: each vertex of a tetrahedron is related with another tetrahedron.
Those are :
   *Feldspars: there are those
       - Rich in potassium: orthoclase, sanidine (typical of high temperatures) and microcline.
       - Those of the Na-Ca series: plagioclase. Na and Ca can easily replace one finds a progressive series of minerals according to their percentage of Na and Ca. albite, Na is the pole, and the pole anorthite Ca.
   *The Feldspathoids: composition similar to feldspar, they are much less rich in silica. Leucite is rich in potassium while nepheline is rich in sodium (mainly is found in phonolites).
   Phyllosilicates: on the four peaks, one is not in connection with another tetrahedron. This structure thus forms a flat network at home:
   *Micas: Their appearance is lamellar or fibrous.
      - Muscovite mica or white, rich in K essentially,
      - Black mica or biotite-rich K, Mg and Fe. It's not found in sedimentary rocks.
   *Chlorites and Serpentines : which originate from weathering of other minerals.
talc, characteristic of metamorphic rocks.
clay minerals (illite, kaolin ..).
4 Summits, only two are linked to other tetrahedra. Those are :
   - The cyclosilicates (beryl and emerald, cordierite, tourmaline). They are characteristic of pegmatites (igneous rock with big crystals).
   - Inosilicates:
          Pyroxene: They are characteristic of volcanic rocks, but can be found in deeper rocks basic or ultrabasic.
(Ex: enstatite, augites)
          Amphibole: Mineral needles. (Ex: hornblende, glaucophane). They may result from alteration of pyroxene by ouralitisation.
    - Sorosilicates: only one oxygen is shared between two tetrahedra. These are the epidote. They result from the alteration of other minerals.
    - Nesosilicates: tetrahedra linked together, not by their oxygen, but by intermediate atoms (divalent cations) such as Mg2 + or Fe2 +. Those are :
peridot olivine as the characteristics of mantle rocks,
    Garnets: common in metamorphic rocks.
    Alumina silicates (andalusite, kyanite and sillimanite, staurolite) characteristics of metamorphic rocks.                                                                                                                                       
These minerals form under specific conditions of temperature and pressure. Series Bowen: Olivine, pyroxene and anorthite form at high temperature while mica and feldspar pole plagioclase form at low temperature, such as quartz. Orthoclase is indifferent.


                                                                                                                                                                                    Other:

    Non-silicate minerals are less present. They usually found only in sedimentary rocks. The most important are carbonates (calcite, dolomite, aragonite) that make up sedimentary limestone. Next are the phosphates, chlorides (gypsum, salt), sulfates and rutile, corundum and spinels. There are also the minerals.


                                                                                                                                                                                    Crystalline form and other characters:

   When looking at a stone, it is often not practical to use a microscope to determine the molecular structure of a mineral. We will thus prefer to study the crystalline form: a mineral, because of its molecular structure, is developed, in fact, in a particular form and feature. Other characters such as color and hardness in the determination.

Crystalline systems

  There are 7 different basic shapes possible in nature, defined by the symmetry of the crystals. These are crystalline systems:                                                                                                   

This basic form is often complicated by the combination of several minerals them: they are twins.

Calcite crystals

 other characters:

  Crystal fragile areas are also characteristic. These divisions respect the crystalline system, they can not relate to a single plan. Generally as we want to break a crystal it breaks at its areas of cleavages.

 The hardness of the mineral allows its identification: The Mohs scale of hardness consists of 10 levels, each characterized by a particular mineral. The hard lines the less hard.

      1-Talc

        2-Gypsum or salt

<-> Nail

        3-Calcite

        4-Fluorite

<-> Steel

        5-Apatite

        6-Orthose

<-> Glass

        7-Quartz

       8-Topaz

       9-Corundum

      10-Diamond                                                                                                                                          

     Color can be an important factor as well as the brightness, transparency or taste! Another feature is the staining in polarized light.

                                                                                                                                                                                  The main minerals:

     Quartz (SiO2): it is one of the most common minerals. It is found mainly in magmatic rocks, in which it arises. Very hard (7), frequently is found in sedimentary rocks because it resists erosion. Silica sand is composed mainly of quartz crystals. For the same reason it occurs in metamorphic rocks. Often colorless (rock crystal), it can be colored due to impurities in purple (amethyst), pink, yellow (citrine) and even blue! It is in the geodes that the crystalline form is the most beautiful.

In thin quartz can have various shapes, as this is often one of the last mineral to crystallize. It is gray in polarized light.

Quartz crystals

     

     Orthoclase (K [Si3ALO8]): potassic feldspar, it is common in deep igneous rock. It is often large and is visible to the naked eye. Native white color, it is often altered by hematite and therefore has a pink to reddish. Crystalline form often has a twin Carlsbad, clearly visible in polarized light.

Orthoclase 

     Plagioclase (anorthite Ca [Si2Al2O8] in albite Na [Si3AlO8]): sodium feldspars, they are common in igneous and metamorphic rocks. During their growth composition changes (it is always the anorthosite that forms calcium then gradually gives way to sodium), one can thus see growth lines in certain crystals. In polarized light, it is also possible to see certain forms altered or twinned.

    

Plagioclase

Sometimes sodium feldspar and combine over the crystallization then forms a perthite:

   The Peridot (forsterite Mg 2 [SiO 4] to fayalite Fe 2 [SiO 4] through olivine (Fe, Mg) 2 [SiO 4]): If the extreme forms, forsterite and fayalite, are found respectively in the dolomites metamorphosed HT and HP and in eruptive rocks, olivine is present in mantle rocks. In polarized light, olivine has beautiful bright colors.

The Peridot 

    Amphibole (hornblende, Glaucophane ...): These minerals are most often a needle or fiber. They are often rich in mafic. They essentially characterize the metamorphic rocks and some volcanic rocks.

Amphibole 

    Pyroxene (from Enstatite to hypersthène for orthopyroxenes, Diopside, augite, jadeite for clinopyroxene): These minerals have a prismatic shape. They can be found in igneous and metamorphic rocks.

Pyroxene

    Mica (muscovite and biotite): The crystals correspond to a stack of thin strips, colorless to muscovite (white mica) and dark for biotite (black mica) due to the presence of large quantities Ferromagnesian. Muscovite is often associated with granite but are found in metamorphic rocks (mica).

Biotite