Metamorphic rocks have layers


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The world of rocks

 
 A rock is an aggregate of minerals. If the rock is composed of only one type of mineral, one speaks of monomictem Rock. If the rock consists of several types of mineral, it is referred to as polymiktes Rock. 
 A mineral is a solid, natural, inorganic, macroscopically homogeneous component of our earth. Minerals are (mostly) crystalline or (less often) amorphous substances. 
 
 According to their formation, three types of rocks are distinguished: magma rocks (igneous rocks), sedimentary rocks (loose rocks = sediments, solid rocks = sedimentary rocks) and metamorphic rocks (metamorphic rocks).

Igneous rocksSedimentsMetamorphic rocks
  • Deep rock (plutonite) is formed by slow cooling at great depths (in the lower crust or in the upper mantle) and under high pressure. In the process, large crystals are formed, there are no gas-filled cavities.
    Example: granite
  • Igneous rocks or effusion rocks (volcanic rocks) emerge as lava to the surface and are quickly cooled there. Therefore, only microscopic crystals can form and the rocks can have cavities. One example of this is basalt. If the cooling takes place so quickly that no crystallization takes place, volcanic glass is formed.
  • Clastic (mechanical) sediments are created by weathering and erosion of rocks. Gravity, wind, water or glaciers transport the weathering debris (detritus), process it again and deposit it. Example: sandstone.
    So-called loose sediments are formed during the deposition. The material becomes more and more solidified through the loading of further sedimentation layers and / or through chemical cementation processes; this is called diagenesis. The result is solid sedimentary rocks, the sedimentites. The clastic sediments are differentiated according to the size and type of rock debris they contain.
    • Clay - grain sizes <0.002 mm
    • Silt - grain sizes 0.002 - 0.063 mm
    • Sand - grain sizes 0.063 - 2 mm
    • Fine gravel - grain sizes 2 - 6 mm
    • similarly solidified: mudstone, siltstone, sandstone.
      Coarse (> 2 mm) detritic rocks are called conglomerate if the fragments are rounded, breccia if they are angular and fanglomerate if they are rounded.
    • Tuff - volcanic ash and dust
  • One speaks of chemical sediments when water-soluble components are transported through the rivers into the sea and new reactions occur there or the water evaporates and the minerals are deposited. Examples: rock salt, limestone
  • Biogenic sediments are deposits of organogenic substances (shells of shellfish or dead plants). Example: lime, coal, amber.
also transformation rock
  • Paragestone - transformation of sedimentary rock
  • Ortho rock - transformation of igneous rock
due to high pressure and high temperatures deep in the earth's crust. A distinction is made between:
  • contact metamorphic rocks - increased temperature of the rocks due to rising magma
  • Dynamometamorphic rocks - conversion of rocks through kinetic energy
  • regional metamorphic rocks - due to the rock complexes above, the rocks sink into deeper and hotter areas of the earth's crust, where high pressures are present and are converted. Example: phyllite, mica slate, gneiss.
 
 
 

The cycle of rocks

 
 
 The rocks of our earth are in a constant cycle. Rocks in the upper crust of the earth are subject to constant physical and chemical weathering and are deposited as sediments. These in turn are covered and solidified by new sediments (diagenesis). The deeper the rocks sink, the more the pressure and temperature increase. Metamorphosis or melting (magma formation) of the rocks takes place in the deeper layers. Then they are raised to the surface of the earth again by mountain formations or volcanism, weather there again and the cycle begins again.
 modified from Press & Sievers (1995)
 
 
 

Breakdown of the minerals

 
 The minerals are named after important people (goethite FeOOH), after finding points (zinnwaldite-lithium iron mica), according to the chemical composition (argentite) or according to physical properties (rose quartz-pink color). 
 Max von Laue first divided minerals into so-called mineral classes in 1912. Then the approx. 2500 - 3000 different minerals are divided into 9 different mineral classes.
  1. Element - minerals
    Example: gold (Au), silver (Ag), copper (Cu), diamond (C)
  2. Sulfides (sulfur compounds)
    Example: Pyrite FeS2
  3. Oxides and hydroxides
    Example: Goethite FeOOH, Hematite Fe2O3
  4. Halides - compounds with elements of main group VII
    Example: Halite (rock salt) NaCl
  5. Silicates - SiO4-Tetrahedron
    Example: Topas (Al2SiO4(F, OH)2), Garnet almandine (Fe3Al2(Si04)3, Feldspar orthoclase (KAlSi3O8)
  1. Phosphates - compounds of metals with PO4
    Example: fluoroapatite (approx5(PO4)3F)
  2. Sulphates - compounds of metals with SO4
    Example: plaster of paris (CaSO4 x H2O), barite (BaSO4 )
  3. Carbonates - compounds of metals with CO3
    Example: Calcite (CaCO3), Malachite (Cu22+(CO3(OH)2
  4. Borates / nitrates
    Example: Potash nitrate (K [NO3]), Boracite (Mg [Cl / B7O13]
 
 
 

The rock-forming minerals

 
  Main elemental composition of the earth (% by weight)
elementTotal earthcorecoatoceanic crustcontinental crust
loweruppertotal
O32,44,144,849,546,447,947,2
Si17,27,421,523,127,130,328,8
Al1,5 2,24,18,27,78,0
Fe28,279,45,86,85,73,14,3
Mg15,9 22,84,63,21,62,2
Approx1,6 2,38,94,92,93,9
N / A0,3 0,31,02,12,62,4
K0,02 0,030,11,32,92,1
       
       
total97,198,199,798,198,999,098,9
Table according to ALLEGRE (1995)
 
 
 The table shows that the elements oxygen (O) and silicon (Si) mainly occur in our earth's crust. About 95% of the crust consists of igneous rocks and their metamorphic derivatives and only 5% of sedimentary rocks. This sedimentary rock covers a large part of the earth's surface. 
 

The most important minerals in the earth's crust

Igneous rocksSedimentsMetamorphic rocks
Feldspar
e.g. Albite: Na [AlSi3O8]
Quartz, SiO2Feldspar
quartzClay minerals
e.g. kaolinite: Al4[Si4O10](OH)8
quartz
mica
e.g. muscovite K2Al4[Si6Al2O20] (OH, F)4
Feldsparmica
Pyroxene
e.g. Augite (Ca, Mg, Fe, Al)2[(Si, Al)2O6]
Calcite* CaCO3garnet
e.g. grossular approx3Al2[Si3O12]
Amphibole
e.g. hornblende approx2(Mg, Fe)4Al [Si7AlO22](OH)2
dolomite* CaMg (CO3)2Pyroxene
Olivine (Mg, Fe)2[SiO4]plaster* CaSO42H2OCordierite (Mg, Fe)2Al3[Si5AlO18] nH2O
Spinel Group *
e.g. Chromite FeCr2O4
Rock salt* NaClDisthene Al2SiO5
(*) non-silicate minerals
Table data from H.BAHLBURG, C.BREITKREUZ "Fundamentals of Geology" according to DEER, HOWIE & ZUSSMAN (1992)
 
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