Mica

As mica group or shortly mica refers to a group of phyllosilicates with the chemical composition:

In this formula:

  • D: 12-fold coordinated cations ( K, Na, Ca, Ba, Rb, Cs, NH4 )
  • G: 6-fold coordinated cations ( Li, Mg, Fe 2 , Mn, Zn, Al, Fe3 , Cr, V, Ti)
  • D: 4-fold coordinated cation (Si, Al, Fe 3 , B, Be)
  • X: anion (OH -, F -, Cl -, O2 -, S2 - )

The coordination of the cation in this context means the number and type of nearest neighbors. A 12-fold coordinated cation, for example, in mica is surrounded by 12 oxygen atoms.

Highlighted in bold are the dominant ions, respectively. The ions in brackets can be represented in any combination, but always stand in the same relation to the other groups of atoms (substitution).

Structurally, the mica is characterized by layers of TO4 tetrahedra and octahedra GO6. An octahedron is hereby incorporated by two tetrahedral layers. Among themselves these TOT sandwiches are connected only very weakly on large low -charged interlayer cations.

Characteristic of the minerals of the mica group, the perfect cleavage parallel to this layer packets. They have a low hardness of 2 ( parallel to the planes of the layers ) to 4 (all other directions). Their color varies from white to brown, black and rare green or pink. The stroke color is white. For many technical applications of mica their very low electrical conductivity is crucial.

Mica among the most common rock-forming minerals and are important constituents of many igneous ( granites, diorites, pegmatites, ...) and metamorphic ( mica schist, gneiss ) rocks.

  • 4.1 silicate anion
  • 4.2 octahedron
  • 4.3 Linking the layers

Etymology and history

Micas is called glow or shine weak. But from time immemorial, it was thought by the name of a blender, does not keep his promises. Therefore, some types of mica are pejoratively referred to as cat silver or fool's gold. In English, this means that mineral mica, mica from the Latin ( common occurrence in small leaves ) or micare, glitter '.

Mica were mentioned in 1546 by the mineralogist Georgius Agricola. In the 20th century mica were first studied by Charles -Victor Mauguin with X-rays.

Classification and Nomenclature

According to the classification of the mica Dana belong to the phyllosilicates (Class 71) with silicate layers of six-membered rings and a ratio of silicate to octahedral sheets of 2:1 ( Dana 71.1 ). In it the mica are represented by the subgroups 71.2.2a ( Muskovituntergruppe ) 71.2.2.b ( Biotituntergruppe ) 1.2.2.C ( Margarituntergruppe ) and 71.2.2.d ( hydromica ).

Strunz assigns the mica to the phyllosilicates (Class VIII / H) and divided them into groups VIII/H.10 ( mica group Muskovitreihe ) VIII/H.11 ( mica group Biotitreihe ) VIIIH.12 ( mica group Lepidolithreihe ) and VIII / H .13 ( mica group Glaukonitreihe ).

The current classification of the mica was submitted by a Working Group of the IMA Commission on New Minerals, Nomenclature and Classification. They divided the mica group based on the occupation of the D position, which is the cation position between the TOT sandwiches, into three sub- groups:

  • Genuine mica: mica with more than 50% of monovalent cations on the D position
  • Brittle mica: Mica with more than 50% of divalent cations to the D position
  • Interlayer -deficient mica: mica with less than 0.85 positive charges per formula unit to the D position

These subgroups are subdivided according to the occupation of the octahedral G- position:

  • Dioctahedral mica: Mica with less than 2.5 cations on the G- position
  • Trioctahedral mica: mica with more than 2.5 cations on the G- position

Later, this classification was supplemented by further sub-groups. The classification is based on the cations on the D position (Na, Rb, Cs, NH4 instead of K ) and the primary occupation of the G -T and X positions with unusual for mica ions (eg, Mn, Cr, V instead of Fe or Mg in the M locations, O or F instead of OH).

Below are the different mica of the individual subgroups are listed with their idealized compositions. Strunz, Dana and the IMA shall proceed to a different allocation of the mica minerals to the groups in individual cases. Here, the classification of the IMA is reproduced.

Genuine mica

Ordinary potassium mica

Muscovite - celadonite series ( dioctahedral )

  • Muscovite: K Al2 [ AlSi3O10 (OH ) 2]
  • Aluminoceladonit: K Al (Mg, Fe2 ) [ Si4O10 (OH ) 2] with Mg / (Mg VIFe2 ) > 0.5
  • Ferro - Aluminoceladonit: K Al (Mg, Fe2 ) [ Si4O10 (OH ) 2] with Mg / (Mg VIFe2 ) < 0.5
  • Celadonite: K Fe3 ( Mg, Fe2 ) [ Si4O10 (OH ) 2] with Mg / (Mg VIFe2 ) > 0.5
  • Ferroceladonit: K Fe3 ( Mg, Fe2 ) [ Si4O10 (OH ) 2] with Mg / (Mg VIFe2 ) < 0.5

Phlogopite Annite series ( trioctahedral )

  • Annite: K Fe2 3 [ AlSi3O10 (OH ) 2]
  • Phlogopite: K Mg2 3 [ AlSi3O10 (OH ) 2]

Siderophyllit - polylithionite series ( trioctahedral )

  • Siderophyllit: K Fe2 2 Al [ Al2Si2O10 (OH ) 2]
  • Polylithionite: K Li2 Al [ Si4O10F2 ]

Tainiolit Group

  • Tainiolith: K Li Mg 2 [ Si4O10F2 ]

Unusual potassium mica

Dioctahedral

  • Roscoelite K V2 [ AlSi3O10 (OH ) 2]
  • Chromphyllit: K Cr2 [ Al Si3O10 (OH ) 2]
  • Boromuskovit: K Al2 [ BSi3O10 (OH ) 2]

Trioctahedral

  • Eastonit: K Mg2 2 Al [ Al2Si2O10 (OH ) 2]
  • Hendricksit: K Zn2 3 [ AlSi3O10 (OH ) 2]
  • Montdorit: K Fe2 Mn2 1.5 0.5 Mg0, 5 [ Si4O10F2 ]
  • Trilithionite: K Li 1, 5 Al1, 5 [ AlSi3O10F2 ]
  • Masutomilith: K Li Mn2 Al [ AlSi3O10F2 ]
  • Norrishit: K Li Mn3 2 [ Si4O10O2 ]
  • Tetra- ferri- Annite: K Fe2 3 [ Fe3 Si3O10 (OH ) 2]
  • Tetra Ferriphlogopit: K Mg2 3 [ Fe3 Si3O10 (OH ) 2]

Non - potassium mica

Na - mica

  • Aspidolith: Na Mg2 3 [ AlSi3O10 (OH ) 2]
  • Preiswerkit: Na Mg2 2 Al [ Al2Si2O10 (OH ) 2]
  • Ephestit: Na Li Al2 [ Al2Si2O10 (OH ) 2]
  • Paragonite: Well Al2 [ AlSi3O10 (OH ) 2]

Cs- mica

  • Nanpingit: Cs Al2 [ AlSi3O10 (OH ) 2]
  • Sokolovait: Cs Li2 Al [ Si4O10F2 ]

NH4- mica

  • Tobelith: ( NH4) Al2 [ AlSi3O10 (OH ) 2]

Brittle mica: ( Dana: Margarituntergruppe; Strunz: Lepidolithreihe )

Ordinary brittle mica

Trioctahedral

  • Clintonite: Ca Mg2Al [ Al3Si O10 (OH ) 2]
  • Ferrokinoshitalith: Ba Fe2 3 [ Al2Si2 O10 (OH ) 2]
  • Kinoshitalith: Ba Mg3 [ Al2Si2 O10 (OH ) 2]

Dioctahedral

  • Margarit: Ca Al2 [ Al2Si2O10 (OH ) 2]
  • Ganterit: Ba0.5 (Na, K ) 0.5 Al2 [ Al1.5Si2.5O10 (OH ) 2]

Unusual brittle mica

Trioctahedral

  • Bityit: Ca LiAl2 [ BeAlSi2 O10 (OH ) 2]
  • Anandit: Ba Fe2 3 [ Fe3 Si3 O10 (OH ) 2]

Dioctahedral

  • Chernykhit: Ba V2 [ Al2Si2O10 (OH ) 2]
  • Oxykinoshitalith: Ba Mg2Ti [ Al2Si2 O10O2 ]

Interlayer Loss- mica: ( Dana: hydromica; Strunz: Glaukonitreihe )

Dioctahedral

  • Illite ( series): K0.65 Al 2 [ Al 0 65Si3, 35O10 (OH ) 2]
  • Glauconite ( series): K0.8 R3 1.33 0.67 R2 [ Al 0 13Si3, 87O10 (OH ) 2]
  • Brammallit ( series): Na0.65 Al 2 [ Al 0 65Si3, 35O10 (OH ) 2]

Trioctahedral

  • Wonesit: Na0.5 Mg2, 5Al0, 5 [ Al Si3O10 (OH ) 2]

Series name

Some traditional names are permitted as labels for the mixed crystal composition, if a more accurate characterization is possible.

  • Biotite: Dark lithium mica free with compositions between Annite, phlogopite, and Siderophyllit Eastonit.
  • Glauconite: dioctahedral interlayer - Loss- mica with more than 15 % of divalent cations on the M position and predominantly Fe3 as the trivalent cation on the M position
  • Illite: dioctahedral interlayer - Loss- mica with less than 25 % of divalent cations on the M position and mainly Al as trivalent cation on the M position
  • Lepidolite: Lithium rich trioctahedral micas with compositions between Trilithionite and polylithionite
  • Zinnwaldite: Dark lithium bearing mica with compositions between Siderophyllit and polylithionite.

Occurrence

Mica are common constituents of igneous, metamorphic and sedimentary rocks. The variety muscovite found, for example very often in quartz-rich granites or pegmatites, besides also in metamorphic rocks such as phyllite. As a very efflorescence resistant variety it also occurs in sedimentary rocks such as sandstone. Biotite weathered much easier and therefore more likely to be found in granite or diorite.

Major producers are the United States and the People's Republic of China.

Structure

Silicate anion

Mica belonging to the group of the phyllosilicates. The Si4 ions form four very strong covalent bonds to four O2 - ions, the Si ion is surrounded tetrahedrally. The oxygen ions sit at the corners of the coordination tetrahedron and the silicon is in its center. On the Structural illustrations only these coordination polyhedra are shown for clarity and not the atoms themselves

This SiO4 tetrahedra are connected via corners ( common oxygens ) in a theoretically unlimited layers. The layered structure of mica is characterized in that each SiO4 tetrahedron is connected via three common vertices ( oxygens ) with three other SiO4 tetrahedra and the free fourth peaks all the tetrahedra of a layer in the same direction ( see Figure 1). The resulting Silicatanionenkomplex has the empirical formula [ Si4O10 ] 4 -.

Octahedral

The divalent and trivalent cations of the G- position are octahedrally surrounded by six oxygens. This GO6 octahedra are connected by edges ( two common oxygen atoms ) and also form theoretically unlimited layers. In the dioctahedral micas are only the M2 octahedra occupied by cations ( Fig. 2a) whereas in trioctahedral micas octahedron all of these layers are occupied by cations ( Fig. 2b).

Connection between the layers

It is characteristic of the mica structure that these silicate and octahedral layers are joined together so that each octahedron is enclosed by two silicate layers. Here, the Silicattetraeder associated with its free tip (oxygen ) with the octahedral layer. This unit is comparable to the I- beams of the pyroxenes, amphiboles and other Biopyribole. The charges are largely offset within this module. The final oxygens to the outward-facing base areas of the SiO4 tetrahedra are all bonded to two Si ions, and have virtually no free bonding valences more. Among themselves these mica structural units are therefore connected only by weak ionic bonds with the interlayer cations of the D position. This is the structural explanation for the excellent leaved cleavage of mica.

This mica structural units, also known as TOT or 2:1 - referred layers are stacked along the crystallographic c- axis (Fig. 3) and can thereby rotated about the c axis with n * 60 ° to each his (0 ≤ n ≤ 5). Different stacking sequences of differently oriented mica structural units receive various Glimmerpolytype with different symmetry (monoclinic, orthorhombic, trigonal ). The ordered distribution of different cations on the octahedral positions of symmetry G of the polytype is sometimes lowered, for example, from C2 / m ( monoclinic) of C-1 ( triclinic ).

The Glimmerpolytype can be divided into three subfamilies:

  • A polytype: rotation of the TOT layers just to 2n * 60 ° (0 °, 120 °, 240 °). In this subfamily are the most common mica polyps 1M, 2M1 and 3T
  • Polytype B: Rotation of TOT - layers only (2n 1) * 60 ° (60 °, 180 °, 300 °). From this group, only the rare polytype 2M2 and 2O have been found in nature so far.
  • Mixed polytype: Both 2n * 60 ° as well as (2n 1) * 60 ° - rotation of the mica layers ( 1MD )

The names of the polytype are essentially made ​​up of the number of differently oriented units (numbers, d for ' disordered - disordered ') and the crystal class (upper case M for monoclinic, T for trigonal, O for orthorhombic, H for hexagonal).

Use

Even in prehistoric times mica schist was mined and the mica obtained therefrom probably used for cosmetic purposes. Even today, find mica - under the INCI name Mica (CI 77019 ) - use in decorative cosmetics, such as in powder to achieve a shimmering effect.

Celadonite and glauconite (green earth ) were used as a green pigment, for example, in Roman, Byzantine and Japanese murals.

With inorganic interference layers, such as silicon dioxide and titanium dioxide coated mica is used since the mid- 1980s as a pearlescent or interference pigment including in car paint and cosmetics.

Because of the easy cleavage along the layer planes of mica can be split into thin transparent slices. Where mica and easily available at reasonable prices, glass, however, was too expensive, the mineral was used in particular in rural areas for the windows.

Due to the high melting point of the mineral before it was distribution of refractory glass, inter alia, use as an inspection window in room heaters, as a replacement for glass inspection window in industrial furnaces or protective glass of lanterns.

Mica and mica is as an electrical insulator and as the substrate material used for the heat ( soldering iron, toaster, electric stove ). Mica withstand temperatures of over 600 ° C., 400-500 ° C. mica

Mica sheets are used as insulating between power semiconductor components and their heat sinks. Coated mica sheets with punched holes can be used in electron tubes for construction of the electrode system.

Furthermore mica as a dielectric for low loss mica capacitors for high frequencies and power, as a window material of counter tubes in Geiger counters and - used as cover in microwave ovens - in the form of mica.

The membrane of the soundbox gramophones in mica has been used until they are replaced by metals such as aluminum or copper until the 1940s.

The plate material, the material in shipbuilding, building construction and in the production of fireplaces is used.

Since mica has a very smooth surface after the cleavage, it is also used as a substrate, self -organizing monolayers, and as a matrix in the atomic force microscopy.

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