Pyroxene

As pyroxene is known minerals from the mineral class Silicates and Germanates, as well as the Department of Kettensilicate whose crystal structure is characterized by single chains of corner-sharing SiO4 tetrahedra and complying with the composition of the following generalized empirical formula:

M1 M2 T2O6.

In the structural formula representing M1, M2 and T different positions in the Pyroxenstruktur. They are mainly occupied by the following cations:

  • M1: Mg2 , Fe2 , Mn2 , Al3 , Fe3 , Ti4 , Ti3 , V3 , Sc3 , Cr3 , Zr 4 , Zn2 ,
  • M2: Mg2 , Fe2 , Mn2 , Ca2 , Na , Li
  • T: Si4 , Al3 , Fe3

Highlighted in bold are the dominant cations are on the individual items.

Pyroxenes show a large chemical variability and occur worldwide in many different paragenesis and geological environment. They are an important part of both igneous as well as metamorphic rocks of different composition and formation conditions.

Pyroxenes have a hardness between 5 and 6.5 and a pale green to brownish green or bronze color. Line color is a greenish white.

A very similar group of minerals are the amphiboles; Pyroxene differs from these, however, in the cleavage; the gap angle at pyroxenes are at 90 degrees, while they are 120 degrees in amphibole. Pyroxenes forming short columns, amphiboles, however, usually long. Euhedral pyroxenes have two head faces, amphiboles, however, three.

  • 4.1 silicate anion
  • 4.2 octahedra
  • 4.3 I- Beams
  • 4.4 clino - and orthopyroxenes

Etymology and history

The name pyroxene comes from the Greek pyros (fire) and xenos ( stranger ). He alludes to the fact that pyroxenes mainly occur in volcanic lava, where they can be found as crystal inclusions of volcanic glass; earlier it was assumed that it was just to impurities in the glass, so that the name " fire strangers " was born. In fact, it is in the pyroxenes to minerals that crystallize before Lava Burst.

Classification and Nomenclature

Based on a correct designation of a pyroxene is a complete chemical analysis and the application of a predetermined calculation scheme that normalizes the exact contents of the individual elements and the individual positions (M1, M2, T) be split.

The International Mineralogical Association (IMA ) shares the pyroxenes according to their composition in 6 groups:

In these six groups of 20 base name for pyroxenes are defined. Major deviations from the compositions listed at the end is by preceding adjectives (titanium rich, rich in iron, ...) into account.

In the structural formulas given below in brackets, the atoms can be represented in any combination by substitution, but always stand in the same relation to the other groups of atoms. Here only the idealized compositions of the various pyroxenes are listed. The validity of the mineral name extends over a larger range of composition. For example, all Ca - poor Mg -Fe pyroxenes are denoted by Mg contents of 0 to 1 Fe2 as enstatite or Klinoenstatit.

Mg -Fe- pyroxene

Mg -Fe- pyroxene occur both with orthorhombic as well as monoclinic symmetry. The following end-members form the boundaries of the Mg -Fe pyroxenes:

  • Enstatite ( orthorhombic ) Mg2Si2O6
  • Ferrosillit ( orthorhombic ) Fe2 2 Si2O6
  • Klinoenstatit ( monoclinic) Mg2Si2O6
  • Klinoferrosillit ( monoclinic) Fe2 2 Si2AlO6
  • Pigeonite ( monoclinic) (Mg, Fe 2 , Ca) 2Si2AlO6

Mn-Mg - pyroxene

Mg -Mn- pyroxenes occur both with orthorhombic as well as monoclinic symmetry.

  • Donpeacorite ( orthorhombic ): (Mn 2 , Mg) MgSi2O6
  • Kanoit ( monoclinic): Mn2 MgSi2O6

Ca pyroxene

All Ca pyroxene crystallize with monoclinic symmetry.

  • Diopside: CaMgSi2O6
  • Hedenbergite: CaFe2 Si2O6
  • Augite ( Ca, Mg, Fe2 ) 2Si2O6
  • Johannsenite: CaMnSi2O6
  • Petedunnit: CaZnSi2O6
  • Esseneit: CaFe3 AlSiO6

Ca-Na pyroxenes

  • Omphacite: (Ca, Na) (R2 Al ) Si2O6
  • Ägirinaugit: (Ca, Na) (R2 Fe3 ) Si2O6

Na - pyroxenes

  • Jadeite: NaAlSi2O6
  • Aegirine: NaFe3 Si2O6
  • Kosmochlor: NaCr3 Si2O6
  • Jervisit: NaSc3 Si2O6
  • Namansilit: NaMn3 Si2O6
  • Natalyit: NaV3 Si2O6

Li- pyroxenes

The compositions of naturally occurring pyroxenes are often between the idealized compositions of these groups. Accordingly, a more coarser division has been developed that takes account of M1 and M2 of this complex cations on the miscibility of positions. After four chemical groups:

  • Quaternary pyroxene (Fig. 1): Ca -Mg -Fe pyroxenes with less than 0.2 Na ( enstatite, ferrosilite, pigeonite, Klinoenstatit, Klinoferrosilit, diopside, hedenbergite, augite ). The symmetry of the Ca - rich pyroxene quaternary differs at room temperature in detail from that of the Ca - poor pyroxene. The distinction between augite ( in the monoclinic space group C2 / m) and Pigenonit ( in the monoclinic space group P21 / c) is based on this change in symmetry. Augite and Pigeonite also differ in their Ca contents. Therefore, the simple composition to be determined is usually used for naming. The Ca contents of Pigeonite ( (Mg, Fe2 ) 1.9 to 1.6 Ca0 0.1-0, 4Si2AlO6 ) below 0.4 atoms per formula unit ( apfu ) and that of augite ( Ca0 0.4-0, 9 ( mg, Fe2 ) 1.6 to 1.1 Si2O6 ) about it.
  • Ca-Na pyroxene (Fig. 2): Ca -Mg -Fe- pyroxene with 0.2 to 0.8 Na ( omphacite, aegirine - augite ).
  • Na - pyroxene (Fig. 2 ): Al -Fe- pyroxene with 0.8 to 1.0 Na ( jadeite, aegirine ).
  • Other pyroxenes: This group contains all other pyroxenes, of which often only a few occurrences are known with extraordinary compositions less variability ( spodumene, Essenit, Johannsenite, Petedunnit, Kanoit, Donpeacorite ).

According to the symmetry of pyroxene be further divided into two groups:

  • Clinopyroxene: pyroxene monoclinic symmetry
  • Orthopyroxenes: pyroxene with an orthorhombic symmetry

Education and Locations

Pyroxenes are both solid, available in granular form as well as the most dark, short, prismatic crystals. As rock-forming minerals they are often found in quartz- poor igneous rocks such as basalt, gabbro and pyroxenite. Calcium-rich clinopyroxene are also proved beneficial in metamorphisiertem limestone ( skarn = ) included, while found in stony meteorites mainly orthopyroxenes. In the southern highlands of Mars, the results of spectroscopic studies on the occurrence of pyroxene and olivine close, the volcanic rocks are taken.

Structure

The variation of the chemical composition of the pyroxenes is explained by their structure. It has the cation positions of very different size and shape, providing a variety of cations of different size and charge space. In all these cation positions, the cations are surrounded by oxygen anions. The different positions are different in the number of the surrounding anions ( coordination number ), the distance to the cation and arrangement around the cation. Generally speaking, the more a cation surrounded anions, the larger the average distance from the cation to the anion position, the weaker the individual bonds and the greater the ionic character of the bonds.

The Pyroxenstruktur has cation positions with 3 different coordination numbers.

  • Tetrahedral positions (T): 4 oxygen ions ( O2 - ) surrounding a cation tetrahedron- shaped. This position offers small cations with high charge mostly space ( Si4 , Ti4 , Al3 ). The short cation-anion bonds have a high covalent ratio (atomic bonds). Atomic bonds are highly directional. Therefore, the geometry of the atomic orbitals must be as good as possible binding in accordance with the arrangement of the surrounding anions. Will meet these geometric boundary condition eg sp3 -hybridized cations such as Si4 . In this electron configuration unites one outer s orbital with the three outer p orbitals to four tetrahedrally oriented sp3 hybrid orbitals.
  • Octahedral (M1 ): 6 oxygen ions ( O2 - ) surrounding an octahedral cation. This position offers medium-sized, mostly di-and trivalent cations space ( Mg2 , Fe2 , Mn2 , Al3 , Fe3 ). The bonds are primarily non-directional ionic.
  • 6 to 8 -fold coordinated courses (M2 ): 6-8 oxygen ions ( O2 - ) surrounding a cation. This position offers medium to large one to divalent cations space ( Mg2 , Fe2 or Na , Ca2 ). The bonds are weak and predominantly ionic. Cations medium size ( Mg2 , Fe2 ) are 6-fold coordinated, larger (Na , Ca2 ) 8-fold.

The adjacent structure diagrams show the sake of clarity only the faces of these polyhedra. The oxygens and cations themselves are not shown. The oxygen anions are located on the corners of the polyhedron, the cations in the center of the polyhedron.

Silicate anion

The structural characteristic of all pyroxene is a single chain of SiO4 tetrahedra with the empirical formula [ Si2O6 ] 4 -. Herein, the SiO4 tetrahedra are connected via two oxygens to ideally infinite chains.

After Silicatklassifikation by F. Liebau the pyroxenes belong to the group of unbranched chains of two single - silicates. Within a chain repeats the orientation of the Silicattetraeder with every second tetrahedron ( two chain ). The chains are not interconnected directly with each other (single chain) and chain branch no further tetrahedron from ( unbranched).

The SiO4 tetrahedra are arranged in chains so that all tetrahedra have a chain with a tetrahedron point in the same direction. Accordingly, all tetrahedron with a face in the opposite direction. The figure shows a section of a SiO4 two single chain with a view of the tetrahedra tips.

Octahedra

On the M1 position, the smaller cations (mainly Mg2 , Fe2 , Mn2 , Al3 ) are octahedrally coordinated by six oxygens. The octahedra are linked via common edges to form zigzag chains.

The M2 polyhedra are three common Did connected to three M1 octahedra in a chain. In the case of 8-fold greater coordinated cations M2, such as Ca2 or diopside Hedenbergid the polyhedra are connected by a common Knew with the M2 polyhedrons of an adjacent octahedra. For smaller hexacoordinate cations M2, such as Mg2 in enstatite, there is no such link.

I- Beams

The two chains of tetrahedra are connected via their free tips with the top or bottom of a Oktaederbandes. This sandwich -like assembly is also referred to as I-beam because of their resembling the capital letters I cross-section.

These I- beams are connected to each other via the SiO4 tetrahedron - octahedron and M2.

Clino - and orthopyroxenes

The pyroxenes are divided into two groups according to their symmetry:

  • Clinopyroxene: pyroxene monoclinic symmetry (space group C2 / c). These include, for example, all Na - and Ca - pyroxene
  • Orthopyroxenes: pyroxene with an orthorhombic symmetry ( space group Pbca, Pbcn ). These include, for example, the pyroxenes of enstatite ferrosilite series ( space group Pbca ) and, for example, the high-temperature form of Enstatits ( protoenstatite, space group Pbcn ).

The Pyroxenstrukturen the different space groups differ in the stacking of the Oktaederlagen in the direction of the crystallographic a- axis ( see figure).

In clinopyroxenes octahedron, all in the same orientation. In a direction successive tetrahedral and Oktaederlagen are offset in the c direction something to each other. For this misalignment results in the pyroxenes the oblique angle of the monoclinic symmetry.

The Orthopyroxenen the orientation of the octahedron changes periodically in a direction. The displacement of the successive layers is in the a- direction in this case balanced, and the result is an orthorhombic elementary cell.

The diagonal of a Oktaederecke by Oktaederzentrum to the opposite corner has, alternately in the direction of the a- and c- axis ( layers M ) and opposite to the direction of the a- and c- axes ( layers M ). The Oktaederlagen opposite orientation can be mapped ( parallel to the b - and c- axis) to each other by reflection in a Spiegelebne. These ratios equal to unit cell level where the common macroscopic twinning in pyroxenes. Therefore, it describes Orthopyroxenen as polysynthetic twinning on unit cell level.

The orthopyroxenes of the different space groups Pbca and Pbcn differ in the frequency of reversal of Oktaederorientierung. PBCA pyroxene (eg Ferrosollit ) exhibit a periodicity of two, i.e., after every second octahedron changes the orientation of the octahedron ( Oktaederlagen sequence of M-M -M M M-M -...). Pbcn - pyroxenes are characterized by a reversal of the Oktaederorientierung after each layer of ( sequence of Oktaederlagen M -M M -M M -...).

Use

Some pyroxenes are suitable as a precious stone, the green enstatite, which is also green diopside and the red-brown hypersthene.

The most massively occurring jadeite was used because of its very compact structure for the production of ax blades; next to it can be made from very fine jadeite carved jewelry objects.

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