Pyroxene group

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The pyroxene group is a group of minerals from the chain silicate department . Their crystal structure is characterized by single chains of corner-linked SiO 4 tetrahedra and their composition satisfies the following generalized empirical formula:

M1 M2 T 2 O 6 .

In this structural formula, M1, M2 and T represent different positions in the pyroxene structure. They are mainly occupied by the following cations:

  • M1: Mg 2+ , Fe 2+ , Mn 2+ , Al 3+ , Fe 3+ , Ti 4+ , Ti 3+ , V 3+ , Sc 3+ , Cr 3+ , Zr 4+ , Zn 2+ ,
  • M2: Mg 2+ , Fe 2+ , Mn 2+ , Ca 2+ , Na + , Li +
  • T: Si 4+ , Al 3+ , Fe 3+

The dominant cations on the individual positions are highlighted in bold.

The compositions of the pyroxenes are very variable and they occur worldwide in very many different parageneses and geological environments. They are an important component of both igneous and metamorphic rocks of various compositions and formation conditions.

Pyroxenes have a hardness between 5 and 6.5 and a pale green to brownish green or bronze color. The color of the stroke is a greenish white.

A very similar mineral group are the amphibole ; Pyroxene differs from these, however, in its cleavability ; the gap angles for pyroxenes are 90 degrees, while for amphibole they are 120 degrees. Pyroxenes form short columns, while amphiboles are usually long. Idiomorphic pyroxenes have two head surfaces, while amphiboles have three.

Aegirine (black) on feldspar from Mountain Malosa, Zomba District, Malawi
Size: 3.0 × 2.8 × 2.0 cm

Etymology and history

The name pyroxen comes from the Greek pyros (fire) and xenos (foreign). He alludes to the fact that pyroxenes occur mainly in volcanic lava, where they can be found as crystal inclusions of volcanic glass ; it used to be assumed that it was only a matter of impurities in the glass, so that the name "alien to fire" arose. In fact, however, the pyroxenes are minerals that crystallize before the lava eruption.

Classification and nomenclature

The basis of a correct naming of a pyroxene is a complete chemical analysis and the application of a given calculation scheme with which the exact contents of the individual elements are normalized and distributed to the individual items (M1, M2, T).

The International Mineralogical Association (IMA) divides pyroxenes into 6 groups according to their composition:

  1. Mg-Fe-Pyroxene
  2. Mn-Mg pyroxenes
  3. Ca-pyroxene
  4. Ca-Na-Pyroxene
  5. Na-pyroxene
  6. Li-Pyroxene

In these six groups, 20 basic names for pyroxenes are defined. Significant deviations from the compositions listed below are taken into account by preceding adjectives (rich in titanium, rich in iron, ...).

In the structural formulas given below, the atoms in brackets can be substituted in any mixture , but they are always in the same relationship to the other atom groups. Only the idealized compositions of the various pyroxenes are listed here. The validity of the mineral names extends over a larger composition area. So z. B. all low-Ca Mg-Fe-pyroxenes with Mg contents of 0 to 1 Fe 2+ are called enstatite or clinoenstatite.

Strictly speaking, the structural formulas for pigeonite, augite, omphacite and aegirin-augite describe mixed crystals and not minerals. The current guideline for a mineral end link composition allows a mixed occupation of only one lattice position with a maximum of two different ions. In view of the widespread use of these pyroxenes and the widespread use of these mineral names in the literature, they are still listed as independent minerals.

Mg-Fe-Pyroxene

Fig. 1: Quaternary Ca-Mg-Fe pyroxenes.

Mg-Fe-pyroxenes occur with both orthorhombic and monoclinic symmetry.

The following end links form the boundaries of the Mg-Fe-Pyroxene:

Surname M2 2+ M1 2+ T 4+ 2 O 2- 6 Space group annotation
Enstatite Mg 2+ Mg 2+ Si 4+ 2 O 6 Pbca (No. 61)Template: room group / 61
Ferrosilite Fe 2+ Fe 2+ Si 4+ 2 O 6 Pbca (No. 61)Template: room group / 61
Protoenstatite Mg 2+ Mg 2+ Si 4+ 2 O 6 Pbcn (No. 60)Template: room group / 60
Protoferrosilite Fe 2+ Fe 2+ Si 4+ 2 O 6 Pbcn (No. 60)Template: room group / 60 hypothetical, ~ 20% in protoenstatite
Clinoenstatite Mg 2+ Mg 2+ Si 4+ 2 O 6 P 2 1 / c (No. 14)Template: room group / 14
Clinoferrosilite Fe 2+ Fe 2+ Si 4+ 2 O 6 P 2 1 / c (No. 14)Template: room group / 14
Pigeonite (Ca, Mg, Fe) 2+ (Mg, Fe) 2+ Si 4+ 2 O 6 P 2 1 / c (No. 14)Template: room group / 14

Mn-Mg pyroxenes

Mg-Mn-pyroxenes occur with both orthorhombic and monoclinic symmetry.

Surname M2 2+ M1 2+ T 4+ 2 O 2- 6 Space group annotation
Donpeacorite Mn 2+ Mg Si 4+ 2 O 6 Pbca (No. 61)Template: room group / 61
Kanoite Mn 2+ Mg Si 4+ 2 O 6 P 2 1 / c (No. 14)Template: room group / 14

Ca-pyroxene

All Ca-pyroxenes crystallize with monoclinic symmetry.

Surname M2 2+ M1 2+ T 4+ 2 O 2- 6 Space group annotation
Augit ( Ca , Mg, Fe 2+ ) (Mg, Fe 2+ ) Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Diopside Ca 2+ Mg 2+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Hedenbergite Ca 2+ Fe 2+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Johannsenite Ca 2+ Mn 2+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Petedunnite Ca 2+ Zn 2+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
M2 2+ M1 3+ T 3+ T 4+ O 2-
Kushiroite Ca 2+ Al 3+ Al 3+ Si 4+ O 6 C 2 / c (No. 15)Template: room group / 15
Esseneit Ca 2+ Fe 3+ Al 3+ Si 4+ O 6 C 2 / c (No. 15)Template: room group / 15
Burnettite Ca 2+ V 3+ Al 3+ Si 4+ O 6 C 2 / c (No. 15)Template: room group / 15
Davisite Ca 2+ Sc 3+ Al 3+ Si 4+ O 6 C 2 / c (No. 15)Template: room group / 15
Grand manite Ca 2+ Ti 3+ Al 3+ Si 4+ O 6 C 2 / c (No. 15)Template: room group / 15
M2 2+ M1 4+ 0.5 M1 2+ 0.5 T 3+ T 4+ O 2-
Buffonite Ca 2+ Ti 4+ 0.5 Mg 2+ 0.5 Fe 3+ Si 4+ O 6 hypothetical Ti 4+ -Fe 3+ - end link
Al-Buffonite Ca 2+ Ti 4+ 0.5 Mg 2+ 0.5 Al 3+ Si 4+ O 6 hypothetical Ti 4+ -Al 3+ end link
M2 2+ 0.50.5 M1 3+ T 4+ 2 O 2-
Tissintit Ca 2+ 0.50.5 Al 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Chrome Eskola Pyroxene Mg 2+ 0.50.5 Cr 3+ Si 4+ 2 O 6 hypothetical end link

Ca-Na-Pyroxene

Fig. 2: Ca-Na-Pyroxene

Omphacite occurs naturally in two different symmetries. P 2 / n (No. 13, position 2) -Omphacites differ structurally from the other pyroxenes and can be viewed as a separate species. C 2 / c (No. 15) -Omphacites can be considered as jadeite-diopside-hedenbergite mixed crystal. Template: room group / 13.2Template: room group / 15

So far, aegirine-augite has only been found with the space group C 2 / c (space group no. 15) and can be regarded as aegirine-diopside-hedenbergite mixed crystal. Template: room group / 15

Surname M2 2+ 0.5 M2 + 0.5 M1 2+ 0.5 M1 3+ 0.5 T 4+ 2 O 2- 6 Space group annotation
Omphacit Ca 2+ 0.5 Na + 0.5 (Mg, Fe) 2+ 0.5 Al 3+ 0.5 Si 4+ 2 O 6 C 2 / c (No. 15) , P 2 / n (No. 13, position 2)Template: room group / 15Template: room group / 13.2
Aegirine Augite Ca 2+ 0.5 Na + 0.5 (Mg, Fe) 2+ 0.5 Fe 3+ 0.5 Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15

Na-pyroxene

Surname M2 + M1 3+ T 4+ 2 O 2- 6 Space group annotation
Jadeite Na + Al 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Aegirine Na + Fe 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Cosmochlor Na + Cr 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Jervisit Na + Sc 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Namansilite Na + Mn 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15
Natalyite Na + V 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15

Li-Pyroxene

Surname M2 + M1 3+ T 4+ 2 O 2- 6 Space group annotation
Spodumene Li + Al 3+ Si 4+ 2 O 6 C 2 / c (No. 15)Template: room group / 15

The compositions of naturally occurring pyroxenes are often between the idealized compositions of these groups. A further, more coarse classification was developed accordingly, which takes this complex miscibility of cations in positions M1 and M2 into account. According to this, four chemical groups are distinguished:

  • Quaternary pyroxenes (Fig. 1): All Ca-Mg-Fe-pyroxenes with less than 0.2 Na (enstatite, ferrosilite, pigeonite, clinoenstatite, clinoferrosilite, diopside, hedenbergite, augite). The symmetry of the Ca-rich quaternary pyroxenes differs in detail from that of the Ca-poor pyroxenes at room temperature. The distinction between augite (monoclinic in space group C2 / m) and pigenonite (monoclinic in space group P2 1 / c) is based on this change in symmetry. Augite and pigeonite also differ in their Ca contents. Therefore, the easier to determine composition is usually used for naming. The Ca content of Pigeonite ((Mg, Fe 2+ ) 1.9–1.6 Ca 0.1–0.4 Si 2 AlO 6 ) is below 0.4 atoms per formula unit (apfu) and that of Augite ( Ca 0.4-0.9 (Mg, Fe 2+ ) 1.6-1.1 Si 2 O 6 ) above.
  • Ca-Na-Pyroxene (Fig. 2): Ca-Mg-Fe-Pyroxene with 0.2 to 0.8 Na (Omphacit, Aegirin-Augit).
  • 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 with unusual compositions with little variability are known (spodumene, essenite, Johannsenite, Petedunnite, Kanoite, Donpeacorite).

According to symmetry, pyroxenes are further divided into two groups:

Education and Locations

Pyroxenes occur both massive, in granular form and as mostly dark, short, prism-shaped crystals . As rock-forming minerals, they are often found in low-quartz igneous rocks such as basalt , gabbro and pyroxenite . Calcium-rich clinopyroxenes are also contained in metamorphosed limestone (= skarn ), while orthopyroxenes, i.e. low-calcium pyroxenes, primarily occur in stone meteorites . In the southern highlands of Mars , the results of the spectroscopic investigations suggest the occurrence of pyroxene and olivine , which originate from volcanic rocks.

structure

The range of variation in the chemical composition of the pyroxenes is explained in their structure. It has cation positions of very different sizes and shapes and thus offers space for a multitude of cations of different sizes and charges. In all these cation positions, the cations are surrounded by oxygen anions. The different positions differ in the number of surrounding anions (coordination number), their distance to the cation and their arrangement around the cation. In general, the following applies: The more anions surround a cation, the greater the mean distance from the cation position to the anions, the weaker the individual bonds and the greater the ionic character of the bonds.

The pyroxene structure has cation positions with 3 different coordination numbers.

  • Tetrahedral positions (T): 4 oxygen ions (O 2− ) surround a cation in a tetrahedral shape. This position offers space for small cations, usually with a high charge (Si 4+ , Ti 4+ , Al 3+ ). The short cation-anion bonds have a high covalent content (atomic bonds). Atomic bonds are strongly directed. Therefore, the geometry of the binding atomic orbitals must match the arrangement of the surrounding anions as closely as possible. This geometric boundary condition is fulfilled z. B. of sp3-hybridized cations such as Si 4+ . In this electron configuration, the one outer s orbital unites with the three outer p orbitals to form 4 tetrahedrally aligned sp 3 hybrid orbitals.
  • Octahedral position (M1): 6 oxygen ions (O 2− ) surround a cation in an octahedral shape. This position offers space for medium-sized, mostly divalent and trivalent cations (Mg 2+ , Fe 2+ , Mn 2+ , Al 3+ , Fe 3+ ). The bonds are predominantly non-directional ionic.
  • 6 to 8-fold coordinated sites (M2): 6–8 oxygen ions (O 2− ) surround a cation. This position offers space for medium-sized to large monovalent to divalent cations (Mg 2+ , Fe 2+ or Na + , Ca 2+ ). The bonds are weak and predominantly ionic. Medium-sized cations (Mg 2+ , Fe 2+ ) are 6-fold coordinated, larger cations (Na + , Ca 2+ ) 8-fold.

For the sake of clarity, the structure diagrams on the left show only the surfaces of these coordination polyhedra. The oxygen and cations themselves are not shown. The oxygen anions are on the corners of the polyhedra, the cations in the center of the polyhedra.

Silicate anion complex

Pyroxene structure: two single SiO 4 tetrahedron
chains parallel to the crystallographic c-axis; Blue rectangle: unit cell

The structural characteristic of all pyroxenes is the single chain of SiO 4 tetrahedra with the empirical formula [Si 2 O 6 ] 4− . Here, the SiO 4 tetrahedra are connected to ideally infinite chains via two oxygen atoms.

According to F. Liebau's silicate classification, pyroxenes belong to the group of unbranched two single-chain silicates. Within a chain, the orientation of the silicate tetrahedron is repeated with every second tetrahedron (two chains). The chains are not directly connected to each other (single chain) and no further tetrahedra branch off from the chain (unbranched).

The SiO 4 tetrahedra are arranged in the chains in such a way that all tetrahedra in a chain with a tetrahedron point point in the same direction. Correspondingly, all tetrahedra with a face point in the opposite direction. The adjacent figure shows a section of a SiO 4 double single chain with a view of the tetrahedron tips.

Octahedron chain

Pyroxene structure: zigzag chain of the M1 and M2 octaves parallel to the crystallographic c-axis; Blue rectangle: unit cell

At the M1 position, the smaller cations (mainly Mg 2+ , Fe 2+ , Mn 2+ , Al 3+ ) are octahedrally coordinated by six oxygenates. The octahedra are linked to form zigzag chains via common edges.

The M2 coordination polyhedra are connected to three M1 octahedra of a chain via three common edges. In the case of 8-fold coordinated larger cations on M2, e.g. B. Ca 2+ in the Diopside or Hedenbergid, the polyhedra are connected to the M2 polyhedra of an adjacent octahedron chain via a common edge. For smaller six-coordinate cations on M2, such as B. Mg 2+ in enstatite, there is no such link.

I-beams

Pyroxene structure
: sandwich-like structural unit made of SiO 4 - MO 6 - SiO 4 layers (I-beams); Blue rectangle: unit cell
Pyroxene structure: Linking the I-Beams; Blue rectangle: unit cell

Two tetrahedron chains are connected to the top and bottom of an octahedral ribbon via their free tips. This sandwich-like structural unit is also referred to as an I-beam because of its cross-section reminiscent of the capital letter I.

These I-beams are connected to one another via the SiO 4 tetrahedron and M2 octahedron.

Clino- and orthopyroxenes

Pyroxene structure: sequence of tetrahedral and octahedral
layers in clino- and orthopyroxenes; Blue rectangles: unit cell

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

  • Clinopyroxenes: pyroxenes with monoclinic symmetry (space group C2 / c). These include B. all Na and Ca pyroxenes
  • Orthopyroxenes: Pyroxenes with orthorhombic symmetry (space groups Pbca, Pbcn). These include B. the pyroxenes of the enstatite ferrosilite series (space group Pbca) and z. B. the high temperature form of the enstatite (protoenstatite, space group Pbcn).

The pyroxene structures of the various space groups differ in the stacking of the octahedral layers in the direction of the crystallographic a-axis (see figure).

In the clinopyroxenes, all octahedra have the same orientation. Tetrahedral and octahedral layers that follow one another in the a-direction are each slightly offset from one another in the c-direction. This offset results in the oblique angle of monoclinic symmetry in the pyroxenes.

In the orthopyroxene the orientation of the octahedra alternates periodically in the a direction. The offset of the layers following one another in a-directions is compensated for and an orthorhombic unit cell results.

The diagonal from one octahedron through the octahedron center to the opposite corner points alternately in the direction of the a- and c-axes (positions M +) and against the direction of the a- and c-axes (positions M−). The octahedral layers of opposite orientations can be mapped onto each other by mirroring them on a mirror plane (parallel to the b and c axes). At the unit cell level, these relationships resemble those of the common macroscopic twin formation in pyroxenes. Therefore, orthopyroxenes are also described as polysynthetic twinning on the unit cell level.

The orthopyroxenes of the various space groups Pbca and Pbcn differ in the periodicity of the reversal of the octahedral orientation. Pbca-pyroxenes (e.g. ferrosollite) have a periodicity of two; H. after every second octahedron layer the orientation of the octahedron changes (sequence of octahedron layers M− M− M + M + M− M−…). Pbcn-pyroxenes are characterized by a reversal of the octahedral orientation after each position (sequence of octahedron positions M− M + M− M + M−…).

use

Some pyroxenes are suitable as gemstones , such as the green enstatite, the likewise green diopside and the red-brown hypersthene.

The mostly massive jadeite was used because of its very compact structure for the production of ax blades; In addition, very finely carved jewelry objects can be made from jadeite.

See also

literature

  • M. Cameron, JJ Papike: Crystal Chemestry of Silicate Pyroxenes (1980), in Reviews in Mineralogy Vol. 7 pp. 5 - 87; Mineralogical Society of America
  • Edition Dörfler: Mineralien Enzyklopädie , Nebel Verlag, ISBN 3-89555-076-0
  • Martin Okrusch, Siegfried Matthes: Mineralogy . 7th edition. Springer Verlag, Berlin 2005, ISBN 3-540-23812-3
  • Stefan Weiß: The large Lapis mineral directory . 4th edition. Christian Weise Verlag, Munich 2002, ISBN 3-921656-17-6

Web links

Commons : Pyroxene  - collection of images, videos, and audio files
Wiktionary: Pyroxene  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. a b c d Subcommite on Pyroxenes, CNMMN; Nobuo Morimoto: Nomenclature of Pyroxenes . In: The Canadian Mineralogist . tape 27 , 1989, pp. 143–156 ( mineralogicalassociation.ca [PDF; 1.6 MB ; accessed on March 30, 2019]).
  2. Hawthorne FC: The Use Of End-Member Charge-Arrangements In Defining New Mineral Species And Heterovalent Substitutions In Complex Minerals. In: The Canadian Mineralogist. 40, 2002, pp. 699-710. ( PDF (309 kB) )
  3. Huifang Xu, Tina R. Hill, Hiromi Konishi, Gabriela Farfan: Protoenstatite: A new mineral in Oregon sunstones with “watermelon” colors . In: American Mineralogist . tape 102 , 2017, p. 2146–2149 ( minsocam.org [PDF; 1.1 MB ; accessed on May 18, 2019]).
  4. ^ A b Richard O. Sack, Mark S. Ghiorso: Thermodynamics of multicomponent pyroxenes: III. Calibration of Fe2 + (Mg) -1, TiAl2 (MgSi2) -1, TiFe3 + 2 (MgSi2) -1, AlFe3 + (MgSi) -1, NaAI (CaMg) -1, Al2 (MgSi) -1 and Ca (Mg) -1 exchange reactions between pyroxenes and silicate melts . In: Contributions to Mineralogy Petrology . tape 118 , 1994, pp. 271–296 ( springer.com [PDF; 253 kB ; accessed on January 6, 2018] preview).
  5. CA Goodrich & GE Harlow: Knorringite-Uvarovite Garnet and Cr-Eskola Pyroxene in Ureilite LEW 88774 . In: Meteoritics & Planetary Science . 36, Supplement, 2002, pp. A68 , bibcode : 2001M & PSA..36R..68G .
  6. The wet phase of Mars was global. In: www.raumfahrer.net. Raumfahrer Net eV, accessed on September 19, 2019 .