Clinoenstatite

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Clinoenstatite
General and classification
chemical formula Mg 2 Si 2 O 6
Mineral class
(and possibly department) 		Silicates and Germanates
System no. to Strunz
and to Dana 		9.DA.10 ( 8th edition : 8 / F.01-10)
65.1.1.1
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group P 2 1 / c (No. 14)Template: room group / 14
Lattice parameters a  = 9.6065 (11)  Å ; b  = 8.8146 (7) Å; c  = 5.1688 (6) Å
α  = 90 °; β  = 108.335 (9) °; γ  = 90 °
Formula units Z  = 4
Twinning polysynthetic according to (100)
Physical Properties
Mohs hardness 5-6
Density (g / cm 3 ) synthetic: 3.210
Cleavage good after {110}
colour colorless, brown
Line color White
transparency transparent
shine Please complete!
radioactivity -
magnetism -
Crystal optics
Refractive indices n α  = 1.651
n β  = 1.654
n γ  = 1.660
Birefringence δ = 0.009
Optical character biaxial positive
Axis angle 2V = 53.5 °

The mineral clinoenstatite is a rare chain silicate from the pyroxene group with the idealized chemical composition Mg 2+ 2 Si 2 O 6 .

Clinoenstatite crystallizes with monoclinic symmetry at temperatures below ~ 700 ° C and pressures below 70 kbar.

In nature, clinoenstatite occurs primarily in meteorites , especially in enstatite chondrites and in some basic igneous rocks, e.g. B. Boninites before. It occurs in the form of colorless to brownish or greenish crystals from a few mm to cm in size. A characteristic is a lamellar twinning after a prism surface, which can be seen as surface stripes.

Etymology and history

Since the middle of the 19th century, low-calcium, monoclinic pyroxenes from basic igneous rocks and meteorites have been described, which did not fit well into the then common division according to Gustav Tschermak . In his dissertation presented at the University of Helsinki in May 1906 , Walter Wahl compiled the descriptions of these clinopyroxenes known at the time and introduced new names for this group of "enstatitaugites". For the clinopyroxene with the lowest calcium levels, he followed the name of orthopyroxene at the time and assigned the names "Klinohypersthen", "Klinobronzit" and "Klinoenstatit" for increasing magnesium contents. The name Klinoenstatit is the only one still in use today.

Until the 1950s it was assumed that clinoenstatite had the same structure as diopside ( space group C 2 / c (space group no. 15) ). Based on considerations by T. Ito from 1950, Nobuo Morimoto from the University of Tokyo was able to show the differences to the diopside structure in 1956 and describe the structure of low-calcium clinopyroxenes in space group P 2 1 / c (space group no. 14) for the first time . Independently of Morimoto, Bown & Gay at the University of Cambridge came to the same conclusion for Pigeonit in 1957 . During a stay at the Geophysical Laboratory of the Carnegie Institution of Washington , Morimoto was finally able to clarify and refine the structure of pigeonite and clinoenstatite in 1959. Template: room group / 15 Template: room group / 14

classification

In the structural classification of the International Mineralogical Association (IMA), clinoenstatite, together with enstatite , protoenstatite , ferrosilite , clinoferrosilite and pigeonite, belongs to the magnesium-iron proxenes (Mg-Fe-pyroxenes) in the pyroxene group .

In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the clinoenstatite belonged to the department of " chain and band silicates (inosilicates) ", where it together with aegirine , augite , diopside , esseneit , hedenbergite , jadeite , jervisite , johannsenite , kanoite , Klinoferrosilit, Kosmochlor , Namansilit , Natalyit , omphacite , Petedunnit , pigeonite and spodumene the "pyroxene, clinopyroxene subgroup" with the system no. VIII / F.01 within the pyroxene group .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns the Klinoenstatit to the class of "silicates and germanates" and there in the department of "chain and band silicates (inosilicates)" a. This department is further subdivided according to the structure of the silicate chains as well as belonging to larger mineral families, so that the mineral according to its composition and structure is classified in the subdivision “Chain and band silicates with 2-periodic single chains Si 2 O 6 ; Pyroxen-Familie "is to be found, where together with Kanoite, Klinoferrosilit, and Pigeonit the" Mg, Fe, Mn-Klinopyroxene - Klinoenstatite group "with the system no. 9.DA.10 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns clinoenstatite to the class of "silicates and Germanates" and there in the department of "chain silicate minerals". Here it is together with clinoferrosilite, kanoite and pigeonite in the group of "P2 / c clinopyroxenes" with system no. 65.01.01 can be found in the subsection " Chain Silicates: Simple unbranched chains, W = 1 with chains P = 2 ".

Chemism

Clinoenstatite has the idealized composition [M2] Mg [M1] Mg [T] Si 2 O 6 , where [M2], [M1] and [T] are the positions in the pyroxene structure .

There is complete miscibility with clinoferrosilite , according to the exchange reaction

  • M1.2 Mg 2+ = M1.2 Fe 2+ (clinoferrosilite)

and limited miscibility with diopside , depending on the exchange reaction

  • M2 Mg 2+ = M2 Ca 2+ (diopside).

Furthermore, clinoenstatite can contain small amounts of aluminum, which z. B. is incorporated via the Tschermaks substitution :

  • M1.2 Mg 2+ + T Si 4+ = M1.2 Al 3+ + T Al 3+ (Magnesuim-Tschermaks).

Crystal structure

Clinoenstatite crystallizes with monoclinic symmetry in the space group P 2 1 / c (space group no. 14) with 4 formula units per unit cell . The synthetic end  link has the lattice parameters a = 9.6065 (11)  Å , b  = 8.8146 (7) Å, c  = 5.1688 (6) Å and ß = 108.335 (9). Template: room group / 14

The structure is that of pyroxene and differs in some details from that of the centrosymmetric clinopyroxene with the space group C 2 / c (space group no. 15) (e.g. diopside). Calcium is surrounded by eight oxygen in the diopside, while magnesium (Mg 2+ ) is octahedrally surrounded by six oxygen on the M1 and M2 positions of the clinoenstatite structure . Silicon (Si 4+ ) occupies the tetrahedral T-positions surrounded by 4 oxygen ions. Mg 2+ is a significantly smaller cation than Ca 2+ and the SiO 4 tetrahedron chains are therefore somewhat compressed in the clinoenstatite. This shortening is achieved by turning the SiO 4 tetrahedron slightly . In contrast to the centrosymmetrical clinopyroxenes of the diopside-hedenbergite series, the clinoenstatite structure has two symmetrically different silicate chains with oppositely rotated silicate tetrahedra. Template: room group / 15

Modifications

Pressure-temperature phase diagram for the compound MgSiO 3 according to Presnell 1995

Magnesium metasilicate MgSiO 3 is polymorphic and can occur with different structure types and symmetries.

Pyroxenes

Klinoenstatite denotes MgSiO 3 with a pyroxene structure in the monoclinic symmetry described above with the space group P 2 1 / c (No. 14) . It is stable at temperatures below ~ 700 ° C and pressures up to ~ 7 GPa. At higher pressure, clinoenstatite transforms into high clinoenstatite with the structure of diopside in space group C 2 / c (no. 15) . The orthopyroxene enstatite ( Pbca (No. 61) ) is stable at higher temperatures , and at low pressure above ~ 1000 ° C it transforms into the orthorhombic protoenstatite ( Pbcn (No. 60) ). With rapid cooling, protoenstatite does not change into enstatite, but metastable to clinoenstatite at ~ 865 ° C. Template: room group / 14Template: room group / 15Template: room group / 61Template: room group / 60

High pressure phases

The pyroxene structure of MgSiO 3 is no longer stable above 17–18 GPa and the magnesium metasilicate is in the structure of garnet ( majorite ) at temperatures above ~ 1600 ° C and in the ilmenite structure ( akimotoite ) below 1600 to 2000 ° C. At extremely high pressures above ~ 22 GPa, MgSiO 3 changes into the perovskite structure ( bridgmanite ).

Education and Locations

Most natural clinoenstatites were not formed in their stability field, but metastable when the high-temperature modification protoenstatite cooled down .

A type locality is not given for Klinoenstatit. As early as the middle of the 19th century, clinoenstatite was described under various names in meteorites and some basic igneous rocks.

Boninite

Clinoenstatite is very rare on the earth's surface. Most of the known occurrences are in the western Pacific Ring of Fire . Clinoenstatite is found here in the form of mm to cm-sized crystals in boninites , magnesium-rich, ultrabasic magmatites, where they occur together with enstatite , glass, secondary zeolites and chromite .

Enstatite chondrites

Most clinoenstatite sites are meteorites . The meteorites from the group of enstatite chondrites consist mainly of enstatite and clinoenstatite, kamacite , troilite and plagioclase , whereby the enstatite chondrites of the type EH contain clinoenstatite, whereas the EL chondrites do not.

Clinoenstatite was also detected in the material of comet 81P / Wild 2 .

Mantle

Under the conditions of the upper mantle , enstatite is transformed into high clinoenstatite, the centrosymmetrical modification of clion statite with the structure of diopside and the space group C 2 / c (No. 15) . High clinoenstatite, together with diopside-jadeite mixed crystals, forsterite and pyrope, is an essential component of the upper mantle. In the area of ​​the mantle transition zone at a depth of ~ 400 km, clinopyroxene as a majorite component increasingly dissolves in pyrope and, under the conditions of the lower earth mantle, is present in the structure of perovskite as bridgmanite . Template: room group / 15

Segregation

Separation lamellae of clinoenstatite have been observed in diopside from ultrabasic rocks in Hokkaidō , Japan.

In addition to diopside, plagioclase can also show segregation of pyroxenes. In the labradorite of the Dust Devil Mine near Plush in Lake County (Oregon) , USA, clinoenstatite occurs in the form of submicroscopic segregation together with protoenstatite and copper nanocrystals. This segregation is responsible for the green color of the watermelon-like zoned kernels of this plagioclase.

Web links

Individual evidence

  1. ↑ a b c Clinoenstatite. In: mindat.org. Hudson Institute of Mineralogy, accessed May 19, 2019 .
  2. ↑ a b c d e Clinoenstatite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 75  kB ; accessed on June 15, 2019]).
  3. ↑ a b c d e f g DA Stephenson, CB Sclar, JV Smith: Unit cell volumes of synthetic orthoenstatite and low clinoenstatite . In: Mineralogical Magazine . tape 35 , 1966, pp. 838-846 ( citeseerx.ist.psu.edu [PDF; 468 kB ; accessed on June 15, 2019]).
  4. ^ A b Joseph R. Smyth and Tamsin C. McCormick: Crystallographic Data For Minerals . In: Mineral physics and crystallography: a handbook of physical constants . 1995, p. 1–17 ( citeseerx.ist.psu.edu [PDF; 24.1 MB ; accessed on May 19, 2019]).
  5. ↑ a b Walter Wahl: The Enstatitaugite . 1906, p. 146 ( rruff.info [PDF; 9.7 MB ; accessed on June 3, 2019]).
  6. ↑ a b 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]).
  7. ↑ Hisashi Kuno and Harry Hammond Hess: Unit cell dimensions of clinoenstatite and pigeonite in relation to other common clinopyroxenes . In: American Journal of Science . tape 251 , 1953, pp. 741-752 , doi : 10.2475 / ajs.251.10.741 .
  8. ^ A b Nobuo Morimoto: The Existence of Monoclinic Pyroxenes with the Space Group C52h - P21 / c . In: Proceedings of the Japan Academy . tape 32 , 1956, pp. 750-752 ( jst.go.jp [PDF; 547 kB ; accessed on June 3, 2019]).
  9. ↑ MC Bown and P. Gay: Observations on pigeonite . In: Acta Crystallographica . tape 10 , 1957, pp. 440-441 , doi : 10.1107 / S0365110X57001462 .
  10. ^ A b Nobuo Morimoto, Daniel E. Appelman, Howard T. Evans Jr .: The crystal structure of clinoenstatit and pigenite . In: Journal of Crystallography . tape 114 , 1960, pp. 120–147 ( rruff.info [PDF; 2.1 MB ; accessed on June 3, 2019]).
  11. ↑ a b Dean C. Presnall: Phase Diagrams of Earth-Forming Minerals . In: Mineral physics and crystallography: a handbook of physical constants . 1995, p. 252-273 ( citeseerx.ist.psu.edu [PDF; 24.1 MB ; accessed on May 19, 2019]).
  12. ^ Leon Atlas: The Polymorphism of MgSiO3 and Solid-State Equilibria in the System MgSiO 3 -CaMgSi 2 O 6 . In: The Journal of Geology . tape 60 , 1952, pp. 125-147 , doi : 10.1086 / 625944 .
  13. ^ JF Sarver, FA Hummel: Stability Relations of Magnesium Metasilicate Polymorphs . In: Journal of the American Ceramic Society . tape 45 , 1962, pp. 152-156 , doi : 10.1111 / j.1151-2916.1962.tb11110.x .
  14. ^ A b WB Dallwitz, DH Green, JE Thompson: Clinoenstatite in a Volcanic Rock from the Cape Vogel Area, Papua . In: Journal of Petrology . tape 7 , 1966, pp. 375-403 ( d28rz98at9flks.cloudfront.net [PDF; 32.9 MB ; accessed on May 19, 2019]).
  15. ↑ a b Sylvia Schmitz, Frank E. Brenker: Microstructural Indications for Protoenstatite Precursor of Cometary MgSiO 3 Pyroxene: A Further High-Temperature Component of Comet Wild 2 . In: The Astrophysical Journal . tape 681 , 2008, p. L105-L108 ( researchgate.net [PDF; 1.7 MB ; accessed on May 18, 2019]).
  16. ↑ Keiichi Shiraki, Naoshi Kuroda, Hayaomi Urano, Shigenori Manuyama: Clinoenstatite in boninites from the Bonin Islands, Japan . In: Nature . tape 285 , 1980, pp. 31-32 , doi : 10.1038 / 285031a0 .
  17. ↑ T. Sameshima, J.-P. Paris, Philippa M. Black, RF Herring: Clinoenstatite-bearing lava from Nepoui, New Caledonia . In: American Mineralogist . tape 68 , 1983, pp. 1076-1082 ( minsocam.org [PDF; 733 kB ; accessed on May 19, 2019]).
  18. ↑ Find location list for clinoenstatite at the Mineralienatlas and at Mindat
  19. ↑ Derek W. Sears, Gregory W. Kallemeyn, Jhon T. Wasson: The compositional classification of chondrites: II The enstatit chondrit groups . In: Geochimica et Cosmochimica Acta . tape 46 , 1982, pp. 597–608 ( dsears.hosted.uark.edu [PDF; 2.1 MB ; accessed on June 19, 2019]).
  20. ↑ RJ Angel, A. & Chopelas NL Ross: Clinoenstatite in boninites from the Bonin Islands, Japan . In: Nature . tape 358 , 1992, pp. 322-324 , doi : 10.1038 / 358322a0 .
  21. ^ Yoshiaki Yamaguchi, Katsutoshi Tomita: Clinoenstatite as an Exsolution Phase in Diopside . In: Memoirs of the Faculty of Science, Kyoto University. Series of geology and mineralogy . tape 37 , 1970, pp. 173-180 ( core.ac.uk [PDF; 1.8 MB ; accessed on June 19, 2019]).
  22. ↑ 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]).