Protoenstatite

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Protoenstatite
Sunstone2.jpg
Sunstone from Oregon
The watermelon-like coloration of the feldspar is caused by the finest, oriented inclusions of copper and protoenstatite.
General and classification
other names

IMA2016-117

chemical formula Mg 2 Si 2 O 6
Mineral class
(and possibly department) 		Silicates and Germanates
System no. according to Strunz 9.DA
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol mmmTemplate: crystal class / unknown crystal class
Space group Pbcn (No. 60)Template: room group / 60
Lattice parameters a  = natural: 9.25; synthetic: 9.25  Å ; b  = natural: 8.78; synthetic: 8.74 Å; c  = natural: 5.32; synthetic: 5.32 Å
α  = 90 °; β  = 90 °; γ  = 90 ° natural :, synthetic:
Formula units Z  = 4 natural :, synthetic:
Physical Properties
Mohs hardness not determined
Density (g / cm 3 ) natural: 3.30, synthetic: 3.30
Cleavage not determined
Break ; Tenacity not determined
colour not determined
Line color not determined
transparency not determined
shine not determined
radioactivity -
Crystal optics
Refractive index n  = not determined

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

Protoenstatite crystallizes with orthorhombic symmetry at low pressure and temperatures above ~ 1000 ° C. When cooled, protoenstatite transforms into clinoenstatite . Only very small crystals of less than one µm in size, which have a very large surface-to-volume ratio, can remain metastable .

The type locality and the only known natural occurrence so far (2019) is the Dust Devil Mine near Plush in Lake County (Oregon) , USA. Protoenstatite occurs here in the form of tiny inclusions in labradorite , which is mined as a gemstone with the trade name "Oregon Sunstone" (sun stone). The water-clear feldspars from this site have a watermelon-like zoned green and red core. The green color is attributed to submicroscopic segregation of protoenstatite and clinoenstatite.

Etymology and history

Magnesium silicates are of key technical and geological importance and were investigated experimentally as early as the 1930s. The existence of protoenstatite was initially overlooked as it transforms into clinoenstatite when it cools. Wilfried D. Foster, a petrologist who did research in the ceramics department of the "Champion Spark Plug Company", an American spark plug manufacturer , examined the system MgO - SiO 2 at high temperatures and was the first to show in 1951 that protoenstatite formed a thermodynamically stable compound high temperature. The following year, Leon M. Atlas from the Illinois Institute of Technology published a systematic study of the stability ranges of the various MgSiO 3 phases. He localized the phase transition from enstatite to protoenstatite at 990 ° C (1 bar) and made first structural proposals for proto- and clinoenstatite. The crystal structure of protoenstatite was elucidated 7 years later by JV Smith at Pennsylvania State University .

For a long time there was no direct evidence that protoenstatite occurs in nature. From the frequent occurrence of certain lattice construction defects of natural enstatites, it was concluded that protoenstatite was the precursor and correspondingly high formation temperatures. B. Clinoenstatite from boninites or material from comet 81P / Wild 2 . It was not until 2017 that protoenstatite nanocrystals were detected in plagioclases , which presumably remained metastable due to their small size of less than 200 nm.

classification

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

In the now outdated 8th edition of the mineral classification according to Strunz , the protoenstatite is not yet recorded.

The 9th edition of Strunz's mineral systematics, which has been valid since 2001 and was updated by the International Mineralogical Association (IMA) until 2009 , does not yet list the protoenstatite. Here it would be in the class of "Silicates and Germanates" and there in the department of "Chain and band silicates (inosilicates)". 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 ribbon silicates with 2-periodic single chains Si 2 O 6 ; Pyroxen Family ”with the system number 9.DA.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , does not yet know the protoenstatite. It would belong to the class of "silicates and germanates" and there to the department of "chain silicate minerals", sub-department " chain silicates: simple unbranched chains, W = 1 with chains P = 2 ".

Chemism

Protoenstatite 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 .

The protoenstatite from the type locality has the composition:

  • (Mg 1.17 Fe 0.43 Al 0.26 Ca 0.03 Na 0.10 Ti 0.01 ) ∑2.00 (Si 1.83 Al 0.17 ) ∑2.00 O 6

The iron content shows a row of mixed crystals with a hypothetical protoferrosilite, corresponding to the exchange reaction

  • M1.2 Mg 2+ = M1.2 Fe 2+ ("protoferrosilite")

and the aluminum contents can be explained by a combination of two coupled substitutions :

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

The calcium levels are low, as with all Fe Mg pyroxene.

For synthetic protoenstatite, maximum aluminum contents of 1.62% by weight Al 2 O 3 and 4.10% by weight CaO were determined at 1220 to 1390 ° C. That is around four times as much calcium as in the natural protoenstatite and only 1/10 of its Al 2 O 3 content.

Crystal structure

Protoenstatite crystallizes with orthrhombic symmetry in the space group Pbcn (space group no. 60) with 4 formula units per unit cell . The synthetic end link has the lattice parameters a  = 9.25  Å , b  = 8.74 Å and c  = 5.32 Å. Template: room group / 60

The structure is that of orthopyroxene , with layers with opposite octahedron orientations alternating after each layer and not after two layers, e.g. B. enstatite or ferrosilite. Silicon (Si 4+ ) occupies the tetrahedral T-positions surrounded by 4 oxygen ions and magnesium (Mg 2+ ) the octahedral M1 and M2-positions surrounded by 6 oxygen.

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

Protoenstatite denotes MgSiO 3 with a pyroxene structure and the previously described orthorhombic symmetry in the space group Pbcn (No. 60) and is stable at low pressure between 1 bar and ~ 10 kBar and temperatures between ~ 1000 ° C and 1600 ° C. At higher temperatures there is a small stability field of the high-temperature clinopyroxene with the space group P 2 / c (No. 13) . Enstatite ( Pbca (No. 61) ) is stable at lower temperatures between ~ 600 ° C and 1000 ° C and pressures from ~ 0–1 GPa to ~ 7–10 GPa. At temperatures below ~ 600 ° C - ~ 800 ° C at less than ~ 7 GPa, MgSiO 3 is present in the monoclinic structure of the clinoenstatite with the space group P 2 1 / c (No. 14) . With rapid cooling, protoenstatite does not change into enstatite, but metastable to clinoenstatite at ~ 865 ° C. Template: room group / 60Template: room group / 13Template: room group / 61Template: room group / 14

At pressures above ~ 7 GPa, enstatite or clinoenstatite transforms into high clinoenstatite with the space group P 2 / c (No. 13) . Template: room group / 13

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 1600–2000 ° C. At extremely high pressures above ~ 22 GPa, MgSiO 3 changes into the perovskite structure ( bridgmanite ).

Education and Locations

Protoenstatite is formed at low pressure and very high temperatures above 1000 ° C and spontaneously transforms into clinoenstatite when it cools down. For clinoenstatites from boninites , e.g. B. near Népoui in New Caledonia , Cape Vogel on Papua New Guinea , the Bonin Islands or from sites on the Mariana Trench , a formation through the transformation of protoenstatite is assumed.

The type locality of protoenstatite and the only known natural occurrence so far (2019) is the Dust Devil Mine near Plush in Lake County (Oregon) , USA. Protoenstatite occurs here in the form of submicroscopic segregation in labradorite . The protoenstatite inclusions, which are less than 200 nm in size, appear together with clinoenstatite and copper nanocrystals and give the otherwise water-clear plagioclases a watermelon-like, green and red core.

Web links

Individual evidence

  1. ↑ a b Protoenstatites. In: mindat.org. Hudson Institute of Mineralogy, accessed May 19, 2019 .
  2. ↑ a b c d e f g 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]).
  3. ↑ a b c d e JV Smith: The crystal structure of proto-enstatite, MgSiO 3 . In: Acta Crystallographica . tape 12 , 1959, pp. 515-519 , doi : 10.1107 / S0365110X59001554 .
  4. ↑ a b List of sites for protoenstatite in the Mineralienatlas and Mindat
  5. ↑ Wilfried R. Foster: High-Temperature X-Ray Diffraction Study of the Polymorphism of MgSiO 3 . In: Journal of the American Ceramic Society . tape 34 , 1951, pp. 255-259 , doi : 10.1111 / j.1151-2916.1951.tb09127.x .
  6. ^ A b 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 .
  7. ^ 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]).
  8. ↑ a b 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 .
  9. ↑ a b 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]).
  10. ↑ 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]).
  11. ↑ 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]).
  12. ↑ Ernest H. Nickel, Monte C. Nichols: IMA / CNMNC List of Minerals 2009. (PDF 1703 kB) In: cnmnc.main.jp. IMA / CNMNC, January 2009, accessed June 1, 2019 .
  13. ↑ Gordon M. Biggar, D. Barrie Clarke: Protoenstatite solid solution in the system CaO-MgO-Al 2 O 3 -SiO 2 . In: Lithos . tape 5 , 1972, p. 125-129 , doi : 10.1016 / 0024-4937 (72) 90064-3 .
  14. ↑ 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]).
  15. ^ 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 .