Natural mullites: chemical composition, crystal structure, and optical properties

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Marco E. Ciriotti
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Natural mullites: chemical composition, crystal structure, and optical properties

Messaggio da Marco E. Ciriotti » gio 27 set, 2018 14:08

Prossima pubblicazione.

Referenza:
▪ Lenz, S., Birkenstock, J., Fischer, L.A., Schüller, W., Schneider, H., Fischer, R.X. (2019): Natural mullites: chemical composition, crystal structure, and optical properties. European Journal of Mineralogy, 31, (in press).

NdR: mullite group
Boralsilite Al16B6O30(Si2O7)
Boromullite Al4.5SiB0.5O9.5
Mullite AlAl[Al2+2xSi2-2x]O10-x (x = 0.125; 0.250; 0.375)
Vránaite Al16B4Si4O38
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 31645
Iscritto il: ven 25 giu, 2004 11:31
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Re: Natural mullites: chemical composition, crystal structure, and optical properties

Messaggio da Marco E. Ciriotti » ven 09 nov, 2018 14:31

Prossima pubblicazione.

Referenza:
▪ Lenz, S., Birkenstock, J., Fischer, L.A., Schüller, W., Schneider, H., Fischer, R.X. (2019): Natural mullites: chemical composition, crystal structure, and optical properties. European Journal of Mineralogy, 31, (in press).

Abstract:
Synthetic mullite (Al2[Al2+2xSi2-2x]O10-x) is a very well characterised material. Due to its outstanding properties it is used for a vast range of technical applications representing one of the most important groups of ceramic materials. In contrast to the synthetic product, the mineral mullite is only poorly investigated. In this study crystal-structure refinements of natural mullites are presented for the first time. Three mullite single-crystals from the Ettringer Bellerberg, Eifel area, Germany (ME1 & ME2) and from Kladno, Czech Republic (MK1) have been examined by single-crystal X-ray diffraction methods, Electron Microprobe Analyses (EMPA), and the spindle-stage method to determine the refractive indices. Chemical analyses revealed an incorporation of Fe and Ti cations and yielded the following compositions per formula unit with Z = 1: Al4.34Fe0.12Si1.49Ti0.05O9.77 for ME1 corresponding to an x-value of x = 0.23(4); ME2 consists of two different parts with compositions of Al4.406Fe0.103Si1.455Ti0.037O9.746 and Al4.15Fe0.107Si1.68Ti0.06O9.872 corresponding to x = 0.254(2) and x = 0.128(14), respectively; MK1 has a composition of Al4.40Fe0.018Si1.58O9.791 with x = 0.209(15). The crystal structure of natural mullite is similar to its synthetic counterpart with chains of edge-sharing (Al, Fe)O6 octahedra parallel c crosslinked by T2O7 groups with T = Al,Si and a number of oxygen vacancies corresponding to the x-value in the general composition. Mullite crystallises in the orthorhombic space group Pbam with a = 7.5542(3) Å, b = 7.6998(3) Å, c = 2.8935(1) Å, V = 168.31(2) Å3 for ME1; a = 7.5353(3) Å, b = 7.7018(3) Å, c = 2.8928(3) Å, V = 167.89(4) Å3 for ME2 representing average values for the two parts; a = 7.5439(3) Å, b = 7.6958(3) Å, c = 2.8879(1) Å, V = 167.66(1) Å3 for MK1. The refinements give clear evidence that iron predominantly enters the octahedral sites, which is an explanation for the slight increase in lattice parameters c. It could be demonstrated that the incorporation of foreign cations increases the refractive indices (nx ≈ ny = 1.654(2) and nz = 1.668(2) for ME1; nx ≈ ny = 1.648(4) and nz = 1.662(4) for MK1). Our results confirm previous findings, that mullite minerals are commonly formed on the SiO2-rich side (x < 0.25) of the Al2O3–SiO2 system with compositions not yet observed for synthetic mullites, which usually range between about 0.25 ≤ x ≤ 0.4.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 31645
Iscritto il: ven 25 giu, 2004 11:31
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Re: Natural mullites: chemical composition, crystal structure, and optical properties

Messaggio da Marco E. Ciriotti » ven 15 mar, 2019 16:38

Referenza:
▪ Klar, P.B., Etxebarria, I., Madariaga, G. (2019): Characterizing modulated structures with first-principles calculations: a unified superspace scheme of ordering in mullite. Acta Crystallographica Section A, 75, 260-272.

Abstract:
The benefit of computational methods applying density functional theory for the description and understanding of modulated crystal structures is investigated. A method is presented which allows one to establish, improve and test superspace models including displacive and occupational modulation functions from first-principles calculations on commensurate structures. The total energies of different configurations allow one to distinguish stable and less stable structure models. The study is based on a series of geometrically optimized superstructures of mullite (Al4+2xSi2−2xO10−x) derived from the superspace group Pbam(α0½)0ss. Despite the disordered and structurally complex nature of mullite, the calculations on ordered superstructures are very useful for determining the ideal Al/Si ordering in mullite, extracting atomic modulation functions as well as understanding the SiO2–Al2O3 phase diagram. The results are compared with experimentally established models which confirm the validity and utility of the presented method.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 31645
Iscritto il: ven 25 giu, 2004 11:31
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Re: Natural mullites: chemical composition, crystal structure, and optical properties

Messaggio da Marco E. Ciriotti » mer 22 mag, 2019 10:19

Pubblicazione effettuata.

Referenza:
▪ Lenz, S., Birkenstock, J., Fischer, L.A., Schüller, W., Schneider, H., Fischer, R.X. (2019): Natural mullites: chemical composition, crystal structure, and optical properties. European Journal of Mineralogy, 31, 353-367.

Abstract:
Synthetic mullite (Al2[Al2+2xSi2-2x]O10-x) is a very well characterised material. Due to its outstanding properties it is used for a vast range of technical applications representing one of the most important groups of ceramic materials. In contrast to the synthetic product, the mineral mullite is only poorly investigated. In this study crystal-structure refinements of natural mullites are presented for the first time. Three mullite single-crystals from the Ettringer Bellerberg, Eifel area, Germany (ME1 & ME2) and from Kladno, Czech Republic (MK1) have been examined by single-crystal X-ray diffraction methods, Electron Microprobe Analyses (EMPA), and the spindle-stage method to determine the refractive indices. Chemical analyses revealed an incorporation of Fe and Ti cations and yielded the following compositions per formula unit with Z = 1: Al4.34Fe0.12Si1.49Ti0.05O9.77 for ME1 corresponding to an x-value of x = 0.23(4); ME2 consists of two different parts with compositions of Al4.406Fe0.103Si1.455Ti0.037O9.746 and Al4.15Fe0.107Si1.68Ti0.06O9.872 corresponding to x = 0.254(2) and x = 0.128(14), respectively; MK1 has a composition of Al4.40Fe0.018Si1.58O9.791 with x = 0.209(15). The crystal structure of natural mullite is similar to its synthetic counterpart with chains of edge-sharing (Al, Fe)O6 octahedra parallel c crosslinked by T2O7 groups with T = Al,Si and a number of oxygen vacancies corresponding to the x-value in the general composition. Mullite crystallises in the orthorhombic space group Pbam with a = 7.5542(3) Å, b = 7.6998(3) Å, c = 2.8935(1) Å, V = 168.31(2) Å3 for ME1; a = 7.5353(3) Å, b = 7.7018(3) Å, c = 2.8928(3) Å, V = 167.89(4) Å3 for ME2 representing average values for the two parts; a = 7.5439(3) Å, b = 7.6958(3) Å, c = 2.8879(1) Å, V = 167.66(1) Å3 for MK1. The refinements give clear evidence that iron predominantly enters the octahedral sites, which is an explanation for the slight increase in lattice parameters c. It could be demonstrated that the incorporation of foreign cations increases the refractive indices (nx ≈ ny = 1.654(2) and nz = 1.668(2) for ME1; nx ≈ ny = 1.648(4) and nz = 1.662(4) for MK1). Our results confirm previous findings, that mullite minerals are commonly formed on the SiO2-rich side (x < 0.25) of the Al2O3–SiO2 system with compositions not yet observed for synthetic mullites, which usually range between about 0.25 ≤ x ≤ 0.4.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 31645
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Natural mullites: chemical composition, crystal structure, and optical properties

Messaggio da Marco E. Ciriotti » gio 17 dic, 2020 10:57

Referenza:
▪ Lenz, S., Birkenstock, J., Fischer, L.A., Schüller, W., Schneider, H., Fischer, R.X. (2020): Corrigendum to “Natural mullites: chemical composition, crystal structure, and optical properties” published in Eur. J. Mineral., 31, 353–367, 2019. European Journal of Mineralogy, 32, 673-673.

The universal mineral formula for natural mullites presented here contains a typing error. The mineral formula, given on p. 358 (Sect. 4.1), in the heading of Table 7a (p. 361) and at the end of Sect. 4.2 (p. 363), is stated as M + y Mg 2+ z M 2+ w M 3+ 4+2x+y+w−2z M 4+ 2−2x−y−2w+z O10−x.
However, the indexing of M3+ is erroneous. The parameter w must contain a factor of 2. The correct formula is M + y Mg 2+ z M 2+ w M 3 4+2x+y+2w−2z M 4+ 2−2x−y−2w+z O10−x.
Consequently, also the formula NayMgzCawAl 4.5+y+w−2z Si 1.5−y−2w+z O9.75 given on p. 359 must be stated as NayMgzCawAl 4.5+y+2w−2z Si 1.5−y−2w+z O9.75.
The citation “Angel and Prewitt (1987)” on p. 357 is incorrect. It is “Angel and Prewitt (1986)”.
Marco E. Ciriotti

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