ALBERT

Description: A mineral intermediate between sillimanite and mullite, tentatively designated as "sillimullite," was studied by electron microprobe analyses and single-crystal X-ray diffraction methods. The chemical compositions derived from the microprobe results and the crystal-structure refinement are Al 7.84 Fe 0.18 Ti 0.03 Mg 0.03 Si 3.92 O 19.96 and Al 8.28 Fe 0.20 Si 3.52 O 19.76 (Fe is Fe 3+ ) corresponding to x -values of 0.02 and 0.12, respectively, in the solid-solution series Al 8+4x Si 4–4x O 20–2x assigning Fe 3+ , Ti, and Mg to the Al site. The composition derived from microprobe analysis is very close to a stoichiometric sillimanite (with Fe 3+ ,Ti, and Mg assigned to Al sites), while the composition derived from diffraction data is midway between sillimanite and Si-rich mullites. The discrepancy is assumed to be caused by the occurrence of amorphous nano-sized SiO 2 inclusions in the aluminosilicate phase not affecting the diffraction data but detected in the microprobe analysis. "Sillimullite" crystallizes in the orthorhombic space group Pnam with a = 7.5127(4), b = 7.6823(4), c = 5.785(3) Å, V = 333.88(4) Å 3 , Z = 1. It has a complete Si/Al ordering at tetrahedral sites like sillimanite but with neighboring double chains of SiO 4 and AlO 4 tetrahedra being offset by 1/2 unit cell parallel to c relative to each other causing the change of the space-group setting from Pbnm (sillimanite) to Pnam . Difference Fourier calculations and refinements with anisotropic displacement parameters revealed the formation of oxygen vacancies and triclusters as known in the crystal structures of mullite. Final refinements converged at R 1 = 5.9% for 1024 unique reflections with F o 〉 4( F o ). Fe was found to reside predominantly in the octahedral site and with minor amounts in one of the T* sites. Mg and Ti were not considered in the refinements. The crystal studied here is considered to represent a new mineral intermediate between sillimanite and mullite, named "sillimullite."

Description: 〈span〉Synthetic mullite (Al〈sub〉2〈/sub〉[Al〈sub〉2+2〈span〉x〈/span〉〈/sub〉Si〈sub〉2-2〈span〉x〈/span〉〈/sub〉]O〈sub〉10-〈span〉x〈/span〉〈/sub〉) 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 〈span〉Z〈/span〉 = 1: Al〈sub〉4.34〈/sub〉Fe〈sub〉0.12〈/sub〉Si〈sub〉1.49〈/sub〉Ti〈sub〉0.05〈/sub〉O〈sub〉9.77〈/sub〉 for ME1 corresponding to an 〈span〉x〈/span〉-value of 〈span〉x〈/span〉 = 0.23(4); ME2 consists of two different parts with compositions of Al〈sub〉4.406〈/sub〉Fe〈sub〉0.103〈/sub〉Si〈sub〉1.455〈/sub〉Ti〈sub〉0.037〈/sub〉O〈sub〉9.746〈/sub〉 and Al〈sub〉4.15〈/sub〉Fe〈sub〉0.107〈/sub〉Si〈sub〉1.68〈/sub〉Ti〈sub〉0.06〈/sub〉O〈sub〉9.872〈/sub〉 corresponding to 〈span〉x〈/span〉 = 0.254(2) and 〈span〉x〈/span〉 = 0.128(14), respectively; MK1 has a composition of Al〈sub〉4.40〈/sub〉Fe〈sub〉0.018〈/sub〉Si〈sub〉1.58〈/sub〉O〈sub〉9.791〈/sub〉 with 〈span〉x〈/span〉 = 0.209(15). The crystal structure of natural mullite is similar to its synthetic counterpart with chains of edge-sharing (Al, Fe)O〈sub〉6〈/sub〉 octahedra parallel 〈strong〉c〈/strong〉 crosslinked by T〈sub〉2〈/sub〉O〈sub〉7〈/sub〉 groups with T = Al,Si and a number of oxygen vacancies corresponding to the 〈span〉x〈/span〉-value in the general composition. Mullite crystals belong to the orthorhombic space group 〈span〉Pbam〈/span〉 with 〈span〉a〈/span〉 = 7.5542(3) Å, 〈span〉b〈/span〉 = 7.6998(3) Å, 〈span〉c〈/span〉 = 2.8935(1) Å, 〈span〉V〈/span〉 = 168.31(2) Å〈sup〉3〈/sup〉 for ME1; 〈span〉a〈/span〉 = 7.5353(3) Å, 〈span〉b〈/span〉 = 7.7018(3) Å, 〈span〉c〈/span〉 = 2.8928(3) Å, 〈span〉V〈/span〉 = 167.89(4) Å〈sup〉3〈/sup〉 for ME2 representing average values for the two parts; 〈span〉a〈/span〉 = 7.5439(3) Å, 〈span〉b〈/span〉 = 7.6958(3) Å, 〈span〉c〈/span〉 = 2.8879(1) Å, 〈span〉V〈/span〉 = 167.66(1) Å〈sup〉3〈/sup〉 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 〈span〉c〈/span〉. It could be demonstrated that the incorporation of the foreign cations (Fe, Ti) increases the refractive indices (〈span〉n〈/span〉〈sub〉〈span〉x〈/span〉〈/sub〉 ≈ 〈span〉n〈/span〉〈sub〉〈span〉y〈/span〉〈/sub〉 = 1.654(2) and 〈span〉n〈/span〉〈sub〉〈span〉z〈/span〉〈/sub〉 = 1.668(2) for ME1; 〈span〉n〈/span〉〈sub〉〈span〉x〈/span〉〈/sub〉 ≈ 〈span〉n〈/span〉〈sub〉〈span〉y〈/span〉〈/sub〉 = 1.648(4) and 〈span〉n〈/span〉〈sub〉〈span〉z〈/span〉〈/sub〉 = 1.662(4) for MK1). Our results confirm previous findings, that mullite minerals are commonly formed on the SiO〈sub〉2〈/sub〉-rich side (〈span〉x〈/span〉 〈 0.25) of the Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉–SiO〈sub〉2〈/sub〉 system with compositions not yet observed for synthetic mullites, which usually range between about 0.25 ≤ 〈span〉x〈/span〉 ≤ 0.4.〈/span〉

Description: 〈span〉Synthetic mullite (Al〈sub〉2〈/sub〉[Al〈sub〉2+2〈span〉x〈/span〉〈/sub〉Si〈sub〉2-2〈span〉x〈/span〉〈/sub〉]O〈sub〉10-〈span〉x〈/span〉〈/sub〉) 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 〈span〉Z〈/span〉 = 1: Al〈sub〉4.34〈/sub〉Fe〈sub〉0.12〈/sub〉Si〈sub〉1.49〈/sub〉Ti〈sub〉0.05〈/sub〉O〈sub〉9.77〈/sub〉 for ME1 corresponding to an 〈span〉x〈/span〉-value of 〈span〉x〈/span〉 = 0.23(4); ME2 consists of two different parts with compositions of Al〈sub〉4.406〈/sub〉Fe〈sub〉0.103〈/sub〉Si〈sub〉1.455〈/sub〉Ti〈sub〉0.037〈/sub〉O〈sub〉9.746〈/sub〉 and Al〈sub〉4.15〈/sub〉Fe〈sub〉0.107〈/sub〉Si〈sub〉1.68〈/sub〉Ti〈sub〉0.06〈/sub〉O〈sub〉9.872〈/sub〉 corresponding to 〈span〉x〈/span〉 = 0.254(2) and 〈span〉x〈/span〉 = 0.128(14), respectively; MK1 has a composition of Al〈sub〉4.40〈/sub〉Fe〈sub〉0.018〈/sub〉Si〈sub〉1.58〈/sub〉O〈sub〉9.791〈/sub〉 with 〈span〉x〈/span〉 = 0.209(15). The crystal structure of natural mullite is similar to its synthetic counterpart with chains of edge-sharing (Al, Fe)O〈sub〉6〈/sub〉 octahedra parallel 〈strong〉c〈/strong〉 crosslinked by T〈sub〉2〈/sub〉O〈sub〉7〈/sub〉 groups with T = Al,Si and a number of oxygen vacancies corresponding to the 〈span〉x〈/span〉-value in the general composition. Mullite crystals belong to the orthorhombic space group 〈span〉Pbam〈/span〉 with 〈span〉a〈/span〉 = 7.5542(3) Å, 〈span〉b〈/span〉 = 7.6998(3) Å, 〈span〉c〈/span〉 = 2.8935(1) Å, 〈span〉V〈/span〉 = 168.31(2) Å〈sup〉3〈/sup〉 for ME1; 〈span〉a〈/span〉 = 7.5353(3) Å, 〈span〉b〈/span〉 = 7.7018(3) Å, 〈span〉c〈/span〉 = 2.8928(3) Å, 〈span〉V〈/span〉 = 167.89(4) Å〈sup〉3〈/sup〉 for ME2 representing average values for the two parts; 〈span〉a〈/span〉 = 7.5439(3) Å, 〈span〉b〈/span〉 = 7.6958(3) Å, 〈span〉c〈/span〉 = 2.8879(1) Å, 〈span〉V〈/span〉 = 167.66(1) Å〈sup〉3〈/sup〉 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 〈span〉c〈/span〉. It could be demonstrated that the incorporation of the foreign cations (Fe, Ti) increases the refractive indices (〈span〉n〈/span〉〈sub〉〈span〉x〈/span〉〈/sub〉 ≈ 〈span〉n〈/span〉〈sub〉〈span〉y〈/span〉〈/sub〉 = 1.654(2) and 〈span〉n〈/span〉〈sub〉〈span〉z〈/span〉〈/sub〉 = 1.668(2) for ME1; 〈span〉n〈/span〉〈sub〉〈span〉x〈/span〉〈/sub〉 ≈ 〈span〉n〈/span〉〈sub〉〈span〉y〈/span〉〈/sub〉 = 1.648(4) and 〈span〉n〈/span〉〈sub〉〈span〉z〈/span〉〈/sub〉 = 1.662(4) for MK1). Our results confirm previous findings, that mullite minerals are commonly formed on the SiO〈sub〉2〈/sub〉-rich side (〈span〉x〈/span〉 〈 0.25) of the Al〈sub〉2〈/sub〉O〈sub〉3〈/sub〉–SiO〈sub〉2〈/sub〉 system with compositions not yet observed for synthetic mullites, which usually range between about 0.25 ≤ 〈span〉x〈/span〉 ≤ 0.4.〈/span〉