Mullite (Mul) formation after high-T muscovite (Ms) breakdown has been studied in phyllosilicaterich bricks. At T ≥ 900 ∞C Ms dehydroxylation is followed by partial melting that triggers the nucleation and growth of Mul acicular crystals. An analytical electron microscopy study reveals that the Mul is a 3:2-type with a [6] (Al1.686Ti0.031Fe0.159Mg0.134) [4] (Al2.360Si1.649)O9.82 formula and an O atom vacancy of x = 0.18. This is consistent with X-ray diffraction results [i.e., unit-cell parameters: a = 7.553(7), b = 7.694(7), and c = 2.881(1) A, V = 167.45 A 3 ]. The initial stage of the process resulting in Mul growth followed the balanced reaction Ms AE 0.275Mul + 0.667Melt + 0.244K2O + 0.01Na2O + 0.125H2O, yielding an alkali-poor peraluminous melt. H 2O with K (and Na), which are lost along the (001) planes of dehydroxylated Ms, play a significant role as melting agents. The c-axes of the Mul crystals are oriented parallel to [010] ms or to the symmetrically equivalent ms zone axis, while the (120)mul or (210)mul planes are subparallel to (001) ms (TEM results). These systematic orientations point to epitaxial Mul nucleation and growth on the remaining Ms substrate, which acts as a template for Mul heterogeneous nucleation. Randomly oriented Mul growth is also observed during the late stages of the process (i.e., melt cooling). The epitaxial nature of Mul growth after dehydroxylated Ms melting minimizes the energy requirement for nucleation. In addition, the water released after Ms breakdown and the multicomponent nature of the melt enable this high- T aluminum silicate to grow at T ~ 900 ∞C, almost 100 ∞C below the SiO2-Al2O3-K2O ternary system eutectic (after a melt with an end-member Ms composition).

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TEM study of mullite growth after muscovite breakdown

Mullite grown from fired andalusite grains: the role of impurities and of the high temperature liquid phase on the kinetics of mullitization and consequences on thermal shocks resistance

X-ray diffraction, differential thermal analysis, and29Si and27Al MAS NMR spectroscopy were used to characterize the formation of mullite from non-crystalline precursors. The precursors were produced by the sol-gel route from organic starting compounds (SGM), and by coprecipitation of inorganic starting materials (CM). There is NMR evidence that both types of mullite precursor consist of Si-O and Al-O tetrahedra, and of Al-O octahedra. Furthermore, fivefold-coordinated Al-O polyhedra, or alternatively strongly distorted Al-O tetrahedra, do occur in the precursors. SGM and CM precursors transform to mullite in multi-step reactions with intermediate formation ofγ-Al2O3, and a non-crystalline nearly pure SiO2 phase. Theγ-Al2O3 compound has a highly distorted spinel structure, and may contain some minor amount of Si. The diffusion-controlled decomposition of the precursors toγ-Al2O3 + SiO2 at low temperature (⩽1000°C) suggests that the precursors are diphasic, consisting of nanometre-sized, long-range-disordered Al2O3- and SiO2-rich domains. The reaction ofγ-Al2O3 + SiO2 above about 1000°C initially produces Al2O3-rich mullite. This is explained by a possible nucleation of mullite on the surfaces ofγ-Al2O3 crystallites. The gradual transformation of the unstable, low-temperature (⩾1000 to ⩽1400°C) Al2O3-rich mullites to the stable high-temperature (⩾ 1400°C) 3/2-type mullites (3Al2O3 · 2SiO2) is essentially controlled by the annealing temperatures, whereas annealing times play a minor role.

Mullite formation from non-crystalline precursors

Compatibility tests between Solar Salt and thermal storage ceramics from inorganic industrial wastes

Andalusite is a well known alumino-silicate mineral largely employed in the refractory industry in order to provide high mechanical properties and thermal shock resistance. This work is devoted to the study of the thermal expansion behaviour of andalusite aggregates used in refractory castables. Anisotropic thermal expansion behaviour has been underlined by dilatometric measurements on andalusite single crystals according to their crystallographic axes. This behaviour can be magnified by the presence of quartz, even in small quantity, as impurities in the andalusite particle. The anisotropy of dilation is preserved after mullitisation of andalusite. These important results make it possible to explain the origin of the diffuse damage observed in andalusite-based castables which is most probably responsible for the enhancement of the strain to rupture required for the good thermal shock performance of such material.

Anisotropic behaviour of andalusite particles used as aggregates on refractory castables

The high-temperature transformation of andalusite (Al2SiO5) into 3/2-mullite (3A2O32SiO2) plus vitreous silica (SiO2) has been studied within the temperature interval from 1300 to 1600 °C by means of X-ray powder and single-crystal techniques and by infrared spectroscopy. Results are interpreted in terms of a topotactic transformation in which (011) and (01) planes of andalusite transform into (20) and (201) planes of 3/2-mullite; in this way the a- and b-axes of the two phases are interchanged. From the structural and orientational relationship between the andalusite and 3/2-mullite latrices it is concluded that the aluminium-oxygen octahedral chains, running in both structures parallel to the crystallographic c-axis, are preserved during phase transformation.

Wolfgang Pannhorst

Papers

The High-Temperature Transformation of Andalusite (A12SiO5) into 3/2-Mullite (3Al2O32SiO2) and vitreous silica (SiO2)