Cordierite crystallization

FIGURE 13.1.3 (a) Orientation of cordierite crystals, (b) Anisotropy of cordierite crystals, (c) SEM photograph of cordierite crystals. 

Beall (1992) reported on typical coast-and-island microstructures in cordierite (Section 2.2.5) and pollucite (Section 2.2.4) glass-ceramics. The cordierite crystallized from grain boundaries and became the predominant crystalline phase. In pollucite glass-ceramics, the muUite-type remains of the glass matrix became enveloped by the pollucite matrix. 

At temperatures in the range of 790°- 1000°C, curved micas demonstrating a cabbage-head microstructure were formed. A typical example of this microstructure is shown in Fig. 3-14b. The growth of a second crystal phase in the form of a secondary phase was observed in the final stage of the formation of curved micas in different glass-ceramics of this type. This phase was located between the mica crystals (Fig. 3-15a), X-ray diffraction investigations have shown the phase to be composed of cordierite crystals. The formation of... 

Winter W, Berger A., Muller G., and Pannhorst W, "TEM Investigation of Cordierite Crystallization from a Glass Powder with Composition Mg2Al4Sijj03(j,"/. Eur. Ceram. Soc., 15, 65-70 (1995). 

Commercial, electronic ceramic-type cor-dierites range in color from off-white to light brown. Cordierite crystals can be developed in certain bodies for example, about 30% talc in a kaolin body produces cordierite as the chief crystalline compound in the fired piece. 

Phase composition XRD studies revealed that modification of cordierite with manganese oxide results in formation of mainly cordierite-like phase at calcination temperature about 1150°C, magnesium silicate, quartz and Mn203 being the minor phases (Figure 1). It should be mentioned that the usual ceramic preparation procedure does not lead to cordierite crystallization at this temperature. The preliminary mechanical treatment of the raw materials considerably reduces the temperature and time of... 

The series of MnOx-modified cordierite-like catalysts 2(Mgi xMnx)0-2Al203-5Si02 (x=0-l) was prepared by mechanochemical method. It was found that the increase of calcination time and temperature, the decrease of particle size duriug the mechanical treatment, the modification with manganese oxide (up to x=0.5) leads to better cordierite crystallization followed by reduction of specific surface area and internal pore volume of the samples. The fully substituted catalysts were shown to have the highest selectivity to NO at studied conditions, most probably, due to the higher content of Mn20s (active oxide) on the catalysts surface. 

To delay crystallization, an additional element can be added, e.g. yttrium or magnesium76. Thus, Laine and coworkers80 have shown that ammonolysis of a cordierite (2Mg0 2Al203 5Si02) precursor system provides access to MgSiAlON materials. The development of a cordierite precursor is described below in the sections on silicate precursors. 

The Crystal Structures of Cordierite and Indialite [Mg2Al3(AlSis0i8) H20]... 

The mineral beryls, cordierite, and tourmaline, collectively designated as cyclosilicates, have large open channels in their crystal structures. These channels are large enough to accommodate atoms and molecules extraneous to the structure, and these minerals are well known as hosts of such extraneous elements as noble gases. Beryl and tourmaline are characteristic minerals of pegmatites, and cordierite is a common metamorphic mineral. 

There has been a considerable effort in the physics and chemistry communities to use INS methods to study magnetic dynamics, which can often be described as spin waves. Measurements of spin wave dispersion curves can provide information about the interactions between atomic magnetic moments, the so-called exchange interactions. There have been comparatively few INS measurements on magnetic minerals. INS methods have been used to produce spin wave dispersion curves for hematite. Crystal field magnetic transitions in cobalt bearing cordierite, and spinel phases have also been studied by INS. ... 

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