Mullite-forming minerals

Transmission electron micrographs show hectorite and nontronite as elongated, lath-shaped units, whereas the other smectite clays appear more nearly equidimensional. A broken surface of smectite clays typically shows a "com flakes" or "oak leaf surface texture (54). High temperature minerals formed upon heating smectites vary considerably with the compositions of the clays. Spinels commonly appear at 800—1000°C, and dissolve at higher temperatures. Quartz, especially cristobalite, appears and mullite forms if the content of aluminum is adequate (38). 

Much of the quartz in the fly ash originates from the coal as silt- and sand-sized particles, and it remains in the ash because it survives thermal transformation during the combustion process (Helmuth 1987). Small amounts of volatilized Si may also oxidize to form very fine crystals of quartz within the fly ash glass (Diamond 1984 Hubbard et al. 1984). Although bituminous coal ash may contain more than 50 wt% analytical Si02, only 5-10 wt% of it is present in the form of quartz (McCarthy et al. 1990). Some Si is present in the mineral mullite, but the majority of it is in the amorphous glass phase. 

Bauxitic Kaolins andMullites. Deposits of bauxitic kaolins, kaolins having aluminous minerals, have been discovered that have alumina contents between 50 and 70%. These materials are made into refractory aggregates called calcines, grog, clinker, or grain. In addition to sdectivdy mined deposits, synthetic compositions can be prepared from kaolin and alumina and other minerals to produce compositions of desired alumina and mineralogical content. These synthetic mullites are readily available in the form of sintered and fused aggregates. 

The reactions of several other minerals which thermally decompose to form mullite have been studied by Si and Al NMR. These include the mica mineral muscovite, which also contained sufficient iron to permit a complementary Fe Mossbauer study (MacKenzie et al. 1987), the hydroxyfluoride mineral topaz (Day et al. 1995) and the semi-amorphous aluminosilicate minerals allophane (MacKenzie et al. 1991) and imogolite (MacKenzie et al. 1989). The same combination of NMR nuclei has been used to study the thermal decomposition of other aluminosilicates including an illite-rich clay (Roch et al. 1998), montmorillonite (Brown et al. 1987), and a related mineral, Fuller s Earth (Drachman et al. 1997). NMR has also been used to study the effect of water vapour on the thermal decomposition of montmorillonite clay compacts (Temuujin et al. 2000a). 

However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at > 1400°C (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form P-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie er a/. 1994a). 

The fired ceramic body is a mixture of two phases mullite and silica. Mullite, a rare mineral in nature, takes the form of needlelike crystals that interpenetrate and confer strength on the ceramic. When the temperature is above 1470°C, the silica phase forms as minute grains of cristobalite, one of the several crystalline forms of Si02. 

In the second phase of his study, Yamaguchi [28] investigated the corrosion of certain burned refractories by sodium carbonate vapor. He suspended the test piece with a platinum wire from the bottom of an alumina crucible placed upside down. The entire assembly was heated at 1200°C for various times. Included in this part of the study was a fireclay refractory composed of mullite and silica minerals. Mullite reacted with NazO to form nepheline and alumina. The nepheline increased in amount as the remaining soda vapor reacted with the newly formed alumina and the preexistent silica. For refractories composed of mullite and corundum, carnegieite solid solution was the major reaction product. The formation Yamaguchi described resulted when Na2C03 vapor reacted with the alumina liberated from mullite and preexistent as corundum, forming... 

The three sillimanite minerals are structurally similar and have structures that are related to that of mullite. It is not surprising that they all form mullite upon decomposition. Kyanite crystallizes in the triclinic system, while sillimanite, andalusite, as well as mullite have orthorhombic crystal structures. In these structures, all the Si4+ cations are in fourfold coordination with 02 anions, but the Al3+ cations exist in four-, five-, and sixfold coordination with 02 anions, and therein lie the structural differences. The fivefold coordination of some Al3+ cations within A105 polyhedra is rather unusual, perhaps the result of formation at high pressures. The other structural differences among the three minerals are quite small. They are associated with the double chain structures of these three minerals and the linkages of the chains to one another by different alumina and silica polyhedra. Those concepts are readily extended to mullite. 

Aluminum Silicate. Al,05 i moi wt 162.05. Ai 33.30%, O 49.37%, Si 17.33%. AljSiOj. Usually contains some water. Polymorphous the three naturally occurring forms are andalusite cyanite. sillimanite. Other aluminum silicate mineral are anauxite, dickite. kaoUttile. kochite, mullite. newtonite, pyrophyllite, takizolite. termierite ton. For prepn and properties see Omelin s Aluminum (8th ed.) 35B, p 313-317 (1934). 

When montmorillonite is heated, water that has been absorbed between the silica layers is evolved first, between 100° and 200°C. About 700°C, the clay mineral breaks down, giving up its water of constitution, and an amorphous mass of silica, alumina and magnesia remains, corresponding to meta-kaolin . On being heated to about 1200°C, mullite, cristobalite, cordierite and spinel form. 

Sillimanite, kyanite, and andalusite are non-plastic and so are processed by fine grinding, with the addition of a binder or plasticizer. When heated at about ISSO C, all three minerals decompose to form mullite and cristobaUte ... 

If firing is carried out above 1595 C, the highly refractory mineral muttite then forms (see mullite) with an additional liberation of free sihca that melts according to the following chemical reaction ... 

In the manufacture of refractory and porcelain bodies, the desirable mullite is formed from the chemically similar minerals andalusite, kyanite and dumortierite, which decompose into mullite and silica at cone 13, cone 12 and cone 6, respectively. To break up siUimanite in the same way, cone 20 is required. Sintered and electro-fused synthetic mullites (including zirconium mullite) are used in kiln furniture and refractories for the glass and steel industries. The theoretical chemical analysis (wt%) of mullite and five typical mullite materials are as follows ... 

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