Decomposition of the Sillimanite Minerals
As the sillimanite minerals are geologically formed at high pressures, they decompose or undergo a structural decomposition when heated to elevated temperatures in air at 1 atm pressure [7-10]. The decomposition reaction can be written as
3 Al2O3 • SiO2 = 3 Al2SiO5 ^ 3Al2O3 • 2SiO2 + SiO2 (2)
where the 3:2 stoichiometric mullite and silica are the decomposition products. The previously mentioned 2:1 mullite does not form during the decomposition of the sillimanites. The form of the silica varies for the three polymorphs. For kyanite, the decomposition product is cristobalite, while for andalusite and sillimanite, it is amorphous. It is worth noting that the temperature ranges of the decompositions are compatible with cristobalite formation. The decompositions of kyanite and andalusite are topotactical, but for sillimanite, the process involves multiple steps with the intermediate formation of a disordered sillimanite structure. Numerous similarities and differences exist for the decomposition reactions.
Extensive attrition milling of the original minerals to the nanosize range alters the form of the resulting silica to cristobalite. In all instances, the resulting silica is highly reactive. Because of this state of the silica, addition of reactive aluminas to the sillimanite minerals before heating to the decomposition temperature results in the immediate reaction with the rejected silica to form a secondary mullite in addition to the primary mullite from the original sillimanite mineral. Complete mullitization of a pure sillimanite mineral requires the addition of 31.42 wt% alumina. It is possible to produce pure single-phase mullite bodies through this technical approach. When fired properly, they can be sintered to a dense, fine grain size ceramic body.
The decompositions of the three sillimanite minerals occur over different temperature ranges and with different volumetric expansions. As the sillimanites are formed at high pressures, it is natural that they exhibit large volumetric expansions when they decompose at 1 atm pressure. As expected, kyanite, the highest-pressure form, undergoes the largest volumetric expansion (about 15%). The P-T formation densities can be viewed as the driving force for the decompositions. Kyanite also initiates its decomposition at the lowest temperature of the three.
As expected for any kinetic process, the sillimanite decompositions are both temperature and time dependent. Typically, when slowly heated, kyanite begins to decompose at ~1,150°C and has completely decomposed by ~1,350°C. Andalusite starts its decomposition at ~1,250°C and is fully transformed by ~1,500°C. Sillimanite is less prone to decomposition and first begins to decompose at ~1,400°C and is not fully decomposed until ~1,700°C. The temperature intervals over which the decompositions occur increase in the following order: kyanite, andalusite, sillimanite. They are approximately 200°C, 250°C, and 300°C, respectively. The decomposition temperatures and intervals are specified as “about” or “approximate” in every instance. The three minerals will themselves vary slightly depending on the specific geological origins that determine their exact location within the original P-T fields in Fig. 1. For that reason, the densities and the driving force for their decompositions vary, even for the same mineral from different locations.
The volume expansions of the decompositions can be beneficial and at the same time a hindrance to their industrial applications. Kyanite has the largest +А V% and is often as large as 15% or even slightly greater. Firing kyanite by itself will result in the individual crystal prisms of the mineral bloating and cracking severely. Of course, this macrostructural destruction of the crystals is highly beneficial to any subsequent milling and homogenization in technical ceramic bodies and refractories. It does, however, somewhat restrict the utilization of “pure” kyanite, as mined, for a high temperature ceramic body. The decomposition volume expansions of andalusite and sillimanite are both somewhat less than that of kyanite, ascribing to the relative pressure levels of their equilibrium formation. The volume expansion of andalusite is only about 4%, while that of sillimanite is larger at about 8%.
That the three sillimanites should exhibit large volume expansions upon decomposition is not surprising when the densities of the products of the decomposition are considered. Mullite, the major product, has a density of about 3.2 Mg m-3, similar to that of andalusite. Thus, andalusite experiences the lowest of the volume expansions during its decomposition. The crystalline silica phase that results from the kyanite decomposition is cristobalite with a density of 2.2-2.3 Mg m-3 and the amorphous silica would be expected to be even less; thus, it is easy to explain the volume expansion and cracking of the sillimanite minerals when they decompose, purely on the basis of the densities of the decomposition products compared with that of the original sillimanite mineral.
When the sillimanite minerals are milled into the nanoparticle regime, the decompositions are strongly modified in several ways. The first effect is that the temperature range of the decomposition to mullite and silica is reduced significantly, often by several hundred degrees Celsius. This decrease in temperature has potential energy savings for subsequent industrial firing processes. Second, the volume expansion, A V%, is reduced, eventually achieving a situation where the sintering of the nanofine particles supercedes the volume expansion of the decomposition and shrinkage occurs upon heating. Finally, the products of the decomposition are altered, as the amorphous silica of the andalusite and sillimanite decompositions becomes cristobalite comparable to the decomposition of kyanite. Although there have not been high-resolution transmission electron microscopy studies of the effects of fine milling on the decomposition, it is highly probable that the detailed mechanisms of the decomposition of very fine sillimanite mineral particles may be altered from those of coarse particles usually produced industrially.