3 The Sillimanite Minerals: Andalusite, Kyanite, and Sillimanite 45decomposition 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 tempera-
ture 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 decom-
position 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 decomposi-
tions 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 +∆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