The formation of silica high temperature polymorphs from quartz : influence of grain size and mineralising agents

The formation of high-temperature silica polymorphs in presence of Na and K has been studied at various temperatures and soaking times, starting from quartzes of different grain size, ex situ as well as in situ. The results show that cristobalite and tridymite formation is strongly influenced by the nature and the amount of mineraliser added. In particular, K seems to discriminate more between the two structures, as it produces the largest observed amount of cristobalite. The disappearance of quartz can be controlled by the proper combination of mineraliser/temperature/time, which in turn control the amount and the type of polymorph formed, together with the amount of amorphous matter. Cristobalite is always the first to form, in agreement with the lower defect formation energy calculated by means of a periodic Linear Combination of Atomic Orbitals hybrid approach (Hartree-Fock and DFT), and tridymite follows, at its expenses. Bearing in mind that the interaction of Na and K with a complete ceramic mixture at high temperature is still largely unknown, this paper demonstrates that the high temperature silica polymorphs formation from quartz is easily controllable by means of the addition of known amount of mineralisers.