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

Abstract

a b s t r a c tThe 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.

Silicic volcanism on Mars evidenced by tridymite in high-SiO 2 sedimentary rock at Gale crater

“…16 and 17) and is thus metastable under ambient terrestrial (and martian) surface conditions. It is readily produced under laboratory conditions within its stability field, especially with alkali fluxes (18)(19)(20)(21)(22)(23)(24)(25)(26)(27). We are not aware of any geologically reasonable, low-temperature formation pathway for preparation of tridymite outside of its thermochemical stability field.…”

Cold sintering of diatomaceous earth

“…54 However, it is usually reported that cristobalite preferentially crystallizes from silicate glasses even below 1400°C, [55][56][57] and this can be largely affected by the presence of even low amounts of alkalis. 58 Furthermore, the formation of cristobalite during high-temperature treatments of diatomaceous earth has already been abundantly documented in the literature. 59,60 The FTIR spectra of conventionally sintered diatomite samples are reported in Figure 4B.…”

Hot water treatment effect in the elephant grass ashes calcinated at different temperatures

“…The increase in the burning temperature (900°C) also increases the dissociation constant (kps) but promotes an increase in the plane 100 of the quartz, raising the crystallinity, thus the hot water treatment become ineffective for the removal of ions in coordination sphere of potassium, which requires much energy because the stability of the structure [41].…”