A TRANSMISSION ELECTRON-MICROSCOPY STUDY ON THE DECOMPOSITION OF SYNTHETIC HILLEBRANDITE (CA2SIO4-CENTER-DOT-H2O)

The decomposition mechanism of hillebrandite (Ca2SiO4 . H2O) to larnit e (beta-Ca2SiO4) was studied by conventional and in situ hot-stage TEM methods. The hillebrandite structure is suggested to be a composition of parawollastonite (CaSiO3) and portlandite [Ca(OH)(2)] adjoining on {001} planes. The larnite fibers showed occasional bending and kinkin g as well as internal defects such as dislocations and domains. No tra nsformation-related microstructures such as twinning were observed. Th e preferred orientations in the larnite fibers were not distinct. The decomposition mechanism of hillebrandite in air and in a vacuum may be different. In air, the CaO from the decomposed portlandite layer dire ctly participates in the process of SiO4 chain breaking to form larnit e. In the TEM this CaO is removed and apparently reprecipitated, so th at hillebrandite converts directly to parawollastonite. A possible lat tice correspondence between hillebrandite (C2SH) and larnite (beta-C2S ), based on crystallographic considerations, is proposed to be b(C2SH) parallel to b(beta-C2S), alpha(C2sH) parallel to approximate to [10(2 ) over bar](beta-C2S), and C-C2SH parallel to approximate to [401](bet a-C2S).