Structural characterization of the thermal transformation of halloysite by solid state NMR

Abstract

Structural changes on thermal decomposition of the clay mineral halloysite (A12Si207 9 4H20 ) with progressive heating up to 1400"C have been followed by a combinat.ion of 29Si and 27A1 so[id state NMtL X-ray diffraction and transmission electron microscopy. Thermal transformations closely follow those previously outlined for the related mineral kaolinite. Clear evidence is presented here that the major exotherm at 1000*C is principaUy associated with the formation of ,/-AlzO 3 and results in phase separation of alumina-rich and silicafich regions on a scale of < 5 nm. On further heating initially an extremely alumina-rich mullite folms. This progressively reacts with further silica, increasing the silica-content of the mullite, finally producing 3 : 2 muUite with the residual silica crystallising as cristobalite.

Solid-state NMR characterisation of the thermal transformation of a Hungarian white illite

“…There are two AlO 6 peaks, one of which at 4 ppm is probably from mullite. This mullite formation appears to occur before the 1200°C found for halloysite [25] and kaolinite [16,20,22]. However there is certainly another AlO 6 -containing phase which could be a γ-alumina or spinel based phase present in this illite at these temperatures corresponding to the peak at ~12…”

“…There have been various multinuclear NMR reports (especially 29 Si and 27 Al) of the thermal decomposition of clay minerals. The most comprehensive NMR data set from clays has been for 1:1 clay minerals, such as kaolinite [14][15][16][17][18][19][20][21][22][23][24] and related phases such as halloysite [25]. For 2:1 type clay minerals pyrophyllite is a cation-free clay that has no iron and it shows different thermal breakdown characteristics from the cation-free 1:1 clay minerals, with much more AlO 5 in the intermediate dehydroxylate state [26][27][28].…”

Changes in Structure, Morphology, Porosity, and Surface Activity of Mesoporous Halloysite Nanotubes Under Heating

“…The XRD pattern of Hal-400 was like that of Hal-120 (Figure 1c), but the intensities of the halloysite reflections from Hal-500 were markedly less ( Figure 1d). The patterns of Hal-600 to Hal-900 showed broad diffraction maxima ( Figure 1eÀh) owing to dehydroxylation and the formation of an X-ray amorphous product, metahalloysite (Smith et al, 1993).…”

“…The reaction mechanism under heating (after dehydration) of halloysite had been postulated to be analogous to that of kaolinite because halloysite is structurally and chemically similar to kaolinite. This assumption was supported by Smith et al (1993), but the halloysite sample used in their work contained considerable impurities (quartz and cristobalite), which inevitably affected the interpretation of the spectroscopic characterization and, thus, decreased the reliability of the results obtained.…”

“…As summarized in the outline by Smith et al (1993), the reaction scheme for the calcination of kaolinite initially involves dehydroxylation between 600 and 850ºC, where most of the OH groups are removed, leading to reduced coordination of originally octahedral aluminum. The metakaolinite formed in this step is X-ray amorphous, but the ab order is preserved while some disorder is introduced along the c axis (Bergaya et al, 1996).…”