Thermal Reactions of Pyrophyllite Studied by High‐Resolution Solid‐state ²⁷Al and ²⁹Si Nuclear Magnetic Resonance Spectroscopy

Abstract

Water is lost in two overlapping steps from well-crystallized pyrophyllite from Coromandel, New Zealand. The pyrophyllite structure survives the loss of the first 30% of the total water content, but the loss of a further 60% water in the second step results in the formation of pyrophyllite dehydroxylate, with corresponding changes in both the 29Si and 27AI solid-state NMR spectra. Detailed analysis of the z9Si chemical shift of the dehydroxylate has allowed the silicate layer structure of this phase to be refined. A similarly detailed interpretation of the A1 spectra is not possible because of electric field gradient effects which result in the loss of -90% of the A1 spectral intensity due to the formation of five-coordinate A1 on dehydroxylation. The loss of further water from the dehydroxylate on further heating results in the formation of mullite and cristobalite and is accompanied by changes in the 29Si and Al spectra which can be accounted for in terms of coordination changes in the structural regions which contained the residual hydroxyls.

Structure and Thermal Transformations of Imogolite Studied by 29Si and 27Al High-Resolution Solid-State Nuclear Magnetic Resonance

“…After dehydroxylation, but prior to recrystallization, the amount of A1 visible was small and resulted in poor signal-to-noise, but peaks appeared to be present at about 37 and 59 ppm ( Figure 3J). Similar results were found for both dehydroxylated pyrophyllite and metakaolinite by MacKenzie et al (1985aMacKenzie et al ( , 1985b, who suggested that these two peaks represented A1 in slightly different tetrahedral environments, although the peak at 37 ppm could possibly have been due to a more regular 5-coordinated A1 site (Alemany and Kirker, 1986). The separation of these sites was lost on recrystallization of the sample ( Figure 3K), which was accompanied by a further increase in the proportion of detectable tetrahedral AI ( Figure 4B) and a change in the octahedral chemical shift to -15 ppm ( Figure 3L).…”

“…Calibration of the integrated peak intensities with respect to the intensity produced by a known amount of A1 in an external standard of ot-A120 3 indicated that the NMR experiment detected all the A1 present in the unheated sample, but that the loss of both hydration and hydroxyl water decreased the amount of A1 detected in the heated samples (Figure 4). Recrystallization at higher temperatures only slightly improved the A1 detection level, consistent with the dehydroxylation behavior of other hydrated aluminosilicates studied by solid-state NMR (MacKenzie et aL, 1985a(MacKenzie et aL, , 1985bBrown et al, 1987).…”

“…Previous work on several aluminosilicate minerals (e.g., MacKenzie et al, 1985aMacKenzie et al, , 1985b …”

“…Furthermore, because the mean tetrahedral angle 0 is used, the method is suitable for systems in which more than one A1 atom is coordinated through oxygen to the silicon concerned (MacKenzie et al, 1985a). This method has predicted within about 2 ppm the 295i chemical shifts of several minerals for which precise crystal structures are available, and it has also been used to deduce structural information for noncrystalline dehydroxylated aluminosilicates from their NMR chemical shifts (MacKenzie et al, 1985a(MacKenzie et al, , 1985b. Based on the imogolite structure proposed by Cradwick et al (1972), computer simulations were made of both the 20-unit (C2oh) and 24-unit (C24h) tubes, from which the mean tetrahedral angles were computed.…”

Comparison of Structure and Properties of Differently Treated Illite Clay and Products

“…One of more investigated solid alumosilicate under highly alkaline conditions is the 1:1 layer lattice alumosilicate mineral -kaolinite [6].Although the mainly used in the preparation of 1:1 layer lattice alumosilicates, i.e.,geopolymers, it is also of interest 2:1minerals such as 2:1 analogue of kaolinite -pyrophillite with unitstructure Al 2 AlSi 3 O 10 (OH) 2 , in which the octahedral Al-O sheet isenclosed above and below by two tetrahedral Si-O sheets to form a repeating unit [7]. It is shown that attempts to produce fully reacted alumosilicategeopolymers from the crystalline 2:1 lattice mineral, pyrophillite, was unsuccessful.…”