Synthesis of clay minerals

Hydrothermal syntheses were performed at various pH values and temperatures to induce variability in kaolinite defect density. Temperature of synthesis ranged from 200 to 240~ for 21 d. Initial pH at room temperature ranged from 0.5 to 14. The starting material was a hyd~othermally treated gel, with an atomic Si/A1 ratio of 0.93, partly transformed into kaolinite. Kaolinite was obtained for a wide range of pH. Although no influence of temperature on "crystallinity" (i.e., defect density) was observed, the effect of pH was important. A continuous series was obtained from a low-defect kaolinite, with high thermal stability and a hexagonal morphology for the most acidic final pH, to a high-defect kaolinite, with low thermal stability and lath shape for the most basic final pH. These variations of kaolinite properties appear related to the pH dependence of kaolinite surface specia- tion. Increasing pH value results in increased cation adsorption on the kaolinite external surfaces and increases in the elongation of particles.

Influence of Synthesis pH on Kaolinite ``Crystallinity'' and Surface Properties

Publisher Summary This chapter describes synthetic clay minerals and purification of natural clays. A complete purification cycle is time consuming. However, not all the purification steps are always necessary; steps can be omitted depending on the purpose of the investigation. In contrast to soil clays, clays from geological deposits (such as bentonites, and kaolins) commonly contain only small amounts of organic materials. As such, oxidation with H 2 O 2 or peroxodisulphate may not be required and the simplified procedure may be satisfactory. Similarly, silica does not always act as a cementing agent, and it can be removed without chemical treatment. Even small amounts of carbonates, (hydr)oxides, and organic materials can strongly influence the colloidal and rheological properties of clay pastes, slips, and dispersions. The chemical reactions involved in clay mineral purification may also cause changes to the clay mineral structure. In selecting a given purification procedure, the effect of different reactions must be duly considered.

Chapter 4 Synthetic Clay Minerals and Purification of Natural Clays

Experimental study of the hydrothermal formation of kaolinite

The crystal structure of synthetic deuterated kaolinite was refined by Rietveld analysis using time-of-flight (TOF) neutron powder diffraction data. For non-hydrogen atoms, C1 symmetry was as- sumed. Starting models were tested in which only the direction of O-D vectors was varied. The constraints were introduced to all A1-O, Si-O and O-D bonds. The refinement adopting the former gives PI(C1), a = 5.169(1) /k, b = 8.960(2) ,&, c = 7.410(2) ~, ct = 91.26(2) ~ (3 = 104.99(2) ~ ~/ = 89.93(1) ~ Rwp = 3.17%, R; = 5.78% and S = 1.34 with constraints of/(A1-O) = 1.93 + 0.05 /~, /(Si-O) = 1.62 + 0.03 ,~ and /(D-O) = 0.95 + 0.15 ,~. The inner O-D vector points toward the tetrahedral sheet. All inner- surface O-D groups form H bonding with basal O atoms in the next kaolinite layers. The results agreed with those obtained from natural kaolinite.

Structure Refinement of Synthetic Deuterated Kaolinite by Rietveld Analysis Using Time-of-Flight Neutron Powder Diffraction Data

The hydrothermal synthesis of kaolinite was examined in the Al2O3-SiO2-H2O system to study inhibitory effects of additional ions on the formation of kaolinite. Syntheses were carried out with amorphous starting materials and salt solutions of various concentrations in Teflon pressure vessels at 220°C for 5 days. The reaction products were characterized by XRD, IR, DTA-TG, NMR and TEM. In all of the runs using solutions with cation concentrations less than 0.001 M, no significant effect on the formation of kaolinite was observed. The inhibitory effect of the univalent cations Li+, Na+ or K+ was less than that of divalent cations such as Mg2+ or Ca2+. The addition of trivalent Fe3+ or excess Al3+ ions interfered with the formation of kaolinite significantly. Sulfate and acetate solutions interfered with the formation of kaolinite more than chlorides and nitrates. No crystalline product was obtained using a 1.0 M basic solution of carbonate or hydroxide. The addition of the lithium ion to the system affected the crystallization of kaolinite only slightly. The use of 0.1 M LiCl and LiNO3 solutions for the syntheses improved crystallization of kaolinite along the [001] direction.

Effects of solution chemistry on the hydrothermal synthesis of kaolinite

The formation process of kaolinite from the amorphous mixture of silica and alumina by hydrothermal reaction was studied by XRD, IR, DTA-TG, TEM, and high-resolution solid state 29 Si- and 27 Al-MAS/NMR. The amorphous mixture of the starting material prepared with commercially available silica-sol and alumina-sol was hydrothermally treated with distilled water in a Teflon pressure vessel at 220°C for a reaction time varying from 1 to 144 hours. The Q 4 state component of the amorphous starting material changed into an intermediate amorphous Q 3 state. The intermediate amorphous phase converted into crystalline kaolinite with spherical shape (Q 3 state). The yield and crystallinity of kaolinite increased with an increase in the reaction time

Soichiro Samejima

Papers

Effects of solution chemistry on the hydrothermal synthesis of kaolinite

Formation process of kaolinite from the amorphous mixture of silica and alumina