Thermal Stability of Azurite and Malachite in Relation to the Formation of Mediaeval Glass and Glazes

Azurite and malachite have been extensively used as pigments in ancient and medieval manuscripts, glasses and glazes. The thermal stability of naturally occurring azurite and malachite was determined using a combination of controlled rate thermal analysis combined with mass spectrometry and infrared emission spectroscopy. Both azurite and malachite thermally decompose in six overlapping stages but the behaviour is different for the two minerals. These stages occur around 282, 328, 350, 369, 384 and 840 degrees Celsius for azurite and 250, 321, 332, 345, 362 and 842 degrees Celsius for malachite. The first two stages are associated with the loss of water, whereas stages 3 and 4 result from the simultaneous loss of water and carbon dioxide. The sixth stage is associated with reduction of cupric oxide to cuprous oxide and finally to copper. Infrared emission spectroscopy shows that dehydroxylation occurs before the loss of carbonate and that the thermal decomposition is complete by 375 degrees Celsius. The implication of this research is that in the preparation of glass or glazes using these two hydroxy-carbonate minerals of copper the samples will decompose at low temperatures and any colour formation in the glass is not due to azurite or malachite

Homogeneous Forced Hydrolysis of Aluminium Through the Thermal Decomposition of Urea

Homogeneous hydrolysis of aluminum by decomposition of urea in solution was achieved because the urea coordinates to the Al3+ in solution, forming [Al(H2O)5(urea)]3+ and to a lesser extent [Al(H2O)4(urea)2]3+. Upon hydrolysis more hydrolyzed monomeric species, [Al(H2O)5(OH)]2+, [Al(H2O)4(OH)2]+, [Al(H2O)4(urea)(OH)]2+, and [Al(H2O)3(urea)(OH)2]+, were formed, followed by trimeric species and the Al13 Keggin complex [AlO4Al12(OH)24(H2O)12]7+. The 27Al NMR spectra indicated the formation of other complexes in addition to the Al13 at the end of the hydrolysis reaction

Near Infrared Spectroscopic Study of Nontronites and Ferruginous Smectite

The existence of life on planets such as Mars depend upon the presence of water. This water may not necessarily be as liquid or crystalline water but may be as interlayer water such as is found in smectitic clays. One group of smectites, relevant to the search for interplanetary life are those which have a high iron content, known as nontronites. Near-IR reflectance spectroscopy has been used to show the presence of water and hydroxyl units in these minerals. Three near –IR spectral regions are identified: (a) the high frequency region between 6400 and 7400 cm-1 attributed to the first overtone of the hydroxyl stretching mode (b) the 4800-5400 cm-1 region attributed to water combination modes and (c) the 4000-4800 cm-1 region attributed to the combination of the stretching and deformation modes of the FeFeOH units of nontronite. Two types of hydroxyl groups were identified using near-IR spectroscopy: hydroxyl units coordinated to the iron and hydroxyl groups from water in the nontronite structure. The first hydroxyls are characterised by several bands: firstly in the 7055 to 7098 cm-1 region assigned to the first overtone of the AlFeOH stretching unit, secondly in the 6958-6878 cm-1 region attributed to the FeFeOH unit. The overtone of the hydroxyl stretching frequency of water was observed at around 6800 cm-1. These observations show that nontronites can be a source of water that may support life

Thermal Decomposition of Bauxite Minerals: Infrared Emission Spectroscopy of Gibbsite, Boehmite and Diaspore

Infrared emission spectroscopy has been used to study the dehydroxylation behavior over the temperature range from 200 to 750 degrees Celsius of three major Al-minerals in bauxite: gibbsite (synthetic and natural), boehmite (synthetic and natural) and diaspore. A good agreement is found with the thermal analysis and differential thermal analysis curves of these minerals. Loss in intensity of especially the hydroxyl-stretching modes of gibbsite, boehmite and diaspore as function of temperature correspond well with the observed changes in the TGA/DTA patterns. The DTA pattern of gibbsite clearly indicates the formation of boehmite as an intermediate shown by a endotherm around 500 degrees Celsius. Dehydroxylation of gibbsite is followed by a loss of intensity of the 3620 and 3351 cm-1 OH-stretching bands and the corresponding deformation band around 1024 cm-1. Dehydroxylation starts around 220 degrees Celsius and is complete around 350 degrees Celsius. Similar observations were made for boehmite and diaspore. For boehmite dehydroxylation was observed to commence around 250 degrees Celsius and could be followed by especially the loss in intensity of the bands around 3319 and 3129 cm-1. The DTA pattern of diaspore is more complex with overlapping endotherms around 622 and 650 degrees Celsius. The dehydroxylation can be followed by the decrease in intensity of the OH-stretching bands around 3667, 3215 and 2972 cm-1. Above 550 degrees Celsius only a single band is observed that disappears after heating above 600 degrees Celsius corresponding to the two endotherms around 622 and 650 degrees Celsius in the DT

The Effects of Synthesis pH and Hydrothermal Treatment on the Formation of Zinc Aluminium Hydrotalcites

Zn/Al hydrotalcites were synthesised by coprecipitation at increasing pH from 6.0 – 14.0, followed by hydrothermal treatment at 150 oC for 7 days. The materials were characterised by XRD, STEM, ICP-AES, thermal analysis, infrared spectroscopy and Raman spectroscopy. The XRD analysis for the samples prepared between pH 9.0 – 12.0 showed a pattern typical of hydrotalcite, with a c-axis distance of ~22.6 Å. STEM showed that the pH of preparation affected the stability of the hydrotalcite and that instability, observed at pH 9.0, favoured the formation of mixed phases when treated hydrothermally. It was also shown that treatment of a stable starting material increased the crystallinity and resulted in the formation of hexagonal plate shaped particles. ICP-AES and thermal analysis showed that the Zn/Al ratio and thermal stability increased with pH. Thermal analysis showed three major weight losses corresponding to the loss of interparticle water, interlayer water and dehydroxylation of the hydroxide layers and decarbonisation of the interlayer region

Catalytic Activity of Synthetic Saponite Clays: Effects of Tetrahedral and Octahedral Composition

This paper describes the catalytic characteristics of synthetic saponites with well-known chemical composition, thermal stability and acidity in three catalytic reactions: 1) catalytic cracking of n-dodecane, 2) hydro-isomerization of n-heptane and 3) Friedel-Crafts alkylation of benzene. Saponites with Mg in the octahedral position was by far the best catalyst for the catalytic cracking of n-dodecane, which can be explained by the higher surface area of these saponites compared to saponites with other compositions. All saponites performed better in the hydro-isomerization reaction of n-heptane and Friedel-Crafts alkylation compared to commercially available catalysts such as HZSM-5 and ASA. The shape selectivity in the Friedel-Crafts alkylation of benzene towards p- and o-DIOPB was remarkably high for the synthetic saponite

Vibrational Spectroscopic Study of the Nitrate Containing Hydrotalcite Mbobomkulite

Raman spectroscopy has been used to study the nitrate hydrotalcite mbobomkulite NiAl2(OH)16(NO3).4H2O. Mbobomkulite along with hydrombobomkulite and sveite are known as ‘cave’ minerals as these hydrotalcites are only found in caves. Two types of nitrate anion are observed using Raman spectroscopy namely free or non-hydrogen bonded nitrate and nitrate hydrogen bonded to the interlayer water and to the ‘brucite-like’ hydroxyl surface. Two bands are observed in the Raman spectrum of Ni-Mbobomkulite at 3576 and 3647 cm-1 with an intensity ratio of 3.36/7.37 and are attributed to the Ni3OH and Al3OH stretching vibrations. The observation of multiple water stretching vibrations implies that there are different types of water present in the hydrotalcite structure. Such types of water would result from different hydrogen bond structures

The Garfield and Uley nontronites - an infrared spectroscopic comparison

FTIR and Infrared emission spectroscopy (IES) has been used to characterise the Uley (Australian) and Garfield nontronites. These clay minerals are characterised by a strong emission band at 3570 cm-1 attributed to the FeFeOH unit. Dehydroxylation is followed by the loss of intensity of this band as a function of temperature. Dehydroxylation is also followed by the loss of intensity of the FeFeOH deformation vibration at 843 cm-1. IES shows that the dehydroxylation occurs as a continuous process in comparison to DTA/TGA studies where the dehydroxylation occurs abruptly at 425 degrees Celsius. Water in these high iron bearing smectites have been observed through the stretching mode at 3430 cm-1 and the bending mode at 1630 cm-1. Different types of water are identified in the nontronite structure by the analysis of the band profile in the 1590 to 1680 cm-1 region. Low frequency vibrations show that the Uley green nontronite is similar to the Garfield nontronite. The brown Uley nontronite is closer to the Hohen-Hagen nontronite. The Uley nontronites may therefore be used spectroscopically to replace other nontronites as a reference clay mineral

XPS Study of Basic Aluminum Sulphate and Basic Aluminium Nitrate

Basic aluminium sulphate and nitrate crystals were prepared by forced hydrolysis of aluminium salt solution followed by precipitation with a sulphate solution or by evaporation for the basic aluminium nitrate. X-ray Photoelectron Spectroscopy (XPS) confirms the chemical composition determined by ICP-AES in earlier work. High resolution XPS scans of the individual elements allow the identification of both the central IVAlO4 group and the 12 aluminium octahedra in the [IVAlO4AlVI(OH)24(H2O)12] building unit by two Al 2p transitions with binding energies of 73.7 and 74.2 eV in both the basic aluminium sulphate and nitrate. Four different types of oxygen atoms were identified in the basic aluminium sulphate associated with the central AlO4, OH, H2O and SO4 groups in the crystal structure with transitions at 529.4, 530.1, 530.7 and 531.8 eV, respectively