Thermal decomposition of hydronium jarosite (H3O)Fe3(SO4)2(OH)6

Thermogravimetry combined with mass spectrometry has been used to study the thermal decomposition of a synthetic hydronium jarosite. Five mass loss steps are observed at 262, 294, 385, 557 and 619°C. The mass loss step at 557°C is sharp and marks a sharp loss of sulphate as SO₃ from the hydronium jarosite. Mass spectrometry through evolved gases confirms the first three mass loss steps to dehydroxylation, the fourth to a mass loss of the hydrated proton and a sulphate and the final step to the loss of the remaining sulphate. Changes in the molecular structure of the hydronium jarosite were followed by infrared emission spectroscopy. This technique allows the infrared spectrum at the elevated temperatures to be obtained. Infrared emission spectroscopy confirms the dehydroxylation has taken place by 400 and the sulphate loss by 650°C. Jarosites are a group of minerals formed in evaporite deposits and form a component of the efflorescence. The minerals can function as cation and heavy metal collectors. Hydronium jarosite has the potential to act as a cation collector by the replacement of the proton with a heavy metal cation.     

Thermal analysis of beaverite in comparisonwith plumbojarosite

The thermal decomposition of beaverite and plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. The mineral beaverite Pb(Fe,Cu)₃(SO₄)₂(OH)₆ decomposes in three stages attributed to dehydroxylation, loss of sulphate and loss of oxygen, which take place at 376 and 420, 539 and 844°C. In comparison three thermal decomposition steps are observed for plumbojarosite PbFe₆(SO₄)₄(OH)₁₂ at 376, 420 and 502°C attributed to dehydroxylation; loss of sulphate occurs at 599°C; and loss of oxygen and formation of lead occurs at 844 and 953°C. The temperatures of the thermal decomposition of the natural plumbojarosite were found to be less than that for the synthetic jarosite. A comparison of the thermal decomposition of plumbojarosite with argentojarosite is made. The understanding of the chemistry of the thermal decomposition of minerals such as beaverite, argentojarosite and plumbojarosite and related minerals is of vital importance in the study known as ‘archeochemistry’.     

Thermal decomposition of the hydrotalcite

Summary A combination of thermogravimetry and hot stage Raman spectroscopy has been used to study the thermal decomposition of the synthesised zinc substituted takovite Zn₆Al₂CO₃(OH)₁₆·4H₂O. Thermogravimetry reveals seven mass loss steps at 52, 135, 174, 237, 265, 590 and ~780°C. MS shows that the first two mass loss steps are due to dehydration, the next two to dehydroxylation and the mass loss step at 265°C to combined dehydroxylation and decarbonation. The two higher mass loss steps are attributed to decarbonation. Raman spectra of the hydroxyl stretching region over the 25 to 200°C temperature range, enable identification of bands attributed to water stretching vibrations, MOH stretching modes and strongly hydrogen bonded CO₃^2--water bands. CO₃^2- symmetric stretching modes are observed at 1077 and 1060 cm^-1. One possible model is that the band at 1077 cm^-1is ascribed to the CO₃^2- units bonded to one OH unit and the band at 1092 cm^-1is due to the CO₃^2- units bonded to two OH units from the Zn-takovite surface. Thermogravimetric analysis when combined with hot stage Raman spectroscopy forms a very powerful technique for the study of the thermal decomposition of minerals such as hydrotalcites.</o:p>     

Thermal decomposition of natural iowaite

The thermal decomposition of natural iowaite of formula Mg₆Fe₂(Cl,(CO₃)_0.5)(OH)₁₆·4H₂O was studied by using a combination of thermogravimetry and evolved gas mass spectrometry. Thermal decomposition occurs over a number of mass loss steps at 60°C attributed to dehydration, 266 and 308°C assigned to dehydroxylation of ferric ions, at 551°C attributed to decarbonation and dehydroxylation, and 644, 703 and 761°C attributed to further dehydroxylation. The mass spectrum of carbon dioxide exhibits a maximum at 523°C. The use of TG coupled to MS shows the complexity of the thermal decomposition of iowaite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal analysis of goethite

Differential scanning calorimetry shows two endotherms at 75 and 225°C for synthetic goethite. The latter endotherm is strongly asymmetric on the low temperature side. The endotherms were attributed to the loss of water and the dehydroxylation of the goethite. The temperature of the endotherms and the enthalpy of the phase change were found to be linear functions of the percentage of aluminium substitution into the goethite. High-resolution thermogravimetric analysis of goethite showed three mass loss steps, occurring at ~175, 196 and 263°C. The temperatures of these mass loss steps and the percentage of mass loss were also linearly related to the degree of Al substitution. The use of infrared emission spectroscopy confirmed the temperature of dehydroxylation. The observation of the low temperature dehydroxylation of goethite and its relation to ancient aboriginal cave art is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamic and controlled rate thermal analysis of hydrozincite and smithsonite

The understanding of the thermal stability of zinc carbonates and the relative stability of hydrous carbonates including hydrozincite and hydromagnesite is extremely important to the sequestration process for the removal of atmospheric CO₂. The hydration-carbonation or hydration-and-carbonation reaction path in the ZnO-CO₂-H₂O system at ambient temperature and atmospheric CO₂ is of environmental significance from the standpoint of carbon balance and the removal of green house gases from the atmosphere. The dynamic thermal analysis of hydrozincite shows a 22.1% mass loss at 247°C. The controlled rate thermal analysis (CRTA) pattern of hydrozincite shows dehydration at 38°C, some dehydroxylation at 170°C and dehydroxylation and decarbonation in a long isothermal step at 190°C. The CRTA pattern of smithsonite shows a long isothermal decomposition with loss of CO₂ at 226°C. CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of zinc carbonate minerals via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. The CRTA technology offers a mechanism for the study of the thermal decomposition and relative stability of minerals such as hydrozincite and smithsonite.     

Thermal decomposition of stichtite

The mineral stichtite was synthesised and its thermal decomposition measured using thermogravimetry coupled to an evolved gas mass spectrometer. Mass loss steps were observed at 52, 294, 550 and 670°C attributed to dehydration, dehydroxylation and loss of carbonate. The loss of carbonate occurred at higher temperatures than dehydroxylation.     

Thermal decomposition of ammonium jarosite (NH4)Fe3(SO4)2(OH)6

Thermogravimetry combined with mass spectrometry has been used to study the thermal decomposition of a synthetic ammonium jarosite. Five mass loss steps are observed at 120, 260, 389, 510 and 541°C. Mass spectrometry through evolved gases confirms these steps as loss of water, dehydroxylation, loss of ammonia and loss of sulphate in two steps. Changes in the molecular structure of the ammonium jarosite were followed by infrared emission spectroscopy (IES). This technique allows the infrared spectrum at the elevated temperatures to be obtained. IES confirms the dehydroxylation to have taken place by 300°C and the ammonia loss by 450°C. Loss of the sulphate is observed by changes in band position and intensity after 500°C.     

Thermal decomposition of the composite hydrotalcites of iowaite and woodallite

The thermal stability and thermal decomposition pathways for synthesized composite iowaite/woodallite have been determined using thermogravimetry analysis in conjunction with evolved gas mass spectrometry. Dehydration of the hydrotalcites occurred over a range of 56–70°C. The first dehydroxylation step occurred at around 255°C and, with the substitution of more iron(III) for chromium(III) this temperature increased to an upper limit of 312°C. This trend was observed throughout all decomposition steps. The release of carbonate ions as carbon dioxide gas initialised at just above 300°C and was always accompanied by loss of hydroxyl units as water molecules. The initial loss of the anion in this case the chloride ion was consistently observed to occur at about 450°C with final traces evolved at 535 to 780°C depending of the Fe:Cr ratio and was detected as HCl (m/z=36). Thus for this to occur, hydroxyl units must have been retained in the structure at temperatures upwards of 750°C. Experimentally it was found difficult to keep CO₂ from reacting with the compounds and in this way the synthesized iowaite-woodallite series somewhat resembled the natural minerals.     

Thermal decomposition of the layered double hydroxides of formula Cu6Al2(OH)16CO3 and Zn6Al2(OH)16CO3

Zn-Al hydrotalcites and Cu-Al hydrotalcites were synthesised by coprecipitation method and analysed by X-ray diffraction (XRD) and thermal analysis coupled with mass spectroscopy. These methods provide a measure of the thermal stability of the hydrotalcite. The XRD patterns demonstrate similar patterns to that of the reference patterns but present impurities attributed to Zn(OH)₂ and Cu(OH)₂. The analysis shows that the d003 peak for the Zn-Al hydrotalcite gives a spacing in the interlayer of 7.59 ? and the estimation of the particle size by using the Debye-Scherrer equation and the width of the d003 peak is 590 ?. In the case of the Cu-Al hydrotalcite, the d003 spacing is 7.57 ? and the size of the diffracting particles was determined to be 225 ?. The thermal decomposition steps can be broken down into 4 sections for both of these hydrotalcites. The first step decomposition below 100°C is caused by the dehydration of some water absorbed. The second stage shows two major steps attributed to the dehydroxylation of the hydrotalcite. In the next stage, the gas CO₂ is liberated over a temperature range of 150°C. The last reactions occur over 400°C and involved CO₂ evolution in the decomposition of the compounds produced during the dehydroxylation of the hydrotalcite.     

Thermal analysis and Hot-stage Raman spectroscopy of the basic copper arsenate mineral

The thermal analysis of euchroite shows two mass loss steps in the temperature range 100–105 °C and 185–205 °C. These mass loss steps are attributed to dehydration and dehydroxylation of the mineral. Hot-stage Raman spectroscopy (HSRS) has been used to study the thermal stability of the mineral euchroite, a mineral involved in a complex set of equilibria between the copper hydroxy arsenates: euchroite Cu₂(AsO₄)(OH)·3H₂O → olivenite Cu₂(AsO₄)(OH) → strashimirite Cu₈(AsO₄)₄(OH)₄·5H₂O → arhbarite Cu₂Mg(AsO₄)(OH)₃. HSRS inolves the collection of Raman spectra as a function of the temperature. HSRS shows that the mineral euchroite decomposes between 125 and 175 °C with the loss of water. At 125 °C, Raman bands are observed at 858 cm^?1 assigned to the ν₁ AsO₄ ^3? symmetric stretching vibration and 801, 822, and 871 cm^?1 assigned to the ν₃ AsO₄ ^3? (A₁) antisymmetric stretching vibrations. A distinct band shift is observed upon heating to 275 °C. At 275 °C, the four Raman bands are resolved at 762, 810, 837, and 862 cm^?1. Further heating results in the diminution of the intensity in the Raman spectra, and this is attributed to sublimation of the arsenate mineral. HSRS is the most useful technique for studying the thermal stability of minerals, especially when only very small amounts of mineral are available.     

Dynamic and controlled rate thermal analysis of halotrichite

Three halotrichites namely halotrichite Fe²⁺SO₄·Al₂(SO₄)₃·22H₂O, apjohnite Mn²⁺SO₄·Al₂(SO₄)₃·22H₂O and dietrichite ZnSO₄·Al₂(SO₄)₃·22H₂O, were analysed by both dynamic, controlled rate thermogravimetric and differential thermogravimetric analysis. Because of the time limitation in the controlled rate experiment of 900 min, two experiments were undertaken (a) from ambient to 430 °C and (b) from 430 to 980 °C. For halotrichite in the dynamic experiment mass losses due to dehydration were observed at 80, 102, 319 and 343 °C. Three higher temperature mass losses occurred at 621, 750 and 805 °C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 82 and 97 °C followed by a non-isothermal dehydration step at 328 °C. For apjohnite in the dynamic experiment mass losses due to dehydration were observed at 99, 116, 256, 271 and 304 °C. Two higher temperature mass losses occurred at 781 and 922 °C. In the controlled rate thermal analysis experiment three isothermal dehydration steps are observed at 57, 77 and 183 °C followed by a non-isothermal dehydration step at 294 °C. For dietrichite in the dynamic experiment mass losses due to dehydration were observed at 115, 173, 251, 276 and 342 °C. One higher temperature mass loss occurred at 746 °C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 78 and 102 °C followed by three non-isothermal dehydration steps at 228, 243 and 323 °C. In the CRTA experiment a long isothermal step at 636 °C attributed to de-sulphation is observed.     

Thermal analysis of synthetic reevesite and cobalt substituted reevesite (Ni,Co)<Subscript>6</Subscript>Fe<Subscript>2</Subscript>(OH)<Subscript>16</Subscript>(CO<Subscript>3</Subscript>) · 4H<Subscript>2</Subscript>O

The mineral reevesite and the cobalt substituted reevesite have been synthesised and studied by thermal analysis and X-ray diffraction. The d(003) spacings of the minerals ranged from 7.54 to 7.95 Å. The maximum d(003) value occurred at around Ni:Co 0.4:0.6. This maximum in interlayer distance is proposed to be due to a greater number of carbonate anions and water molecules intercalated into the structure. This increase in carbonate anion content is attributed to an increase in surface charge on the brucite like layers. The maximum temperature of the reevesite decomposition occurs for the unsubstituted reevesite at around 220 °C. The effect of cobalt substitution results in a decrease in thermal stability of the reevesites. Four thermal decomposition steps are observed and are attributed to dehydration, dehydroxylation and decarbonation, decomposition of the formed carbonate and oxygen loss at ~807 °C. A mechanism for the thermal decomposition of the reevesite and the cobalt substituted reevesite is proposed.     

Contribution to the Study of the Solid-state Reaction of Mercury With Pure Rhodium

Thermogravimetry (TG) and other analysis techniques (EDX, SEM, Mapping surface, X-ray diffraction, inductively coupled argon plasma emission spectroscopy and atomic spectrometry with cold vapor generation) were used to study the reaction of Hg with Rh. The results permitted the suggestion that, when subjected to heat, an electrodeposited Hg film reacts with Rh to form intermetallic products with different stabilities, as indicated by at least three mass loss steps. In the first step, between room temperature and 160°C, only the bulk Hg is removed. From this temperature up to about 175°C, the mass loss can be attributed to the desorption of a film of metallic Hg. The last step, from 175 to 240°C, can be ascribed to the removal of Hg from a thin dark film of RhHg₂ . This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermogravimetric analysis of organoclays intercalated with the surfactant octadecyltrimethylammonium bromide

Summary High resolution thermogravimetric analysis has been used to study the thermal decomposition of montmorillonite modified with octadecyltrimethylammonium bromide. Thermal decomposition occurs in 4 steps.The first step of mass loss is observed from ambient to 100°C temperature range and is attributed to dehydration of adsorbed water. The second step of mass loss occurs between 87.9 to 135.5°C temperature range and is also attributed to dehydration of water hydrating metal cations such as Na⁺. The third mass loss occurs between 179.0 and 384.5°C; it is assigned to the loss of surfactant. The fourth step is ascribed to the loss of OH units due to dehydroxylation of the montmorillonite and takes place between 556.0 and 636.3°C temperature range. These TG steps are related to the arrangement of the surfactant molecules intercalating the montmorillonite. Changes in the basal spacing of the clay with surfactant are followed by X-ray diffraction. Thermal analysis provides an indication of the stability of the organo-clay.     

Thermal decomposition of natural and synthetic plumbojarosites: Importance in ‘archeochemistry’

Plumbojarosite and argentoplumbojarosite were sources of lead and silver in ancient and medieval times. The understanding of the chemistry of the thermal decomposition of these minerals is of vital importance in ‘archeochemistry’. The thermal decomposition of plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. Three mass loss steps are observed at 376, 420 and 502 °C. These are attributed to dehydroxylation, loss of sulphate occurs at 599 °C, and loss of oxygen and formation of lead occurs at 844 and 953 °C. The temperatures of the thermal decomposition of the natural jarosite were found to be less than that for the synthetic jarosite. This is attributed to a depression of freezing point effect induced by impurities in the natural jarosite. Raman spectroscopy was used to study the structure of plumbojarosite. Plumbojarosites are characterised by stretching bands at 1176, 1108, 1019 and 1003 cm⁻¹ and bending modes at 623 and 582 cm⁻¹. Changes in the molecular structure during thermal decomposition were followed by infrared emission spectroscopy. The technique shows the loss of intensity in the hydroxyl stretching region attributed to dehydroxylation. Loss of sulphate only occurs after dehydroxylation. Lead is formed at higher temperatures through oxygen evolution.     

Controlled rate thermal analysis of hydromagnesite

The reaction of magnesium minerals such as brucite with CO₂ is important in the sequestration of CO₂. The study of the thermal stability of hydromagnesite and diagenetically related compounds is of fundamental importance to this sequestration. The understanding of the thermal stability of magnesium carbonates and the relative metastability of hydrous carbonates including hydromagnesite, artinite, nesquehonite, barringtonite and lansfordite is extremely important to the sequestration process for the removal of atmospheric CO₂. This work makes a comparison of the dynamic and controlled rate thermal analysis of hydromagnesite and nesquehonite. The dynamic thermal analysis of synthetic hydromagnesite proves that dehydration takes place in two steps at 135 and 184°C, dehydroxylation at 412°C and decarbonation at 474°C. Controlled rate thermal analysis shows the first dehydration step is isothermal and the second quasi-isothermal at 108 and 145°C, respectively. In the CRTA experiment both water and carbon dioxide are evolved in an isothermal decomposition at 376°C. CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of magnesium carbonates such as nesquehonite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal partial nesquehonite structure.     

Synthesis and thermal analysis of indium-based hydrotalcites of formula Mg<Subscript>6</Subscript>In<Subscript>2</Subscript>(CO<Subscript>3</Subscript>)(OH)<Subscript>16</Subscript>·4H<Subscript>2</Subscript>O

Insight into the unique structure of layered double hydroxides (LDHs) has been obtained using a combination of X-ray diffraction and thermal analysis. Indium containing hydrotalcites of formula Mg₄In₂(CO₃)(OH)₁₂·4H₂O (2:1 In-LDH) through to Mg₈In₂(CO₃)(OH)₁₈·4H₂O (4:1 In-LDH) with variation in the Mg:In ratio have been successfully synthesised. The d(003) spacing varied from 7.83 Å for the 2:1 LDH to 8.15 Å for the 3:1 indium containing LDH. Distinct mass loss steps attributed to dehydration, dehydroxylation and decarbonation are observed for the indium containing hydrotalcite. Dehydration occurs over the temperature range ambient to 205 °C. Dehydroxylation takes place in a series of steps over the 238–277 °C temperature range. Decarbonation occurs between 763 and 795 °C. The dehydroxylation and decarbonation steps depend upon the Mg:In ratio. The formation of indium containing hydrotalcites and their thermal activation provides a method for the synthesis of indium oxide-based catalysts.     

Study of the MS response by TG–MS in an acid mine drainage efflorescence

The aim of this study is to employ a thermogravimetric analyzer coupled to a mass spectrometer to research into the influence of heating rate and sample mass on the response of the detector. That response is examined by means of a particular efflorescence taken from an acid mine drainage environment. This mixture of weathered products is mainly composed by secondary sulfate minerals, which are formed in evaporation conditions, appearing as efflorescence salts. Thermogravimetry coupled to mass spectrometry has been used to analyze the three main loss steps that happen when this combination of minerals is heated from 30 to 1,100 °C. This inorganic material is based on a mixture of hexahydrite, zinc sulfate hexahydrate, apjonite, gypsum, plumbojarosite, calcite, quartz, and magnetite. While heating, three main effluent gases evolved from this efflorescence. At a standard heating rate of 10 °C/min, loss of water (dehydration) occurred over 30–500 °C in four major steps, loss of carbon dioxide (decarbonisation) occurred over 200–800 °C in three steps, and loss of sulfur trioxide (desulfation) occurred over 400–1,100 °C in three steps. According to the results, thermal analysis is an excellent technique for the study of decomposition in these systems.     

Thermal decomposition of hydrotalcites with variable cationic ratios

Thermal analysis complimented with evolved gas mass spectrometry has been applied to hydrotalcites containing carbonate prepared by coprecipitation and with varying divalent/trivalent cation ratios. The resulting materials were characterised by XRD, and TG/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg₄(Fe,Al)₂(OH)₁₂(CO₃)·4H₂O, Mg₆(Fe,Al)₂(OH)₁₆(CO₃)·5H₂O, and Mg₈(Fe,Al)₂(OH)₂₀(CO₃)·8H₂O were formed by intercalation with the carbonate anion as a function of the divalent/trivalent cationic ratio. XRD showed slight variations in the d-spacing between the hydrotalcites. The thermal decomposition of carbonate hydrotalcites consists of two decomposition steps between 300 and 400°C, attributed to the simultaneous dehydroxylation and decarbonation of the hydrotalcite lattice. Water loss ascribed to dehydroxylation occurs in two decomposition steps, where the first step is due to the partial dehydroxylation of the lattice, while the second step is due to the loss of water interacting with the interlayer anions. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal oxides and spinels, including MgO, MgAl₂O₄, and MgFeAlO₄.