Matt L. Weier

Molecular structure of the uranyl mineral zippeite - an XRD, SEM and Raman spectroscopic

Raman spectra at 298 and 77 K and infrared spectra of the uranyl sulfate mineral zippeite, K2[(UO2)6(SO4)3O(OH)6]. 4 H2O, were studied. Observed bands were tentatively attributed to the (UO2)2+ and (SO4)2- stretching and bending vibrations, the OH stretching vibrations of water molecules and hydroxyls, H2O bending vibrations and libration modes, and ￤ U-OH bending vibrations. Empirical relations were used for calculation of U-O bond lengths in uranyl R = f(￮3 or ￮1 (UO2)2+) A. This was found in agreement with U-O bond lengths from the single crystal structure analysis. The number of observed bands supports the conclusion from single crystal structure analysis that at least two symmetrically distinct U6+ (in uranyl) and S6+ (in sulfate), and water molecules and hydroxyls may be present in the zippeite crystal structure. Some O-H…O bond lengths were attributed to the hydrogen-bonding network in zippeite crystal structure.

Thermal decomposition of the natural hydrotalcites carrboydite and hydrohonessite

A combination of high resolution thermogravimetric analysis coupled to a gas evolution mass spectrometer combined with infrared emission spectroscopy has been used to study the thermal decomposition of two Australian hydrotalcites carrboydite (Ni6Al2(SO4,CO3)(OH)16.4H2O) and hydrohonessite (Ni6Fe2(SO4,CO3)(OH)16.7H2O). High resolution thermal analysis shows the decomposition takes place in 5 steps. Each step is related to the loss of water, hydroxyl units, carbonate and sulphate. Infrared emission spectroscopy clearly identifies the presence of these molecular species and the temperature at which they are lost. Minor amounts of carbonate are observed in the minerals. Water is in two states in the structure, namely weakly hydrogen bonded and strongly hydrogen bonded. The symmetry of the sulphate anion is reduced through this hydrogen bonding.

Infrared spectroscopic study of natural hydrotalcites carrboydite and hydrohonessite

Infrared spectroscopy has proven most useful for the study of anions in the interlayer of natural hydrotalcites. A suite of naturally occurring hydrotalcites including carrboydite, hydrohonessite, reevesite, motukoreaite and takovite were analysed. Variation in the hydroxyl stretching region was observed and the band profile is a continuum of states resulting from the OH stretching of the hydroxyl and water units. Infrared spectroscopy identifies some isomorphic substitution of sulphate for carbonate through an anion exchange mechanism for the minerals carrboydite and hydrohonessite. The infrared spectra of the CO3 and SO4 stretching region of takovite is complex because of band overlap. For this mineral some sulphate has replaced the carbonate in the structure. In the spectra of takovites, a band is observed at 1346 cm(-1) and is attributed to the carbonate anion hydrogen bonded to water in the interlayer. Infrared spectroscopy has proven most useful for the study of the interlayer structure of these natural hydrotalcites.

Raman spectroscopy of selected arsenates—implications for soil remediation

The contamination of soils with heavy metals such as As, Cr and Cu is of great importance; the remediation of such soils even more so. Arsenic compounds are prevalent in soils either through leaching of mine tailings, the use of Cu/Cr/As as a wood preservative or through the use of arsenic in cattle dips. The arsenic compounds in soils and leachates can be highly reactive and mobile, resulting in the formation of metal arsenate compounds. Of these compounds, one such set of minerals that can be formed is the vivianite arsenate minerals. Raman spectroscopy has been used to characterise the vivianite arsenates and to identify arsenic contaminants in a soil.

THERMAL DECOMPOSITION OF METATORBERNITE - A CONTROLLED RATE THERMAL ANALYSIS STUDY

SummaryThe mineral metatorbernite, Cu[(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)]<Subscript>2</Subscript>·8H<Subscript>2</Subscript>O, has been studied using a combination of energy dispersive X-ray analysis, X-ray diffraction, dynamic and controlled rate thermal analysis techniques. X-ray diffraction shows that the starting material in the thermal decomposition is metatorbernite and the product of the thermal treatment is copper uranyl phosphate. Three steps are observed for the dehydration of metatorbernite. These occur at 138°C with the loss of 1.5 moles of water, 155°C with the loss of 4.5 moles of water, 291°C with the loss of an additional 2 moles of water. These mass losses result in the formation of four phases namely meta(II)torbernite, meta(III)torbernite, meta(IV)torbernite and anhydrous hydrogen uranium copper pyrophosphate. The use of a combination of dynamic and controlled rate thermal analysis techniques enabled a definitive study of the thermal decomposition of metatorbernite. While the temperature ranges and the mass losses vary from author to author due to the different experimental conditions, the results of the CRTA analysis should be considered as standard data due to the quasi-equilibrium nature of the thermal decomposition process.

In situ observation of the thermal decomposition of weddelite by heating stage environmental scanning electron microscopy

Abstract The morphological and chemical changes occurring during the thermal decomposition of weddelite, CaC2O4·2H2O, have been followed in real time in a heating stage attached to an Environmental Scanning Electron Microscope operating at a pressure of 2 Torr, with a heating rate of 10 °C/min and an equilibration time of approximately 10 min. The dehydration step around 120 °C and the loss of CO around 425 °C do not involve changes in morphology, but changes in the composition were observed. The final reaction of CaCO3 to CaO while evolving CO2 around 600 °C involved the formation of chains of very small oxide particles pseudomorphic to the original oxalate crystals. The change in chemical composition could only be observed after cooling the sample to 350 °C because of the effects of thermal radiation.

A Raman spectroscopic study of thermally treated glushinskite—the natural magnesium oxalate dihydrate

Raman spectroscopy has been used to study the thermal transformations of natural magnesium oxalate dihydrate known in mineralogy as glushinskite. The data obtained by Raman spectroscopy was supplemented with that of infrared emission spectroscopy. The vibrational spectroscopic data was complimented with high resolution thermogravimetric analysis combined with evolved gas mass spectrometry. TG-MS identified two mass loss steps at 146 and 397 degrees C. In the first mass loss step water is evolved only, in the second step carbon dioxide is evolved. The combination of Raman microscopy and a thermal stage clearly identifies the changes in the molecular structure with thermal treatment. Glushinskite is the dihydrate phase in the temperature range up to the pre-dehydration temperature of 146 degrees C. Above 397 degrees C, magnesium oxide is formed. Infrared emission spectroscopy shows that this mineral decomposes at around 400 degrees C. Changes in the position and intensity of the CO and CC stretching vibrations in the Raman spectra indicate the temperature range at which these phase changes occur.

Ultraviolet-visible, near infrared and mid infrared reflectance spectroscopy of turquoise

Near infrared (NIR) spectroscopy of turquoise minerals from Arizona and Senegal with a formula of Cu(Al6–x,Fex)(PO4)4(OH)8.4H2O has been studied and a comparison of their NIR, ultraviolet-visible (UV-vis) and mid-IR spectra made. UV-vis and NIR reflectance spectroscopy is the most effective approach for the detection and determination of transition metal ions that cause colour to minerals and semi-precious gems such as turquoise. The UV-vis spectra of the two turquoise minerals (blue and green) reported in this research are distinctly different. The spectra confirm the difference in the origin of colour of the minerals. The paramagnetism of the blue mineral arises mainly from divalent copper. Turquoise from Senegal, West Africa has a replacement of some Al(III) by Fe(III) in the structure. NIR spectroscopy identifies the bands attributed to the hydroxyl units. The NIR spectroscopic technique is most sensitive in detecting Fe2+, even though it is present in very low concentrations in turquoise from Senegal. Mid-IR spectra of the two mineral samples are very similar in the 1200 to 900 cm−1 region but strong differences are observed in the 900 to 100 cm−1 region. The effect of substitution of Fe for Al in turquoise from Senegal shifts the bands to lower wavenumbers. Factor group analysis (FGA) implies four OH stretching vibrations for both the water and hydroxyl units. Two bands ascribed to water in these minerals at ∼ 3280 and 3070 cm−1 and three hydroxyl stretching vibrations are observed. The combination of these bands and their fundamental overtones give rise to many of the features in the NIR spectra.

Raman spectroscopic study of vivianites of different origins

Raman spectroscopy has been used to study a selection of vivianites from different origins. A band is identified at around 3480 cm-1 whose intensity is sample dependent. The band is attributed to the stretching vibration of Fe3+ OH units which are formed through the autooxidation of the vivianite minerals either by self-oxidation or by photocatalytic oxidation according to the reaction: (Fe2+)3(PO4)2·8H2O + 1/2O2 (Fe2+)3– x(Fe3+)x(PO4)2(OH)x·(8–x)H2O in which some of the water of crystallization is converted to hydroxyl anions. Complexity of the OH stretching region through the overlap of broad bands is reflected in the water HOH deformation modes at 1660 cm–1. Using the infrared bands at 3281, 3105 and 3025 cm–1, hydrogen bond distances of 2.734(5), 2.675(2) and 2.655(2) A are calculated. Vivianites are characterised by an intense band at 950 cm–1 assigned to the PO4 symmetric stretching vibration. Low Raman intensity bands are observed at ~1077, ~1050, 1015 and ~ 985 cm–1 assigned to the phosphate PO4 antisymmetric stretching vibrations. Multiple antisymmetric stretching vibrations are due to the reduced tetrahedral symmetry. This loss of degeneracy is also reflected in the bending modes. Two bands are observed at ~ 423 and ~ 456 cm–1 assigned to the2bending modes. For the vivianites four bands are observed at ~ 584, ~ 571, ~ 545 and ~ 525 cm–1 assigned to the 4modes of vivianite.

Identification of mixite minerals – an SEM and Raman spectroscopic analysis

Abstract Two mixites from Boss Tweed Mine, Tintic District, Juab County, Utah and Tin Stope, Majuba Hill, Pershing County, Nevada, USA, were analysed by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and by Raman spectroscopy. The SEM images show the mixite crystals to be elongated fibres up to 200 μm long and 2 μm wide. Detailed images of the mixite crystals show the mineral to be composed of bundles of fibres. The EDX analyses depend on the crystal studied, though the Majuba mixite gave analyses which matched the formula BiCu6(AsO4)3(OH)6·3H2O. Raman bands observed in the 880-910 cm-1 and 867-870 cm-1 regions are assigned to the AsO-stretching vibrations of (HAsO4)2- and (H2AsO4)- units, whilst bands at 803 and 833 cm-1 are assigned to the stretching vibrations of uncomplexed (AsO4)3- units. Intense bands observed at 473.7 and 475.4 cm-1 are assigned to the ν4 bending mode of AsO4 units. Bands observed at 386.5, 395.3 and 423.1 cm-1 are assigned to the ν2 bending modes of the HAsO4 (434 and 400 cm-1) and the AsO4 groups (324 cm-1). Raman spectroscopy lends itself to the identification of minerals on host matrices and is especially useful for the identification of mixites.