1,721,031 research outputs found
Raman spectroscopic study of vivianites of different origins
No full textRaman 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:\ud
\ud
(Fe2+)3(PO4)2·8H2O + 1/2O2 (Fe2+)3– x(Fe3+)x(PO4)2(OH)x·(8–x)H2O\ud
\ud
in which some of the water of crystallization is converted to hydroxyl anions.\ud
\ud
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) Å 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
Raman spectroscopy of natural oxalates at 298 and 77 K
No full textA comparative study of a suite of natural oxalates including weddellite, whewellite, moolooite, humboldtine, glushinskite, natroxalate and oxammite was undertaken using Raman spectroscopy at 298 and 77 K. Oxalates are found as films on host rocks as a result of heavy metal expulsion by primitive plants such as lichens and fungi. The minerals are characterized by the Raman position of the CO stretching vibration, which is cation sensitive. The band is observed at 1468 cm-1 for weddellite, 1464 cm-1 for whewellite, 1489 cm-1 for moolooite, 1489 cm-1 for humboldtine, 1471 cm-1 for glushinskite, 1456 cm-1 for natroxalate and 1473 cm-1 for oxammite. Except for oxammite, the infrared and Raman spectra are mutually exclusive, indicating that the minerals are bidentate and planar. Significant differences are observed in the CC stretching region and in the OCO bending region centred upon 910 and 860 cm-1, respectively. The significance of this work rests with the ability of Raman spectroscopy to identify oxalates which often occur as a film on a host rock. As such, Raman spectroscopy has the potential to identify the existence or pre-existence of life forms on planets such as Mars
Thermal treatment of whewellite-a thermal analysis and Raman spectroscopic study
No full textThermal transformations of natural calcium oxalate monohydrate known in mineralogy as whewellite have been undertaken using a combination of thermal analysis and Raman microscopy with the use of a thermal stage. High resolution thermogravimetry shows that three mass loss steps occur at 162, 479 and 684 degrees Celsius. \ud
Evolved gas mass spectrometry shows that water is evolved in the first step and carbon dioxide in the second and third mass loss steps. The changes in the molecular structure of whewellite can be followed by the use of the in-situ Raman spectroscopy of whewellite at the elevated temperatures. The whewellite is stable up to around 161 degrees Celsius, above which temperature the anhydrous calcium oxalate is formed. At 479 degrees Celsius, the oxalate transforms to calcium carbonate with loss of carbon dioxide. Above 684 degrees Celsius, calcium oxide is formed
Raman microscopy of autunite minerals at liquid nitrogen temperature
No full textUranyl micas are based upon (UO2 PO4)- units in layered structures with hydrated counter cations between the interlayers. Uranyl micas also known as the autunite minerals are of general formula M(UO2)2(XO4)2.8-12H2O where M may be Ba, Ca, Cu, Fe2+, Mg, Mn2+ or ½(HAl) and X is As, or P. The structures of these minerals have been studied using Raman microscopy at 298 and 77 K. Six hydroxyl stretching bands are observed of which three are highly polarised. The hydroxyl stretching vibrations are related to the strength of hydrogen bonding of the water OH units. Bands in the Raman spectrum of autunite at 998, 842 and 820 cm-1 are highly polarised. Low intensity band at 915 cm-1 is attributed to the ν3 antisymmetric stretching vibration of (UO2)2+ units. The band at 820 cm-1 is attributed to the ν1 symmetric stretching mode of the (UO2)2+ units. The (UO2)2+ bending modes are found at 295 and 222 cm-1. The presence of phosphate and arsenate anions and their isomorphic substitution are readily determined by Raman spectroscopy. The collection of Raman spectra at 77 K enables excellent band separation
Using Raman spectroscopy to identify mixite minerals
No full textRaman spectroscopy has been used to identify whether or not a selection of minerals labelled as mixites (formula BiCu6(AsO4)3(OH)6.3H2O) are correctly marked. Of the four samples, two samples are shown to be potentially mixites because of the presence of the characteristic Raman spectra of (AsO4)3- units and (HAsO4)- units, characterised by bands at around 803 and 833 cm-1. Two of the minerals are shown to be predominantly carbonates. Bands are observed at 3473.9 and 3470.3 \ud
cm-1 for the two mixite samples. Bands observed in the region 880 to 910 cm-1 and in the 867 to 870 cm-1 region are assigned to the AsO stretching vibrations of (HAsO4)2- and (H2AsO4)- units. Whilst bands at around 803 and 833 cm-1 are assigned to the stretching vibrations of uncomplexed (AsO4)3- units. Intense bands are observed at 473.7 and 475.4 cm-1 are assigned to the ν4 bending mode of AsO4 units. Bands observed at around 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
Raman microscopy of selected tungstate minerals
No full textA series of tungstate bearing minerals including scheelite, stolzite ferberite, hübnerite, wolframite, russellite, tungstenian wulfenite and cuprotungstite have been analysed by Raman microscopy. The results of the Raman spectroscopic analysis are compared with published data. These minerals are closely related and often have related paragenesis. Raman microscopy enables the selection of individual crystals of these minerals for spectroscopic analysis even though several of the minerals can be found in the same matrix because of the pargenetic relationships between the minerals. The Raman spectra are assigned according to factor group analysis and related to the structure of the minerals. These minerals have characteristically different Raman spectra. The ￮1(Ag) band is observed at 909 cm-1 and although the corresponding ￮1(Bu) vibration should be inactive a minor band is observed around 894 cm-1. The bands at 790 and 881 cm-1 are associated with the antisymmetric and symmetric Ag modes of terminal WO2. The band at 695 cm-1 is interpreted as an antisymmetric bridging mode associated with the tungstate chain. The ￮4(Eg) band was absent for scheelite. The bands at 353 and 401 cm-1 are assigned as either deformation modes or as r(Bg) and ￤(Ag) modes of terminal WO2. The band at \ud
462 cm-1 has an equivalent band in the infrared at 455 cm-1 assigned as ￤as(Au) of the (W2O4)n chain. The band at 508 cm-1 is assigned as ￮sym(Bg) of the (W2O4)n chain
The'cave' mineral oxammite: a high resolution thermogravimetry and Raman spectroscopic study
No full textThe thermal decomposition of natural ammonium oxalate known as oxammite has been studied using a combination of high resolution thermogravimetry coupled to an evolved gas mass spectrometer and Raman spectroscopy coupled to a thermal stage. Three mass loss steps were found at 57, 175 and 188°C attributed to dehydration, ammonia evolution and carbon dioxide evolution respectively. Raman spectroscopy shows two bands at 3235 and 3030 cm-1 attributed to the OH stretching vibrations and three bands at 2995, 2900 and 2879 cm-1, attributed to the NH vibrational modes. The thermal degradation of oxammite may be followed by the loss of intensity of these bands. No intensity remains in the OH stretching bands at 100°C and the NH stretching bands show no intensity at 200°C. Multiple CO symmetric stretching bands are observed at 1473, 1454, 1447 and 1431cm-1, suggesting that the mineral oxammite is composed of a mixture of chemicals including ammonium oxalate dihydrate, ammonium oxalate monohydrate and anhydrous ammonium oxalate
Raman and infrared spectroscopy of the manganese arsenate mineral allactite
No full textThe mineral allactite [Mn7(AsO4)2(OH)8]is a basic manganese arsenate which is highly pleochroic. The use of the 633 nm excitation line enables quality spectra of to be obtained irrespective of the crystal orientation. The mineral is characterised by a set of sharp bands in the 770 to 885 cm-1 region. Intense and sharp Raman bands are observed at 883, 858, 834, 827, 808 and 779 cm-1. Collecting the spectral data at 77 K enabled better band separation with narrower bandwidths. The observation of multiple AsO4 stretching bands indicates the non equivalence of the arsenate anions in the allactite structure. In comparison the infrared spectrum shows a broad spectral profile with a series of difficult to define overlapping bands. The low wavenumber region sets of bands which are assigned to the ν2 modes (361 and 359 cm-1), the ν4 modes (471, 452 and 422 cm-1), AsO stretching vibrations at 331 and 324 cm-1, and bands at 289 and 271 cm-1 which may be ascribed to MnO stretching modes. The observation of multiple bands shows the loss of symmetry of the AsO4 units and the non equivalence of these units in the allactite structure. The study shows that highly pleochroic minerals can be studied by Raman spectroscopy
Thermal decomposition of metatorbernite - A controlled rate thermal analysis study
No full textThe mineral metatorbernite, Cu[(UO2)2(PO4)]2•8H2O, 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 degrees Celsius with the loss of 1.5 moles of water, 155 degrees Celsius with the loss of 4.5 moles of water, 291 degrees Celsius 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
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