Spectrochimica Acta Part A 62 (2005) 176-180 Raman spectroscopy of halotrichite from Jaroso, Spain

Abstract Raman spectroscopy complimented with infrared ATR spectroscopy has been used to characterise a halotrichite FeSO 4 ·Al 2 (SO 4 ) 3 ·22H 2 O from The Jaroso Ravine, Almeria, Spain. Halotrichites form a continuous solid solution series with pickingerite and chemical analysis shows that the jarosite contains 6% Mg 2+ . Halotrichite is characterised by four infrared bands at 3569.5, 3485.7, 3371.4 and 3239.0 cm −1 . Using Libowitsky type relationships, hydrogen bond distances of 3.08, 2.876, 2.780 and 2.718Å were determined. Two intense Raman bands are observed at 987.7 and 984.4 cm −1 and are assigned to the ν 1 symmetric stretching vibrations of the sulphate bonded to the Fe 2+ and the water units in the structure. Three sulphate bands are observed at 77 K at 1000.0, 991.3 and 985.0 cm −1 suggesting further differentiation of the sulphate units. Raman spectrum of the ν 2 and ν 4 region of halotrichite at 298 K shows two bands at 445.1 and 466.9 cm −1 , and 624.2 and 605.5 cm −1 , respectively, confirming the reduction of symmetry of the sulphate in halotrichite

Vibrational Spectroscopy of Selected Natural Uranyl Vanadates

Raman spectroscopy has been used to study a selection of uranyl vanadate minerals including carnotite, curienite, francevillite, tyuyamunite and metatyuyamunite. The minerals are characterised by an intense band in the 800 to 824 cm-1 region, assigned to the ν1 symmetric stretching vibrations of the (UO2)2+ units. A second intense band is observed in the 965 to 985 cm-1 range and is attributed to the ν1 (VO3) symmetric stretching vibrations in the (V2O8) units. This band is split with a second component observed at around 963 cm-1. A band of very low intensity is observed around 948 cm-1 and is assigned to the ν3 antisymmetric stretching vibrations of the (VO3) units. Bands in the range 608-655 cm-1 may be attributed to molecular water librational modes or the stretching modes ￮(V2O2) units. Bands in the range 573-583 cm-1 may be connected with the ￮ (U-Oequatorial) vibrations or ￮ (V2O2) units. Bands located in the range 467-539 cm-1 may be also attributed to the ￮ (U-Oequatorial) units vibrations. The bending modes of the (VO3) units are observed in the 463 to 480 cm-1 range – there may be some coincidence with ￮ (U-Oequatorial). The bending modes of the (V2O2) in the (V2O8) units are located in a series of bands around 407, 365 and 347 cm-1 (ν2). Two intense bands are observed in the 304 to 312 cm-1 range and 241 to 264 cm-1 range and are assigned to the doubly degenerate ν2 modes of the (UO2)2+ units. The study of the vibrational spectroscopy of uranyl vanadates is complicated by the overlap of bands from the (VO3) and (UO2)2+ units. Raman spectroscopy has proven most useful in assigning bands to these two units since Raman bands are sharp and well separated as compared with infrared bands. The uranyl vanadate minerals are often found as crystals on a host matrix and Raman spectroscopy enables their in-situ characterisation without sample preparation

Thermal decomposition of metatorbernite - A controlled rate thermal analysis study

The 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

Use of Infrared Spectroscopy for the Determination of Electronegativity of Rare Earth Elements

Infrared spectroscopy has been used to study a series of synthetic agardite minerals. Four OH stretching bands are observed at around 3568, 3482, 3362, and 3296 cm⁻¹. The first band is assigned to zeolitic, non-hydrogen-bonded water. The band at 3296 cm⁻¹ is assigned to strongly hydrogen-bonded water with an H bond distance of 2.72 Å. The water in agardites is better described as structured water and not as zeolitic water. Two bands at around 999 and 975 cm⁻¹ are assigned to OH deformation modes. Two sets of AsO symmetric stretching vibrations were found and assigned to the vibrational modes of AsO₄ and HAsO₄ units. Linear relationships between positions of infrared bands associated with bonding to the OH units and the electronegativity of the rare earth elements were derived, with correlation coefficients >0.92. These linear functions were then used to calculate the electronegativity of Eu, for which a value of 1.1808 on the Pauling scale was found

Thermal Decomposition of Agardites (REE) - Relationship Between Dehydroxylation Temperature and Electronegativity

The thermal decomposition of a suite of synthetic agardites of formula ACu6(AsO4)2(OH)6.3H2O where A is given by a rare earth element has been studied using thermogravimetric analysis techniques. Dehydration of the agardites occurs at low temperatures and over an extended temperature range from ambient to around 60 degrees Celsius. This loss of water is attributed to the loss of zeolitic water. The mass loss of water indicates 3 moles of zeolitic water in the structure. Dehydroxylation occurs in steps over a wide range of temperatures from 235 to 456 degrees Celsius. The mass loss during dehydroxylation shows the number of moles of hydroxyl units is six. There is a linear relationship between the first dehydroxylation temperature and the electronegativity of the REE

Studies of Natural and Synthetic Agardites

Agardite of formula [(Al,Nd,REE)Cu6(AsO4)3(OH)6.3H2O] has been discovered at Cobar, New South Wales, Australia. A series of synthetic agardites were analysed by X-ray diffraction and a correlation exists between the effective ionic radius of the REE3+ in the M site and the unit cell size for each respective agardite mineral. No value for the effective ionic radius of 9-coordinate Bi3+ has been reported but a value of approximately 115.5 pm is estimated from this correlation. The results of the TGA analyses show that the synthetic agardites are all fully hydrated, i.e., n = 3. Near infrared spectroscopy and mid-infrared spectroscopy has been used to characterise a group of synthetic agardites of formula ACu6(AsO4)2(OH)6.3H2O where A is a rare earth element. The hydroxyl stretching region is characterised by four bands observed at around 3568, 3489, 3382 and 3290 cm-1. The first two bands are attributed to the stretching mode of hydroxyl units and the last two bands to water stretching vibrations. The position of these bands indicates strongly hydrogen bonded water. The water in agardites is zeolitic type water. Near-IR spectroscopy shows a series of bands at 7242, 7007, 6809, 6770 and 6579 cm-1 attributed to the first overtones of the hydroxyl fundamentals. The NIR spectrum of agardite (Sm) is different and may be affected by electronic bands. Combination bands are observed at around 4404, 4343, 4340, 4294 and 4263 cm-1. Bands attributed to water combination modes are found at around 5200, 5173, 5082 and 4837 cm-1. Agardites are a group of minerals known for their REE content and have been rarely studied. NIR spectroscopy is an excellent technique for the characterisation and ready identification of these minerals

Raman and infrared spectroscopy of the manganese arsenate mineral allactite

The 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

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