Synthesis and Raman spectroscopic characterisation of hydrotalcites based on the formula Ca6Al2(CO3)(OH)16· 4H2O

We have successfully synthesised hydrotalcites (HTs) containing calcium, which are naturally occurring minerals. Insight into the unique structure of HTs has been obtained using a combination of X‐ray diffraction (XRD) as well as infrared and Raman spectroscopies. Calcium‐containing hydrotalcites (Ca‐HTs) of the formula Ca₄Al₂(CO₃)(OH)₁₂·4H₂O (2:1 Ca‐HT) to Ca₈Al₂(CO₃)(OH)₂₀· 4H₂O (4:1 Ca‐HT) have been successfully synthesised and characterised by XRD and Raman spectroscopy. XRD has shown that 3:1 calcium HTs have the largest interlayer distance. Raman spectroscopy complemented with selected infrared data has been used to characterise the synthesised Ca‐HTs. The Raman bands observed at around 1086 and 1077 cm⁻¹ were attributed to the ν₁ symmetric stretching modes of the (CO₃²⁻) units of calcite and carbonate intercalated into the HT interlayer. The corresponding ν₃ CO₃²⁻ antisymmetric stretching modes are found at around 1410 and 1475 cm⁻¹. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and Raman spectroscopy of indium‐based hydrotalcites of formula Mg6In2(CO3)(OH)16· 4H2O

Insight into the unique structure of layered double hydroxides has been obtained using a combination of X‐ray diffraction and Raman spectroscopy. Indium‐containing hydrotalcites of formula Mg₄In₂(CO₃)(OH)₁₂· 4H₂O [2:1 In‐LDH (layered double hydroxides)] through to Mg₈In₂(CO₃)(OH)₁₈· 4H₂O (4:1 In‐LDH) with variation in the Mg : In ratio have been successfully synthesized. The d(003) spacing varied from 7.83 Å for the 2:1 LDH to 8.15 Å for the 3:1 indium‐containing layered double hydroxide. Raman spectroscopy complemented with selected infrared data has been used to characterize the synthesized indium‐containing layered double hydroxides of formula Mg₆In₂(CO₃)(OH)₁₆· 4H₂O. Raman bands observed at around 1058, 1075 and 1115 cm⁻¹ are attributed to the symmetric stretching modes of the CO₃²⁻ units. Multiple ν₃ CO₃²⁻ antisymmetric stretching modes are found at around 1348, 1373, 1429 and 1488 cm⁻¹ in the infrared spectra. The splitting of this mode indicates that the carbonate anion is in a perturbed state. Raman bands observed at 690 and 700 cm⁻¹ assigned to the ν₄ CO₃²⁻ modes support the concept of multiple carbonate species in the interlayer. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopy of gallium‐ and zinc‐based hydrotalcites

Insight into the unique structure of hydrotalcites (HTs) has been obtained using Raman spectroscopy. Gallium‐containing HTs of formula Zn₄ Ga₂(CO₃)(OH)₁₂ · xH₂O (2:1 ZnGa‐HT), Zn₆ Ga₂(CO₃)(OH)₁₆ · xH₂O (3:1 ZnGa‐HT) and Zn₈ Ga₂(CO₃)(OH)₁₈ · xH₂O (4:1 ZnGa‐HT) have been successfully synthesised and characterised by X‐ray diffraction (XRD) and Raman spectroscopy. The d(003) spacing varies from 7.62 Å for the 2:1 ZnGa‐HT to 7.64 Å for the 3:1 ZnGa‐HT. The 4:1 ZnGa‐HT showed a decrease in the d(003) spacing, compared to the 2:1 and 3:1 compounds. Raman spectroscopy complemented with selected infrared data has been used to characterise the synthesised gallium‐containing HTs. Raman bands observed at around 1050, 1060 and 1067 cm⁻¹ are attributed to the symmetric stretching modes of the (CO₃²⁻) units. Multiple ν₃ (CO₃²⁻) antisymmetric stretching modes are found between 1350 and 1520 cm⁻¹, confirming multiple carbonate species in the HT structure. The splitting of this mode indicates that the carbonate anion is in a perturbed state. Raman bands observed at 710 and 717 cm⁻¹ and assigned to the ν₄ (CO₃²⁻) modes support the concept of multiple carbonate species in the interlayer. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman microscopy of haidingerite Ca(AsO3OH)·H2O and brassite Mg(AsO3OH)·4H2O

Raman spectroscopy has been used to study the arsenate minerals haidingerite Ca(AsO₃OH)·H₂O and brassite Mg(AsO₃OH)·4H₂O. Intense Raman bands in the haidingerite spectrum observed at 745 and 855 cm⁻¹ are assigned to the (AsO₃OH)²⁻ν₃ antisymmetric stretching and ν₁ symmetric stretching vibrational modes. For brassite, two similarly assigned intense bands are found at 809 and 862 cm⁻¹. The observation of multiple Raman bands in the (AsO₃OH)²⁻ stretching and bending regions suggests that the arsenate tetrahedrons in the crystal structures of both minerals studied are strongly distorted. Broad Raman bands observed at 2842 cm⁻¹ for haidingerite and 3035 cm⁻¹ for brassite indicate strong hydrogen bonding of water molecules in the structure of these minerals. OH···O hydrogen‐bond lengths were calculated from the Raman spectra based on empirical relations. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermo‐Raman spectroscopy of selected layered double hydroxides of formula Cu6Al2(OH)16CO3 and Zn6Al2(OH)16CO3

Raman spectroscopy using a hot stage was used to characterise layered double hydroxides (LDHs) of the formula (Cu,Zn)₆Al₂(OH)₁₆(CO₃)^·4H₂O. The spectra were used to assess the molecular assembly of the cations in the LDH structure. The sharp band at 1058 cm⁻¹for the Zn₆Al₂(OH)₁₆(CO₃)^·4H₂O is assigned to the ν₁CO₃²⁻ symmetric stretching mode. This band shifts to higher wavenumbers and is observed at 1103 cm⁻¹at 600 °C. It is proposed that metal carbonate species formed during the decomposition of the hydrotalcite structure is responsible for the increase in the band position. The Cu–Al hydrotalcite did not show the same trend. The symmetric stretching mode of carbonate is observed at around 1110 cm⁻¹, and at temperatures above 200 °C a shoulder appears at around 1210 cm⁻¹, suggested to be due to CuCO₃. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman spectroscopic study of synthetic reevesite and cobalt substituted reevesite (Ni,Co)6Fe2(OH)16(CO3)·4H2O

Raman spectroscopy, complemented with infrared spectroscopy of compounds equivalent to reevesite, formula (Ni,Co)₆Fe₂(OH)₁₆(CO₃)·4H₂O, with the ratio of Ni/Co ranging from 0 to 1, have been synthesised and characterised based on the molecular structure of the synthesised mineral. The combination of Raman spectroscopy with infrared spectroscopy enables an assessment of bands attributable to water stretching and brucite‐like surface hydroxyl units to be obtained. Raman spectroscopy shows a reduction in the symmetry of the carbonate anion, leading to the conclusion that the carbonate anion is bonded to the brucite‐like hydroxyl surface and to the water in the interlayer. Variation in the position of the carbonate anion stretching vibrations occurs and is dependent on the Ni/Co ratio. Water bending modes are identified in both the Raman and infrared spectra at positions greater than 1620 cm⁻¹, indicating that water is strongly hydrogen bonded to both the interlayer anions and the hydrotalcite surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopy of hydrogen‐arsenate group (AsO3OH) in solid‐state compounds: cobalt mineral phase burgessite Co2(H2O)4[AsO3OH]2·H2O

Raman spectrum of burgessite, Co₂(H₂O)₄[AsO₃OH]₂· H₂O, was studied, interpreted and compared with its infrared spectrum. The stretching and bending vibrations of (AsO₃) and As‐OH units, as well as the stretching, bending and libration modes of water molecules and hydroxyl ions were assigned. The range of O H···O hydrogen bond lengths was inferred from the Raman and infrared spectra of burgessite. The presence of (AsO₃OH)²⁻ units in the crystal structure of burgessite was proved, which is in agreement with its recently solved crystal structure. Raman and infrared spectra of erythrite inferred from the RRUFF database are used for comparison. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the magnesium carbonate mineral hydromagnesite (Mg5[(CO3)4(OH)2]· 4H2O)

Magnesium minerals are important for understanding the concept of geosequestration. One method of studying the hydrated hydroxy magnesium carbonate minerals is through vibrational spectroscopy. A combination of Raman and infrared spectroscopy has been used to study the mineral hydromagnesite. An intense band is observed at 1121 cm⁻¹, attributed to the CO₃²⁻ν₁ symmetric stretching mode. A series of infrared bands at 1387, 1413 and 1474 cm⁻¹ are assigned to the CO₃²⁻ν₃ antisymmetric stretching modes. The CO₃²⁻ν₃ antisymmetric stretching vibrations are extremely weak in the Raman spectrum and are observed at 1404, 1451, 1490 and 1520 cm⁻¹. A series of Raman bands at 708, 716, 728 and 758 cm⁻¹ are assigned to the CO₃²⁻ν₂ in‐plane bending mode. The Raman spectrum in the OH stretching region is characterized by bands at 3416, 3516 and 3447 cm⁻¹. In the infrared spectrum, a broad band is found at 2940 cm⁻¹, which is assigned to water stretching vibrations. Infrared bands at 3430, 3446, 3511, 2648 and 3685 cm⁻¹ are attributed to MgOH stretching modes. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the antimonate mineral brandholzite Mg[Sb2(OH)12]·6H2O

Raman spectra of brandholzite Mg[Sb₂(OH)₁₂]·6H₂O were studied, complemented with infrared spectra, and related to the structure of the mineral. An intense Raman sharp band at 618 cm⁻¹ is attributed to the SbO symmetric stretching mode. The low‐intensity band at 730 cm⁻¹ is ascribed to the SbO antisymmetric stretching vibration. Low‐intensity Raman bands were found at 503, 526 and 578 cm⁻¹. Corresponding infrared bands were observed at 527, 600, 637, 693, 741 and 788 cm⁻¹. Four Raman bands observed at 1043, 1092, 1160 and 1189 cm⁻¹ and eight infrared bands at 963, 1027, 1055, 1075, 1108, 1128, 1156 and 1196 cm⁻¹ are assigned to δ SbOH deformation modes. A complex pattern resulting from the overlapping band of the water and hydroxyl units is observed. Raman bands are observed at 3240, 3383, 3466, 3483 and 3552 cm⁻¹; infrared bands at 3248, 3434 and 3565 cm⁻¹. The Raman bands at 3240 and 3383 cm⁻¹ and the infrared band at 3248 cm⁻¹ are assigned to water‐stretching vibrations. The two higher wavenumber Raman bands observed at 3466 and 3552 cm⁻¹ and two infrared bands at 3434 and 3565 cm⁻¹ are assigned to the stretching vibrations of the hydroxyl units. Observed Raman and infrared bands in the OH stretching region are associated with O‐H···O hydrogen bonds and their lengths 2.72, 2.79, 2.86, 2.88 and 3.0 Å (Raman) and 2.73, 2.83 and 3.07 Å (infrared). Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the antimonate mineral bottinoite Ni[Sb2(OH)12]·6H2O and in comparison with brandholzite Mg[Sb5+2(OH)12]·6H2O

Raman spectroscopy was used to study the mineral bottinoite and a comparison with the Raman spectra of brandholzite was made. An intense sharp Raman band at 618 cm⁻¹ is attributed to the SbO symmetric stretching mode. The low intensity band at 735 cm⁻¹ is ascribed to the SbO antisymmetric stretching vibration. Low intensity Raman bands were found at 501, 516 and 578 cm⁻¹. Four Raman bands observed at 1045, 1080, 1111 and 1163 cm⁻¹ are assigned to δ SbOH deformation modes. A complex pattern resulting from the overlapping band of the water and hydroxyl units is observed. Raman bands are observed at 3223, 3228, 3368, 3291, 3458 and 3510 cm⁻¹. The first two Raman bands are assigned to water stretching vibrations. The two higher wavenumber Raman bands observed at 3466 and 3552 cm⁻¹ and two infrared bands at 3434 and 3565 cm⁻¹ are assigned to the stretching vibrations of the hydroxyl units. Observed Raman and infrared bands are connected with O H···O hydrogen bonds and their lengths 2.72, 2.79, 2.86, 2.88 and 3.0 Å (Raman) and 2.73, 2.83 and 3.07 Å (infrared). Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopy of dawsonite NaAl(CO3)(OH)2

Raman spectroscopy both at 298 and 77 K complemented with infrared spectroscopy was used to study the structure of dawsonite. Previous crystallographic studies concluded that the structure of dawsonite was a simple one; however, both Raman and infrared spectroscopy show that this conclusion is incorrect. Multiple bands are observed in both the Raman and infrared spectra in the antisymmetric stretching and bending regions, showing that the symmetry of the carbonate anion is reduced and in all probability the carbonate anions are not equivalent in the dawsonite structure. Multiple OH deformation vibrations centred around 950 cm⁻¹ in both Raman and infrared spectra show that the OH units in the dawsonite structure are non‐equivalent. Calculations using the position of the Raman and infrared OH stretching vibrations enabled estimates of the hydrogen‐bond distances of 0.2735 and 0.27219 pm at 298 K, and 0.27315 and 0.2713 pm at 77 K to be made. This indicates strong hydrogen bonding of the OH units in the dawsonite structure. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the mixed anion mineral yecoraite Bi5Fe3O9(Te4+O3)(Te6+O4)2·9H2O

Tellurates are rare minerals as the tellurate anion is readily reduced to the tellurite ion. Often minerals with both tellurate and tellurite anions are found. An example of such a mineral containing tellurate and tellurite is yecoraite. Raman spectroscopy has been used to study this mineral, the exact structure of which is unknown. Two Raman bands at 796 and 808 cm⁻¹ are assigned to the ν₁(TeO₄)²⁻ symmetric and ν₃(TeO₃)²⁻ antisymmetric stretching modes and Raman bands at 699 cm⁻¹ are attributed to the ν₃(TeO₄)²⁻ antisymmetric stretching mode and the band at 690 cm⁻¹ to the ν₁(TeO₃)²⁻ symmetric stretching mode. The intense band at 465 cm⁻¹ with a shoulder at 470 cm⁻¹ is assigned the (TeO₄)²⁻ and (TeO₃)²⁻ bending modes. Prominent Raman bands are observed at 2878, 2936, 3180 and 3400 cm⁻¹. The band at 3936 cm⁻¹ appears quite distinct and the observation of multiple bands indicates the water molecules in the yecoraite structure are not equivalent. The values for the OH stretching vibrations listed provide hydrogen bond distances of 2.625 Å (2878 cm⁻¹), 2.636 Å (2936 cm⁻¹), 2.697 Å (3180 cm⁻¹) and 2.798 Å (3400 cm⁻¹). This range of hydrogen bonding contributes to the stability of the mineral. A comparison of the Raman spectra of yecoraite with that of tellurate containing minerals kuranakhite, tlapallite and xocomecatlite is made. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the hydroxy‐arsenate‐sulfate mineral chalcophyllite Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O

The mixed anion mineral chalcophyllite Cu₁₈Al₂(AsO₄)₄(SO₄)₃(OH)₂₄·36H₂O has been studied by using Raman and infrared spectroscopies. Characteristic bands associated with arsenate, sulfate and hydroxyl units are identified. Broad bands in the OH stretching region are observed and are resolved into component bands. Estimates of hydrogen bond distances were made using a Libowitzky function. Both short and long hydrogen bonds were identified. Two intense bands at 841 and ∼814 cm⁻¹ are assigned to the ν₁ (AsO₄)³⁻ symmetric stretching and ν₃ (AsO₄)³⁻ antisymmetric stretching modes. The comparatively sharp band at 980 cm⁻¹ is assigned to the ν₁ (SO₄)²⁻ symmetric stretching mode, and a broad spectral profile centred upon 1100 cm⁻¹ is attributed to the ν₃ (SO₄)²⁻ antisymmetric stretching mode. A comparison of the Raman spectra is made with other arsenate‐bearing minerals such as carminite, clinotyrolite, kankite, tilasite and pharmacosiderite. Copyright © 2010 John Wiley & Sons, Ltd.     

Single‐crystal Raman spectroscopy of brandholzite Mg[Sb(OH)6]2•6H2O and bottinoite Ni[Sb(OH)6]2• 6H2O and the polycrystalline Raman spectrum of mopungite Na[Sb(OH)6]

The single‐crystal Raman spectra of minerals brandholzite and bottinoite, formula M[Sb(OH)₆]₂•6H₂O, where M is Mg⁺² and Ni⁺², respectively, and the non‐aligned Raman spectrum of mopungite, formula Na[Sb(OH)₆], are presented for the first time. The mixed metal minerals comprise alternating layers of [Sb(OH)₆]⁻¹ octahedra and mixed [M(H₂O)₆]⁺²/[Sb(OH)₆]⁻¹ octahedra. Mopungite comprises hydrogen‐bonded layers of [Sb(OH)₆]⁻¹ octahedra linked within the layer by Na⁺ ions. The spectra of the three minerals were dominated by the Sb O symmetric stretch of the [Sb(OH)₆]⁻¹ octahedron, which occurs at approximately 620 cm⁻¹. The Raman spectrum of mopungite showed many similarities to spectra of the di‐octahedral minerals, supporting the view that the Sb octahedra give rise to most of the Raman bands observed, particularly below 1200 cm⁻¹. Assignments have been proposed on the basis of the spectral comparison between the minerals, prior literature and density functional theory (DFT) calculations of the vibrational spectra of the free [Sb(OH)₆]⁻¹ and [M(H₂O)₆]⁺² octahedra by a model chemistry of B3LYP/6‐31G(d) and lanl2dz for the Sb atom. The single‐crystal spectra showed good mode separation, allowing most of the bands to be assigned to the symmetry species A or E. Copyright © 2010 John Wiley & Sons, Ltd.     

A Raman spectroscopic study of copiapites Fe2+Fe3+(SO4)6(OH)2 · 20H2O: environmental implications

Raman spectroscopy has been used to study selected mineral samples of the copiapite group. Copiapite (Fe²⁺Fe³⁺(SO₄)₆(OH)₂ · 20H₂O) is a secondary mineral formed through the oxidation of pyrite. Minerals of the copiapite group have the general formula AFe₄(SO₄)₆(OH)₂ · 20H₂O, where A has a + 2 charge and can be either magnesium, iron, copper, calcium and/or zinc. The formula can also be B_2/3Fe₄(SO₄)₆(OH)₂ · 20H₂O, where B has a + 3 charge and may be either aluminium or iron. For each mineral, two Raman bands are observed at around 992 and 1029 cm⁻¹, assigned to the (SO₄)²⁻ν₁ symmetric stretching mode. The observation of two bands provides evidence for the existence of two non‐equivalent sulfate anions in the mineral structure. Three Raman bands at 1112, 1142 and 1161 cm⁻¹ are observed in the Raman spectrum of copiapites, indicating a reduction of symmetry of the sulfate anion in the copiapite structure. This reduction in symmetry is supported by multiple bands in the ν₂ and ν₄(SO₄)²⁻ spectral regions. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopy of hydrogen arsenate group (AsO3OH)2− in solid‐state compounds: cobalt‐containing zinc arsenate mineral,koritnigite (Zn,Co)(AsO3OH)·H2O

Raman spectra of two well‐defined types of koritnigite crystals from the Jáchymov ore district, Czech Republic, were recorded and interpreted. No substantial differences were observed between both crystal types. The observed Raman bands were attributed to the (AsO₃OH)²⁻ stretching and bending vibrations as well as stretching and bending vibrations of water molecules and hydroxyl ions. The non‐interpreted Raman spectra of koritnigite from the RRUFF database and the published infrared spectra of cobaltkoritnigite were used for comparison. The O H···O hydrogen bond lengths in the crystal structure of koritnigite were inferred from the Raman spectra and compared with those derived from the X‐ray single‐crystal refinement. The presence of (AsO₃OH)²⁻ units in the crystal structure of koritnigite was proved from the Raman spectra, which supports the conclusions of the X‐ray structure analysis. Copyright © 2010 John Wiley & Sons, Ltd.     

A Raman spectroscopic study of the ‘cave’ mineral ardealite Ca2(HPO4)(SO4)·4H2O

The mineral ardealite Ca₂(HPO₄)(SO₄)·4H₂O is a ‘cave’ mineral and is formed through the reaction of calcite with bat guano. The mineral shows disorder and the composition varies depending on the origin of the mineral. Raman spectroscopy complimented with infrared spectroscopy has been used to characterise the mineral ardealite. The Raman spectrum is very different from that of gypsum. Bands are assigned to SO₄²⁻ and HPO₄²⁻ stretching and bending modes. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman spectroscopic study of the minerals diadochite and destinezite Fe3+2(PO4,SO4)2(OH)· 6H2O: implications for soil science

The two minerals diadochite and destinezite of formula Fe₂(PO₄,SO₄)₂(OH)· 6H₂O have been characterised by Raman spectroscopy and complemented with infrared spectroscopy. Both these minerals are found in soils and are identical except for their morphology. Diadochite is amorphous whereas destinezite is highly crystalline. The spectra of diadochite are broad and ill defined, whereas the spectra of destinezite are intense and well defined. Bands are assigned to phosphate and sulfate stretching and bending modes. Two symmetric stretching modes for both phosphate and sulfate support the concept of non‐equivalent phosphate and sulfate units in the mineral structure. Multiple water bending and stretching modes imply that non‐equivalent water molecules in the structure exist with different hydrogen‐bond strengths. Copyright © 2011 John Wiley & Sons, Ltd.     

A Raman spectroscopic study of bukovskýite Fe2(AsO4)(SO4)(OH)· 7H2O,a mineral phase with a significant role in arsenic migration

The Raman spectrum of bukovskýite [Fe³⁺₂(OH)(SO₄)(AsO₄)· 7H₂O] has been studied and compared with that of an amorphous gel containing specifically Fe, As and S, which is understood to be an intermediate product in the formation of bukovskýite. The observed bands are assigned to the stretching and bending vibrations of (SO₄)²⁻ and (AsO₄)³⁻ units, stretching and bending vibrations and vibrational modes of hydrogen‐bonded water molecules, stretching and bending vibrations of hydrogen‐bonded (OH)⁻ ions and Fe³⁺ (O,OH) units. The approximate range of O H···O hydrogen bond lengths was inferred from the Raman spectra. Raman spectra of crystalline bukovskýite and of the amorphous gel differ in that the bukovskýite spectrum is more complex, the observed bands are sharp and the degenerate bands of (SO₄)²⁻ and (AsO₄)³⁻ are split and more intense. Lower wavenumbers of δ H₂O bending vibrations in the spectrum of the amorphous gel may indicate the presence of weaker hydrogen bonds compared to those in bukovskýite. Copyright © 2011 John Wiley & Sons, Ltd.