Raman Spectroscopic Study of the Vivianite Arsenate Minerals

The molecular structures of the vivianite-type arsenate minerals were studied using a combination of Raman and infrared spectroscopy. The Raman spectra of the hydroxyl-stretching regions are complex with overlapping bands at 3419, 3209, 3185 and 3010 cm-1. This complexity is reflected in the water HOH bending modes with strong infrared bands in the 1660-1685 cm-1 region indicating strong hydrogen bonding to arsenate anions in adjacent layers. The Raman arsenate AsO stretching region shows strong similarity between the vivianite arsenate minerals. In the infrared spectra complexity exists with multiple antisymmetric stretching vibrations observed, indicating a reduction of symmetry. Strong infrared bands around 700 and 560 cm-1 are attributed to librational modes of water. Vibrational spectra enable the structure of the minerals to be determined and, whilst similarities exist in the spectral patterns, sufficient differences exist to determine the identification of the minerals. In particular, Raman spectroscopy assists in the identification of the complex isomorphous substitution in these vivianite arsenate minerals

Peisleyite an Unusual Mixed Anion Mineral - A Vibrational Spectroscopic Study

The mineral peisleyite has been studied using a combination of electron microscopy and vibrational spectroscopy. SEM photomicrographs reveal that the peisleyite morphology consists of an array of small needle like crystals of around 1 μm in length with a thickness of less than 0.1 μm. Raman spectroscopy in the hydroxyl stretching region shows an intense band at 3506 cm-1 assigned to the symmetric stretching mode of the OH units. Four bands are observed at 3564, 3404, 3250 and 3135 cm-1 in the infrared spectrum. These wavenumbers enable an estimation of the hydrogen bond distances 3.052(5), 2.801(0), 2.705(6) and 2.683(6) Å. Two intense Raman bands are observed at 1023 and 989 cm-1 and are assigned to the SO4 and PO4 symmetric stretching modes. Other bands are observed at 1356, 1252, 1235, 1152, 1128, 1098 and 1067 cm-1. The bands at 1067 is attributed to AlOH deformation vibrations. Bands in the low wavenumber region are assigned to the ν4 and ν2 out of plane bending modes of the SO4 and PO4 units. Raman spectroscopy is a useful tool in determining the vibrational spectroscopy of mixed hydrated multianion minerals such as peisleyite. Information on such a mineral would be difficult to obtain by other means

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 in the mineral 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 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

Raman Spectroscopic Study of the Basic Copper Sulphates - Implications for Copper Corrosion and Bronze Disease

The basic copper sulphates are of interest because of their appearance in many environmental situations such as copper pipe corrosion, restoration of brass and bronze objects, leaching from waste mineral dumps and the restoration of frescoes. The Raman spectra of the basic copper sulphate minerals antlerite, brochantite, posnjakite and langite are reported using a Nd-Yag laser operating at an excitation wavelength of 780 nm. In line with their crystal structures, each basic copper sulphate mineral has its own characteristic Raman spectrum, which enables their identification. Except for brochantite multiple bands are observed for the SO stretching vibration. Similarly multiple bands are observed for the antisymmetric SO and OSO bending regions. Hydroxyl deformation modes in the 730 to 790 cm-1 region are observed. The use of the HeNe laser operating at an excitation wavelength of 633 nm enabled the hydroxyl stretching bands of the minerals to be obtained. Antlerite and brochantite are characterised by hydroxyl stretching bands at 3580 and 3488 cm-1. The minerals posnjakite and langite display Raman hydroxyl-stretching vibrations at 3588 and 3564 cm-1. The Raman spectra of these two minerals show water OH-stretching bands at 3405, 3372 and 3260 cm-1. Raman spectroscopy allows the ready identification of these minerals

Raman Spectroscopy of Gerhardtite at 2968 and 77 K

Raman spectroscopy has been used to study the nature of gerhardtite, a naturally occurring basic copper(II) nitrate of formula Cu2NO3(OH)3. Raman spectra at 77 K show four bands at 3557, 3548, 3467 and 3409 cm-1 in accordance with the crystal structure with three non-equivalent hydroxyl groups. Estimations of hydrogen bond distances of 0.3019, 0.2987, 0.2855 and 0.2804 pm are obtained. Cooling to liquid nitrogen temperatures did not affect these values. Two bands are observed at 1048 and 1052 cm-1 in the NO3 stretching region. Cooling to liquid nitrogen temperature altered the ratio of these nitrates from 3/1 to 1/3. Two sets of bands in the antisymmetric stretching region of the nitrate in the 77 K spectra suggest that there are two non-equivalent nitrate anions in the gerhardtite structure at liquid nitrogen temperature. X-ray crystallography suggests three independent oxygens in the one nitrate group. The shift in band positions on obtaining Raman spectra at 298 and 77 K indicates a phase change is occurring for the mineral upon cooling to 77 K

Raman Spectroscopy of Dawsonite NaAI(CO3)(OH)2

Raman spectroscopy at both 298 and 77 K complimented with infrared spectroscopy has been 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 probablity the carbonate anions are not equivalent in the dawsonite structure. Multiple OH deformation vibrations centred upon 950 cm-1 in both the 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, 0.27219 pm at 298 K and 0.27315, 0.2713 pm at 77 K to be made. This indicates strong hydrogen bonding of the OH units in the dawsonite structure

Raman Spectroscopy of Likasite at 298 and 77 K

Raman spectroscopy at 298 and 77 K has been used to study the structure of likasite, a naturally occurring basic copper(II)nitrate of formula Cu3NO3(OH)5.2H2O. An intense sharp band is observed at 3522 cm-1 at 298 K which splits into two bands at 3522 and 3505 cm-1 at 77 K and is assigned to the OH stretching mode. The two OH stretching bands at 3522 and 3505 provide estimates of the hydrogen bond distances of these units as 2.9315 and 2.9028 Å. The significance of this result is that equivalent OH units in the 298 K spectrum become two non-equivalent OH units at 77 K suggesting a structural change by cooling to liquid nitrogen temperature. A number of broad bands are observed in the 298 K spectrum at 3452, 3338, 3281 and 3040 cm-1 assigned to H2O stretching vibrations with estimates of the hydrogen bond distances of 2.8231, 2.7639, 2.7358 and 2.6436 Å. Three sharp bands are observed at 77 K at 1052, 1050 and 1048 cm-1 attributed to the ν1 symmetric stretching mode of the NO3 units. Only a single band at 1050 cm-1 is observed at 298 K, suggesting the non-equivalence of the NO3 units at 77 K, confirming structural changes in likasite by cooling to 77 K

Raman spectroscopy of newberyite Mg(PO3OH).3H2O: A cave mineral

Newberyite Mg(PO3OH)•3H2O is a mineral found in caves such as from Moorba cave, Jurien Bay, Western Australia, the Skipton Lava tubes (SW of Ballarat, Victoria, Australia) and in the Petrogale Cave (Madura , Eucla, Western Australia). Because these minerals contain oxyanions, hydroxyl units and water, the minerals lend themselves to spectroscopic analysis. Raman spectroscopy can investigate the complex paragenetic relationships existing between a number of ‘cave’ minerals. The intense sharp band at 982 cm-1 is assigned to the PO43- ν1 symmetric stretching mode. Low intensity Raman bands at 1152, 1263 and 1277 cm-1 are assigned to the PO43- ν3 antisymmetric stretching vibrations. Raman bands at 497 and 552 cm-1 are attributed to the PO43- ν4 bending modes. An intense Raman band for newberyite at 398 cm-1 with a shoulder band at 413 cm-1 is assigned to the PO43- ν2 bending modes. The values for the OH stretching vibrations provide hydrogen bond distances of 2.728Å (3267 cm-1), 2.781Å (3374cm-1), 2.868Å (3479 cm-1), and 2.918Å (3515 cm-1). Such hydrogen bond distances are typical of secondary minerals. Estimates of the hydrogen-bond distances have been made from the position of the OH stretching vibrations and show a wide range in both strong and weak bonds

Raman spectroscopic study of the tellurite minerals: Graemite CuTeO3 H2O and teineite CuTeO3 2H2O

Tellurites may be subdivided according to formula and structure. There are five groups based upon the formulae (a) A(XO3), (b) A(XO3).xH2O, (c) A2(XO3)3.xH2O, (d) A2(X2O5) and (e) A(X3O8). Raman spectroscopy has been used to study the tellurite minerals teineite and graemite; both contain water as an essential element of their stability. The tellurite ion should show a maximum of six bands. The free tellurite ion will have C3v symmetry and four modes, 2A1 and 2E. Raman bands for teineite at 739 and 778 cm-1 and for graemite at 768 and 793 cm-1 are assigned to the ν1 (TeO3)2- symmetric stretching mode whilst bands at 667 and 701 cm-1 for teineite and 676 and 708 cm-1 for graemite are attributed to the the ν3 (TeO3)2- antisymmetric stretching mode. The intense Raman band at 509 cm-1 for both teineite and graemite is assigned to the water librational mode. Raman bands for teineite at 318 and 347 cm-1 are assigned to the (TeO3)2- ν2 (A1) bending mode and the two bands for teineite at 384 and 458 cm-1 may be assigned to the (TeO3)2- ν4 (E) bending mode. Prominent Raman bands, observed at 2286, 2854, 3040 and 3495 cm-1, are attributed to OH stretching vibrations. The values for these OH stretching vibrations provide hydrogen bond distances of 2.550(6) Å (2341 cm-1), 2.610(3) Å (2796 cm-1) and 2.623(2) Å (2870 cm-1) which are comparatively short for secondary minerals

Raman spectroscopic study of the uranyl mineral pseudojohannite Cu6.5[(UO2)4O4(SO4)2]2(OH)5.25H2O

Raman spectra of pseudojohannite were studied and related to the structure of the mineral. Observed bands were assigned to the stretching and bending vibrations of (UO2)2+ and (SO4)2- units and of water molecules. The published formula of pseudojohannite is Cu6.5(UO2)8\[O8](OH)5\[(SO4)4].25H2O; however Raman spectroscopy does not detect any hydroxyl units. Raman bands at 805 and 810 cm-1 are assigned to (UO2)2+ stretching modes. The Raman bands at 1017 and 1100 cm-1 are assigned to the (SO4)2- symmetric and antisymmetric stretching vibrations. The three Raman bands at 423, 465 and 496 cm-1 are assigned to the (SO4)2- ν2 bending modes. The bands at 210 and 279 cm-1 are assigned to the doubly degenerate ν2 bending vibration of the (UO2)2+ units. U-O bond lengths in uranyl and O-H…O hydrogen bond lengths were calculated from the Raman and infrared spectra