Vibrational spectroscopic study of the minerals cavansite and pentagonite Ca(V4+O)Si4O10·4H2O

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 263–269

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Vibrational spectroscopic study of the minerals cavansiteand pentagonite Ca(V4+O)Si4O10�4H2O

Ray L. Frost ⇑, Yunfei XiSchool of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia

a r t i c l e i n f o

Article history:Received 3 January 2012Received in revised form 12 March 2012Accepted 3 April 2012Available online 23 April 2012

Keywords:CavansitePentagoniteSilicateRaman spectroscopyInfrared spectroscopy

1386-1425/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.saa.2012.04.021

⇑ Corresponding author. Tel.: +61 7 3138 2407; faxE-mail address: [email protected] (R.L. Frost).

a b s t r a c t

The bright blue minerals cavansite and pentagonite, a calcium vanadium silicate Ca(V4+O)Si4O10�4H2O,have been studied by UV–Visible, Raman and infrared spectroscopy. Cavansite shows an open porousstructure with very small micron sized holes. Strong UV–Visible absorption bands are observed at around403, 614 and 789 nm for cavansite and pentagonite.

The Raman spectrum of cavansite is dominated by an intense band at 981 cm�1 and pentagonite by aband at 971 cm�1 attributed to the stretching vibrations of (SiO3)n units. Cavansite is characterised bytwo intense bands at 574 and 672 cm�1 whereas pentagonite by a single band at 651 cm�1. The Ramanspectrum of cavansite in the hydroxyl stretching region shows bands at 3504, 3546, 3577, 3604 and3654 cm�1 whereas pentagonite is a single band at 3532 cm�1. These bands are attributed to watercoordinated to calcium and vanadium. XPS studies show that bond energy of oxygen in oxides is530 eV, and in hydroxides �531.5 eV and for water �533.5 eV. XPS studies show a strong peak at531.5 eV for cavansite, indicating some OH units in the structure of cavansite.

� 2012 Elsevier B.V. All rights reserved.

Introduction

Cavansite Ca(V4+O)Si4O10�4H2O, the name of which is derivedfrom its chemical composition, calcium vanadium silicate, is a deepblue hydrous calcium vanadium phyllosilicate mineral (see supple-mentary information for images), occurring as a secondary mineralin basaltic and andesitic rocks along with a variety of zeolite min-erals [1]. Cavansite is a relatively rare mineral. It is polymorphicwith the even rarer mineral, pentagonite [2]. Ishida et al. [3] sug-gest that cavansite is a low-temperature form and pentagonite ahigh-temperature form of the same mineral. Cavansite is a distinc-tive mineral and forms crystal aggregates, generally in the form ofballs, up to a couple centimeters in size. Crystal aggregates consistof spherical rosettes with jutting pointed crystals. Sometimes theballs are coarse enough to allow the individual crystals to be seen.The colour of cavansite is distinctive, almost always a rich, brightblue. The colour is the same as its dimorph, pentagonite, but thelatter is generally sharper with bladed crystals. Finally, the associ-ated minerals are useful for identification, as cavansite is fre-quently found sitting atop a matrix of zeolites or apophyllites.The colour is due to the V(IV) ion, which is always blue.

The mineral cavansite is orthorhombic with point group 2/m.The Cell Data for cavansite is Space Group: Pcmn with a =9.792(2), b = 13.644(3), c = 9.629(2) and Z = 4. Pentagonite is also

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orthorhombic with point group: mm2. Cell Data for pentagoniteis space group: Ccm21, a = 10.298(4), b = 13.999(7), c = 8.891(2)and Z = 4. Cavansite and pentagonite belong to the vanadium sili-cate minerals which show an open porous structure with verysmall micron sized holes. Cavansite and pentagonite, dimorphs ofCa(VO)(Si4O10)�4H2O represent novel, layered-silicate structuretypes. Zigzag pyroxene-like (SiO3)n chains, joined laterally intosheets parallel to the a–c plane, are present in both minerals withtetrahedral apices pointed alternately plus and minus along the baxes [2]. The lateral linkage in cavansite results in a network of4-fold and 8-fold rings, but in pentagonite the network is entirelymade up of 6-fold rings [2]. The vanadyl groups VO2+ and Ca2+ ionslie in mirror planes between the silicate layers and are coordinatedalternately to pairs of tetrahedral apices along the chains on oppo-site sides of the mirror planes. V is in square-pyramid coordination,and Ca is in seven fold coordination in both structures.

Some X-ray and infrared studies of cavansite have been under-taken [4]. Prasad and Prasad have discussed the Raman spectrumof cavansite but not pentagonite [5]. Raman spectroscopy has pro-ven very useful for the study of minerals [6–13]. Indeed Ramanspectroscopy has proven most useful for the study of diageneticallyrelated minerals as often occurs with minerals containing silicategroups such as cavansite and pentagonite. The objective of this re-search is to report the vibrational spectra of cavansite and pentag-onite and to relate the spectra to the molecular structure of theminerals.

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Fig. 1. UV–Visible spectra of cavansite (lower spectrum) and pentagonite (upperspectrum).

264 R.L. Frost, Y. Xi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 263–269

Experimental

Mineral

The minerals used in this study were supplied by TheMineralogical Research Company. The two cavansite samples orig-inated from (a) Wagholi Quarry, near Pune (Poonah) Maharashtra

Fig. 2. (a) Raman spectrum of cavansite from Poonah, India and (b) cavansite from

State, India and (b) Owyhee Dam, Lake Owyhee State Park, MalheurCounty, Oregon (TL). The pentagonite mineral came from Pune Dis-trict (Poonah District), Maharashtra State, India.

Details of the mineral have been published (page 118 and 636)[14].

Raman spectroscopy

Crystals of cavansite or pentagonite were placed on a polishedmetal surface on the stage of an Olympus BHSM microscope, whichis equipped with 10�, 20�, and 50� objectives. The microscope ispart of a Renishaw 1000 Raman microscope system, which also in-cludes a monochromator, a filter system and a CCD detector (1024pixels). The Raman spectra were excited by a Spectra-Physics model127 He-Ne laser producing highly polarised light at 633 nm and col-lected at a nominal resolution of 2 cm�1 and a precision of ±1 cm�1

in the range between 200 and 4000 cm�1. Repeated acquisitions onthe crystals using the highest magnification (50�) were accumu-lated to improve the signal to noise ratio of the spectra. Because ofthe lack of signal, over 1200 scans were made. The spectra werecollected over night. Low incident power was used. Raman Spectrawere calibrated using the 520.5 cm�1 line of a silicon wafer. The Ra-man spectrum of at least 10 crystals was collected to ensure theconsistency of the spectra.

Infrared spectroscopy

Infrared spectra were obtained using a Nicolet Nexus 870 FTIRspectrometer with a smart endurance single bounce diamondATR cell. Spectra over the 4000–525 cm�1 range were obtainedby the co-addition of 128 scans with a resolution of 4 cm�1 and amirror velocity of 0.6329 cm/s. Spectra were co-added to improvethe signal to noise ratio.

Spectral manipulation such as baseline correction/adjustmentand smoothing were performed using the Spectracalc softwarepackage GRAMS (Galactic Industries Corporation, NH, USA). Bandcomponent analysis was undertaken using the Jandel ‘Peakfit’ soft-ware package that enabled the type of fitting function to be selectedand allows specific parameters to be fixed or varied accordingly.

Oregon over the 100–4000 cm�1 region (c) pentagonite from Poonah, India.

Fig. 3. Infrared spectrum of cavansite from Poonah, India over the 500–4000 cm�1

region.

R.L. Frost, Y. Xi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 263–269 265

Band fitting was done using a Lorentzian–Gaussian cross-productfunction with the minimum number of component bands used forthe fitting process. Band fitting is undertaken using the minimumnumber of bands required to obtain a decent fit. The Gaussian–Lorentzian ratio was maintained at values greater than 0.7 and

Fig. 4. (a) Raman spectra of cavansite from Poonah, India and (b) cavansite from

fitting was undertaken until reproducible results were obtainedwith squared correlations of r2 greater than 0.995.

Results and discussion

UV–Visible spectroscopy of cavansite

The minerals cavansite and pentagonite are an intense brightblue colour. The UV–Visible spectrum of cavansite and pentagoniteis illustrated in Fig. 1. It is noteworthy that intense absorptionpeaks occurs at 403 nm for cavansite and 400 nm for penatgonite.A second absorption band is observed at 614 nm for cavansite and607 nm for penatgonite. A further absorption maxima is observedaround 788 nm. The question arises as to the origin of thesecolours. Cavansite contains vanadium in the tetravalent state.Vanadium in its tetravalent state invariably exists as the molecularVO(II) ion (vanadyl). The VO(II) ion has a single d electron whichgives rise to the free ion term 2D. In a crystal field of octahedralsymmetry, this electron occupies the lowest t2g orbital and givesrise to ground state term 2T2g. When the electron absorbs energy,it is excited to the upper eg orbital and gives rise to the 2Eg term.Accordingly in octahedral symmetry, only one band correspondingto the transition 2T2g ?

2E is expected. Because of the non-sym-metrical alignment of the V@O bond along the symmetry axis,the site symmetry in general is lowered to tetragonal (C4V) orrhombic (C2V). In C4V site symmetry 2T2g splits into 2B2 and 2E,where as 2Eg splits into 2B1, 2A1. Accordingly three bands are ex-pected. The degeneracy of 2E is also removed in rhombic symmetryresulting in four bands. For VO(II) three d–d transition bands areexpected for VO(II) in C4v symmetry in the range 11,000–14,000,14,800–19,000 and 20,000–31,250 cm�1.

Raman spectroscopy of cavansite and pentagonite

The crystallographic structure (symmetry) of cavansite is cen-trosymmetric, and that of pentagonite is non-centrosymmetric[2]. A phonon in centrosymmetric structure is active for eitherRaman scattering or infrared absorption, but not both. On the other

Oregon (c) pentagonite from Poonah, India, over the 800–1400 cm�1 region.


region.


hand, one phonon in non-centrosymmetric structure is active forboth Raman and infrared spectroscopy. The Raman spectra of cav-ansite from the two different origins and of pentagonite in the100–4000 cm�1 range are displayed in Fig. 2. The infrared spectraof cavansite from India in the 500–4000 cm�1 range are displayedin Fig. 3. These spectra illustrate the relative intensities of the


bands and the position of the peaks. It is obvious that there arelarge sections of the spectra where no intensity is observed. Thusthe spectra are subdivided into sections dependent upon the par-ticular vibration being studied. The spectra of the two cavansitesare quite similar, but differences in intensity and peak width arefound. The Raman spectra of cavansite and pentagonite in the800–1400 cm�1 are reported in Fig. 4. The infrared spectrum ofcavansite in the 500–1300 cm�1 range is reported in Fig. 5.

The cavansite mineral is composed of pyroxene-like (SiO3)n

chains. Dowty calculated the band positions for the different idealsilicate units. Dowty showed that the –SiO3 units had a uniqueband position of 980 cm�1 [15] (see Figs. 2 and 4 of this reference).The Raman spectrum of cavansite shows an intense sharp band at981 cm�1 (sample a) and 984 cm�1 (sample b) (Fig. 4). The Ramanspectrum of pentagonite displays an intense sharp band at971 cm�1. Another possible assignment of this intense sharp Ra-man peak is to VO5 units.

The authors [16] have studied many vanadates containing min-erals especially uranyl vanaadates. For the minerals carnotite, curi-enite, francevillite, tyuyamunite and metatyuyamunite, an intenseRaman band is observed in the 965–985 cm�1 range and is attrib-uted to the m1 (VO3) symmetric stretching vibrations in the (V2O8)units. This band is split with a second component observed ataround 963 cm�1. Raman spectrum of pascoite is characterisedby two intense bands at 991 and 965 cm�1. Raman bands are ob-served at 991, 965, 958 and 905 cm�1 and originate from four dis-tinct VO6 sites in the mineral structure [17,18].

Raman bands of sample (a) of significantly lower intensity areobserved at 823, 842, 935, 954 and 973 cm�1 on the lower wave-number band of the 981 cm�1 band and bands at 1043, 1072,1088 and 1109 cm�1 on the higher wavenumber side of the981 cm�1 band. The large number of bands of low intensity createsdifficult in the assignment of such a large number of bands. Forsample (b), Raman bands on the lower wavenumber side of the984 cm�1 peak are observed at 822, 840, 930, 952 and 968 cm�1.Raman bands on the higher wavenumber side of the 984 cm�1

peak are observed at 1017, 1040, 1073 and 1087 cm�1. The Ramanspectra of the two cavansite samples are similar but not exactly thesame. The higher wavenumber bands (1043, 1072, 1088 and



1109 cm�1 sample a) and (1017, 1040, 1073 and 1087 cm�1 sampleb) are attributed to the SiO3 antisymmetric stretching vibrations.Raman bands at 823, 842, 935, 954 and 973 cm�1 for sample 1and 822, 840, 930, 952 and 968 cm�1 for sample b, are related tothe vanadate stretching vibrations.

Raman bands of pentagonite of lower intensity are found at 1047,1089, 1153 and 1191 cm�1. The Raman spectrum of pentagonitehas not been reported previously. Prasad and Prasad reported thedehydration of natural cavansite using in situ Raman spectroscopy[5]. These researchers reported a band at around 980 cm�1 andshowed the band was stable up to 720 K [5]. These researchers as-signed this band to the symmetric stretching modes of VO5 units.



However this does not seem correct as the theoretical studies ofDowty showed that the band is due to the stretching mode of SiO3

units.The infrared spectrum of cavansite shows an intense broader

band centred upon 989 cm�1 (Fig. 5). The band profile is broadand component bands may be resolved at 918, 940, 961, 1010,and 1032 cm�1. Infrared bands on the higher wavenumber sideare observed at 1076 and 1117 cm�1. Infrared bands on the lowwavenumber side are observed at 603, 678, 699, 724, 733, 782and 792 cm�1. The band at 603 cm�1 is broad and is attributed towater librational modes. Dowty calculated the position of infraredbands for silicate structures and also showed measured spectra of




region.


the equivalent theoretical system. The bands in the 678–799 cm�1

are assigned to OSiO bending modes of the (SiO3)n chains. Theinfrared spectra as displayed by Dowty show considerable widthin the peaks. The spectral resolution is apparently better in thiswork. Powar and Byrappa reported the XRD, thermal analysis andinfrared spectra of cavansite [4]. These researchers attributed


bands in the 600–800 cm�1 region to water librational modes. Inthis work, the infrared bands are quite sharp and are more likelyto be due to SiO3 bending modes.

The Raman spectra of cavansite and pentagonite in the 300–800 cm�1 region are reported in Fig. 6. The spectra of the two cav-ansite samples are almost identical. Intense Raman bands are ob-served at 574 and 672 cm�1 (sample a) and 573 and 669 cm�1

(sample b). The Raman spectrum of pentagonite shows an intenseband at 651 cm�1. These bands are assigned to the OSiO bendingmodes. Dowty calculated the band position of these bendingmodes for different siloxane units [15]. Dowty demonstrated theband position of the bending modes for SiO3 units at around650 cm�1. Other bands of moderate intensity are observed at 350and 437 cm�1 (sample a) and 336 and 427 cm�1 (sample b). Thesebands may be attributed to metal–oxygen stretching vibrations.Prasad and Prasad reported the Raman spectrum of cavansite asa function of temperature [5]. It is difficult to compare the spectraof these workers with this work as the peaks were not labelled andno table of peak positions was given. Raman bands in the far lowwavenumber region are shown in Fig. 7. The spectra of the twosamples are very similar. Raman bands are observed at 113, 131,194, 230, 251 and 291 cm�1 for sample (a), and 133, 163, 194,234, 256 and 294 cm�1 for sample (b). Raman bands of pentagoniteare observed at 140, 206, 230, 261 and 288 cm�1. These bands aresimply described as lattice vibrations.

The Raman spectra of cavansite and pentagonite in the 2600–3800 cm�1 region are displayed in Fig. 8. Raman bands are ob-served at 3504, 3546, 3604 and 3654 cm�1 for cavansite sample(a) and 3517, 3552 and 3599 cm�1 for cavansite sample (b). TheRaman spectrum of pentagonite shows a broad band at3532 cm�1. These bands are assigned to the symmetric stretchingvibrations of water. These bands are due to water coordinated tothe vanadium and calcium ions. Prasad and Prasad [5] reported Ra-man bands at 300 K at 3478, 3520 and 3574 cm�1. The position ofthese bands differs from that reported in this work. The infraredspectrum in the 2400–3800 cm�1 region of sample (a) is shownin Fig. 9. Infrared bands are observed at 3244, 3439, 3501, 3548,3601 and 3652 cm�1. Prasad and Prasad [5] tabulated infraredbands at 3186, 3247, 3504, 3552, 3606 and 3654 cm�1. Apart from



the first band of Prasad, the values agree with the band positionsreported in this work. Powar and Byrappa [4] reported infraredbands for cavansite at 3250, 3440, 3548, 3592 and 3648 cm�1.The position of the bands and the width of the bands in Fig. 8a indi-cate that the OH units in the structure of the mineral cavansite. Inthe Raman spectrum of xonotlite, Raman bands are observed insimilar positions to the bands of cavansite. Raman bands for xonot-lite are observed at 3578, 3611, 3627 and 3665 cm�1 and thesebands are attributed to the stretching vibrations of the OH unitsand not water stretching vibrations. The Raman spectrum of cavan-site in Fig. 8b looks different and the bands look more like waterstretching vibrations. XPS studies show that bond energy of oxygenin oxides is 530 eV, and in hydroxides �531.5 eV and for water�533.5 eV. XPS studies show a strong XPS peak at 531.5 eV for cav-ansite, thus indicating some OH units in the structure. This wouldresult in bands in the 3000–3550 cm�1 in the Raman spectra. Thisrequires further research.

The Raman spectra of cavansite and pentagonite in the 1400–1800 cm�1 region is shown in Fig. 10. Raman bands are observedat 1610, 1628 and 1637 cm�1 for cavansite sample (a) and at1626 and 1634 cm�1 for cavansite sample (b). The Raman spec-trum of pentagonite shows peaks at 1612 and 1634 cm�1 in thisspectral region. These bands are attributed to water bendingmodes. The observation of three bands supports the concept ofwater in different hydrogen bonding environments.

Prasad and Prasad [5] reported a single band at 1616 cm�1. Theinfrared spectrum of cavansite over the 1300–1800 cm�1 region isdisplayed in Fig. 11. Three bands are found at 1610, 1632 and1646 cm�1. These bands are attributed to water bending modes.Prasad and Prasad [5] reported infrared bands at 1610, 1635 and1647 cm�1. The results of our infrared spectrum are in good agree-ment with that reported by Prasad and Prasad [5]. However con-

Fig. 11. Infrared spectrum of cavansite from Poonah, India over the 1300–1800 cm�1 region.

siderable differences are observed in the position of bands in theRaman spectra reported in this work and that of Prasad and Prasad.This suggests that the cavansite spectra are dependent upon theorigin of the minerals.

Conclusions

Cavansite is a bright blue mineral based upon a calcium silicatewhich includes a V4+ ion. The mineral is related to zeolite materialsand has an open porous structure with very small micron sizedholes. Pentagonite is a polymorph of cavansite and has even astronger darker blue colour. The presence of the tetravalent vana-dium ion is the reason for the colours of the mineral. A combina-tion of Raman and infrared spectroscopy has been used todetermine details of the molecular structure of cavansite andpentagonite.

The Raman spectrum of cavansite is dominated by an intenseband at 981 cm�1 attributed to the stretching vibrations of (SiO3)n

units. Pentagonite is characterised by a band at 971 cm�1. Cavan-site is characterised by two intense bands at 574 and 672 cm�1

whereas pentagonite by a band at 651 cm�1. These bands areattributed to OSiO bending modes. The Raman spectrum of cavan-site in the hydroxyl stretching region shows bands at 3504, 3546,3577, 3604 and 3654 cm�1 whereas the Raman spectrum of pen-tagonite is characterised by a single band at 3532 cm�1. Thesebands are attributed to stretching vibrations of water coordinatedto the Ca2+ and V4+ ions. XPS studies appear to show a bond energyattributable to OH units, suggesting that hydroxyl units are in-volved in the structure.

Acknowledgments

The financial and infra-structure support of the School of Chem-istry, Physics and Mechanical Engineering, Science and EngineeringFaculty, Queensland University of Technology, GPO Box 2434,Brisbane Queensland 4001, Australia is gratefully acknowledged.The Australian Research Council (ARC) is thanked for funding theinstrumentation.

References

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Documents

Vibrational spectroscopic study of the minerals cavansite and pentagonite Ca(V4+O)Si4O10·4H2O