Infrared Spectroscopic Study of Phase Transitions in Natural Scolecite

Indian Journal of Pure & Applied Physics Vol. 43, February 2005, pp. 79-82

Infrared spectroscopic study of phase transitions in natural scolecite

K Shiva Prasad, P S R Prasad* & S R Sharma National Geophysical Research Institute, Hyderabad 500 007 Received 12 May 2004; revised 26 July 2004; accepted 10 December 2004

In-situ infrared spectroscopic (FTIR) studies are carried out on a naturally occurring scolecite, from Deccan traps, in the temperature range 300-850 K. Five bands are observed at 3589, 3507, 3409, 3327, and 3232 cm−1 in the (OH) stretching mode region, and three bands at 1665, 1649 and 1591 cm−1 in the bending mode region indicating a more complex molecular interaction between H2O and network cations in scolecite. At around 500 K, stretching mode region is replaced with three band structure, and the bending mode region with a band around 1658 cm−1, indicating partial dehydration in scolecite. At around 720 K complete disappearance of infrared modes due to water molecules indicates total dehydration. X- ray diffraction (XRD) of the sample corroborates the transformation to amorphous phase at higher temperatures. [Keywords: Zeolites, Scolecite, Dehydration, FTIR] IPC Code: G01J 3/00

1 Introduction dehydration induced phase transitions in scolecite and A fundamental property that characterizes most of the dehydration studies are based on thermal framework of an alumino-silicate mineral as zeolite is analysis2. Rykl6 reported using thermal analysis, its ability to dehydrate and rehydrate at higher (150- X-ray diffraction and infrared spectroscopy, the 400°C) temperatures. It also shows a lot of order- transformations to metascolecite and to amorphous disorder changes in their framework cations like Si, phase induced due to two-step dehydration process, Al. Particularly, their property of dehydration and while Joshi7 reports no changes in OH stretching and rehydration is attractive for their utility in ion- H2O bending modes below 300°C. However, they exchange processes1,2. The infrared spectroscopy is a reported variations for these modes only above very useful and versatile technique to probe the 300°C, contrary to the mechanism proposed for the structural status and dynamics of water molecules in transformations8. minerals, and the present paper is a part of such The dehydration induced phase transition sequence investigations3-5. in natural scolecite, in the temperature range 300-850 The fibrous zeolites form under diverse geological K, using in-situ FTIR spectroscopy has been environments such as low-grade metamorphic basaltic investigated. The infrared spectroscopy is a more lavas, pegmatitic assemblages and as alteration powerful tool to investigate transitions induced due to products in alkaline intrusive complexes1-2. The loss/gain of water molecules. natrolite groups of zeolites namely natrolite, tetranatrolite, paranatrolite, mesolite, scolecite are 2 Experimental Details part of this group and are important rock forming The samples are from Poona, Mumbai India. This minerals. The natrolite group of zeolites shows a wide scolecite was acicular in shape. This scolecite was range of structural phase transformations and in finely to powder and a pellet was made with KBr for particular the phase transitions driven at lower the FTIR studies. temperatures (<400°C) are due to dehydration The X-ray diffraction patterns obtained were process. A general phase transition sequence reported recorded by using Philips PW-1830 powder for natural scolecite, has been that it first transforms diffractometer with Ni-filter, θ-2θ scan. The radiation to metascolecite and then into amorphous phase and used was Cu-Kα radiation. The 2θ scan was from 2,6 finally decomposes to anortite and quartz . To the 10°-80°. The scanning rate was 3 degrees per minute. best of our knowledge there are no true in-situ Infrared spectroscopy studies were carried out on infrared spectroscopic investigations monitoring the NEXUS FTIR spectrometer from Thermo-Nicolet, ______using a thermo electrically cooled deuterated tri- E-mail: [email protected] glycene sulphate (DTGS) detector, extended range 80 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005

KBr (XT-KBr) beam splitter, capable of working in and amorphous phases. The scolecite phase spectrum the wavenumber range 375-12,500 cm−1. Spectra at broadly matches with that reported by Gottardi and above ambient temperatures were collected using Galli1. As can be seen from Table 1 that the specially fabricated environmental chamber. This framework modes namely Si(Al)-O stretching and chamber can be used to increase the temperature of bending modes below 1200 cm−1 show marginal the sample under study up to about 950 K with an variations in various phases of scolecite. However, the accuracy of ±2°. Running tap water is used to cool the variations for OH stretching and H2O bending modes outer jacket of the chamber, which houses the infrared are very prominent. The thermal evolution of these transparent windows. Conventional KBr pellet modes is shown in Fig. 2. As can be seen from the technique was followed to record FTIR spectra and infrared spectrum of scolecite, the OH stretching the spectra reported are the resultant of 128 scans, mode region consisting of at least five bands around resolution of 2 cm−1 and aperture of 37. Typical 3589; 3507; 3409; 3327; and 3232 cm−1. Two more uncertainties in the peak positions are about 2 cm−1 shoulder peaks around 3140 and 3040 are also for the sharper and stronger modes, while that for the observed in the lower wavenumber side. On the other weak and overlapping modes could be about 5 cm−1. hand, at least three bands are observed around 1665; −1 1649 and 1591 cm . All these observations correlate 3 Results and Discussion with six different O…O bond distances of the water Faith and Hansen9 and Smith10 refined the molecules in scolecite recently proposed by structural details of scolecite and more recently by Stuckenschmidt11. Stuckenschmidt11. The number of Ca atoms in The stretching and bending modes of water scolecite is half the number of Na atoms in natrolite; molecules of scolecite have shown very interesting and the number of water molecules per aluminium variations. We have recorded the spectrum at regular atom are 1.5 and 1 respectively, for scolecite and temperature intervals of 25° in the range 300-800 K natrolite. This additional water molecules result in and more close intervals of 10° near to the transition more complex molecular interactions. Two Na+ and temperature. Some representative traces at eight H2O groups fill the channel space in natrolite, different temperatures are shown in Fig. 2. It is +2 whereas in scolecite one Ca and three H2O groups interesting to note that the fine structure in (OH) occupy it. This additional water molecule introduces stretching mode region has gradually transformed into more complexity in infrared spectra. three modes, and the three modes observed in bending In Fig. 1, we show recorded spectra (in-situ) at mode region transformed into one. These spectral three different temperatures namely 300, 573 and 773 features observed at around 573 K (as shown in K, and these traces represent scolecite, meta-scolecite Fig. 1) are very similar to the spectrum of natrolite. It

Fig. 1—Background corrected FTIR spectra of scolecite at various temperatures as indicated along with the corresponding spectrum PRASAD et al.: INFRARED STUDY OF NATURAL SCOLECITE 81

is also seen that the spectral bands characteristic of The samples at 300 K and 773 K were further water completely disappeared at higher temperatures characterized by XRD. Fig. 3 shows the XRD patterns (>725 K). We failed to get back to the starting of the sample at 300 K and the one heated to 773 K. spectrum even after cooling the sample for over 60 The natural scolecite has a monoclinic structure with hrs. The rehydration behaviour needs further space group Cc and parameters10 a=6.5222 Å, investigation. b=18.9678 Å, c=9.8398 Å and β=109.972. The XRD patterns and hkl (calculated using checkcell software −1 Table 1—Peak positions of infrared spectra in cm at with the above parameters) of the sample at 300 K temperatures 300, 573 and 773 K and their assignment 10 (sh=shoulder) match well with the reported data . The presence of broad diffused peaks in the XRD patterns of the Measured peak positions in cm−1 Assignment sample heated up to 773 K indicates transformation to At 300 K At 573 K At 773 K amorphous phase. The d spacing of the broad peaks 3589 3572 observed in our data (6.48, 5.81 Å) match well with 3507 that of the reported6. These XRD results corroborate 3409 ν(OH) 3327 3334 (sh) with the IR results. The XRD patterns in conjunction 3232 3212 with framework modes in the IR spectrum at 773 K 3140 (sh) implies that this temperature is well before the 3040 (sh) formation of mixture of anortite and quartz. 1665 1658 Libowitzky and Rossman12 have shown that the use 1649 (sh) δ(OH) 1591 of integrated absorbance values (band energy) results 1099 1101 in a linear correlation with water content, whereas, 1068 1049 1035 linear absorbance data (peak heights) is not. Thus, the 1047 temperature-induced variations of integrated 1034 1020 ν [Si(Al)-O] absorbance of water modes could be used to 988 978 970 understand the process of dehydration. In Fig. 3, we 952 show the measured variations of integrated 939 absorbance (normalized to room temperature) of 931 929 721 703 668 stretching and bending modes at various temperatures. 692 673 The integrated absorbance measured for these modes 671 in the wavenumber range 4000-2969 and 1800-1560 632 610 610 δ[O-Si(Al)-O + cm−1 respectively with a linear background. It is very 604 Si-O-Al] 497 501 501 clear that the integrated absorbance of bending modes

422 422 422

Fig. 2—Thermal evolution of OH-stretching (A) and bending (B) modes of water molecules in natural scolecite. The corresponding temperatures at which the spectra are recorded from top are 300, 473, 523, 573, 623, 673, 723 and 773 K 82 INDIAN J PURE & APPL PHYS, VOL 43, FEBRUARY 2005

Fig. 4—Temperature induced variations of total peak areas of OH-stretching (+) and OH bending (−) modes of natural scolecite. Vertical arrows indicate the transition temperatures

Fig. 3—The XRD patterns of the scolecite at 300 K and heated to characteristic sharp and strong peaks of scolecite 773 K is shown along with the hkl plotted against the peaks indicates the transformation to amorphous phase. showed a two-step variation. At around 500 K, it Acknowledgement dropped to about 75 (±10) %, and almost became zero Authors sincerely thank the Director, Dr V P around 720 K. Interestingly, these are the transition Dimri, National Geophysical Research Institute, temperatures reported earlier for meta-scolecite and Hyderabad, for his encouragement, keen interest in amorphous phase. The channel space in scolecite is this program and for permission to publish this work. filled by one Ca and three H2O molecules. The water molecules having interaction with Ca atoms i.e., References 11 1 Gottardi G & Galli E, Minerals and rocks, natural zeolites, Ca…H2O, are different from the other two . The first (Springer-Verlag, Berlin Heidelberg), 1985. stage of dehydration is due to the former. In such 2 Bish D L & William Carey J, Reviews in Mineral & case, the total peak area of infrared modes should Geochemi, 45, (2001), 403. drop to about 66%. As shown in Fig. 3, the 3 Ertl A, Hughes J M, Brandstatter F, Dyar M D & Prasad P S dehydration is in two steps and the drop in stage 1 is R, Can Mineral, 41 (2003) 1363. consistent with the mechanism proposed by 4 Prasad P S R & Gowd T N, J Geol Soc India, 61 (2003) 202. 11 5 Prasad P S R & Sarma L P, Am Mineral, 89 (2004) 1056. Stuckenschmidt . 6 Rykl D, Chalupsky V & Pechar F, Collection Czechoslovak In summary, we studied thermally induced Chem Commun, 51 (1986) 516. dehydration process in a naturally occurring scolecite 7 Joshi M S, Choudhari A L, Mohan Rao P & Kanitkar R G, by in-situ infrared spectroscopy and X-ray diffraction. Thermochimica Acta, 64 (1983) 39. 8 Stahl K & Hanson J C, J Appl Cryst, 27 (1994) 543. Step-wise dehydration is clearly observed in the 9 Faith I & Hansen S, Acta Crystallographica, B35 (1979) variations of total peak areas of bending and 1877. stretching modes of water molecules. Partial 10 Smith J V, Pluth J J, Artioli G & Ross F K, Proceedings of the 6th International Zeolite Conference, (1984) 842. dehydration due to Ca…H2O molecules in scolecite 11 Stuckenschmidt E, Joswig W, Baur W H & Hofmeister W, causes the transformation to meta-scolecite, whereas, Phys Chem Minerals, 24 (1997) 403. the total dehydration is observed in its amorphous 12 Libowitzky E & George Rossman R, Am Mineral, 82 (1997) phase. The diffused XRD pattern replacing 1111.