Clay Minerals (1986) 21, 377-388

I N T E R P R E T A T I O N OF T H E IR S P E C T R A OF C E L A D O N I T E S A N D G L A U C O N I T E S IN THE

R E G I O N OF O H - S T R E T C H I N G F R E Q U E N C I E S

M. V. S L O N I M S K A Y A , G. B E S S O N * , L. G. D A I N Y A K , C. T C H O U B A R * AND V. A. D R I T S

Geological Institute of Geology, Pijevsky Street 7, Moscow, US SR, and *Laboratoire de Cristallographie (UA 810), Universitd d'Orldans, Rue de Chartres, BP 6057, 45067 Orldans Cddex 2, France

(Received 13 March 1986)

A B S T R A C T: A method is given for quantitative analysis of IR spectra of dioctahedral micas in the OH-stretching region, which involves spectral decomposition and correlation of octahedral cation contents with integrated optical densities of the corresponding bands. It provides a basis for the study of order-disorder in these minerals and has allowed revision of the crystal- lOchemical formulae of some glauconites and celadonites.

The IR spectra of dioctahedral micaceous minerals and, in particular, glauconites have been studied by a number of workers (Vlasova et al., 1976; Osherovitch & Nikitina, 1975; Pliusnina, 1982; Slonimskaya et al., 1978; Yukhnevitch, 1970; Farmer, 1974; Rouxhet, 1970; Saksena, 1964; Vedder, 1964; Vedder & Wilkins 1969). However, these have not established any reliable correlation between the nearest cationic environment of the OH groups and the OH stretching frequencies. The interpretation of Vedder (1969), which is still used by some workers (Osherovitch & Nikitina, 1975; Rouxhet, 1970), does not always correspond to the relationship between the effective charge and mass of ions on the one hand, and their vibration frequencies on the other. Farmer (1974), when interpreting IR spectra of glauconites, considered only the R2+R 3+ groups. Nevertheless, IR spectro- scopy may prove extremely useful in structural studies of glauconites, with respect to composition and the distribution of the isomorphic cations.

The present investigation was carried out with the intention of developing a method for the analysis of IR spectra of celadonite and glauconite in the region of OH stretching frequencies. It is known that the OH groups in the structure of dioctahedral mica are coordinated by two octahedral cations. Thus the individual OH stretching frequencies depend on the type of the cations nearest to the OH group. The integrated optical density of each band is determined by the number of absorption centres of the given type (i.e., of the OH groups with the given cationic environment) and by the absorption coefficient. Therefore if the experimental values of the integrated optical densities for each IR band can be determined, and also which of the cation arrangements the latter corresponds to, this would allow analysis of the composition and the distribution of the octahedral cations. It is evident that the basis for the interpretation of the OH stretching frequencies of celadonites and glauconites should be the study of IR spectra of samples with reliably known cationic compositions of their octahedral sheets. At the same time the majority of papers on the

�9 1986 The Mineralogical Society

378 M . V . Slonimskaya et al.

interpretation of the OH stretching frequencies of micas do not give any chemical compositions of the samples (Vlasova et al., 1976; Farmer, 1974; Saksena, 1964).

C H O I C E O F S A M P L E S A N D A P P A R A T U S

I R spectra of samples studied by diffraction methods and with known chemical compositions were analysed. The chosen collections embraced the whole compositional range from the Fe 3+ end-members (celadonites) to the A1 end-members (leucophyllites) with the minimum Al-for-Si substitutions, as well as glauconites with substantially different contents of Fe ~+, AI, Fe 2+ and Mg in octahedra. Monomineralic samples without fluorine were chosen as standards: celadonite (Zavalye), leucophyllite (31) and glauconite (Bolshoi Patom). The choice was determined by the simpler compositions of these samples and well-resolved IR spectra. The standard samples were first studied by a number of methods, such as X-ray and electron diffraction, chemical analyses and thermogravimetry (Dainyak et al., 1984; Sokolova et al., 1976; Tsipursky, 1982). The diffraction methods showed that the trans octahedra in the structure of all the samples are vacant. The crystallochemical formulae of these samples are given in Table 1. Spectra of these samples have been studied extensively. Initially, also, the spectra o f a considerable number of celadonites, leucophyllites and glauconites were analysed qualitatively.

IR spectra in the region of the OH stretching vibrations were obtained with a Perkin-Elmer 180 double-beam spectrophotometer. The resolution in the 3 800--3400 cm -1

TABLE 1. Crystallochemical formulae of different samples of celadonites, leucophillites and glauconites. (I) MALKOVA K.M. (1956) On the celadonite of Pobuzhye. Collected Papers on Mineralogy. Lvov Geol. Soc. 10, 305-318. (If, XII) PAVLISHIN V.I., PLATONOV A.N., POLISHIN E.V., SEMENOVA T.F. & STAROVA G.K. (1978) Micas with iron in quadrupole coordination. Trans. All-Union Min. Soc. 107, 165 (in Russian). (III) RASSKAZOV A.A. (1984) Clay Minerals in Potassium Deposits. Nauka, Moscow. (IV, V, VII) NIKOLAEVA I.V. (1977) Minerals of the Glauconite Group in Sedimentary Formations. Nauka, Moscow. (VI) KIMBARA K. & SHIMODA S. (1973) A ferric celadonite in amygdales of dolerite at Talheizan, Akita Prefecture, Japan. Clay Sci. 4, 143-150. (VIII) Sample presented by T.A. Ivanovskaya, Geological Institute of the USSR Academy of Science. (IX) KOYAMA N., SrnMODA S. & SUDO T. (1971) Celadonites in the tuff of Oya, Tochigi Prefecture, Japan. Mineral. J. 6, 29-312. (X, X1) Samples presented by M.I. Lipkina,

Pacific Ocean Study Institute of the Far-East Department of the USSR Academy of Science.

_ . ~ p l e Zavalye 69G 31 B. Patom 655 Taiheisan E8/2 5/1 Oya 933/3 132 133G

Cation ~ (I) (II) (III) (IV) (V) (VI) (VII) (VIII) (IX) (X) (XI) (XII)

Si 3.96 3.94 3.94 3.46 3.64 3.72 3.65 3-69 3.99 3.88 3.96 3.84 AI w 0.04 0.06 0.06 0.54 0.36 0.28 0.35 0.31 0.01 0.02 0.04 - - Fei3v + 0.10 0.16

Air1 0.05 0.05 1.10 1.11 0-08 0.16 0.68 0.13 0.93 Fe~w + 0-96 1.15 0-17 0.41 1-08 1.07 0.79 1-30 0.25 1.47 1-32 0.75 Fe 2+ 0.26 0.36 0.07 0.13 0.12 0.14 0.10 0.07 0.44 0.07 1.05 Mg 0.73 0.41 0.64 0.35 0.66 0.67 0.43 0.50 0.31 0.53 0.54 0.55

K 0.89 0.83 0.91 0.74 0.90 0.82 0.78 0-78 0.73 0.43 0.65 0.59 Na 0.01 0.07 0.01 0.18 0.01 0.18 0.08 0.02 0-03 Ca 0.10 0.03 0.06 0.02 0.02 0.01 Mg 0.07 0.02 0.05

IR of celadonite and glauconite 379

region was less than 2 cm -1. The alkali halide pressed disk technique was used: 3 mg of sample o f particle size <2 ~tm was added to ~200 mg of KBr in a steel capsule with steel balls and ground and mixed for 2 min in a v ibra tory grinder. The disk obtained after pressing was heated at ~ 170 ~ C to remove most o f the absorbed water.

I N T E R P R E T A T I O N O F I N D I V I D U A L B A N D S A N D T H E D E C O M P O S I T I O N O F S P E C T R A

Qualitative analysis of the IR spect ra of the large collection of samples of celadonites, leucophyUites and glauconites suggested a prel iminary interpretat ion of the individual bands. This interpretat ion was later refined by quantitative analysis of the spectra of s tandard samples, and the final version is presented in Table 2. The basis for this correlation is discussed below.

The bands at 3560-3605 cm -1, related to MgFe 3+ and MgA1 respectively, are strong and clearly resolved in the I R spectra of celadonite (Zavalye) (Fig. 1) and leucophyllite (31) (Fig. 3); their location and interpretat ion are unambiguous. The band at 3580 cm -1 is strongest in the spectrum of celadonite (Oya) (Fig. 9b) and according to the composi t ion o f the octahedral sheet is unambiguously related to A1Fe 2+.

The absorpt ion band at 3620 cm -1 (AIA1) is characteris t ic of muscovites (Farmer , 1974). Fur thermore , a band at 3640 cm -~ has also been observed in the muscovite spectrum by many workers and also related to AIA1 (Viasova et al., 1976; S lonimskaya et al., 1978; Saksena, 1964). Having applied theoretical point-group analysis, Sutherland (1955)

reached the conclusion that there should be at least two bands in the region of the O H stretching frequencies of the spectrum of muscovite. In the spectra of the present samples, the band at 3640 cm -~ is present most ly in those cases when the A1 content in the oc tahedra is > 1 per half unit-cell.

The Fe 2+ Fe 3+ absorpt ion band at 3528 cm -1 is reliably resolved in the spectrum for a sample with a high Fe 2+ content in octahedra l sites (133G, Table 2, Fig. 9a); in the spectra

TABLE 2. Correlations between OH stretching frequencies and octahedral cations to which the hydroxyl groups are coordinated, and integrated optical densities of IR band (%).

Wave number Zavalye 69G 31 B. Patom 655 Taiheizan E8/2 5/1 (cm -1) Cation pair (I) (II) (III) (IV) (V) (VI) (VII) (VIII)

3495 Fe2+Fe 2+ - - 6.5 . . . . . . 3505 MgMg 9-1 3.0 2-9 - - 8.4 9.9 - - 5.0

Fe2+Fe3+ "~ 3531" Fe3+Fe3+ j 30.0 49.8 3.2 20.2 28.0 30.1 14.9 35.9

3545 Fe3+Fe 3+ - - 3.6 - - - - 4.3 - - - - 11.5 3560 Mg Fe 3+ 54.0 30.0 16.4 6.9 38.3 35.6 32.8 23.1 3580 Fe2+AI 2.6 - - 1.7 8.1 6.8 11.4 7.6 8.9 3605 Mg A1 4.3 6.8 46.8 26.7 10.4 8.8 12.9 12.0 3620 A1AI - - - - 12.8 22.6 3.2 3.3 27-6 3.6 3640 A1 A1 - - - - 16.2 15.2 0.6 0.9 4-2 - - X 2 1.55 0.65 0.23 0.29 0.12 0.75 1.01 0.17

~'3528 Fe2+Fe 3+ 21.1 23.0 3.2 6.0 6.3 2.1 0.9 - - * ~, 3534 Fe3+Fe 3+ 8.9 26.8 - - 14.2 21.7 28.0 14.0 35.9

380 M . V . Slonimskaya et al.

of other samples it is superimposed by a stronger band at 3534 cm -1. An unambiguous assignment to this band to Fe3+Fe 3+ follows from a qualitative treatment of the spectrum for sample 132 (Fig. 9d) and a number of other samples having a similar composition, where CFe,+ ~ 1.5 atomic unit (a.u.) per half unit-cell, as well as from decomposition of the spectra for samples 5/1 and 655. Since the difference between the frequencies for the Fe 2+ Fe 3+ and Fe 3+ Fe 3+ bands is close to the sum of the instrumental error and the error resulting from variation of the position of the peak during decomposition, these bands have been sought mainly in the form of a resultant band at 3531 cm -1. The IR spectra of the samples with CFe'+ > 1 a.u. (Table 1, samples 69G, 655, 5/1, 933/3, 132) contain an additional band at 3545 cm -~ which is also related to Fe 3+ Fe 3+. This feature will be discussed below in more detail. The bands at 3495 and 3505 cm -~ related to Fe2+Fe 2+ and MgMg are weak. They have been included to obtain a good fit of the calculated spectrum to the experimental one (min 2,2). The interpretation of these bands is consistent with the mode of dependence of the OH stretching frequencies on the effective charges and masses of the nearest cations. Further study of the standard samples has shown that quantitative correlation of the IR spectrum to the chemical composition of the octahedra cannot be achieved without including these bands in the decomposition.

Note that Kodama et al. (1974) assigned the absorption band at 3595 cm -~ in the spectra of a synthetic mica KMg2.sSi40~o(OH)2 to MgMgOH. The difference between this frequency and the one reported in this paper (Table 2) may be associated with a different distribution of vacancies over cis and trans octahedral sites. Trans octahedra in the samples studied here are known to be vacant while in the Mg-mica they are partly occupied by cations. A similar difference is also observed for positions of MgA1OH bands in the spectra of smectite (3687 cm -~) and glauconite and celadonite (3600 cm -~) (Farmer, 1974). This may be again associated with occupancy of trans octahedra in smectite.

The interpretation of the individual bands was used as the basis for decomposition of the IR spectra of the samples in the OH stretching frequency region. It is known that the wide band of molecular water with its absorption peak at 3400 cm -1 overlaps on its high-frequency side the spectrum of the structural OH stretching vibrations of minerals of the glauconite-celadonite-leucophyllite group. The contribution of this band to the spectrum being studied should be taken into account when this is being decomposed. Previous workers, in similar cases, either interpolated the base line in the region of superposition (Osherovitch & Nikitina, 1975) or graphically subtracted the water band (Lapides & Valeton, 1979) beforehand. A more accurate method is used here, this consisting of the inclusion of a band at 3400 cm -~ in the spectrum being decomposed as one of the components. The spectrum was digitized at 2.5 cm -~ intervals; each spectrum contained between 150 and 200 points. The absorption intensity ~" for each frequency was transformed to optical density ~ according to the expression ~ = In ~7"0/J, ~'0 being the intensity of the incident radiation. The number and the location of the bands for the decompositions were assumed from the interpretation model adopted and the specific form of the given spectrum. The principles of the choice of standard samples have already been mentioned. Here, it will only be emphasized that it is the analysis of the samples with certain cations dominant in the octahedral sheets and therefore with only two or three strong and well-resolved bands in the IR spectrum of the OH stretching vibrations that give reliable foundations for the interpretations proposed, as the errors of determination of integrated densities for the strong bands are small. Decomposition of the spectra was

IR of celadonite and glauconite 381

carried out with the help of a computing program (Dainyak, 1980) assuming a Lorentz form of each component. The variable parameters were the location, half-width and intensity of each component, including the absorption band of molecular water. The value of Z 2 and agreement of the areas of components with the cationic composition of the octahedral sheets served as criteria for the quality of the decomposition. For the decomposition, the areas of the bands transformed to opitcal densities (integrated optical densities) were used rather than the maximum intensities as in some papers (Lapides & Valetov, 1979; Osherovitch & Nikitina, 1975). It is the areas that are proportional to the number of absorbing centres of each type. The absorption coefficient has been assumed to be equal for all the bands*.

R E S U L T S A N D D I S C U S S I O N

Results of decomposition of the spectra of the standard samples will be considered first. Figs 1-4 present the decomposition of these spectra. Table 2 contains the experimental values of the integrated optical densities of the band (~nt) obtained after decomposition of the spectra of the standard samples.

o o o o (D ir

C M - 1 o~ r I I

Fro. 1. Decomposition of the IR s

= - 3 5 6 0 V- )ectrum of Zavalye celad0nite.

O

C M -1

3531

FIo. 2. Decomposition of the IR spectrum of celadonite 69G.

* The absorption coefficients for individual bands in the region of OH stretching vibrations for dioctahedral micas were not determined. However, the mean absorption coefficients for all such bands as well as for V-bands in trioctahedral micas (Rouxhet, 1970) have close values notwithstanding the different octahedral cation compositions. This is an indirect indication of small differences in the values of absorption coefficients for separate bands in the glauconite IR spectra region involved.

0 0 0 0

M. V. Slonimskaya et al.

CM-1

FIG. 3. Decomposition of the IR spectrum of leucophyllite "31.

1 ~ ~l" C M - ~

382

FIG. 4. Decomposition of the IR spectrum of Bolshoi Patom glauconite.

As each OH group is coordinated by two cations, 100% of the integrated optical densities implies 200% of the concentrations of the octahedral cations: CA~ + CFe-+ C Fe~§ + Crag = 2.0 where CAI, CFe,+, CFe2+ and Crag are the concentrations of the corresponding cations in the octahedral sheets. This implies that the content of each octahedral cation (C~) should be equal to the sum of the contribution of this cation to the integrated optical densities Wt~ of the bands determined by those OH groups that contain the given cation in their coordination sphere, e.g.

CAI = (2WAIAI)cal c q- (WAIFe2+)cal r 4- (WAIFe,+)ealc -'b (WAIMg)cal r

As a result, the A1, Mg and (Fe 2+ + Fe 3+) contents can be obtained. The contents of the octahedral cations thus estimated were compared with those obtained from chemical analysis (Tables 3, 4). For all the spectra studied, CIR - Cchem < 0"05 a.u. per half-cell, i.e. the error does not exceed 2.5%.

Satisfactory fitting of the spectrum of 69G celadonite in terms of Z 2 required the inclusion of an additional component at 3545 cm -1. This could be related to Fe3+Fe 3+ along with the Fe3+Fe 3+ band at 3534 cm-1; these bands are analoguous to the two AIAI bands in the spectra of Al-rich samples. The second Fe3+Fe 3+ band was observed in the IR spectra of samples with Fe 3+ contents > 1 per half unit-cell. For example, in the IR spectra of glauconites 132 (Fig. 9d) and 933/3 (formulae in Table 1) and a number of glauconites with similar chemical compositions containing only Fe 3+ and Mg in octahedra with C F~. '~ ~

IR of celadonite and glauconite 383

1.5, the presence of only two bands would be predicted: at 3560 cm - t (MgFe 3+) and 3534 cm -1 (Fe3+Fe~+). However, as for the 69G celadonite with CFe3+ = 1" 15, it proved necessary to include the band at 3545 cm -1 which can not be interpreted otherwise than Fe3+Fe 3+. This band was also observed in the spectrum of Fe-rich glauconite sample 5/1 (see below).

The only exception is celadonite (Zavalye), where CR2. ~ CR3~ In its spectrum the intensity of the base band Fe3+Fe 3+ (3534 cm -~) is relatively low despite the high Fe 3+ content in octahedra (Cre3+ = 0.96). This is due to a high degree of ordering in the cation distribution (Drits et al., 1984), which implies predominance of R2+R 3+ pairs. In this case the presence of the second Fe3+Fe 3+ band would hardly be expected.

Figs 1, 2, 3 and 4 and Table 2 contain results of the decomposition of the spectra of celadonite (Zavalye, 69G), leucophyllite (sample 31) and glauconite (Bolshoi Patom). Octahedral cation contents for these samples presented in Table 3 imply that for the standard samples the interpretation proposed (Table 2, column 2) satisfies, with a high degree of accuracy, the criterion of the agreement of the integrated optical density with the composition of the octahedral cations*. The spectra of these samples also confirmed the interpretation of the bands at 3495 and 3505 cm -~ included previously to obtain a satisfactory value of g 2. Table 3 shows that integrated optical densities of these bands as well as those of the strong ones agree with the cationic composition of the octahedral sheets within experimental error.

The interpretation (Table 2, column 2), checked and refined according to the spectra of standard samples, was used as the basis of analysis of the OH stretching vibrations of different glauconite samples.

The four glauconite samples, 655, Taiheisan, E8/2 and 5/1, provide examples of the more complex IR spectra. In the spectra of these samples (Figs 5-8) there is a distinct discrepancy between the distribution of the optical densities at low and high frequencies and the concentrations of Fe 3+ and AI in the crystaUochemical formula (Table 1).

In this respect it is interesting to consider the spectrum of the glauconite sample 655. The IR spectra of this glauconite presented in Fig. 5 contains a strong band at 3605 cm -1 (MgAi) while, according to the conventional crystallochemical formula (Table 1), the octahedra contain practically no A1.

This discrepancy may be due to the fact that, according to the conventional method of

TABLE 3. Cationic contents of octahedral sheets for standard samples (I) according to the crystallochemical formula. (II) derived from decomposition of IR spectrum.

ple Zavalye 69G 31 B. Patom

Cation ~ (I) (It) (I) (II) (I) (II) (I) (II)

Mg 0.73 0.76 0.41 0.43 0.64 0.69 0.35 0.34 Fe 2+ 0-26 0.24 0.36 0.36 0.07 0.05 0.13 0.14 Fe 3+ 0.96 0.93 1.15 1.14 0.17 0.19 0.41 0.41 AI 0.05 0.07 0.05 0.07 1.10 1.07 1.11 1.11

* According to (Fe 2+ + Fe 3+) contents obtained from the IR spectrum and Fe 2+ contents given by the crystallochemical formula, the integrated optical density for the (Fe2+Fe 3+ + Fe3~Fe 3+) band at 3531 cm -t was divided into two components (Table 2).

384 M. 1I. Slonimskaya et al.

TABLE 4. IR spectroscopy refinement of the distribution of isomorphous cations among tetrahedral and octahedral positions in the glauconite structure: (I) according to the conventional crystallochemical formula,

(II) according to decomposition of IR spectrum, (III) according to the revised crystallochemical formula.

e 655 Taiheizan E8/2 5/1

(I) (II) (III) (I) (II) (III) (I) (II) (III) (I) (II) (ili)

Si 3.64 3.64 3.72 3.72 3.65 3.65 3.69 3.69 Ally 0.36 0.22 0.28 0.15 0.35 0.18 0.31 0.18 Fe~v + - - 0.14 - - 0.13 - - 0.17 - - 0.13

Alvx 0.08 0.25 0.22 0.16 0.29 0.29 0.68 0.84 0.85 0.13 0.28 0.26 Fe~ + 1.08 0.96 0.94 1.07 0.94 0.97 0.79 0.61 0.62 1.30 0.61 1.17 Fe 2§ 0.12 0.13 0.12 0.14 0.13 0.14 0-10 0-09 0.10 1.07 0.09 0.07 Mg 0.66 0.66 0.66 0.67 0.64 0.67 0.43 0.46 0.43 0.50 0.45 0.50

O O

CM-1

FIG. 5. Decomposition of the IR spectrum of gauconite 655.

O O

0 0

FIG. 6. Decomposition of the IR spectrum of Taiheisan glauconite.

O O

CM-1 =o

FIG. 7. Decomposition of the IR spectrum of glauconite E8/2.

IR of celadonite and glauconite 385

0 o

0 o

FIG. 8. Decomposition of the IR spectrum ofglauconite 5/1.

calculation of crystallochemical formulae. Si is replaced first of all by AI, and then by Fe 3+ only when AI is lacking. This may not always be correct. For example, the octahedral A1 and Fe a+ contents estimated from the IR spectrum of sample 655 do not coincide with those of the crystallochemical formulae obtained by the conventional method (Table 4), the difference being 0.25-0.08 = 0.17 a.u. for AI and 0.96-1.08 = -0 .12 a.u. for Fe 3+. The average difference is 0.14 a.u. This may imply that 0.14 a.u. of Fe a+ should be placed in the tetrahedral sites instead of 0.14 a.u. of AI, which then should be placed in octahedral positions. The crystallochemical formula of sample 655 thus revised is also presented in Table 4. This formula is confirmed by considering the data on chemical composition and the unit-ceU parameters of sample 655. The value of b t estimated using the conventional crystallochemical formula is lower than that of bex p. In order to obtain a reasonable value of b t a proportion of the Fe 3+ (0.15 a.u.) should be placed in tetrahedra, which is in agreement with the revised crystallochemical formula.

The crystallochemical formulae of the glauconites of Taiheizan, E8/2 and 5/1 were also revised using the above approach (Table 4), which eliminates the difficulties of their crystal structure simulations based on the 'conventional' crystallochemical formulae; the differences between the octahedral cation contents in the revised formulae and those obtained from the IR data did not exceed 0.05 a.u. per half unit-cell (2.5%), as for the standard samples.

C O N C L U S I O N S

A method for interpreting the OH stretching frequencies of glauconites-celadonites- leucophyilites has been devised using the following main components: correlation of the frequencies with the nearest cationic environment of the OH groups, the mode of spectral decomposition, and correlation of octahedral cation contents with integrated optical densities of the corresponding bands.

Qualitative analysis of the IR spectra of a large collection of these minerals, as well as the results of the decomposition of the spectra of standard samples, indicate that the proposed interpretation of the OH stretching frequencies is valid. In all cases the error in determination of octahedral cation contents according to the IR spectrum did not exceed 0.06 a.u. per half unit-cell.

Analysis of the IR spectra of glauconites E8/2 and 5/1 showed that by using this method the crystallochemical formulae of glauconites could be corrected with respect to A1 and Fe 3+ distribution between tetrahedral and octahedral sites. It is evident that the

386 M. V. Slonimskaya et al.

CM-I 360O ! I !

3400 I

b

C

~ / d

FIG. 9. IR spectra of: (a) glauconite 133G. (b) Oya celadonite, (c) Krivoy Rog celadonite,

(d) glauconite 132.

analysis of the IR spectra of glauconites (celadonites, leucophyllites) according to the method proposed provides a basis for the study of order-disorder in the distribution of isomorphous cations over octahedral cis-positions.

Analysis of the spectrum of the Zavalye celadonite was the most convincing demonstration of this conclusion. Previously M6ssbauer spectroscopy and oblique-texture electron diffraction studies were used to establish the degree of order in the distribution of the R 3+ and the R 2+ cations over eis octahedra when the R 3+ cations are surrounded only by the R 2+ ones and vice versa. This strict alternation ofbi- and trivalent cations is violated only in ten of each hundred cation pairs located in neighbouring cis octahedra sharing OH groups (Dainyak et al., 1981). This is the cation arrangement that the distribution of the integrated optical densities in the IR spectrum of the Zavalye celadonite, decomposed according to the present method, correspond to (Table 2).

IR spectra of the minerals of the glauconite group with minor A1 for Si substitutions, such as celadonites and leucophyllites, are usually better resolved than those of glauconites. This might be due to the requirements for a better charge balance. This requirement implies

IR o f celadonite and glauconite 387

only a few possible octahedral cat ion combinat ions, whereas the number of these combinat ions with the same degree of charge balance increases, thus complicat ing the spectrum.

However, it is not always possible to distinguish between celadonite and glauconite on the basis of I R spectra, as suggested by Buckley et aL (1978). Two celadonite samples (Zavalye and Kr ivoy Rog) may serve to illustrate this. These structures have been refined by Drits et al. (1984) and Zhoukhl is tov et al. (1977), respectively, who showed that

octahedral cation distribution is highly ordered in the first structure and completely disordered in the second.

IR spectra of these celadonites differ substantial ly in resolution (Figs 1, 9c); therefore the resolution of an IR spectrum should depend not only on the heterogeneity in chemical composi t ion but also on the degree of ordering in the octahedral cation distribution. This problem will be treated in detail in a subsequent paper.

A C K N O W L E D G E M E N T S

The authors are grateful to everyone who kindly provided samples. Thanks are due to the referee for valuable criticism. Mrs. B. Smoliar translated the manuscript.

REFERENCES

BUCKLEr B., BEVAN I.C., BROWN K.M., JONSON L.R. & FARMER V.C. (1978) Glauconites and celadonites: two separate mineral species. Mineral. Mag. 42, 373-382.

DAYNYAK L.G. (1980) Interpretation of Mossbauer spectra of some Fe 3+ layer silicates using modeling. Doctorat Thesis, Moscow (in Russian).

DAYNYAK L.G., BOOKIN A.S., DFaTS V.A. & TSIPURSKY S.I. (1981) Mossbauer and electron diffraction study of cation distribution in celadonite. Collected abstracts XII Int. Cong. Crystallography, Ottawa.

DAYNYAK L.G., BOOKIN A.S. & DRITS V.A. (1984) Interpretation of the Mossbauer spectra of dioctahedral Fe3+-layer silicates. III. Celadonite. Kristallografia 29, 312-321 (in Russian).

DRITS V.A., TSIPURSKY S.I. & PLANCON A. (1984) A method for calculation of intensity distribution applied to electron diffraction structure analysis. Izvestia A kad. Sci. SSSR. Phys, Series 22, 1708-1713.

FARMER V.C. (1974) The Layer Silicates. Pp. 331-363 in: The Infrared Spectra of Minerals. Mineralogical Society, London.

LAPIDES I.L. & VALETOV T.A. (1979) Cation ordering in the isomorphous series of amphiboles. Pp. 65-88 in: Cation Ordering in the Structures of Minerals. Nauka, Novosibirsk (in Russian).

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