THE AMERICAN MINERALOGIST, VOL 46, JANUARY FEBRUARY, 1961

ISOMORPHOUS SUBSTITUTION AND INFRA-RED SPECTRAOF THE LAYER LATTICE SILICATES*

V. Sruerdex eNo Rusruu F.ou, The Pennsylaan'ia State(I nia er s'ity, U nia er sity P ar k, P enn s yla ania.

Assrnect

Infra-red spectra of series of dioctahedral and trioctahedral synthetic clays in the region

400-5000 cm.-r have been obtained. Characteristic changes in the IR spectra with ionic

substitution have been observed with both types. From these results some general conclu-

sions concerning the variations of IR spectra with ionic substitution in both groups of ciay

minerals can be drau.n. fn many cases quantitative relationships between frequencies or

intensities of the characteristic absorption bands and the extent of ionic substitutions

could be established. The same Darameters can also be used for the location of substituentions.

INrnooucrrorq

The investigation of solids by the absorption of infra-red rays has at-tracted considerable interest in recent years. The same technique has alsobeen applied to the study of minerals. Among them the infra-red spectraof the clays have been reported by several workers. For example, I luntet a l . (1950, 1951, 1953), Adler (1951), and Kel ler and Picket t (1949,1950) surveyed the infra-red spectra of the principal natural clay min-erals between 650-5000 cm.-l. In the paper of Nahin (1955) the differ-ences between infra-red spectra of some clay minerals as well as the un-resolved problems have been pointed out. Further progress in this fieldwas made when Beutelspacher (1956) extended the spectral range ofstudy to 400 cm.-l, obtaining well resolved spectra of many clay min-erals. An analysis of the l iterature concerning this subject shows thatmuch remains to be done before the infra-red absorption technique canbe successfully applied not only for analytical purposes but also for thestudy of the crystal chemistry and the structure of clay minerals. Toattain this aim it is naturally of the greatest importance to study welldefined "pure" specimens, which can rarely be found in nature. On theother hand, synthetic specimens can be prepared in very pure form, andonly with such authenticated specimens can one hope to approach moredirectly the complex relationship between structure or composition andthe infra-red spectra of clay minerals. Whereas the first part of our re-search (Stubidan and Roy, 1959) was concerned with an attempted as-signment of the absorption bands to the proper metal-oxygen bonds, inthe present investigation an attempt has been made to correlate thenature and extent of ionic substitutions with the changes caused in IR

* Contribution No. 59-74 College of Mineral Industries, The Pennsylvania State Uni-

versity, University Park, Pennsylvania.

32

LAYER LATTICE SILICATES 33

spectra. As a matter of fact the results of the latter approach were veryhelpful in the attempted assignments.

E xrenrlrBNrar PnocBounBs

Preparation oJ the synthet'ic minerals

The synthetic clays and micas were prepared from mixtures of gels.The gels were obtained as previously described by Roy (1956), by start-ing with "Ludox" sil ica sol or ethyl orthosil icate, and solutions ofnitrates (Al, Mg). In some cases, to obtain'the composition required bythe mineral formula, an addition of sodium or potassium hydroxide soru-tion was necessary. The mixtures were evaporated to dryness and thenfired to about 500" C. unti l the nitrates were completely decomposed.The syntheses were carried out in small sealed gold or platinum tubes.A small amount of disti l led water was added before the tubes weresealed. The usual equipment (Roy and Tuttle, 1956) was used for thehydrothermal syntheses. The conditions for the syntheses (temperature,pressure) have in general been the same as previously reported from thislaboratory, except that in some cases the time of hydrothermal runs wasprolonged to obtain better crystall inity of specimens. The crystall inityand the purity of each synthesized specimen were checked by r-rays be-fore infra-red spectra were run. A few natural specimens were used, whenanalyses were available. The specimens investigated are l isted in Table r.

Infra-red. Spectra

Measurements were made with a Perkin-Elmer model 21 double beamspectrometer in the spe

Tasm I. SpncnrnNs Usno rN Tnrs INvrsrrceuoN

I. Trioctahedrol' 2:1 JamilY

1. Talc Mea(SiaOro)(OH)z

2.Ni-Mg-Talc NiMez(SiaOro)(OH)z

Saponites3. Mga(Alo tzSi , er)Oro(OH)r 'Nao,z+4. Mes(AIo

"Sir.oz)Oto(OH)2' Nao s:+

5. Mgr(Alo uoSis uo)Oto(OH)r'Na6 s6+6. Mga(Alo ozSi: n)Oro(OH)z'Nao oz+

The Phlogopite-Biotite Series

7. Phlogopite KMgs(AlSL)Oro(OH),

8. Ni-Mg-Phlogopite KNiMgr(AISir)Oro(OH)z

9. Al-Phlogopite K(Mgz sAlo t(Al1 5Si, r)Oro(OH),

10. Al-Biotite I K(Mgz "AD(A1,

oSiz)Oro(OH),

I I. Dioctahedral 2 : 1 JamilY

ll.Pyrophyllite Ab(SLO'o)(OH)'

Beidellites12. Ah(A10 rzSia sa)Oro(OH)z'Nao rz+13. Alr(Alo nSir oz)Oro(OH)2' Nao ::+14. Alr(Al0 roSi:.so)Oro(OH)z' Nao ro*15. Ah(A10 uzSir n)Oro(OH)r'Nao uz+

Montmorillonites16. (AIr esMgo rz)SirOro(OH)z'Nao rz+

17. (Al1 ozMgo s)SiaOro(OH)z'Nao r+18. (A11 sMgo oz)SirOro(OH)z'Nao oz+

The Muscovite-Phensite Series

19. Muscovite KAl:(AISia)Oro(OH)z

20. f Muscovite and

* Phengite K(Alr zaMgo zs)(Al0 TrSL:s)Oro(OH),

21. Phengite K(AIr aoMgo 56)(AIo soSir so)Oro(OH)z

III. Trioctahedratr 1:1 familY

22. Ant igor i te Mgr(SirOa)(OH)q*

23. Chrysotile Mgi(SirO6)(OH)4

Chlorites24. (Mgz ssAlo rz)(Alo usir s8)O6(OH)425. iMgz zsAlo zs)(Alo raSir zs)Or(OH)r26. (Mgz uoAlo uo)(Alo soSir ro)Os(OH)+t27. (Msz36Al0 64)(Al0 orSir m)Os(OH)r28. (Mgz.:5Alo zu) (Alo zaSir s6)Ou(OH)nf29. (Mgz rrAl0 st(Al0 esSit

")Ou(OH)n30. (Mgz ooAlr oo)(Alr ooSit oo)Ou(OH)ot

IY. Dioclahed'rol lil lamilY

31. Kaolinite Ab(sirot(oH)4

32. Halloysite (Meta) Alr(SizO5)(OH)4

* The natural platy antigorite from Vermlands Taberg, Western Sweden'

t These specimens were investigated both, as the 7 A and 14 A polymorphs'

LAYER LAI:TICE SILICATES J J

\ \ /\ \ /

4

ti

2000 1500 t200 rooo 800 700y' cm-l

Frc. 1 (above). Infra-red spectra of kaolinite (full line) and pyrohyllite (dashed line).Frc. 2 (below). fnfra-retl spectra of antigorite (full line) and talc (dashed line).

Assignments for the main bands are shown on the diagrams themselvesand have been discussed by use elsewhere (Stubidan and Roy, 1959).*Starting with each of these end-member spectra one can follow the vari-ous types of substitutions common in clays:

I. Trioctahedral 2:1 fami,ly

Two types of substitution may be distinguished in this as in mostclay mineral families. llhe first involves a simple ion for ion substitu-tion of Ni2+ for Mgz+. 1112+ is used here as a model for the Fe2+ com-mon in nature yet relatively difficult to handle in the laboratory. Talc,

x Assignments for the main absorption bands have been accomplished by studyinglayer lattice silicates where complete replacement of an ion e.g. Mg2+ by Ni2+ and Fe2+,Ala+ by Ga3+ or Fe3+, Ge4+ for Sia+. The bands involvinE various OH-vibrations were lo-cated by complete replacement of OH- by OD-.

y c m

36 V. STUBIEAN AND R. ROY

3MgO'4SiO2' 2H2O may be regarded as the end-member slructure here'

In this case one can see a distinct shift as follows: the band which ap-

pears in talc at 668 cm.-1 moves slightly towards higher frequencies

(with ]Ni and f Mg it is at 675 cm.-l), and the new poorly resolved band

appears at 710 cm.-l. (Fig. 3.)The second type of substitution involves a balanced charge dual sub-

stitution. In talc itself experiments in this laboratory have demon-

strated that the 2Al3+ for (Mgt+*Si') substitution is very l imited.

However, it is of course well known that the replacement of Sia+ by

(Al3+f l{a+ in exchange positions) gives rise to the saponite family of the

3000 2000 t500 1200 looo 800 70o. - ly c m

600 500- - l

v c m

Frc. 3. Infra-red spectra of NiMg-talc No. 2 (full line) and

NiMg-phlogopite No. 8 (dashed line).

montmoril lonoid; and the extent of substitution is very substantial (see

Koisumi and Roy, 1959).

Saponites

Infra-red spectra of four synthetic specimens (Table I) with increas-

ing amounts of aluminum ions in the tetrahedral sites were obtained.

In this case 1 mg. of each specimen was mixed and pressed with 300

mg. KBr.The spectra obtained (Fig. a) are comparable to the spectrum of talc.

Some differences become more pronounced as the number of aluminum

ions in the tetrahedral sites increases. For example, with increasing

alumina content there is an increasing intensity of the weak absorption

band at 877 cm.-1 as well as the gradual appearance of the weak bands

between 800-850 cm.-1. At the same time the absorption bands at 527,

167 , and 427 cm.-\ are becoming more difiuse with slight changes in their

j - - - - - - \ .H

!

\\

\. ll\rsi-o \sio-M9''

LAYER LATTICE SILICATES

Frc. 4. Infra-red spectra of saponites: No. 3 (full line), No. 4(dashed line), No. 6 (dotted line).

position. The most radical change can be observed with the Si-O band oftalc with the frequency center at 668 cm.-r which decreases in intensityand moves towards lower frequencies. To determine more accurately theposition and the intensity of this band for every synthetic saponite, theabsorption spectra in the frequency region between 600-740 cm.-l wererun using a scale factor of 4 cm./p, (Fig.5).In Fig. 6 one can see that

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