THE ALTERNATING LAYER SEQUENCE OF RECTORITE* W. F. Bneotx,u, Illinois State Geological, Suroey, (Jrbana, Iltinois. Assrnecr

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1 THE ALTERNATNG LAYER EQUENCE OF RECTORTE* W. F. Bneotx,u, llinois tate Geological, uroey, (Jrbana, ltinois. Assrnecr Rectorite is a complex layer type hydrous alumino-silicate related to the better knon mica-like minerals. The structural scheme of rectorite consists of contiguous pairs of pyrophyllitejike units separated by pairs of layers of ater molecules. An equally apt description ould be the alternation of one pyrophyllite unit ith one vermiculite unit. The nature of the arrangement is confirmed by the preparation of the equivalent complex in hich ethylene glycol proxies for ater. The recognition that random mixed layer groths occur in nature ith considerable frequency among the clay and clayjike minerals (, ) has raised the question hether minerals might not exist hich ould be characterized by a specific regularly alternating epitactic intergroth. fn a sense this is precisely the case ith ell knon minerals like the chlorites or the chrondrodite series. The present intention is to demonstrate that the principle extends to more complex instances. Rectorite is included among the species of doubtful validity in Dana,s ixth Edition, and has since been omitted. A specimen of the original material (UN #867), obtained by R. E. Grim from W. F. Foshag of the United tates National useum, has been available for study. Analyses cited by Dana for material dried at C. are: ioz AlzOg HrO FezOa CaO O NauO KrO HrO ('C.) r The specimen is preponderantly monomineral, having associated ith it only a small proportion of a kaolinite group mineral. The eak kaolinite lines in the diagrams of Fig. t have simply been excluded from the analysis hich follos. Excessive preferred orientation, occasioned by characteristic matted texture of the sub-parallel aggregates is also evident in the reproductions, and serves to emphasize the outstanding characteristic of the diagrams, hich is the extended series of orders of diffraction from the basal periodicity. The presence of a hexagon al array of prism diffraction lines reminiscent of those of clays immediately suggests that rectorite is some arrangement of the mica-type hydrous alumino-silicate layer, but the magnitude of the fundamental periodicity, about 5 A, exceeds common values. A valuable supplementary source of information in the study of layer silicates has been the observation that ater layers (hen present in llinois. * Published ith the permission of the Chief, llinois tate Geological urvey, lrbana, 59

$ALTERNATNG LAYER EQUENCE OF RECTOKTE 59 such structures) may be displaced by other liquids, as ethylene glycol, ith production of a ne sequence of diffraction orders for an altered fundamental$

2 ALTERNATNG LAYER EQUENCE OF RECTOKTE 59 such structures) may be displaced by other liquids, as ethylene glycol, ith production of a ne sequence of diffraction orders for an altered fundamental periodicity (). The pair of sequences is then more amenable to interpretation than is a single sequence. n Table are summarized the data employed in interpretation of the diagrams of Fig.. From an earlier study of a number of layered associations of montmorillonite ith organic liquids (3), arbitrarily reduced F-factor estimates and signs ere available as comparison standards hich could be assumed to apply in this present case if it ere true that one mica, montmorillonite, or pyrophyllite layer ere involved in the present structure. The numbers in the F columns of Table ere chosen by comparison of the intensities of the rectorite diagrams ith those of reflections at comparable angles for various montmorillonite-liquid complexes. Ta,sr-n. X-Rav Drrrnecrrolq Dere lon Rnctonrtn rn nnns Relative -bstimated ^. ntensltv - lgn lot " Jlgn lot relatlve urder naturat con- octahedral interlayer mnzuratron orrgln orlgrn -' ^-^e mrtance door:l5.ua t t t,3 5 l6 'ti 8 9 coincident ith kaolinite 6 ) U - - Relative Lstrmatro intensitv sign for ign for relatlve r elvcol octahedral interlayer.l-pt"* l:--^ origin origin P{'rtance a,,.':io.oa t W W.3 6 t z il i )omitted J Figure represents the results of one-dimensional Fourier synthesis based on the F-values and signs so chosen. The traces from z:. to ' are typical for a three-component layer. The intervening space is occupied in both cases by five lesser maxima hich lose definition in the glycol reaction. One suggestion is that this space includes a second threecomponent unit unsymmetrically displaced ith respect to the conven-

$. X-ray poder diffraction diagrams of rectorite; (o) natural material, (6) complex ith ethylene glycol, (c) natural material dried at ' c, (d) natural material roasted at 6'C everal$ important orders are marked for reference. tional origin. Rearrangement of signs to conform to an origin at the trough near z:.

important orders are marked for reference. tional origin. Rearrangement of signs to conform to an origin at the trough near z:.

3 W. F. BRADLEY Frc.. X-ray poder diffraction diagrams of rectorite; (o) natural material, (6) complex ith ethylene glycol, (c) natural material dried at ' c, (d) natural material roasted at 6'C everal important orders are marked for reference. tional origin. Rearrangement of signs to conform to an origin at the trough near z:. affords the second pafu of syntheses illustrated in Fig.3. These traces are readily interpreted as contiguous pairs of three component, pyrophyilite-type layers, interleaved in the one case by double layers of ater and in the other by double layers of ethylene glycol. Rectorite does not sell or disperse in ater. pecimens irradiated hile bathed in ater exhibit the same 5 A periodicity as does the

ALTERNATNG LAYER EQUENCE OF RECTORTE 593 natural material. The molecular ater layers are hoever removed by oven drying, after hich a pyrophyllite-like dehydration product remains.

4 ALTERNATNG LAYER EQUENCE OF RECTORTE 593 natural material. The molecular ater layers are hoever removed by oven drying, after hich a pyrophyllite-like dehydration product remains. The condition of the dehydration product can be discussed in some Frc.. Preliminary electron density projections from z: to z:.5 for rectorite (upper) and for the rectorite glycoi complex (loer). The origin lies in an octahedral layer. ta N d z=5 -l- Frc. 3. Electron density projections from z: to z:.5 using interlayer origin for rectorite (upper) and for its glycol complex (loer). Levels of constituent atoms for the ideal structural model are marked. ta detail. Thermal analysis () indicates that the molecular ater is removed in three fractions. To rather arge unresolved endothermal effects center around and 5" C., and a lesser ill defined trend extends to about 3o C. This character, together ith the absence of

$Diffraction diagrams of partially dehydrated material, of hich one is illustrated in Fig., sho distinct lack of homogeneity. Lo orders are relatively sharp and at least ap- 6 proximatelyrational.$

5 59 W, F, BRADLEY selling jusf noted, is suggestive of the vermiculite type of ater layer rather than the montmorillonite type. Diffraction diagrams of partially dehydrated material, of hich one is illustrated in Fig., sho distinct lack of homogeneity. Lo orders are relatively sharp and at least ap- 6 proximatelyrational. Higher orders are complex and appear to consist of relatively sharp rational components ith hich there are associated more or less difiuse scattering maxima. For parallel to sutr-parallel aggregates such as those ith hich e are here concerned, penetration of radiation into the aggregates is less for lo angle difiraction than for the higher orders. The conclusion may therefore be made that for the parq zo}j, Al OH 6 6 OH Al oll 6 HzO a 6 OH Al Frc.. chematic representation of the seouence of atomic iayers in rectorite.

ALTERNATNG LAYER EQUENCE OF RECTORTE 595 tially dehydrated aggregates a surface skin of a certain number of dehydrated layers encloses interior layers ith hich ater either may or may not still be

6 ALTERNATNG LAYER EQUENCE OF RECTORTE 595 tially dehydrated aggregates a surface skin of a certain number of dehydrated layers encloses interior layers ith hich ater either may or may not still be associated, the interior seque-nce being a random array. The surface layer periodicity is about 9.5 A and the interior average periodicity about or A. pecimens heated to 6o C. have again become homogenous, and differ for the anhydrides prepared from montmorillonite or related minerals only in the persistence of a eak first order line for the doubled periodicity; that is about 9.5 A. This order apparently signifies that the "pyrophyllite" neighbors hich ere originally together continue to stand closer to each other than to those neighbors hich ere previously separated by ater, and are probably still separated by the not inconsiderable number of Na ions of the cited analysis hich ere presumably associated ith this ater. Chemically, this assemblage is in agreement ith the analyses quoted by Dana if alloance be made for the or 5 per cent of kaolinite hich as probably present in the analyzed specimens. Optically, the refractivities are essentially the same as for Na-montmorillonite. The mean index ould be about.5-.5 ith birefringence lo. No observed index is cited by Dana. n coordinating the r-ray diffraction information ith the thermal character it can probably best be said that both alumino-silicate layers resemble pyrophyllite and that the ater is in a condition comparable ith that of the vermiculites. The structure then amounts to alternate piling of one pyrophyllite unit on one vermiculite unit. The schematic sequence is illustrated in Fig.. Alternative braces illustrate the ambiguity of origin hich arises accordingly as one considers the layers to be geometrical or chemical entities. The question of hether a study such as this can be said to validate the specific rank of rectorite as a mineral is probably one hich should be deferred. The number of incompletely characterized doubtful species and variety names is too great to fairly estimate ho many might be intergroths of simpler systems. RrlennNcrs. Bneory,W.F.,Diagnosticcriteriaforclayminerals:,n.ineral.,3Or7-73(95).. Guu, R. E. & BnAlr.rv, W. F., The illite clays: nternational, Geological Congress,vol of titles and abstracts, London, pp.7-8 (98). 3. Bnerr,ev, W. F., olecular associations beteen montmorillonite and some polyfunctional organic liquids: -. Am. Chem. oc.,67, (95).. Gnru, R. E. & RoreNn, R. A., Difierential thermal anaiysis of clay minerals and other hydrous materials: Am. inerol,.,7, 8 (9).

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