THE AMERICAN MINERALOGIST, VOL 44, JULY_AUGUST, 1959

HIGH TEMPERATURE PHASES IN SEPIOLITB, ATTAPULGITE AND Gnoncrs Kulnrcrr,* Departmentof Geology, Uniters'ityof lllinois, Urbana,Illinois.

AssrnA.cr

The high temperature reactions of , attapulgite and saponite were studied by continuous high temperatute rc-ray diffraction techniques. The easy formation of enstatite from the fibrous is explained by structural analogy. The reactions of the well crystallized specimens of sepiolite and attapulgite difier somewhat from those of their massive sedimentary varieties. The difierences cannot be ex- plained with the chemical and structural data, suggesting possible variations in some inti- mate details of the framework of these two varieties.

INrnooucuox The nature of the crystalline phases formed by firing the magnesian clay mineralshas beendescribed (2,3,7, I0, 12,15, 16),but only for the well crystallized chlorites (3) have the precise conditions of formation of these phases as well as their structural relationships with the starting material been clearly determined. The three minerals chosenfor this investigation provide difierent struc- tural arrangementsof the same type of lattice in a seriesof Al-Mg hydrous silicates.Saponite and sepiolite have the same bulk composi- tion but they difier in the mode of assemblage,i.e. layers or ribbons, of their structural units. Attapulgite has the same kind of framework as sepiolite, but a large proportion of has been replaced by aluminum or iron. It was thought that, by taking advantage of these features and by using a method of continuous high-tempetature x-ray diffraction analysis,a new contribution to the problem would be possible. Details of the procedurewere describedpreviously (8, 9). Differential thermal curves with a heating range extended to 1400' C' were also obtained. Sanpr-BsINvBsrrcerBp Sepiolite Sepiolite may occur either in large fibrous crystals associated with hydrothermal deposits, or in cryptocrystalline masses of sedimentary origin. Specimensof both types were included in this investigation. Many samples,from various localities were examined and their purity checked by r-ray diffraction. Two apparently homogeneousspecimens were re- tained for high temperature study: an excellent fibrous sepiolite from Ampandandrava (Madagascar) and a fine grained sedimentary one

* Present address: Centre de RecherchesS.N.P.A., Pau (B-P), . 752 SEPIOLITE,ATTAPULGITE AND SAPONITE 753 from Vallejas (). The former containssome free cristobalitewhich could not be separatedbut did not afiect the high temperaturereactions (Fig. 1). Attopulgite Attapulgite also occurs in sedimentary as well as in probable hydro- thermal deposits(mountain leather).A sampleof mountain leather from the Aleutian Islands* appeared to be a very well crystallized and seem- ingly pure attapulgite (Fig. 1), but it was impossible to find a sedi- mentary attapulgite free of other clay minerals. For example, in the courseof an extensivestudy of every clay horizon of the Hawthorne formation, in the Attapulgus, Georgia,area (more than 500samples have been so far examined and determined), attapulgite was always found with variable amounts of other clay minerals:sepiolite, , iilite, or . The selectedsample contained an estimated5 to I07o montmorillonite which was removed by centrifugation (Fig. 1). No chemical analysis of the mountain leather was available, but data published for specimensof similar origin suggesta relatively high alumi- num content (Table 1). The Attapulgus sampleis rich in magnesiumand iron. Saponite The selected sample comes from the bentonite deposits of Ksabi (). It is apparently free of impurities, but its very poor diffrac- T.csr,n1. Cnrurcer,Dare ol R,q,wSlupr,as (Rrcoupurro Wrrnour rnr W,trrn)

SiOz 66.82 tu .51 63.07 66.67 67.28 69.68 Mgo 27.12 21.82 34.12 13.86 8.42 6.93 Alzo: . /o 2.79 1.02 10.72 18.05 r7.r7 FezOa 3.81 2.20 t.19 s.78 2.0+ 2.67 FeO .89 .41 CaO .60 2.62 .01 t.92 3.80 3.55 TiOz .61 NazO .52 KzO .60 .43 PzOs 1.61

1. Sepiolite from Ampandandrava (after Caillere) (5). 2. Sepiolite from Vallejas (after Vivaldi, Cano Ruiz) (11). 3. Saponite from Ksabi (after Millot) (13). 4. Attapulgite from Attapulgus (courtesy of Minerals and Chemical Corp. of America). 5. from Nijni-Novgorod (after Caillere) (5). 6. Palygorskite from Taodeni (after Caillere) (5). * Courtesy of Dr. W. F. Bradley. 754 GEORGESKULBICKI

Deqrees-2e

Frc. 1. X-ray spectrometer traces: A. Sepiolite (Vallejas). B. Sepiolite (Ampandan- drava). C. Attapulgite (Georgia). D. Attapulgite (Aleutian Islands). E. Saponite (Ksabi). Unoriented aggregate. Cu.Ka. 2'/l minute. SEPIOLITE, ATTAPULGITE AND SAPONITE

Degrees- 2 O

Frc.2. X-ray spectrometer traces: A. Sepiolite (Vallejas) heated at 1400' C. B. Sepiolite (Ampandandrava) heated at 850" C. Enstatite lines are indexed. Cr: cristobalite. tion diagram (Fig. 1) suggestsmany dislocationsand lattice irregularities. It is believed to have the same composition as the specimen described by G. i\{illot from the samedeposit (13) which has but little substitutions for magnesium(Table 1). The influenceof exchangedor adsorbedcations on the nature and the conditionsof formation of high temperaturephases in clay mineralswas shown previously (9). Nlost of thesecations can be eliminatedby two 15 minute soakingsin HCI N/10, followed by a filtration. Except when otherwiseindicated, the specimensof this study receivedthis treatment. RBsurrs Sed,iment ar y se piolit e The diffraction pattern of the disappears-around 800' C. (Fig. 3-A). Simultaneously,two broad lines at 3.20 A (27.8",20) and 2.90 A (30.8o,20) arerecorded. These lines grow slowly between1100" C. and 1350" C., and at that temperature they increasesubstantially and sharpen, while others appear, completing a pattern of enstatite (Fig. 2-A). Some B-cristobaliteis formed at the same time (1350'C). When the firing is stopped,at 1500"C., the sampleis sinteredbut not melted, and enstatiteand p-cristobaliteare still present.Below 1400' C. the two /JO GEORGESKULBICK]

Deqrees- C

Frc.3. Intensity of difiraction by high temperature phases in heated specimens of: A. Sepiolite (Vallejas). Treated with HCI. B. Sepiolite (Vallejas). No treatment. C. Sepio- lite (Ampandandrava). Treated rvith HCl. D. Sepiolite (Ampandandrava). No treatment. E. Attapulgite (Georgia). Treated with HCI. F. Attapulgite (Aleutian Island). Treated with HCl. G. Saponite (Ksabi). Treated with HCl. Er and Ez: (610) and (420) oI enstatite. SEPIOLITE,ATTAPULGITB AND SAPONITE 757 main reflectionsof enstatite (610) and (420) have approximately the same intensity; between 1400' C. and 1500oC. the pattern becomes normal and (420) is twice as intenseas (610). The differential thermal analysis curve shows an endothermic peak at 800' C. followed immediately by an exothermicreaction (Fig. 4-A). The endothermalfigure can be related to the departureof water shownon the dehydration curves (10, 11, 12). The exothermalbranch is very sudden and sharp, suggesting,according to Bradley and Grim (2),"the incorpo- ration into new phasesof large articulate units from reactant structures, without catastrophicrearrangements within the units." There is no other peak on the curve but, above 1200"C., the shrinkageof the sampledue to sinteringand, possibly,the slow crystallizationof B-cristobalite,cause the curve to rise gradually. Since there is no differencein the hish temperature reactionsbefore

2O0 4OO 600 8OO |OOO l2oo l4oo Degrees- G

4. D.T.A. curves. Specimenstreated u'ith HCI. A. Sepiolite (Vallejas). B. Atta- pulgite (Georgia). C. Attapulgite (Aleutian Islands). D. Saponite (Ksabi). /Jd GEORGESKULBILKI and after HCI treatment, it can be concludedthat exchangeableMg and extraneousCa cations have no appreciableinfluence on thesereactions (Fig.3-A,B).

H y dr othermal se p'iolite Enstatite beginsto form at 800oC. (Fig. 3-C). Unlike the sedimentary sepiolite, the (610) reflection is more intense than (420). B-cristobalite appearsat 1075oC., developingstepwise at 1300oC. and 1450' C. with correspondingdecrease of enstatite. It is noteworthy that a sample heatedat 1400oC. keepsthe needleshape of its particles,as revealedby microscopicobservation. The removal of exchangeableor extraneouscations by HCI treatment doesnot notably afiect the high temperaturephases (Fig. 3-C,D), The high temperature diagrams of the two sepiolite specimens (Fig. 3-A, C) show notabledifierences in the growing of enstatite,the tempera- ture of initiation and the amount of d-cristobalite. These difierences cannot be accounted for by the size of the crystals in the two specimens nor by the slight differencesin bulk composition since the sepiolite which containsless silica grows more B-cristobaliteand at a lower temperature. No explanationcan be proposedat this time.

Sed,iment ar y alta p ul gil e Like the sepiolite specimens,enstatite is formed when the clay struc- ture is destroyed,at 800o C. (Fig.3-E). The formation of B-quafiz at 1100' C. seemsto accentuatethe enstatite reflections.Both minerals disappear at 1200" C. with the formation of B-cristobalite. The (420) reflectionof enstatite is here 4 times smaller than (610), indicating the preferential growth of the crystals in the o direction. The D.T.A. curve (Fig. 4-B) showsa singlesharp exothermalpeak at 800' C. marking the formation of enstatite. A secondexothermal peak around 1100" C. can be correlatedwith the crystallization oI B-. The other featuresare faint and cannot be interpreted. Various cations(NHa, K, Cs, Al, Ba, Ni . . . ) were fixed on the lattice by baseexchange. They did not afiect visibly the formation of enstatite, but they acted on the silica phases.The efiects were similar to those ob- servedwith (9). The most notable was the absenceof B-quartz and B-cristobalite in the specimenscontaining K or Cs.

Mountain leather Enstatite is formed at the same temperature as for sepiolite but only in a small quantity (Fig.3-F). It is not affected by the produclion of SEPIOLITE,ATTAPULGITE AND SAPONITE 759

B-quartz at 1100o C. At 1200' C. B-quartz inverts to B-cristobalite. Another magnesian phase, cordierite, appears at 1200" C. It must be noted herethat, exceptfor the formation of enstatitebetween 800o C. and 1200oC., this specimenbehaves like a Nlg-rich montmorillonite. Thus, a high temperature diagram like Fig. 3-F, including B-quartz,B-cristoba- lite and cordierite,is typical of a dioctahedralmontmorillonite conlaining a considerableamount of Mg (4-5%) replacingAl (8, 9). Baseexchange treatmentswere also made with various cations,but, unlike sedimentary attapulgite or montmorillonite, no appreciablechange in the high tem- perature phaseswas recorded. Possibly this indicates only the difficulty of introducing significant amounts of calions inside the framework of this largely crystallized specimen. At 1200' C. the material is well agglomeratedand the needleshape of the starting product is no longer recognizableunder the microscope. Above 800oC., the D.T.A. curve differs completely from that of the Attapulgus sample (Fig. a-C). Indeed, it is identical with a curve of I{g-rich dioctahedralmontmorillonite (8, 9). The collapseof the struc- ture seemsto causethe endothermalpeak at 805oC. and the crystalliza- tion of B-quartz correlateswith the exothermicpeak at 100" C.

Saponite The reactionsof saponiteare very similar to those of the well crystal- lized sepiolite,but there is very little enstatite at the beginningand its growth is very slow (Fig.3-G). The relative intensities of (420) and (610) are those of a normal pattern, indicating no preferentialgrowing of the crystals.B-cristobalite appears in appreciableamount at 1200"C. and is accompaniedby a substantialincrease of enstatite.The reactions cannot be closelycorrelated with the thermal curves,probably because they are not sufficientlyabrupt. After heatingat 1500"C. the specimenis not fusedand the two phases are still present. The high temperature diagrams of the specimensinvestigated show some significant differencesbetween the well crystallized sepiolite and attapulgite and their sedimentarymassive varieties. It has been noted for example that the former have many similarities with the Iayer minerals of the montmorillonite group. This is not expected from the compositionnor the existing structural data. It is suggestedthat some intimate structural features may be responsibleand that the distinction between the two varieties of sepiolite or attapulgite is more than an accidental difference in crystal size due to different growing environ- ments. More studiesare neededon this ooint. 7ffi GEORGESKULBICKI

Rnlranrs oN THE FonunrroN ol ENsrarrrn The enstatite nucleation around 800' C. is a common feature of all specimens,which seemsat first to indicate that the reaction is primarily due to the magnesiumcontent, regardlessof the structure of the starting material. Ifowever, it occurs more rapidly and with more intensity in most specimensof the fibrous type minerals, sepiolite and attapulgite, suggestingin turn a favorable effect of this kind of assemblage.This point seemsto be supported by the examination of the different structures. The various structures encounteredin this study have been schemati- cally representedin Figs. 5, 6, 7, 8 and 9. These stylized drawings are projections onto the bc plane for saponite, and ab for the others. It should

Frc. 5. Idealized structure for attapulgite projected onto 001 (after Bradley).

Frc.6. Idealized structure for sepiolite projected onto 001 (after Brauner-Preisinger). SEPIOLITE, ATTAPULGITE AN D SAPONITE 761

Frc. 7. Idealized structure for sepiolite projected onto 001 (after Bradley). be rememberedthat these planes are structurally identical, since the usagehas been to call the 5.2 A period of these silicates,o in the layer structuresand c in the fibrous structures. For the purpose of the discussion, the hexagonal net of tetrahedra making up the silica Iayers of all these minerals can be seenas groupings of silica chains (Siy 06) extendingin a direction normal to the drawing. In fact, if we disregard isomorphous replacements, these structures are identically made of groupsof "basic units" which are formed of two such silica chains linked by octahedral cations. The relative disposition of theseunits in the various structuresis shown in Fie. 9.

Frc. 8. Idealized structure for enstatite projected onto 001. 762 GEORGESRULBICKI

Frc. 9. Schematic structures for the investigated minerals showing the mode of associa- tion of the "basic-chain-units" (dotted lines). A. Sepiolite projected onto 001 (Brauner- Preisinger proposed structure). B. Attapulgite projected onto 001 (Bradley proposed structure). C. Saponite projected onto 100.D. Enstatite projected onto 001. E. Dehydrated sepiolite projected onto 001 (after Preisinger) The angle of tilting is arbitrary.

The close relationships between the enstatite structure and the structures of sepioliteand attapulgite appear clearly. They explain the easeof formation of the pyroxene from the latter minerals: the genera- tion of enstatite can be achieved through the rearrangement of whole "basic units" reacting edgewiseand remaining parallel to their primitive elongation, without any major disturbance within these units. Thus enstatite nuclei will be generated upon breaking of the oxygen-ribbon- SEPIOLITE,ATTAPULGITE AND SAPONITE 763 edgebonds of the sepioliteassemblage and their linking to the I{g of the octahedrallevel (Fig. 6, 8 and 9). These nuclei extend mainly in the a and c directions,resulting in only two diffraction lines (610) and (420) with the predominanceof (610). In another step, the 3 "basic units" making up a sepioliteribbon will be separatedand then complete the enstatite framework. Theseviews are basedon one of the two alternative sepiolitestructures (4, 14, 15) (Fig. 6). In the other one, where the ribbons are linked by double oxygen bonds (Fig. 7), the formation of enstatite as described does not appear immediately possiblesince it necessitatesa more com- plete reorganization of the atoms. It is suggestedthat the differencesin the production of enstatite in various sepiolitespecimens might be due to the coexistenceof both structuresin a given specimen. Preisinger(15) has shown that below 800" C. the ribbons of sepiolite take a tilted position as indicated in Fig. 9. The same thing has since been observedfor attapulgite by W. F. Bradley and the authorx who show that large exchangeablecations like potassium may prevent this tilting to a certain extent. Ifowever, in the experimentsreported here the amount of tilting of the ribbons doesnot seemto affect the formation of enstatite. In the caseof attapulgite, the reaction could be theoritically seenin the same way, the variations in the production of enstatite indicating the existenceof two alternativelinkages of the ribbons (1, 14). However, the difierent compositionof the octahedrallayer may, in addition, cause somenotable difierences. In this mineral,chemical analyses indicate that, in general,only ] to f of the octahedralcations are Mg. If the formation of enstatite necessitatesNIg ions along the edgesof the ribbons, it will vary with the proportion and with the position of this cation; but in every caseit should be lessintense than in a sepiolitespecimen. On the contrary, high temperaturediagrams indicate that, in some instances, enstatite forms more easily than from sepiolite (Fig.3). It is suggested that the separation of the "basic units," which is necessaryfor the building of enstatite, is controlled by the disposition of the cations within the octahedral sheet. Unfortunately, nothing is known of the pattern versuscomposition of the octahedrallevel to predict the behavior of a specimen. In saponite, the slownessof the enstatite crystallizationmay be at- tributed to the layer structure of the mineral. Some nuclei are first realized along the edge-contactsprovided by latice irregularities and dislocationswithin the crystals. Although these are not negligible,the reactionremains limited to a relatively small number of positions.Upon

x Report in preparation. 764 GEORGESKULBICKI

increasedheating, the "basic units" are progressivelyseparated and they contribute to the gradual growing of the pyroxene structure.

AcxNowrBlcMENTS The author wishesto expresshis gratitude to Dr. R' E. Grim for his guidance and aid throughout the course of this investigation' He is also deeply grateful to Dr. W. F. Bradley who contributed valuable criti- cismsand suggestionsin the structural aspectsof the problem. Rrl'rruNces

1. Bterr-nv, W. F. The structural scheme of attapulgite: Am. Mineral.,25' 4105-510, 1940. 2. Bnnor,ov, W. F. aNl Gnru, R. E. Thermal effects of clay and related sediments: Am' Mineral ,36, 182-201, 1951. 3. Bnrsrr-rv, G. W. eNo Lr't, S. Z Thermal transformations in magnesian chlorite: Acto Crist.,3, 25-30, 1950. 4. Bnruwrn, K. eNn PnatsrNcnn, A. Struktur und Entstehung dex sepioliths'. Tschermaks Min. Pet Mit.,l-2, 120-140, 1956 5. Clnlrnn, S Chap. VIII in X-ray identification and structure of clay minerals: Monograph of Min. Soc. London 1951. 6. CAlr,r-rnn, S. ,wn Hrwrw, S. Chap. IX in "X-ray identification and structure of clay minerals": Monograph of Min. Soc. London 1951. 7. C.lrr,r,ana, S. enn Hrrrx, S. Chap. IX in "The difierential thermal investigation of clays": Monograph of Min. Soc. London 1957. 8. Gnru, R. E. exn Kur,nrcrr, G. Etude aux rayons X des reactions des mineraux argileux a haute temperature: Bul'|. Soc.Fr. Cerom.,361 2l-28, 1957. 9. Kur,nrcrr, G. High temperature phases in montmorillonites: 5th Nat. CIay Conf., Urbana, 1956. Nat. Research Coun. 1958. 10. LoNccrraurox, H. Sur certaines caracteristiques de la sepiolite d'Anpandandrava (Madagascar) et la formule des : Bull,' Soc. Min. Fr.,60' 232-276. 1937' 11. M,mflN, Vwar.nr J. L. eNo CaNo, Rurz J. Contribucion ai estudio de Ia sepiolita I. Caracterizacion y propiedades de sepiolites espanolas: Anal'. Ed'afol'. Madri'il, 12, 827-855, 1953. II. "Some considerations regarding the mineralogical formula": Pro- ceedingsIV Nat. Clay Conf. Nat. Research Council, 1956. 12. Mrcnom, G. Sepiolites:BuIl. Soc. Fr. Min.,59, Gl34' 1936. 13. Mrr.r.or, G. C R.Ac.Sc. ff2, lt Jan. 257-259, 1954. 14. Necv, B. euo Bnmlnv, W. F. The structural schemeof sepiolite: Am. Minerol'., 4O, 885-892,1955. 15. PnBrsrxcnn, A, "An X-ray study of the structure of sepiolite": VI Nat. CIay Conf', Berkeley 1957. 16. Tnn-o, E. lNo Roccn, G. Slrukturbericht,T,164' 1939.

Manuscri.pt receiaedNorember 8, 1958.