SEPIOLITE AND RELATED COMPOUNDS: ITS STABILITY AND APPLICATION by

The unit-cell characteristics and structures of sepiolite, ferri-sepiolites (xylotile, mountain wood, gunnbjarnite), loughlinite and palygorskite (attapulgite) and their structural relationships are discussed. The chemical, morphological and thermal properties are explained by their structures. The unit-cell characteristics and structures of sepiolite "anhydride ", loughlinite "anhydride" and palygorskite "anhydride" are given. Under low water vapor pressure the phase transformations sepiolite to sepiolite "anhydride" and palygorskite to palygorskite "anhydride" are reversible. I t was not possible to synthesize sepiolite in the system lYigO-SiO2-H20 under confined pressure within a range of 80 ~ to 300 ~ C The possibility of the application of sepiolite and related compounds in industry, specially in the automobile industry, is referred to. There is much va r i e t y in the occurrences of sepiolite. Sepioli te occurs in the form of lumps (e.g. Eskischir , Turkey) , s ed imen ta ry beds (e.g. Amboschi , Tanganyika) , m a t t e d p l a t e s (so-called m o u n t a i n leather) , or fibers (e.g~ A m p a n d r a n d a v a , Madagascar) . Al l these forms are based on long single c rys ta l fibers wi th th icknesses of only 100/~ to several hundred A. The fel t ing in one, two, or three d imensions of these ex t r eme ly th in single c rys ta l fibers leads to the i r aggrega t ion in to these var ious forms. The s t ruc ture of sepiol i te was de t e rmined b y Brauner and Preis inger (1956). The ideal fo rmula of the s t ruc ture is : 1 (II~O)n. 1 {Mgs(H20)4(OH),[Si12Os0]}; n~< 8. co co I t s uni t -cel l character is t ics are : a = 1 3 . 4 ~ ; b = e 6 . s h ; ~ = 5 e s s z = e; P 2 1 2 e D ~ h n ~ n The minera ls known in the l i t e ra tu re as xylot i le , m o u n t a i n wood and gunnb ja rn i t e are i sos t ruc tura l wi th sepioli te (Brauner and Preisinger, 1956; St runz, 1957). In al l these minera ls the Mg ~+ ion is p a r t l y subs t i t u t ed by the Fe 3+ ion, 365 366 TENTH NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS and the (OH)group by the 02ion. The ideal formula of these ferri-sepiolites (xylotile, mountain wood and gunnbjarnite) is: l(H~O)n, l{(Mg,Fea+)s(H~O)4(OH,O)4[Si120a0]); n ~< 8. co co Very similar to the structure of sepiolite is the structure of the mineral loughlinite, in which some of the Mg ions are substituted by Na ions. The formula of the structure of loughlinite is: l(H20)nl{Na4Mg~(H20)4(OH)4[Sii2Oao]}; n ~< 10. co co I t s unit-cell characteristics are: a = 14.66~; b = 26. 71_~; c = 5.26~; Z = 2; p2X2-2-D~h. ~ b C ~ b The difference between the two structures (Fig. 1) is only tha t the position of the Mg ion a t the end of the complex is substituted by the two Na ions. One of these Na ions substitutes the Mg ion itself; the other is coordinated by 6 H20 molecules. The calculation of the exact parameters is in progress. Fahey, Ross and Axelrod (1960) showed tha t loughlinite can be transformed into sepiolite by treating it with MgCI~ solutions. When looking at these two structures (Fig. 1), one can readily understand the result of the experiment. The Na ions are deposited in the channels surrounded by water and thus are easily exchangeable. A comparison of sepiolite (Brauner and Prcisinger, 1956) and palygorskite (attapulgite; Bradley, 1940) shows a similar build-up (Fig. 2). In the (100) plane there are layers of silicate tetrahedra with unshared corners alternately pointed up or down. In palygorskite they alternate after two chains; in sepiolite they alternate after three chains. In palygorskite the six coordinated cations are Mg and A1, with a ratio ofMg: A1 varying between 3 : 2 and 2 : 3 (Doelter, 1917). The ideal formula for the structure of palygorskite is : l (H20)n, l{Mg~_ax,A12z)(H20)4(OH)2[Sis020]); n ~< 4. co co The unit-cell characteristics of palygorskite from Palygorskaja are: a = 12.65/~; b = 17.9/~; c = 5.26~; Z = 2; P 2 1 2 2 bran D~h" A comparison of sepiolite with Mg-layer silicates, as talc, stevensite, saponite, vermiculite, etc., shows tha t all these possess layers of silicate with unshared corners of tetrahedra, but in all these lat ter minerals the direction of the tips does not alternate within the same sheet. All these fibrous minerals, in which the layers of silicate tetrahedra alternate in orientation of unshared corners, contain three kinds of water molecules: zeolitie water, crystal water and constitution water (OH groups). The water SEPIOLITE AND I~ELIT]~D COMPOUNDS: ITS STABILITY AN]) APPLICATION 367 content of these minerals depends on the humidi ty . The zeolitie water lies in the channels and its loss is reversible. As shown by Preisinger (1959), no change in the unit-cell dimensions of sepiolite takes place up to 380 ~ C. I f sepiolite is