SILICON ALUMINIUM ORDERING IN THE FRAMEWORK OF ZEOLITES GLAUCO GOTTARDI and ALBERTO ALBERTI GOTTARDI, GLAUCO and ALBERTI, ALBERTO, 1985: Silicon aluminium ordering in the framework of zeolites. Bull. Geol. Soc. Finland 57, Part 1—2, 197—206. The (Si, Al) ordering scheme in the framework of zeolites is reviewed, and four classes are recognized: 1 — zeolites with Si: Al = 1 : 1 and a simple order given by the alternation of Si and Al; 2 — zeolites with Si> Al and an ordered distribution; 3 — zeolites with Si>Al and partial Al-enrichment in some tetrahedra; 4 — zeolites with Si>Al and complete disorder, or at least with a framework that in principle favours disorder. A crystallization model requiring the formation of cages of tetrahedra around a hydrated cation acting as a template is suitable for classes 3 and 4, but not for classes 1 and 2, which require the standard crystal growth scheme. Key words: zeolite, framework, (Si, Al) order, crystallization. Glauco Gottardi and Alberto Alberti: Istituto di Mineralogia e Petrologia, Univer- sitå di Modena, via S.Eufemia 19, 41100 Modena, Italy. Introduction Class 1: Zeolites with Si: (Si 4- Al) = 0.50. These obey Loewenstein's rule and exhibit or- The distribution of silicon and aluminium in dered distribution given by the alternation of Si the centre of the tetrahedra of framework sili- and Al in the tetrahedra. An obvious conse- cates may be ordered or disordered, and every quence is that the rings with an odd number of mineralogist knows how hard it is to solve the tetrahedra are prohibited in their frameworks; enigma of the real (Si, Al) order in plagioclases, (Si, Al) disorder is possible only if Si: (Si + Al) concealed in their modulated domain structure. is slightly over 50 °7o. The (Si, Al) distribution in the framework of Class 2: Zeolites with Si: (Si + Al)>0.5, zeolites has certainly not been the subject of in- which normally have nearly perfect (Si, Al) vestigations so detailed as those on feldspars: order; disorder is possible but rare. our paper therefore aims to give the present Class 3: Zeolites with Si: (Si + Al)>0.5 and a status of the subject matter for natural zeolites small but definite Al-enrichment repeatedly and to draw some conclusions on their crystal- found in the same tetrahedra. lization. Class 4: Zeolites with any ratio Si: (Si + Al), From this particular point of view and without in which disorder is highly favoured by their any other purpose, zeolites may be subdivided structural features, although order is possible, into four classes: and has been detected clearly in some cases. 198 Glauco Gottardi and Alberto Alberti The Al fraction in a tetrahedron is normally tetrahedra are occupied by Si. They feature X- deduced from the average T—O distance, for ray powder patterns with a weakening of all instance, using Jones' relation, 1968; here this diffractions with 1 odd, clear evidence of disor- relation will be used in a qualitative way only. der (Wise and Tschernich 1978). When describ- We use T as a code for both the tetrahedral ing the refinement of their thomsonite with Si: node and the tetrahedron. Extraframework cat- (Si 4- Al) = 51 Alberti et al. (1981) showed ions are normally located near the tetrahedra that even with such a small increase in Si, there occupied by Al and hence with a deficit of posi- is a slight tendency to disorder with a detectable tive charge. On the other hand, if a framework increase of Al in one tetrahedron, which should oxygen is at bond distance from an extraframe- be occupied by Si only. In other words, given an work cation, its T—O bond length is increased average Al decrease, the Si increase in the Al (see, for instance, Baur, 1978), but by such a tetrahedra is larger than that strictly necessary small amount as to be negligible for the conclu- to counterbalance the Al deficit, hence some Al sions to be drawn in this article. is shifted to the Si-tetrahedra. This is a conse- quence of the »extended Loewenstein's rule»: if two nearby tetrahedra are both occupied by Si, Class 1 there is a greater likelihood of finding Al in the next tetrahedron. This class contains only a few structurally Garronite has Si: (Si + Al) = 0.60 and prob- well known species: gismondine, amicite, thom- ably the same framework as gismondine. Its sonite and willhendersonite. tetragonal space group gives some evidence for Gismondine (Fischer and Schramm 1971) and disordered zeolite, but the diffuseness of its X- amicite (Alberti and Vezzalini 1979) share the ray diffractions suggests the presence of or- same framework of tetragonal topological sym- dered domains with four orientations (Gottardi metry, and all tetrahedra are topologically and Alberti 1974). equivalent, but the extraframework cations are There is no evidence of order in the X-ray positioned so as to reduce symmetry. The order powder patterns of chabazite, even if only 59 % is dominated by the Al—O—Al avoidance (Loe- of the tetrahedra are occupied by Si (Passaglia, wenstein's) rule: the presence of Ca, which 1970). Unfortunately the samples are not suitable strongly favours order in many cases, is not de- for single crystal work, which could provide cisive here, as the same order is also found in better information. amicite too, which contains only K and Na. Thomsonite (Alberti et al. 1981) has a dif- ferent framework but here, too, the order is Class 2 simply imposed by Loewenstein's rule. Willhendersonite (Peacor et al. 1984; Till- This class includes some fibrous zeolites plus manns and Fisher 1981) has a chabazite frame- laumontite, wairakite and yugawaralite. work, and hence all tetrahedra are topologically Edingtonite: this zeolite may be orthorhomb- equivalent. Nevertheless, Loewenstein's rule ic (Galli 1976; Kvick and Smith 1983) or tetra- imposes ordered alternation of Si and Al. gonal (Mazzi et al. 1984). The average T—O If the ratio Si: (Si + Al) is slightly over 50 % distances (see Table 1) show nearly perfect (Si, in one of these frameworks, the probability of Al) order in the orthorhombic mineral and antiphase domains grows rapidly. There are complete disorder in the tetragonal one. There thus some thomsonites that are microcristalline is no correspondence between the Al fraction in and botryoidal and in which up to 55 % of a tetrahedron and the vicinity of an extra- Silicon aluminium ordering in the framework of zeolites 199 Table 1. Average T—O distances (A) in edingtonites. Sample Symmetry Tl—O T2—O T3—O Reference Bohlet Mine orthorhombic 1.609 1.623 1.752 Galli (1976) New Brunswick orthorhombic 1.617 1.621 1.743 Kvick and Smith (1983) Kilpatrick Hill tetragonal 1.660 1.666 Mazzi et al. (1984) Ice River tetragonal 1.658 1.668 Mazzi et al. (1984) Note: in tetragonal edingtonites T2 is equivalent to T3. framework cation barium; in fact Ba is linked 1.623, 1.620Å, and for the nodes coded All to to three framework oxygens, Ol, 02, 03, all of A13: 1.737, 1.750, 1.736Å. It is obvious that which are vertices of both T2 (Al free) and T3 there is nearly perfect (Si, Al) order. As in the (Al filled). other fibrous zeolites, no relation exists between Natrolite and tetranatrolite: although two the Al fraction in a node and the vicinity of an names are used, the situation is similar to that extraframework cation. Thus Ca is linked to of edingtonite; orthorhombic natrolite is a very 04, 06, 08, Oil, which are part of the tetra- common mineral and is usually perfectly or- hedra Si2, Si3, Si4, All, A12, A13, and Na is dered (see the first three samples in Table 2); as linked to 05, 07, OIO, which are part of the in edingtonite, there is no relation between the tetrahedra Si2, Si3, All, A12. Al fraction in a tetrahedral node and the vicini- Scolecite: there are five non-equivalent tetra- ty of an extraframework cation Na at bond dis- hedral nodes with the following T—O distances tance from 02, 03, 04, all of which are vertices (Smith et al. 1984): 1.616, 1.620, 1.620, 1.743, not only of site Al but also of Si2. The last two 1.749Å. This means nearly perfect (Si, Al) samples in Table 2 are partially disordered, and order. Ca is at bond distance from framework the existence of the tetragonal equivalent (Krog- oxygens 02, 03, 05, 06, which are part not Andersen et al. 1969) termed tetranatrolite only of the tetrahedra All and A12 but also of proves beyond doubt that complete (Si, Al) Si2 and Si3. disorder is also possible in this framework, even Laumontite: according to Bartl (1970), there though this structure has never been refined be- are three non-equivalent tetrahedral nodes. One cause suitable crystals are not available. Tetra- is coded Al with an average T—O distance of natrolite is much less common than orthorhombic 1.750Å; the other two are coded Sil and Si2 natrolite, but it is still not a rare mineral. with T—O distances of 1.611 and 1.621Å. Thus Mesolite: Adiwidjaja (1972) found the fol- there is nearly perfect (Si, Al) order. The extra- lowing average T—O distances for the tetra- framework cation Ca is at bond distance with hedral nodes coded Sil to Si5: 1.619, 1.620, 02 and 03, which are both part of the Al tetra- hedron; the Sil and Si2 tetrahedra each contain one of these two oxygens. Table 2. Average T—O distances (A) in natrolites.