View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Okayama University Scientific Achievement Repository OKAYAMA University Earth Science Reports, Vol. 4, No. 1,4172, (1997) Structural Modification of KAISi04 Minerals Yasuhiko OKAMOTO The board of education of Okayama Prefecture Uchisange 24 6, Okayama 700-8570, Japan Kalsilite, a polymorph of KAlSi04 is an end member of nepheline-kalsilite series and the mineral was syn­ t hesized by hydort hermal methods. The synthetic kalsili te is hexagonal, P6 3 , with a = .5.151 (5), c = 8.690(8) A. The structure was refmed by full-matrix least-squares methods to a R-value 0.084, using 373 observed reflections. The obtained structure agrees well with those of the natural and the alkali-exchanged specimens reported in the previous literatures. The oxygen atoms are disordered at two mirror-equivalent sites, const.ruct.ing t.he domain structure. The average domain structure shows P63 me symmetry and the structural relation between the two P6 3 structun: corresponds to the twinning by merohedry. The domain structure was considered to be caused accompanied with the high-low inversion of the kalslite structure. Heatinp, experiments of kalsilite reveal that the X-ray powder pattern changes at 865°C, and that cell dimensions vary discontinuously at this temperature. It was confirmed that kalsilite underwent a displacive transition like those observed in qnartz or tridymite. The high-form is refered as 'high-kalsilite', and a possible simnlate model is proposed. The struct nre of the high-kalsilite at 9.50°C was refined byfull-matrix least-squares methods to a R­ value IUl9'"i. nsing 11.5 observed reflections. The high-kalsilite is also hexagonal, P63mc or P63/mmc, with a = 5.288( 1). c = 8.628(5) A at 950°(:. The structure almost prefedly coincides with that of the simulated model. Based on the interatomic distances, the distribution of silicon and aluminum atoms is found to be or­ dered and the space p,ronp is determined to be P6;>.mc. l\:aliophilite and the related orthorhombic form, polymorphs of KAISi04 , were synthesized by dry method. The synthetic kaliophilik (kaliophilite-1I2) is hexagonal with a = 5.17(1), c = 8.49(3) A, and the orthorhombic KA1Si04 (kaliophilite-Ol) is orthorhombic with a = 9.01(1), b = 1.5.60(2), c = 8..53(4) A. Detailed examina­ tion of the obtained powder patterns together with that of simulated model indicates that t.he ka1iophilit.e-H2 has a disordered strncture of four t.ypes of the low-kalsilite. The structure was refined by the X-ray powder pattern-fittinp, met.hod (Rietvelt. method) to a R-value 0.121. Keywords: kalsilite, b.liophilite, feldspat.hoid, systheses, crystal structure 1 Introduction Na, or Ca, so it is similar to feldspar. Feldspathoid is charact(:'rized by unique composition with less Minerals of fE'!dspar group are the most abundant silica than that of the corresponding feldspar. constituents of igrwolls rocks and the most im­ Minerals of feldspathoid group are important portant minerals ill the lithosphere. The minerals constituents of alkaline rocks deficient in silica. show wide range in chemical composition and the Among the feldspathoid group, minerals belong­ structures of tl10 minerals is full of variety depend­ ing to the nepheline (NaAISi04 : Ne) - kalsilite ing on the formation condition such as tempera­ (KAISi04 : Ks) series are the most common in ture and pressure toget her with the nature of the the nature. The investigation on the Ne-Ks series related magma (P.g. Ribbe, ]~)84). is very important for clarifying the crystallization mechanism of alkaline rocks. Feldspathoid is aluminosilicates of mainly K, 42 Yasuhiko OKAMOTO The structures of the feldspathoid group have sized higher-temperature orthorhombic KAlSi04 a framework of linked (Si,AI)04 tetrahedra. The obtained through the transition of OI-phase by structures of nepheline and kalsilite consists of the heating. tridymite-like frame\'iOrk whose tetrahedral sites In this report, the structures of the Ne-Ks se­ are occupied by aluminum and silicon atoms one ries, especially KAlSi0 4 minerals (kalsilite and after the other. The excess minus charges are com­ kaliophilite) will be investigated. Special atten­ pensated by Na+ or K+ ions in the cavities of the tion will be paid on the following four points. frameworks constructing six-memebered rings in the structure. In nepheline, the cavities of the 1. Syntheses of kalsilite by hydrothermal method, structure are distorted one after the other from especially syntheses of single crystals fitted that of kalsilite. The detailed structures of these for the structural analyses. minerals are rather complicated, and vary accord­ 2. The structural refinement of synthetic kalsilite ing to chemical composition and/or thermal his­ and the comparison of the structure with tory. However, the structure of kalsilite can be that of natural kalsilite and the synthesized well understood based on the wide comprehen­ kalsilite by alkali exchange methods. sive structural view point including the structure of nepheline. In this paper, detailed structure of 3. Measurement of the high-low inversion point kalsilite in the system of the Ne--Ks series will be of kalsilite and the structural refinement of examined. high temperature form of kalsilite. Furthermore, many modifications are known in 4. The structual refinement of synthetic kalio­ the Ne--Ks series. Numerous investigations on the philite and the comparison of the structure two component system of the Ne-Ks series have with that of kalsilite. been accumulated since 1940's. Among those, the phase relations of this system has been investi­ The general overview of the structures of gated in detail by Smith and Tuttle (1958), who feldspathoid minerals will be described in the next established the phase equilibrium at sub-solidus section and the research histories of the minerals will be mentioned at the beginning of each chap­ temperatures between NaAlSi04 and KAlSi04 components. The two component system has ter. the miscibility gap below approximately 1,000°C 1.1 Feldspathoid groups and the case is similar to that of the sys­ tem NaAISi308(albite)-I~AISh08(orthoclase).In The crystal structures of feldspathoid groups are the course of these investigations, Smith and characterized by three demensional framework, Tuttle (19.57) identified the following phases by in which tetrahedra of (Si,AI)04 are linked to X-ray powder methods and gave their X-ray one another in all directions by shared oxy­ data: high- and low-carnegieite, high- and low­ gens. Feldspathoids are divided into three nepheline, kalsilite, orthorhombic KAISi04(01), groups: leucite group, sodalite-cancrinite group second orthorhombic phase( 02), natural kalio­ and nepheline-kalsilite group (Merlino,1984). philite, anomalous natural kaliophilite, synthetic In addition to the three groups Melilite, kaliophilite and tetrakalsilite(H4). Further, Sa­ (Ca,Na)z(Mg,Fe,AI,Sih07, is generally included hama and Smith (1957) described trikalsilite, and in feldspathoid but the structure belongs to ortho­ Cook, Roth, Parker and Negas (1977) synthe- silicate (Smith, 1953). Structural ~rodification of KAlSi04 Minerals 43 The structures of the three groups will be in leucite, the symmetry of pollucite is cubic (Ia3d, the next briefly summarized. In each subsec­ a = 13.69 A) even at the room temperature. In tion. space groups and cell dimensions of the pollucite, twelve Cs+ ions and four H20 molecules feldspathoid minerals are quoted from those of are located in \V sites. whereas four Na+ ions lo­ ~Ier1ino (19~~~). cated in S sites. 1.1.1 Leucite group Analcime-type structures The framework of the leucite group IS intricate, The structure of anaIcime, NaI6(AI16Si32096)' but the framework can be best expressed in terms 16H20, contains sixteen H20 molecules in W sites of a basic topology shown ill Fig. 1 1(Merlino, and sixteen Na+ ions in S sites. If there is no Al­ 198,1). The basic structure is formed by four­ Si ordering like in the cases of leucite and pollu­ membered rings with ,1 symmetry. Two four­ cite, the average symmetry of analcime is Ia3d. membered rings and two tetrahedra located on But the true symmetry is lowered to I 4dacd, opposite edges are linked. and construct ten- a = c = 13.78 A because of AI-Si ordering in membered ring (Fig. 11 : a and b). The part. ten-membered ring cOllstitutps a structural unit. The structure of wairakite, Cas(Ah6Sb2096)' These units arp conllPded with each other forming 16H 0, contains sixteen H 0 molecules in W sites three-dimensional networks (Fig. 1 1 : c). In the 2 2 and eight Ca2+ ions in S sites. Wairakite is consid­ framework two kinds of cavity sites are present, ered to be the most ordered form of the anaIcime­ which are occupied by cations or water molecules. type structures. Ordering of tetrahedra cations in The cavity sites occupied by cations are called S analcime and wairakite is shown in Fig. 1-2 (Mer­ sites and those occupied by water molecules are lino, 1984). In the wairakaite structure perfect called W sites (Fig. 11 : d). In the leucite­ AI-Si ordering reduces the symmetry to 141/acd. type structure cations occupy only W sites and/or Furthermore, Ca2+ ions in S sites cause distortion both Wand S sites, whl?H'as in the analcime-type in the framework, and the true symmetry is low­ sturucture cations occupy only S sites and water ered to 12/a, a = 13.69, b = 13.68, c = 13.56 A, molecules occupy \V sites. ;3 = 90.5°. Leucite-type structures 1.1.2 Sodalite-cancrinite group Leucite. K]fJAhfiSi;o209G. is tetragonal (f4 1 /a, a = 13.0,1.