The nature of diffraction effects from illite and illite-smectite consisting of interstratified trans-vacant and cis-vacant 2:1 layers; a semiquantitative technique for determination of layer-type content

Previous work has described the structural and diffraction criteria for distinguishing between tv-1M, cv-lM, and m-lM illite varieties, where tv-1M illite corresponds to a structure with trans vacant (tv) 2:1 layers, cv-lM illite consists of cis-vacant (cv) 2:1 layers, and m-lM illites consist of 2:1 layers with random distributions of octahedral cations over trans and cis sites within each layer. Detailed analysis of calculated and experimental X-ray diffraction (XRD) patterns from illites and mixed-layered illite-smectites (I/S) with a range of cisand trans-vacant layer interstratification, and consideration of unit-cell parameters for pure tv-1M and cv-lM, provides insight into the nature of the diffraction effects from the interlayered structures with different types and contents of rotational stacking defects. Structural fragments of the 2M! and 2M2 polytypes exist within IMd structures in which rotational disorder is dominated by n. 120 and n. 60°, respectively. Such structural fragments constitute coherent-scattering domains and affect the peak positions of the diagnostic III reflections for rotationally disordered liS and illites in different ways depending on the proportion of cisand trans-vacant 2:1 layers. Simple techniques that do not require computer programs were developed for the semiquantitative determination of the proportion of tv and cv 2:I layers in liS and illites where such layers are interstratified. These techniques may help to reveal the diversity of liS and illite samples relating to the conditions of their formation and transformation. INTRODUCTION models: (1) tv -1M illite consisting of transvacant (tv) 2: 1 layers, (2) cv-lM illite consisting of cis-vacant (cv) 2: I Until recently the structural characterization of nonlayers, (3) m-lM illite consisting of 2: I layers with ranswelling dioctahedral micas was restricted by difficulties dom distributions of octahedral cations over trans and in determining the details of their polytype modification cis sites within each layer, and (4) tv-cv illite consisting (Srodon and Eberl 1984). One-layer illites and glauconite of randomly interstratified tv and cv 2:I layers, but each are subdivided, depending on their structural perfection, layer is uniformly tv or cv. between the 1M polytype, which has no rotational layer The possible existence of these structures among natdisorder, and the IMd polytype, which contains many ural dioctahedral micaceous minerals was assumed by noncoincident azimuthal layer orientations. analogy with dioctahedral smectites, the actual structures Description of illite-smectite (I/S) has been restricted of which may consist of 2: I layers with different distrito the determination of proportions of illite and smectite butions of octahedral cations over trans and cis sites (Tsiinterlayers and the ordering pattern in their distribution pursky and Drits 1984). The pure cv-lM illite was de(Reynolds and Hower 1970; Drits and Sakharov 1976; scribed by Zvyagin et al. (1985) and Drits et al. (1993), Reynolds 1980). Most researchers have assumed that in and a mixture of cv-IM, tv-1M, and tv-2M! by Reynolds all dioctahedral micas and liS, 2:I layers have tv octaand Thomson (1993) and Drits et al. (1993). Gavrilov hedra. This supposition is based on single-crystal X-ray and Tsipursky (1987) described 1M illite with an almost structure refinements in which octahedral cations in 2: 1 random distribution of Al cations over available trans layers of the various polytypes of muscovite, paragonite, and cis sites. and margarite were shown to occupy only cis sites (Bailey Reynolds (1993), McCarty and Reynolds (1995), and 1984). Drits et al. (1984) considered the hypothetical Drits et al. (1996) showed that in some liS so-called funstructural models for one-layer illites in which 2:1 layers dam ental illite particles are mixed-layer structures in have different distributions of octahedral cations over which tv and cv 2:I layers are randomly interstratified. available trans and cis sites. They described the main Coexistence of tv and cv layers was established in a nonstructural and diffraction features for the following four swelling illite sample from the Amethyst Vein System, 0003-004X/96/0708-0852$05.00 852 DRITS AND McCARTY: DIFFRACTION EFFECTS FROM ILLITE AND liS 853 Creede Mining District, Colorado (Bookin, personal communication). This sample was initially described as 3Tillite (Horton 1983) because of the similarity of X-ray diffraction (XRD) patterns corresponding to cv-1M, tvcv, and 3T illite structures (Drits et al. 1993). To estimate the proportions of tv and cv 2: 1 layers in illite and liS, Reynolds (1993), McCarty and Reynolds (1995), and Bookin (personal communication) calculated diffraction effects of different structural models and then compared them with experimental XRD patterns from natural samples. Computer programs were used for these calculations. Using the same approach, Sakharov et al. (1990) studied a large collection of glauconite samples with different chemical compositions, ages, and degrees of structural perfection. They found that all the glauconite samples studied consisted of tv 2: 1 layers. It seems likely, therefore, that coexistence of tv and cv 2: 1 layers within the same particle or coherent-scattering domain is a characteristic feature of some illites and liS, whereas the structure of glauconite is restricted to the tv type. The mechanism resulting in these respective structural differences is still under investigation. Mica polytypes differ by azimuthal orientations of the successive 2: 1 layers resulting from layer rotations by n. 120° (n is an integer) or n. 60° (n is an odd integer) around the axis normal to the (001) plane and passing through a K cation in mica interlayers. In pure 1M illite all layers have the same azimuthal orientation, whereas in 2M] and 2M2 polytypes the azimuthal orientations of any adjacent layers differ by 120 or 60°, respectively. Three-dimensional periodicity of mica polytypes and, in particular, 1M illite may be completely or partially destroyed because of the presence of rotational stacking faults. For example, so-called semiordered IMd micas may contain some stacking faults owing to layer rotations by :t 120° or by n.60° (Sakharov et al. 1990; Reynolds 1993; McCarty and Reynolds 1995). The aim of this paper is to describe the nature of the diffraction effects from interstratified tv and cv 2: 1 layers in illites and liS that contain rotational stacking faults and to introduce techniques for semiquantitative estimation of layer-type content that do not require computer programs.