Abstract
ABSTRACTDespite many reviews and original articles, the actual crystal structure of tubular halloysites remains unclear. Analysis of the structural features of defect-free kaolinite, refined by Bish & von Dreele (1989), shows that the ordered 1Tc kaolinite structure can be described equally well by the orthogonal layer cell {a0, b0, γ0} (γ0 = 90°) or by two enantiomorphic oblique layer cells {a1, b1, γ1} and {a2, b2, γ2}, related to each other by a mirror plane. To simulate diffraction effects for tubular halloysite, the parameters and atomic coordinates of the orthogonal layer unit cell and the layer-displacement vectors t1 and t2 responsible for formation of the kaolinite enantiomorphs were deduced by transformation of the parameters of the defect-free kaolinite refined by Bish & von Dreele (1989). Modelling X-ray diffraction patterns show that the samples consist of either single, two or three phases, with the number and their structural features depending on the morphology of the particles. Samples formed of prismatic particles consist of halloysite-like structure (HLS), kaolinite-like structure (KLS) and halloysite cylindrical structure (HCS) phases occurring in various proportions. Samples of proper cylindrical tubes consist of a single HCS phase, whilst samples formed by particles having morphologies intermediate between proper cylindrical and well-developed prismatic forms consist of the KLS and HCS phases. The KLS phase is comparable to low-ordered platy kaolinite with identical unit-cell parameters, layer-displacement vectors and arbitrary stacking faults, except that the layer displacements are not random as in kaolinite, but are distributed at R = 1 such that t1 and t2 displacements have a strong tendency to be segregated. Structural parameters describing the HLS and KLS phases are identical, but in the HLS phase there is a strong tendency to the regular alternation of the t1 and t2 displacements, and the HLS phases do not contain arbitrary stacking faults. A characteristic feature of the three-phase prismatic samples is that the stacking of the layers along the c* axis is periodic and the layer thicknesses are similar to those of platy kaolinite. In contrast, in the KLS phase formed in samples with particles of intermediate morphologies, the hydrated 10 and 7.25 Å layers are interstratified. The relationship between the structural and morphological features of the coexisting phases suggests a sequence of phase formation from the centre to the surface of halloysite tubes that progresses from the HCS to the HLS via the KLS phase. The results of this study demonstrate that all kaolinite and halloysite (7 Å) varieties are built by the same fundamental structural units.
Highlights
ABSTR AC T : Despite many reviews and original articles, the actual crystal structure of tubular halloysites remains unclear
The samples formed of prismatic particles consist of the halloysite-like structure (HLS), kaolinite-like structure (KLS) and halloysite cylindrical structure (HCS) phases, populations of proper cylindrical tubes consist of a single HCS phase, whereas those samples formed by particles having morphologies intermediate between proper cylindrical and well-developed prismatic forms consist of the KLS and HCS phases
Samples formed of prismatic particles consist of three phases termed HLS, KLS and HCS, the first two of which are planar and the other cylindrical; populations of proper cylindrical tubes form a single HCS phase, whereas the samples formed by particles having morphologies intermediate between cylindrical and well-developed prismatic forms consist of the KLS and HCS phases
Summary
ABSTR AC T : Despite many reviews and original articles, the actual crystal structure of tubular halloysites remains unclear. Samples formed of prismatic particles consist of halloysite-like structure (HLS), kaolinite-like structure (KLS) and halloysite cylindrical structure (HCS) phases occurring in various proportions. The other distinct difference between kaolinite and halloysite is that kaolinite consists exclusively of platy particles with hexagonal outlines, whereas halloysite has been described in a wide variety of morphologies, most commonly cylindrical and prismatic tubular, as well as spheroidal forms, rare Fe-rich platy forms are known (Joussein et al, 2005; Moon et al, 2015; Churchman et al, 2016; Hillier et al, 2016). The cylindrical and prismatic tubular forms are the most common particle forms in most halloysite samples (Chukhrov & Zvyagin, 1966; Dixon & McKee, 1974; Bailey, 1990; Guggenheim, 2015; Churchman et al, 2016; Hillier et al, 2016)
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