Kaolinite system - disordered kaolinite - metahaloysite - adelite - atapulgite - microscopy and electron diffraction

Abstract

The morphological and structural relationships between kaolinite, disordered kaolinite, methaloisite, and endellite are studied using X-ray diffraction, microscopy, and electron diffraction. Crystals of kaolinite from a refractory clay in the State of Mexico, with hexagonal or irregular morphology, Laue symmetry C6, and a structure free of visible defects by electron diffraction, bend or roll at variable angles of their a and b axes into curved longitudinal crystals, with straight or wavy longitudinal edges, elliptical curved transverse edges, and transverse zones with different electron absorption. The process maintains C6 symmetry, but the b-axis can be at any angle relative to the axis of rolling, and the lattice is normally pseudohexagonal with no displacement of the tetrahedral and octahedral layers or visible tension. When treated with acetate, 001 shifts from 7.35 Å to 13.75 Å, which, when dried at 100°C, reverts to 7.35 Å, stable. The crystals, originally rolled, unroll into irregular, stable plates that no longer roll upon drying, water, or glycol treatment, and their diffraction becomes normal pseudohexagonal C6. This suggests a reaction mechanism and implies that rolling is not permanent nor a consequence of layer translations or structural tensions, but possibly caused by ionic repulsions or dimensional differences between layers.

Metahaloisite from San Luis Potosí is presented as well-formed concentric tubular crystals, frequently displaced along a straight longitudinal axis, exhibiting original crystallization rather than rolling. Its lattice exhibits monoclinic Laue symmetry C2v, with very intense 060 reflections and a shift in the reflections perpendicular to b, following a superlattice or elastic strains parallel to b. This appears at variable angles relative to the longitudinal axis, but predominantly at db 33°. With acetate, the 001 reflection of endellite shifts from 10 Å to 13.70 Å, remaining stable under normal conditions. When dried at 100°C and treated with glycol, it remains at 10 Å. The crystals, originally straight tubular, do not change with the treatment, remaining as such and only deforming slightly. Intensities for electron diffraction of the C2h — 2/m and C8 — m groups are calculated, concluding that the latter is most consistent with the experimental data.

There is no continuous transition from kaolinite to endellite. The kaolinite–disordered kaolinite system is pseudohexagonal C6, with morphologies ranging from hexagonal plates to curved rolled crystals, likely due to ionic repulsion. In the endellite–methaloisite system, methaloisite is monoclinic Cs — m, with an original tubular morphology.