Abstract
Changes in temperature alter the structure and properties of clay minerals. Four temperature ranges are distinguished: (1) Temperatures sufficiently low to cause partial freezing of clay mineral dispersions or pastes: in this temperature range some of the water is converted into ice, but some remains in a liquid or semi-liquid state. (2) Temperatures above dehydration, but below dehydroxylation, when clay minerals lose adsorbed and hydration water: the interlayer spaces collapse, pore space is changed and surface acidity is substantially altered. (3) Temperatures above dehydroxylation, but below those leading to complete breakdown of the structure: the changes occurring in this temperature range vary for different clay mineral groups. Dehydroxylation destroys the layer structure of trioctahedral, but not that of dioctahedral 2:1 type minerals. Kaolinite group minerals become amorphous to X-rays, although some features of the structural framework are preserved. (4) Temperatures at which some minerals turn amorphous, others recrystallise to new phases.
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