Abstract
Magnesium hydroxide has been precipitated at various pH levels at 140°F from magnesium chloride and either ammonia, sodium hydroxide, or calcium hydroxide. Particle morphology, perfection and size were studied by transmission electron microscopy. Constant pH runs with ammonia showed that increase in pH favored growth parallel to the basal plane relative to growth along the c-axis. This resulted in a transition from a plate habit over the pH range 11.0 to 9.0, to dense equiaxed particles at pH 8.75. Anisotropy of growth was also observed in the basal plane at pH 9.0 and 9. 5, giving plates of hexagonal shape, but near pH 10.0 this tended to disappear yielding larger nearly circular plates with indented outline. Substitution of 30% wt/wt calcium chloride for water in the reactor tended to give pyramidal particles. Precipitation in an aqueous solution of calcium hydroxide yielded imperfect plates of poorly developed hexagonal shape. Precipitation with calcium hydroxide in 30% calcium chloride solution yielded very large imperfect plates. It is concluded that under conditions where the solubility limit of calcium hydroxide in water or calcium chloride solution is approached, Ca++ ions retard growth of magnesium hydroxide along the c-axis, probably by adsorption on the basal surface, followed by cationic substitution in the crystal, leading to crystal imperfection.
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