Hydrotalcites having a hexagonal needle-like crystal structure and process for production thereof

A process for producing a hydrotalcite having a hexagonal needle-like structure and having a length-to-diameter ratio, determined by an electron microscope at a magnification of 1000 X, of at least 10, said hydrotalcite having the following formula:

• Mg.sub.1-x.sbsb.1 M.sub.x.sbsb.1.sup.3+ (OH).sub.2+x.sbsb.1.sub.=ny A.sub.y.sup.n-.m.sub.1 H.sub.2 O (1)
• wherein M.sup.3+ represents a trivalent metal cation, A.sup.n- represents an anion having a valence of n, and x.sub.1, y and m.sub.1 are numbers satisfying the following conditions:
• O<x.sub.1 .ltoreq.0.6,
• O<y=0.5,
• O<m.sub.1 .ltoreq.2;
• said process comprising contacting a basic magnesium compound having a needle-like crystal structure and expressed by the following formula (2):
• Mg(OH).sub.2-n'x.sbsb.2 A.sub.x.sbsb.2.sup.n'-.m.sub.2 H.sub.2 O (2)
• wherein A.sup.n'- represents a monovalent or divalent anion, n' is 1 or 2, and x.sub.2 and m.sub.2 are numbers satisfying the following conditions:
• 0.2.ltoreq.x.sub.2 .ltoreq.0.5,
• O<m.sub.2 .ltoreq.2,
• with a compound capable of providing a trivalent metal cation and being soluble in a liquid reaction medium which is chemically inert and is a non-solvent for the basic magnesium compound, the contacting being carried out in said liquid reaction medium under conditions which do not cause a loss of the needle-like crystal form of the basic magnesium compound, while maintaining the ratio of M.sup.3+ to the sum of Mg and M.sup.3+ at O<M.sup.3+ /(Mg+M.sup.3+).ltoreq.0.6 and the pH of the contacting system at not less than 9.