Formation of X-ray Amorphous and Crystalline Aluminium Hydroxides1

Pa Ho Hsu and Thomas F. Bates
Department of Geochemistry and Mineralogy, The Pennsylvania State University, University Park, Pennsylvania, U.S.A.
1Contribution No. 62–50 from the College of Mineral Industries, The Pennsylvania State University.

Summary: Various crystalline and X-ray amorphous forms of aluminium hydroxide prepared from sulphate and chloride solutions by varying the mole ratio of NaOH to Al3+ have been characterized by chemical analysis and X-ray diffraction. When the NaOH/Al ratio is 3 or 3·3, crystalline Al(OH)3 in the form of bayerite, nordstrandite, gibbsite, or mixtures is obtained in several hours. When the ratio is 2·75 or below, no Al(OH)3 is evident and the products are X-ray amorphous and remain so even after ageing for six months. Chemical analysis indicates that the amorphous precipitates are basic aluminium sulphates or chlorides. In the 0 to 2·1 range of NaOH/Al, the composition of the products is constant, being approximately Al(OH)2·2X0·8. In the 2·1 to 2·75 range, a continuous series of basic salts, of composition ranging from Al(OH)2·2X0·8 to Al(OH)2·75X0·25, is obtained.

The following hypothesis is proposed to account for the occurrence of X-ray amorphous versus crystalline forms: When NaOH is added to an aluminium salt solution, the initial reaction yields Al(OH)2+ ions, which polymerize into stable 6-membered ring units of composition Al6(OH)126+ or multiples thereof (double and triple rings of composition Al10(OH)228+ and Al13(OH)309+), all with H2O molecules on the surface. At NaOH/Al = 2 or nearly so, the inclusion of Al3+ in the basic ring units is complete; still further OH then transforms these ring units to larger polymers, giving rise to a continuous series of species. At NaOH/Al = 2·75 and below, the positively charged hydroxy-aluminium polymers repel one another unless joined together by the counter-anions to form basic aluminium salts, which are usually X-ray amorphous because of their highly hydrated state. At NaOH/Al = 3, the net positive charge per aluminium disappears and consequently the repulsion among polymers as well as their water of hydration becomes negligible; therefore, all the polymers cluster together and form crystalline Al(OH)3 in a matter of hours or days, depending on the conditions of crystal growth.

Mineralogical Magazine; March 1964 v. 33; no. 264; p. 749-768; DOI: 10.1180/minmag.1964.033.264.04
© 1964, The Mineralogical Society
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