The Origin of High-alumina Clay Minerals --a Review

High-alumina minerals refer, in this paper, to Al-rich, normal sedimentary phases, including gibbsite, boehmite, diaspore, possibly "proto-diaspore" and "proto-alunite", "Al-chlorite", and corundum, in association with kaolin minerals. They may be derived from any common Al-containing rock. Processes of origin include direct bauxitization of non-clay silicate minerals and rocks, and the desilication of any of the common clay minerals, particularly of the kaolin group. Apparently aluminous gels were formed within certain marshy basins, and/or were transported into basins, giving rise to concretionary masses of high-alumina minerals. Concretionary deposits were iormed by dissolution of A1 and reprecipitation. Alumina is probably more soluble, inherently, than is interpreted from geologic occurrences. Even after A1 is dissolved under appropriate pH, or complexing reactions, its solubility is obscured by Al-immobilization through fixation within 3-layer clay minerals, or absorbed by other phases. A 14~ phase, probably aluminous, is not uncommon in certain soils, and in flint and boehmite-diaspore-containing clays. Subtraction of silica by plants, is accompanied by concentration of alumina. I N T R O D U C T I O N THE t e r m " h i g h a l u m i n a c lays" has been app l i ed to var ious c lays (rocks) t h a t con ta in A120 3 in excess of cer ta in m i n i m u m a m o u n t s t h e m i n i m u m c o m m o n l y has been set b y the use to which the h igh -a lumina c lay was pu t , as for ref rac tor ies , abras ives , a l u m i n u m m e t a l ex t rac t ion , or o ther purposes . P r o b a b l y the lowest a l u m i n a con ten t app l i cab le to high a lumina has been a b o u t 35 per cent A120 a (Wilson a n d Treasher , 1938; Allen, 1946; Wil l iams, 1960) or down to 20 per cent AlsO 3 for h igh-a lumina kaol in i t ic c lay (Mark, 1962), for the purposes of th is p a p e r the low l imi t of h igha l umina c lays will be p laced a r b i t r a r i l y a t 40 per cent , A1203 on the raw c lay basis, i.e., j u s t a b o v e the theore t i ca l a l umina con ten t of kaol in i te (39.5 per cent A1,O3). This requi res t h a t some a l u m i n u m oxide minera l (usual ly h y d r a t e d ) be assoc ia ted w i th the c lay minera ls compr is ing the c lay rocks. The c lay minera l s in such h igh -a lumina c lays c o m m o n l y are in the kaol in group, a n d the c l ay rocks (mixtures) ensuing m a y be those classified as nodu l a r clays, bu r l ey clays, baux i t i c kaolins, t e r ra rosa, and