The natural conversion of ordered kaolinite to halloysite (1 at Burning Mountain near Wingen ' New South Wales

In the vicinity of Burning Mountain, New South Wales, an unusually thick sequence of ka-olinite clayrocks (flint clays) has been considerably altered by heat from the natural com-bustion of coal seams. Recent quarrying operations in the area have exposed excellent sec-tions through the metamorphosed succession and it is evident that in the high temPerature zone above the burnt seamJ ordered kaolinite of the clayrocks has been converted to mullite and cristobalite. Below the seams, however, the temperatures were considerably lower and metakaolin resulted. Much of the metakaolin has subsequently been rehydroxylated by groundwater to yield halloysite (l0A) and this reaction is continuing at the present time. To determine the reason for development of halloysite (l0A) instead of kaolinite as the rehy-droxylated product, a study has been undertaken of the various intermediate phases in the reaction 5"qo"o". ordered kaolinite + metakaolin-+ halloysite 1t0A). Attempts have also been made to reproduce the rehydroxylation reaction in the laboratory by relatively mild hy-drothermal treatment and whereas these have proven unsuccessful to date, it would appear that the upper temperature limit for development of halloysite (l0A) does not lie much above ambient conditions