Halloysite is a common pedogenic clay mineral, often found in young soils developed on volcanic deposits (Dixon, 1989), It is a member of the kaolin group of clays with the same ideal stoichiometric composition as kaolinite [Al2Si2O5(OH)4]. Halloysite, however, often contains water of hydration (i,e., Al2Si2O5(OH)4·nH2O), and is commonly found with a tubular morphology, This “rolling” of halloysite has received a great deal of study because there is no generally agreed upon mechanism for the process and there is no corresponding phenomenon in natural kaolinite (e.g. , Bates et al., 1950; Bailey, 1989; Singh, 1996; Singh and Mackinnon, 1996). The crystal structure of halloysite often shows stacking disorder. This property, combined with a rolled morphology, makes identification by X-ray diffraction (XRD) difficult. The XRD peaks at 7.5, 4.4, and 3.6 Å are often asymmetric with a large width at half peak height (Bailey, 1989).

Kaolinite from the Black Ridge, Clermont, has relatively low δ18O (12.3‰ to 14.8‰) and very low δD values with a large variation (−120‰ to −85‰). Comparison of these data with those from the nearby Denison Trough and elsewhere in eastern Australia, together with previous studies of the mineralogy of the sedimentary rocks, suggests that extensive kaolinization of the “White Section” resulted from weathering during the Late Triassic to Early Jurassic periods. The relatively large variation in δD values of kaolinite probably derives from post-formational isotopic exchange with other fluids.

The similarity between δ18O values of kaolinites from Black Ridge and from the Denison Trough suggests that the small Miclere-Black Ridge basin may have been part of the Denison Trough before the Late-Triassic inversion. The preservation of original δD values in kaolinite at Black Ridge indicates that unlike the Denison Trough, which was reburied at more than 1000 m, the Miclere-Black Ridge basin was not rebuffed at great depth during the Mesozoic period.