Chemistry and structural order in hydrothermal and sedimentary Sardinia kaolinites

Chem., thermal, x-ray analyses, and crystallinity tests were made on kaolinite samples of hydrothermal and sedimentary origin (from 2 Sardinia localities, Italy), to explore correlations among structural order, unit-cell parameter dimensions, compn., and geol. setting. The structural formulae indicate the nature and extent of various types of ionic substitution (prevalently Fe for Al in octahedral sites): the Fe content was 0.005-0.116 in sedimentary samples and 0.052-0.406 in hydrothermal ones. A good linear correlation between Fe and Cr content is shown. The variable crystallinity of these samples is linked prevalently to Fe content. A trend that links the increase in total Fe content to the decrease in the structural order is confirmed; also the temps. of thermal effects are inversely correlated to structural Fe content. The b and c unit-cell parameters increase as structural disorder (or Fe content) increases; a behaves inversely. The unit-cell parameters are affected by the trace element content; this is particularly evident if Y vs. b and c is considered. The sedimentary samples are characterized by a higher structural order than those of hydrothermal origin because of the different Fe content. Statistical anal. results (factor anal.) confirm that the structural order of kaolinites is affected by the overall geochem. of the genetic environment.