CONSTRAINTS ON THE GENESIS OF FERRIAN ILLITE AND ALUMINUM-RICH GLAUCONITE: POTENTIAL IMPACT ON SEDIMENTOLOGY AND ISOTOPIC STUDIES

Abstract

The term glauconite covers a series of iron-rich minerals that form in the upper layer of sediments of the sea bottom in locations where sediment input is low. Because of its potassium content and the process of its formation, it is one of few minerals that can be used in both sequence stratigraphy and in the determination of sedimentation age. Although aluminum-rich glauconite has been identified in several locations, the way it forms remains relatively unknown. A study of the ferrian illite from the Cambrian Anse Maranda Formation shows that, according to the present models for the formation of glauconite and diagenesis, the Al-for-Fe substitution responsible for the genesis of Al-rich glauconite occurs during early burial. In order to maintain charge balance while replacing Fe2+ and Mg2+ by Al3+ at the octahedral site, there is an expulsion of K, as high as 31.6% of the measured K2O. This loss is important when evaluating the time needed to form glauconite and interpreting the occurrence of Fe-rich illite; it must be accounted for when using the K-Ar system, for either dating or in diagenetic studies.