Characteristics of different types of glauconite and their classification systems
Origin and classification of glauconite,a typical kind of sedimentary facies indicator,have long been the hot topics of mineralogy and sedimentology.This paper investigates the characteristics of autochthonous,parautochthonous and allochthonous glauconite,and documents its classification systems and genesis based on previous researches on the genesis of glauconite and a large number of case studies of glauconite worldwide.Several hypotheses have been proposed to interpret the genesis of autochthonous glauconite.The results suggest that the grain verdissement and pseudomorphic replacement hypotheses are applicable for the interpretation of the genesis of granular and autochthonous glauconite in the microscopic pores of shale,but are inapplicable for the interpretation of the formation of glauconite cement in intergranular pores in sandstones.Different types of glauconite show significant differences in elemental composition,maturity,morphology and hosting environment.Autochthonous glauconite tends to accumulate in a thin layer of condensed section,and shows high maturity.Its element-composition features,thus,are symptomatic of depositional environment.Parautochthonous glauconite is characterized by multi-stage involucrums.Detrital glauconite generally accumulates in sandstones with large-scale bedding structure,and is commonly oval or granular.Its element-composition features cannot be used as the indicators of sedimentary environments.New classification systems are proposed to replace the current confused classification systems of glauconite.The glauconites are divided into "primary" vs."secondary" classes and "autochthonous" vs."allochthonous" categories,according to two main criteria (i.e.,genesis and whether removing from the initial formation place).An authigenic precipitation and cementation theory is proposed to interpret the genesis of glauconite that shows shapes of cement,halo edge and involucrum.Moreover,the existence of the authigenic glauconite cement is proved by a large number of case studies.Glauconite,therefore,plays a significant role in diagnosis sedimentary environment,rock dating and improving reservoir physical properties.
autochthonous glauconiteallochthonous glauconitematurityclassification systemauthigenic precipitation and cementation theorysedimentary environment
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