De glauconiarum origine

Abstract

The glauconitic facies is widespread on present‐day continental shelves from 50° S to 65° N and at water depths between 50 and 500 m, and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m. It is also common in many ancient rocks of post‐late Precambrian age. It occurs as sand‐ to pebble‐sized, essentially green particles (granular facies) but also as a surface coating on particles and hardgrounds and as a diffuse impregnation (film and diffuse facies). We suggest the replacement of the term ‘glauconite’, which has been interchangeably used to designate a morphological form and a specific mineral, by glaucony (facies) and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family. The widely accepted model of Burst and Hower for glauconitization requires a degraded, micaceous (2: 1 layer lattice structure) parent clay mineral. However, detailed analysis of numerous samples of Recent glaucony reveals that such a parent substrate is exceptional. The model therefore requires modification. Generally the parent material is carbonate particles, argillaceous (kaolinitic) faecal pellets, infillings of foraminiferal tests, various mineral grains and rock fragments, that pass gradually into the commonly occurring green grains. We show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate, accompanied by progressive alteration and replacement of the substrate. It is this two‐fold evolution that causes the ‘verdissement’of granular substrates, macrofossils and hardgrounds. The authigenic mineral is an iron‐rich and potassium‐poor glauconitic smectite. While new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium, producing decreasingly expandable minerals with a non‐expandable glauconitic mica as the end member. This mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony. Four categories, nascent, little‐evolved, evolved and highly‐evolved glaucony are distinguished. Glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro‐environment and ambient open marine sea water. The optimum conditions for glauconitization are those of semi‐confinement. As a result the interior of a grain is more glauconitized than its less confined periphery. Similarly, for identical substrate types, large grains (500μm) provide more favourable substrates for glauconitization than lesser confined small grains. On a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing. Low accumulation rates expose grains to the open marine environment for sufficiently long times (10 5‐10 6 years for highly‐evolved glaucony).