Compositional variations in glauconite

Abstract

Variation in composition, Fe³⁺/Fe²⁺ ratio, per cent mixed-layering, and d(060) spacing have been determined both between and within grains of glauconite from the Folkestone Beds at 286.5 m in the Tollgate bore, Sandwich, Kent. Grains extracted from the rock were classified on the basis of size into large (> 1 mm) and small (< 1 mm), and on colour into light and dark green. Large grains have a progressive increase in Fe³⁺/Fe²⁺ ratio with increase in total iron, while small grains have variable ratios; the largest variation is in the dark grains. The effect of the range of Fe³⁺/Fe²⁺ ratio variations on structural formula (e.g. R³⁺) is discussed. The per cent mixed-layering is not related to colour. There is a greater variation in Al, Fe, and K contents in and between large grains than small grains. Both sizes of grains may be separated on d(060) spacings determined by X-ray diffraction. Glauconite from coral and gastropod casts has different d(060) spacings than that from bryozoan casts and variegated grains, but is similar to the small grains. Large light grains may have developed from small light grains but it is unlikely that the large dark grains developed from small dark grains. The large grains probably all formed under similar conditions, with a common origin; the small from several sources or from a single source under a variety of conditions.