ABSTRACT

Petrographic study of a glauconitic sandy chalk from the Cretaceous of Northern Ireland reveals medium sand sized microcrystalline "glauconite" pellets of three types: grass green, often with surface shrinkage cracks; pale green; and variegated, that is, mottled in shades of grass green and pale green. The pale green and variegated pellets show no surface shrinkage cracking, but in rare instances display internal dilation cracks reminiscent of those found in septarian nodules. The cracks are lined by colorless or grass green fibrous crystallites (of apatite or the mineral glauconite) which enclose a microcrystalline glauconitic or opaque "hematitic" filling.

Chemical and X-ray analysis of the three types of pellet suggests that the grass green pellets consist of the mineral glauconite; whereas the pale green and variegated pellets are mineral mixtures, pigmented by the mineral glauconite, but chiefly made up of francolite, a carbonate fluorapatite.

The genesis of the pellets is discussed in terms of the hypothesis that they formed by accretion of colloid particles. It is argued that in wave or current stirred bottom environments abrasion would impose a constraint upon such accretionary pellet growth.