Photomicrographs of nonskeletal grains

This photograph shows ooids with well-developed radial and concentric structures. (Ooids are spherical or ellipsoidal grains, less than 2mm in diameter, having regular concentric laminae developed around a uncleus.) The nuclei are micritic carbonate grains. The sample shows a range of ooids, from those with a small nucleus and thick cortex(the oolitic coating), to those with a large nucleus and a single oolitic lamina. The latter are called superficial ooids. The matrix between the ooids is a mixture of carbonate mud and sparry calcite cement.

This photograph illustrate ooids with a rather poorly-preserved concentric structure. The structure may have been partly lost by micritization. The speckled plates with thin micrite coatings are echinoderms(an example can be seen half way up the right-hand edge). The pink-stained cement is non-ferroan sparry(authigenic) quartz replacing calcite.

Those grains composed of micrite and lacking any recognizable internal structure are called 'peloids'. This photograph shows a limestone in which the allochems are mainly peloids, circular to elliptical in cross-section and averaging about 0.1mm in diameter. Such peloids are generally interpreted as faecal in origin and are called 'pellets'. The photograph shows pellets at the lower end of the size range for typical pellets, which extends up to 0.5mm.

This photograph shows a large grain which might be described as a 'coated bioclast'. This is a kind of intraclast, which was once incorporated on the sea-floor of the basin of deposition and was later reworked to form new sediment grains. It comprises a nucleus, which is a fragment of a brachiopod shell, surrounded by a coating of microcrystalline calcite.

This photograph shows aggregate grains. This is made up of irregular aggregates of a small mumber of recognizaable particles cemented together by micrite or fine sparite. The component particles includes ooids(the grain right of center) as well as peloids and a few bioclasts. The opaque material in the top center is bitumen.

This is a photograph of a polished rock surface showing oncoids. Oncoids are presumed to be biogenic, blue-green algae on the grain surface, trapping and binding fine sediment particles. Note the size of the grains, the asymmetrical growth and the wavy nature of many of the laminae, all features characteristic of oncoids. The bluish-grey areas are sparry calcite and the orange-brown areas are stained with iron oxides.

This photograph shows grains which are about 2mm in diameter and whose origin is difficult to interprete. The outer surfaces are not as smooth as most ooids, although the concentric lamination is very regular. Grains in the upoer right show irregular outer coats of micrite and some particles have apparently grown together to form compound grains(e.g. lower left). This latter feature is unlikely to occur in ooids, where precipitated carbonate laminae are formed while the grain is held in suspension. These grains are therefore interpreted as oncoids.

This photograph shows grains with a regular, well-difined concentric layering, in grains up to 5mm in diameter. This is typical of inorganic growth and these grains may be pisoids. Pisoids are commonly fractured or broken. Broken pieces can be seen towards the top right of the photograph.