Ironstone

A sedimentary rock, it contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be confused with native or telluric iron, which is very rare and found in metallic form, dark chocolate in appearance ironstone is customarily restricted to hard, coarsely banded, non-banded, sedimentary rocks of post-Precambrian age.

Petrographic Description and Mineral Examination

Great Tew Ironstone [The Marlstone Rockbed] has a distinctive deep orange-brown colour with occasional patches of white mineralisation/marbleing and is commonly found throughout the counties of North Oxfordshire, Northamptonshire through to Rutland. Overall the stone is fine grained with some particles having a deep iron-rich colour with others being paler.

In thin section the stone had a high content of rounded to angular grainules ranging in colour from dark iron-brown to pale orange. Sometimes the grains were oolitic, with seeds and rims of different composition/texture. These grains were generally well-compacted and often grain-supported. Also present were broken up fossil fragments which appeared to include bryozoan, bivalves, echinoid and crinoid pieces.

A partial carbonate cement is visible throughout the section, with the distribution of this being rather variable. Patches showing a well-developed cement are those areas of the hand specimen appearing white and more resistant. There was also evidence of a fine siliceous/clay matrix in some areas of the section. Based on the mineralogy identified in thin section and the texture seen in hand specimen, the stone has been given the classification of Ferruginous limestone.

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