Change in Colour of Moss Agates

Abstract

THE following observations may perhaps throw light on the colour changes in moss agate and flint noted by Messrs. Whitton and Simmonds in your issues of November 10 and 17. Specimens of the flints from Bournemouth referred to by Mr. Simmonds were brought to this laboratory some months ago, and, though they were not submitted to any very searching examination, it was found that the colouring matter could be removed on boiling a fragment with hydrochloric acid, while the solution gave well marked reactions for iron and phosphoric acid. Now the compound Fe3 (PO4)2.8H2O, whether prepared in the laboratory or occurring as the mineral vivianite, is colourless when pure, but becomes oxidised to ferrosoferric orthophosphate, and turns blue, when exposed to the atmosphere. It seems probable, then, that the change of colour of these flints is due to a layer of vivianite which alters on exposure.