Sur les matières colorantes à base de fer des terrains de sédiment et sur l'origine probable des roches rouges

Spring, W. Recueil des Travaux Chimiques des Pays-Bas (1898), 17, 202-21; SciFinder (Chemical Abstracts Service: Columbus, OH); https://scifinder.cas.org (accessed July 8, 2010).

Ferruginous rocks can be divided into four groups, green, ochre-yellow, wine-red, and black. The author attempts to explain the presence of two or more of these in the same strata, for example, in the Devonian series. It is shown that the yellowish-brown rocks do not owe their colour merely to ferric hydroxide as previously supposed, but to a compound of ferric hydroxide with a colourless oxide such as silica, magnesia, lime, or alumina, and as these compounds are much more stable than ferric hydroxide, they retain their colour when dehydrated, only turning brick-red on calcination, and at the same time becoming magnetic; they also resist the action of saline waters better than the simple hydroxide. The green rocks do not owe their colour to a simple ferrous silicate, but to a ferroso-ferric silicate, they are thus a special group of the black rocks coloured by magnetite. Ferric hydroxide, when in a compact form, retains its water only in an atmosphere the humidity of which is equal to its dissociation tension and at not too high a temperature; in a light form, under water, it crystallises and becomes dehydrated.

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