GLAUCONITE, a green mineral, a hydrous silicate of iron with potassium. It especially occurs in the green sands and muds which are gathering at the present time on the sea bottom at many different places. The wide extension of these sands and muds was first made known by the naturalists of the "Chal lenger," and it is now found that they occur in the Mediter ranean as well as in the open ocean, but they have not been found in the Black sea or in any freshwater lakes. These deposits are not in a true sense abyssal, but are of terrigenous origin, the mud and sand being derived from the wear of the continents, trans ported by marine currents. The depth in which they accumulate varies a good deal, viz., from 20o up to 2,000 fathoms, but as a rule is less than 1,000 fathoms, and it is believed that the most common situations are where the continental shores slope rather steeply into moderate depths of water.
The glauconites, though crystalline, never occur well crystal lized but only as dense clusters of very minute particles which react feebly on polarized light. They have one well-marked char acteristic, inasmuch as they often form rounded lumps. In many cases it is certain that these are casts, which fill up the interior of empty shells of Foraminifera. It is now believed that glauconite is essentially the same as the green iron silicate that forms the primary substance of so many iron ores of marine origin, and that the presence of potash is merely due to colloidal adsorption ; the source of the potash is, however, by no means clear.
In a small number of Tertiary and older rocks glauconite occurs as an essential component. It is found in the Pliocene sands of Holland, the Eocene sands of Paris and the "Molasse" of Swit zerland, but is much more abundant in the Lower Cretaceous rocks of northern Europe, especially in the subdivision known as the Greensand. Rounded lumps and casts like those of the green sands of the present day are plentiful in these rocks, and it is obvious that the mode of formation was in all respects the same. The green sand when weathered is brown or rusty coloured, the glauconite being oxidized to limonite. Calcareous sands or impure limestones with glauconite are also by no means rare, an example being the well-known Kentish Rag. In the chalk-rock and chalk marl of some parts of England glauconite is rather frequent, and glauconitic chalk is known also in the north of France. Among the oldest rocks which contain this mineral are the Ordovician of the Leningrad district and southern Sweden, as well as the basal Cambrian quartzite in Shropshire, but it is very rare in the Palaeozoic formations, possibly because it undergoes crystalline change and is also liable to be oxidized and converted into other ferruginous minerals. It has been suggested that certain deposits of iron ores may owe their origin to deposits of glauconite, as for example those of the Mesabi range, Minnesota. (J. S. F.)