Published online by Cambridge University Press: 01 May 2009
Glauconite may be described as essentially a hydrous silicate of iron and potash, but of very variable composition, and generally containing varying proportions of other bodies, such as alumina, lime, and magnesia. It is found at different horizons in the whole geological series of rocks from the Cambrian up to the most recent Tertiary layers, and, indeed, is particularly interesting in being one of the very few silicates which are in actual process of formation on the sea-bed at the present time. The physical characteristics of glauconite grains are practically the same throughout the series, the normal colour being dark green, but sometimes being yellowish, greyish, or even almost red. These variations of colour, however, mean at least the commencement of decomposition. The size of the grains is usually about one millimetre in diameter, and although much larger masses are sometimes found these are merely agglomerations of the smaller grains. Under the microscope the grains appear quite homogeneous unless some foreign body is enclosed, or, as sometimes occurs, the commencement of decomposition gives a more or less zonary appearance to them.
page 317 note 1 See also an analysis by me of one of the altered Silurian “flags” at Shap (Geol. Mag., January and February, 1894), in which the total alkali is close on 8 per cent.
page 318 note 1 Careful subsequent investigations show that any decomposition which may occur during this preliminary treatment with dilute acid has an inappreciable effect upon the figures of the final analysis. Prolonged treatment with dilute acid tends, however, to remove iron, etc., and to heighten the percentage of silica in the residue. Thus a cold solution containing only 0.5 per cent. hydrochloric acid, acting for 39 1/2 hours, extracted 25 per cent. by weight of the constituents of the glauconite.
page 319 note 1 Report on scientific results of the voyage of H.M.S. “Challenger”; “Deep Sea Deposits” (1891), pp. 458–460.Google Scholar
page 319 note 2 Hintze, “Handbuch der Mineralogie,” p. 850. Analyses I, II, IV–XI, XIII, and XIVGoogle Scholar