GLAUCONITE , a '
See also:green in
See also:colour, and chemically a but it is very rare in the Palaeozoic formations, possibly because it hydrous silicate of iron and potassium . It especially occurs in the undergoes crystalline
See also:change and is also liable to be oxidized green sands and muds which are gathering at the
See also:time on and converted into other ferruginous minerals . It has been the
See also:sea bottom at many different places . The wide extension of suggested that certain deposits of iron ores may owe their origin these sands and muds was first made known by the naturalists of the " Challenger," and it is now found that they occur in the Mediterranean as well as in the open ocean, but they have not been found in the Black Sea or in any fresh-
See also:water lakes . These deposits are not in a true sense abyssal, but are of
See also:terrigenous origin, the mud and sand being derived from the
See also:wear of the continents, transported by marine currents . The greater
See also:part of the mass consists in all cases of minerals such as
See also:felspar (often
See also:chlorite, with more or less
See also:calcite which is probably always derived from shells or other organic
See also:sources . Many
See also:accessory minerals such as
See also:tourmaline and
See also:zircon have been identified also, while
See also:hornblende and other volcanic minerals occur in varying proportion as in all the sediments of the open sea . The
See also:depth in which they accumulate varies a
See also:deal, viz. from 200 up to 2000 fathoms, but as a
See also:rule is less than l000 fathoms, and it is believed that the most
See also:common situations are where the
See also:continental shores slope rather steeply into moderate depths of water . Many of the blue muds, which owe their colour to
See also:fine particles of sulphide of iron, contain also a small quantity of glauconite; in
See also:Globigerina oozes this substance has also been found, and in fact there exists every gradation between the glauconitic deposits and the other types of sands and muds which are found at similar depths . The colouring
See also:matter is believed in every case to be glauconite . Other ingredients, such as lime, alumina and
See also:magnesia are usually shown to be present by the analyses, but may perhaps be regarded as non-essential: it is impossible to isolate this substance in a pure state as it occurs only in fine aggregates, mixed with other minerals . The glauconite, though crystalline, never occurs well crystallized but only as dense clusters of very minute particles which react feebly on polarized
See also:light .
They have one well-marked characteristic inasmuch as they often
See also:form rounded lumps . In many cases it is certain that these are casts, which fill up the interior of empty shells of
See also:Foraminifera . They may be seen occupying these shells, and when the
See also:shell is dissolved away perfect casts of glauconite are set
See also:free . Apparently in some manner not understood, the decaying organic matter in the shell of the dead organism initiated or favoured the chemical reactions by which the glauconite was formed . That the mineral originated on the sea bottom among the sand and mud is quite certainly established by these facts; moreover, since it is so soft and friable that it is easily powdered up by pressure with the fingers, it cannot have been transported from any
See also:great distance by currents . Small rounded glauconite lumps, which are common on the sands but show no trace of having filled the
See also:chambers of Foraminifera, may have arisen by a re-deposit of broken-down casts such as have been described; probably slight
See also:movement of the deposits, occasioned by currents, may have broken up the glauconite casts and scattered the soft material through the water . Films or stains of glauconite on shells, sand grains and phosphate nodules are explained by a similar deposit of fragmental glauconite . In a small number of
See also:Tertiary and older rocks glauconite occurs as an essential component . It is found in the Pliocene sands of
See also:Holland, the Eocene sands of
See also:Paris and the " Molasse " of
See also:Switzerland, but is much more abundant in the
See also:Lower Cretaceous rocks of N .
See also:Europe, especially in the subdivision known as the
See also:Greensand . Rounded lumps and casts like those of the green sands of the present
See also: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
See also:brown or rusty, coloured, the glauconite being oxidized to
See also:limonite .
Calcareous sands or impure limestones with glauconite are also by no means rare, an example being the well-known Kentish Rag . In the
See also:rock and Chalk-marl of some parts of England glauconite is rather frequent, and glauconitic chalk is known also in the
See also:north of France . Among the
See also:oldest rocks which contain this mineral are the Lower
See also:Silurian of the St
See also:district, to deposits of glauconite, as for example those of the Mesabi range,
See also:Minnesota, U.S.A . (J . S .
GLAUCOUS (Gr. yAavK6s, bright, gleaming)
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