Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
GLAUCONITE
Online EncyclopediaEncyclopedia Home :: GEO-GNU
del.icio.us it!
|
GLAUCONITE , a 'See also:mineral, 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 See also:iron and See also: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:present See also:time on and converted into other ferruginous minerals. It has been the See also:sea bottom at many different places. The wide See also: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 See also: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 See also:sand being derived from the See also:wear of the continents, transported by marine currents. The greater See also:part of the See also:mass consists in all cases of minerals such as See also:quartz, See also:felspar (often See also:labradorite), See also:mica, 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:augite, 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:good 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 See also: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 See also:case to be glauconite. Other ingredients, such as See also: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 See also: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 See also: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-See also: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 See also:Pliocene sands of See also: 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:Chalk-See also:rock and Chalk-See also:marl of some parts of See also:England glauconite is rather frequent, and glauconitic chalk is known also in the See also:north of See also:France. Among the See also:oldest rocks which contain this mineral are the Lower See also:Silurian of the St See also:Petersburg See also:district, to deposits of glauconite, as for example those of the Mesabi range, See also:Minnesota, U.S.A. (J. S. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] GLAUCHAU |
[next] GLAUCOUS (Gr. yAavK6s, bright, gleaming) |