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ITACOLUMITE

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Originally appearing in Volume V14, Page 887 of the 1911 Encyclopedia Britannica.
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ITACOLUMITE , the name given to a variety of porous yellow See also:

sandstone or quartzose schist, which occurs at Itacolumi, in thesouthern portion of See also:Minas Geraes, See also:Brazil. This See also:rock is of See also:interest for two reasons; it is believed to be the source of the diamonds which are found in See also:great See also:numbers in the See also:district, and it is the best and most widely known example of a flexible sandstone. Itacolumite is yellow or See also:pale-See also:brown, and splits readily into thin See also:flat slabs. It is a member of a metamorphic See also:series, being accompanied by See also:clay-See also:slate, See also:mica schist, See also:hornblende schist and various types of ferriferous See also:schists. In many places itacolumite is really a coarse grit or See also:fine See also:conglomerate. Other quartzites occur in the district, and there is some doubt whether the diamantiferous sandstones are always itacolumites and also as to the exact manner in which the presence of See also:diamond in these rocks is to be accounted for. Some authorities hold that the diamond has been formed in certain See also:quartz See also:veins which See also:traverse the itacolumite. It is clear, however, that the diamonds are found only in those streams which contain the detritus of this rock. On the split faces of the slabs, scales of greenish mica are visible, but in other respects the rock seems to be remarkably pure. If a piece which is a See also:foot or two See also:long and See also:half an See also:inch thick be sup-ported at its ends it will gradually See also:bend by its own See also:weight. If it then be turned over it will straighten and bend in the opposite direction. Flakes a millimetre or two thick can be See also:bent between the fingers and are said to give out a creaking See also:sound.

It should be noted that specimens showing this See also:

property See also:form only a small See also:part of the whole See also:mass of the rock. Flexible rocks have also been reported and described from See also:North and See also:South Carolina, See also:Georgia, See also:Delhi, and from the north of See also:England (See also:Durham). They are mostly sandstones or quartzites, but the Durham rock is a variety of the magnesian See also:limestone of that district. Some discussion has taken See also:place regarding the cause of the flexibility. At one See also:time it was ascribed to the presence of thin scales of mica which were believed to permit a certain amount of See also:motion between adjacent grains of quartz. More probably, however, it is due to the porous See also:character of the rock together with the inter-locking junctions between the See also:sand grains. The porosity allows interstitial See also:movement, while the See also:hinge-like See also:joints by which the particles are connected hold them together in spite of the displacement. These features are dependent to some extent on weathering, as the rocks contain perishable constituents which are removed and leave open cavities in their place, while at the same time additional See also:silica may have been deposited on the quartz grains fitting their irregular surfaces more 'perfectly together. Most of the known flexible rocks are also fine-grained; in some cases they are said to lose their flexibility after being dried for some time, probably because of the hardening of some interstitial substance, but many specimens kept in a dry See also:atmosphere for years retain this property in a high degree. (J. S.

End of Article: ITACOLUMITE

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