1911 Encyclopædia Britannica/Itacolumite
ITACOLUMITE, the name given to a variety of porous yellow sandstone or quartzose schist, which occurs at Itacolumi, in the southern portion of Minas Geraes, Brazil. This rock is of interest for two reasons; it is believed to be the source of the diamonds which are found in great numbers in the district, and it is the best and most widely known example of a flexible sandstone. Itacolumite is yellow or pale-brown, and splits readily into thin flat slabs. It is a member of a metamorphic series, being accompanied by clay-slate, mica schist, hornblende schist and various types of ferriferous schists. In many places itacolumite is really a coarse grit or fine 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 diamond in these rocks is to be accounted for. Some authorities hold that the diamond has been formed in certain quartz veins which 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 foot or two long and half an inch thick be supported at its ends it will gradually bend by its own weight. If it then be turned over it will straighten and bend in the opposite direction. Flakes a millimetre or two thick can be bent between the fingers and are said to give out a creaking sound. It should be noted that specimens showing this property form only a small part of the whole mass of the rock. Flexible rocks have also been reported and described from North and South Carolina, Georgia, Delhi, and from the north of England (Durham). They are mostly sandstones or quartzites, but the Durham rock is a variety of the magnesian limestone of that district.
Some discussion has taken place regarding the cause of the flexibility. At one time it was ascribed to the presence of thin scales of mica which were believed to permit a certain amount of motion between adjacent grains of quartz. More probably, however it is due to the porous character of the rock together with the interlocking junctions between the sand grains. The porosity allows interstitial movement, while the hinge-like 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 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 atmosphere for years retain this property in a high degree. (J. S. F.)