Itacolumite
Encyclopedia
Itacolumite is a naturally occurring porous, yellow sandstone
that is flexible when cut into thin strips. It occurs in great numbers at Itacolumi, in the southern portion of Minas Gerais
, Brazil
. It is also found in Jhajjar
, India
; Georgia; and Stokes and McDowell Counties North Carolina, USA. It is the best and most widely known example of a flexible sandstone and is a source of diamonds found in the Minas Gerais area.
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 is supported at its ends it will gradually bend by its own weight. If it is then 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, but specimens showing this property form only a small part of the whole mass of the rock.
. 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.
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
that is flexible when cut into thin strips. It occurs in great numbers at Itacolumi, in the southern portion of Minas Gerais
Minas Gerais
Minas Gerais is one of the 26 states of Brazil, of which it is the second most populous, the third richest, and the fourth largest in area. Minas Gerais is the Brazilian state with the largest number of Presidents of Brazil, the current one, Dilma Rousseff, being one of them. The capital is the...
, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. It is also found in Jhajjar
Jhajjar
Jhajjar is a town in Jhajjar district in the Indian state of Haryana.-Geography:Jhajjar is located at . It has an average elevation of 220 metres .-Demographics:...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
; Georgia; and Stokes and McDowell Counties North Carolina, USA. It is the best and most widely known example of a flexible sandstone and is a source of diamonds found in the Minas Gerais area.
On the split faces of the slabs, scales of greenish mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
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 is supported at its ends it will gradually bend by its own weight. If it is then 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, but specimens showing this property form only a small part of the whole mass of the rock.
Cause of flexibility
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 quartzQuartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
. More probably, however, it is due to the porous character of the rock together with the interlocking junctions between the sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
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.