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Banded iron formation from the Barberton Greenstone Belt, South Africa. A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe 3 O 4) or hematite (Fe 2 O 3), alternating with bands of iron-poor chert, often red in color, of similar thickness.
This rock is formed under pressures ranging from 2 to 15 kbar, sometimes even more, and temperatures over 300 °C (572 °F). Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage. Gneisses are common in the ancient crust of continental shields.
An example of a mineral in iron-rich rock containing carbonates is siderite and an example of minerals in an iron-rich rock containing silicate is chamosite. [2] They are often interbedded with limestones, shales, and fine-grained sandstones. They are typically nonbanded, however they can be very coarsely banded on occasion. [1]
Made of banded slate, a material frequently used in bannerstone manufacture. Bannerstones were used in North America for some 3,000 years beginning in the fourth millennium BC. [ 1 ] Bannerstone in use as a weight on a bowstring-style hand drill (re-creation) [ 2 ] Bannerstone, Ferruginous quartz, 2nd millennium BC.
Flow banding is a geological term to describe bands or layers that can sometimes be seen in rock that formed from magma (molten rock). [ 1 ] Flow banding is caused by friction of the viscous magma that is in contact with a solid rock interface, usually the wall rock to an intrusive chamber or, if the magma is erupted, the surface of the Earth ...
Jaspillite (or jaspilite), also called itabirite or jasper taconite, is a chemical rock formed similar to chert, but is generally quite iron-rich. Jaspillite is typically a banded mixture of hematite and quartz common in the banded iron formation rocks of Proterozoic and Archaean age in the Canadian Shield .
The Gunflint Iron Formation is a banded iron formation, composed predominantly of dense chert and slate layers interbedded with ankerite carbonate layers. The chert layers can be subdivided into black layers (containing organic material and pyrite), red layers (containing hematite), and green layers (containing siderite). [5]
Agate (/ ˈ æ ɡ ɪ t / AG-it) is a fibrous, banded variety of chalcedony. [1] Agate stones are characterized by alternating bands of different colored chalcedony and sometimes include macroscopic quartz.