Itabirite

Itabirite is a metamorphosed banded iron formation made of alternating thin layers of quartz and iron oxides, chiefly hematite (often as glittering specularite) and magnetite. The banding gives the rock a striped, often shimmering silvery-grey to reddish appearance.

Named after the Itabira region of Minas Gerais, Brazil, itabirite is one of the world's most important iron ores. It represents an original chemical sediment, a banded iron formation laid down in ancient oceans, that was later recrystallized and foliated by metamorphism.

The rock is hard, dense, and noticeably heavy because of its high iron content.

Itabirite begins as a banded iron formation (BIF), a chemical sedimentary rock deposited in Precambrian oceans, mostly more than 1.8 billion years ago, when iron dissolved in oxygen-poor seawater precipitated in alternating iron-rich and silica-rich layers as the atmosphere and oceans gradually oxygenated.

Later regional metamorphism recrystallized these sediments, coarsening the quartz and converting iron minerals into well-crystallized hematite (specularite) and magnetite, and imparting a foliated or schistose fabric. Weathering and further enrichment can upgrade itabirite into high-grade hematite ore.

Great itabirite deposits occur in the Iron Quadrangle of Minas Gerais, Brazil, and similar metamorphosed BIFs are mined in Australia, India, and elsewhere.

Look for a hard, heavy, banded rock with alternating layers of grey quartz and metallic silvery-grey to black iron oxide, often with sparkling specular hematite and red iron staining. The rock feels dense for its size and may be weakly to strongly magnetic where magnetite is present.

A reddish-brown streak from hematite and the layered metallic banding are good clues. Hardness is high overall due to quartz and iron oxides.

Look-alikes: ordinary quartzite lacks the iron-oxide banding and is lighter; hematite ore is similar but itabirite shows the distinctive alternating quartz-iron banding; jaspilite is a related banded iron rock with red jasper bands rather than grey quartz. Density and banding identify itabirite.

Itabirite is a major iron ore, mined on a vast scale in Brazil and elsewhere to supply the steel industry. Although lower in iron than some direct-shipping ores, it is abundant and is concentrated by crushing and magnetic or gravity separation, and natural weathering enriches some deposits to high-grade hematite.

The specular hematite (specularite) in itabirite is occasionally used decoratively for its sparkle. The rock itself is sometimes cut as an ornamental stone.

Geologically, itabirite and banded iron formations record the oxygenation of Earth's early oceans and atmosphere, making them scientifically important. There is no significant metaphysical tradition for the rock as a whole.