Page 13 - Curriculum Visions Dynamic Book

 A piece of iron ore (haematite). The deep red colour is due to the fact that haematite is iron oxide. Pure iron forms the grey coloured crystals. Haematite is Fe2O3.
Limonite
native metal: a pure form
of a metal, not combined as a compound. Native metal is more common in poorly reactive elements than in those that are very reactive.
ore: a rock containing enough of a useful substance to make mining it worthwhile.
oxidation: a reaction in which the oxidising agent removes electrons. (Note that oxidising agents do not have to contain oxygen.)
Haematite
This is the name for the most widespread form of iron ore.
It is most often found in rocks
once deposited by rivers or the sea.
The deep red colour of some rocks indicates that they contain haematite. Most of these rocks were formed in parts of the tropics with wet and dry seasons. During the wet season the minerals eroded from the land were washed to basins, deltas
or coasts. During the dry period the water evaporated and
the sediments dried out, and iron compounds oxidised to iron oxide. These rocks are often referred to as “red beds” and are usually fine- grained materials such as shales.
Limonite is an iron oxide that forms in cooler climates than haematite. It is usually yellow, orange and brown, rather than red. It contains water molecules, and so is an example of a hydrous (water-containing) iron oxide.
Limonite is often found in marshes and is thus also known as bog iron ore.
The water content influences the colour of the oxide. Heating limonite causes the water to be driven off, thus darkening the colour of the oxide and yielding the paint colour “burnt ochre”. In fact, it was traditionally used as a source of ochre pigments in paint.
13