Iron is one of seven planetary metals to which the alchemists assigned the influence of a planet - the basic chemical process of change, the oxidation of iron in air, is a reaction that was known to the medieval alchemists. It is supposedly dominated by Mars and Karl Wilhelm Scheele (1742-1786) used the planetary symbol to identify iron. Mars is known as the red planet, and the association with the metal makes sense insofar as the reddish-looking surface of the planet means that iron is present. This is Iron (III) oxide dust with the consistency of talcum powder. The red colour is produced by the movement of an electron on to the metal ion from an adjacent oxygen ion in a charge-transfer process.
- Ancient cave painters at Lascaux took their palette from their surroundings: using yellow iron oxide (limonite) for yellow ochre and red iron oxide (haemetite) for red ochre.
- Monument Valley, USA.
- The gradual process of oxidation in the Uluṟu - Kata Tjuṯa National Park gives the outer surface layer of Uluṟu its red colour.
- The village of Roussillon, is located to the south of Mont Ventoux, the top of which is is bare limestone without vegetation or trees, making it appear from a distance to be snow-capped all year round. Roussillon, by contrast, lies at the heart of one of the biggest ochre deposits in the world, surrounded by magnificent red cliffs and ochre quarries.
- Red ochre contains iron mainly in the form of alpha -Fe2O3 and in alumina-type octahedra, while yellow ochre has iron chiefly in the form of alpha -FeOOH.
- The yellow-brown colour of Sienna comes from ferric oxides contained within.
- Iron oxide is a by-product in the manufacture of sulphuric acid, required as a bleach for the textile industry, in the 18th century. a range of Mars pigments including Mars red were produced from the iron oxide.