Ochre

Ochre is a term used to describe natural mineral pigments - commonly red (red ochre) and yellow (yellow ochre).  This is related to yesterday's discussion because both are iron oxide minerals.

Red ochre is derived from hematite (Fe₂O₃) which is oxidized iron and rusty red in color (rust and hematite are essentially the same thing).  The name hematite comes from αἷμα (haima) the Greek word for blood (same root as hemoglobin) because of the color.

Cultures around the world have used hematite to make red ochre pigment since the Paleolithic (Stone Age).  Below is a red ochre (and carbon black) wisent (European bison) in Altamira Cave, Spain.

The neat thing about ochre is that geochemical analysis can often identify the provenance (where it was mined) of the pigment - for example: Iriate, E., et al. 2009. The Origin and Geochemical Characterization of Red Ochres from the Tito Bustillo and Monte Castillo Caves (Northern Spain). Archaeometry 51: 231-251.

Lion Cave, in Swaziland, southern Africa, is one of the oldest hematite ochre mines we know of having been dated back to 43,000 years old.  Women is southern Africa still use ochre to beautify themselves (the woman below is from the Himba tribe of Namibia).

Ochre pigment is common in the New World as well.  The red sandstones of the Colorado Plateau region are red because of hematite and were used as pigment by the Anasazi and other Native Americans.  The pictograph below is from the Grand Gallery deep in Canyonlands National Park of Utah (I love this picture - what are they?  Ghosts?  Shamans? Kachina-like gods?).



I still have a few pair of formerly white socks stained red from Utah red dirt from hiking around Moab in Arches National Park.

Yellow ochre, on the other hand, is a form on hydrated iron(III) hydroxide (FeO(OH) • nH₂O).  It's usually called limonite but limonite is not a true mineral name (although it's commonly used by geologists).  Limonite is actually a mixture of different iron hydroxide minerals which typically form when iron-rich water encounters atmospheric oxygen (and typically helped along by Fe-fixing bacteria) - hence the name "bog iron" for sedimentary deposits of this mineral.



Bog iron was once an important ore of iron in Northern Europe for people like the Vikings (here's an interesting web site about this).  It was also used as a paint pigment.  Below is a yellow and red ochre turtle from Kakadu National Park, Australia.