Professor Barber found that the teeth contain a hard mineral known
as goethite, which forms in the limpet as it grows.
But a range of other rocks appear in the archaeological record, from the yellow
ochre goethite to the often - dramatic specular hematite, sometimes called specularite.
We discovered that the fibres
of goethite are just the right size to make up a resilient composite structure.
Here we present data
for goethites from an ironstone in the Upper Ordovician Neda Formation (Wisconsin, USA) 8, which suggest that 440 Myr ago atmospheric PCo2 was ~ 16 times higher than today.
Studies of the common
mineral goethite (α - FeOOH) have shown that it contains small quantities of a carbonate component (Fe (CO3) OH), the concentration and carbon isotope content of which preserves a record of ambient PCo2at the time of formation4 — 7.
The measurements showed that in the presence of soluble iron and a more stable phase of iron, known
as goethite, the reaction is enhanced.
To complicate matters even more, when heated to at least 480 degrees Fahrenheit, yellow ochre's crystal structure changes and
the goethite can transform into hematite, or red ochre.
The main iron oxide is
goethite (FeOOH), with enough hematite (Fe2O3) to make the soil red.