Introduction

The thermal transformation from goethite to hematite can be 

described by the simple equation:

2 ααα-FeOOH ⇒⇒⇒ ααα-Fe2O3 + H2O

On the other hand, the dehydration mechanism is much more 

complex and depends on the particle size of the goethite needles

(figure 1) [1]. Synthetic goethite pigments (Bayferrox



) of 

different particle size (table 1) were investigated by 

thermogravimetry (TG) and differential scanning calorimetry

(DSC).

The following model for the mechanism of the dehydration 

process was described: at an early stage of the dehydration, 

water leaves the crystal lattice in the [010] direction of the crystal 

by forming dehydration channels parallel to the crystallographic

c-axis [001]. The matrix between the channels is formed by 

hematite (figure 2).

In cases where the crystal dimensions are relatively large (length 

of needles ≥ 0.3 µm), the dehydration channels within the outer 

crystal regions begin to grow together, forming a compact 

hematite layer at the crystal surface before the dehydration front 

within the needle reaches the crystal center, i. e. the dehydration 

process is not finished yet.

The compact surface hematite layer acts as a barrier which 

hampers further dehydration, because an additional  amount of 

enthalpy must be provided to overcome the dehydration barrier 

and thus allow the dehydration front to proceed into the needle 

center until the dehydration is complete. Experimental evidence 

for this model consists of a second peak in the DTG and DSC 

plots of goethite samples.

In cases where the crystal dimensions are relatively small, the 

dehydration is finished before a dehydration barrier can be 

formed.