The serpentine subgroup mineral berthierine was synthesized as a metastable precursor of the chlorite group mineral chamosite in cold seal pressure vessels at 575°C, 0.5 GPa and fO2-conditions of the NNO buffer from a glass of almandine bulk composition. The run products were investigated with X-ray powder diffraction (XRD), Mössbauer spectroscopy and electron microprobe analysis. One run product was also investigated by high-resolution transmission electron microscopy (HRTEM) and its heat capacity measured by heat pulse calorimetry and by differential scanning calorimetry in the temperature range 5–323 K. The XRD and HRTEM investigations clearly showed that the periodicity along the c axis of this sample is 7 Å demonstrating that the serpentine subgroup mineral berthierine of composition (Fe1.832+ Fe0.333+ Al0.67)[Si1.33Al0.67O5](OH)4 has formed in the synthesis experiments.
Integration of our heat capacity data, corrected to the composition (Fe2.5Al0.5)[Si1.5Al0.5O5](OH)4 for end-member berthierine, yields a standard entropy of 284.1 ± 0.3 J mol−1 K−1. The Cp polynomial Cp = 610.72 − 5140.0 × T−0.5 − 5.88486106T−2 + 9.5444 × 108T−3 is recommended for thermodynamic calculations above 298 K involving berthierine.