The heat capacity of the serpentine subgroup mineral berthierine (Fe2.5Al0.5)[Si1.5Al0.5O5](OH)4

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), Mossbauer 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 - 5.88486106 T-2+9.5444×108 T-3 is recommended for thermodynamic calculations above 298 K involving berthierine.