The heat capacity of the serpentine subgroup mineral berthierine (Fe (sub 2.5) Al (sub 0.5) )[Si (sub 1.5) Al (sub 0.5) O (sub 5) ](OH) (sub 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 degrees C, 0.5 GPa and f (sub O2) -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 Aa demonstrating that the serpentine subgroup mineral berthierine of composition (Fe (sub 1.83) (super 2+) Fe (sub 0.33) (super 3+) Al (sub 0.67) )[Si (sub 1.33) Al (sub 0.67) O (sub 5) ](OH) (sub 4) has formed in the synthesis experiments. Integration of our heat capacity data, corrected to the composition (Fe (sub 2.5) Al (sub 0.5) )[Si (sub 1.5) Al (sub 0.5) O (sub 5) ](OH) (sub 4) for end-member berthierine, yields a standard entropy of 284.1+ or -0.3 J mol (super -1) K (super -1) . The C (sub p) polynomial C (sub p) = 610.72-5140.0XT (super -0.5) - 5.8848610 (super 6) T (super -2) +9.5444X10 (super 8) T (super -3) is recommended for thermodynamic calculations above 298 K involving berthierine.