On The Entropy Of Glaucophane Na2Mg3Al2Si8O22(Oh)2

The heat capacity of glaucophane from the Sesia-Lanza region of Italy having the approximate composition (Na1.93Ca0.05Fe0.02) (Mg2.60Fe0.41) (Al1.83 Fe0.15Cr0.01) (Si7.92Al0.08)O22(OH)2 was measured by adiabatic calorimetry between 4.6 and 359.4 K. After correcting the C(p)0 data to values for ideal glaucophane, Na2Mg3Al2Si8O22(OH)2 the third-law entropy S(298)0-S(0)0 was calculated to be 541.2 +/- 3.0 J.mol-1.K-1. Our value for S(298)0 - S(0)0 is 12.0 J.mol-1.K-1 (2.2%) smaller than the value of Likhoydov et al. (1982), 553.2 +/- 3.0, is within 6.2 J.mol-1.K-1 of the value estimated by Holland (1988), and agrees remarkably well with the value calculated by Gillet et al. (1989) from spectroscopic data, 539 J.mol-1.K-1.