Molar volume and thermal expansion of glaucophane

The molar volume of glaucophane [Na2Mg3 Al2Si8O22(OH)(2)] has been determined in this study by correcting synthetic glaucophane- rich amphiboles made in the system Na2O-MgO-Al2O3-SiO2-H2O for very small deviations from ideal glaucophane composition using recent volume data on key amphibole components. The derived unit-cell volume for end-member glaucophane is 862.7 +/- 1.6 angstrom(3), which gives a molar volume of 259.8 +/- 0.5 cm(3)/mol and a calculated density of 3.016 +/- 0.006 g/cm(3). This value has been corroborated through an essentially independent method by correcting the volumes of natural sodic amphiboles reported in the literature for non-glaucophane components, particularly including calcium-rich components, to yield a value of 861.2 +/- 1.9 angstrom(3). The unit-cell volume derived from the synthetic amphiboles, which is considered here to be more reliable, is somewhat smaller than that reported previously in the literature. A thermal expansion (alpha(V)) at 298 K of 1.88 +/- 0.06 x 10(-5)/K was derived from unit-cell volumes measured in the range of 25 - 500 degrees C for a synthetic glaucophane sample, which is noticeably smaller than previously reported.