Garnet–fassaite calc-silicate nodule from La Soufrière, St. Vincent

The mineralogy of an unusual calc-silicate nodule brought to the surface by the 1902 eruption of La Soufrière volcano on St. Vincent (Lesser Antilles arc) is described. It consists of the crystalloblastic mineral assemblage fassaite pyroxene + grandite garnet + wollastonite ± anorthite ± calcite. The pyroxene is more aluminous (Al₂O₃ = 19.66 weight percent) than any previously described from terrestrial rocks, with about 45 mole percent Ca–Al Tschermak´s molecule, 37 mole percent diopside, and 15 mole percent Ca–Fe³⁺ Tschermak's molecule in solid solution. Unit-cell data have been calculated on the basis of X-ray powder diffractometry data for the fassaite (a = 9.704, b = 8.828, c = 5.204A, β = 106.95°; V = 426.49A³) and the grandite (grossular₇₈–andradite₂₂; a = 11.905A; V = 1687A³). We infer a paragenesis related to skarn-type metamorphism of calcareous sediment.