Silica-Rich Garronite-Na From Hirado Island, Nagasaki Prefecture, Japan

Silica-rich garronite-Na was found together with epistilbite in Miocene basaltic rock from Shiratobana, Hirado Island, Nagasaki Prefecture in Japan for the first time. Garronite-Na occurs as an anhedral crystal that covers the center of a small cavity in altered basaltic rock, whereas the epistilbite covers the inside of the cavity. Electron probe microanalysis of the garronite-Na gives an empirical formula of (Na1.99K0.27)Σ2.26Ca1.61(Fe0.01Al5.31Si10.64)Σ15.96O32·14.3H2O on the basis of O = 32. Its Na/Ca molar ratio varies from 1.00 to 1.53, and its unit-cell parameters (space group I2) calculated from X-ray powder diffraction data are a = 9.983(11) Å, b = 10.089(14) Å, c = 10.070(10) Å, and β = 90.223(3)° with a calculated density of 2.183 g/cm3. Garronite-Na from Hirado Island formed from an alkaline high-silica solution in the later stages of hydrothermal zeolitization associated with volcanic activity.