Abstract
Cordierite and cummingtonite occur in tuff blocks in the Nodono Formation. The blocks mainly consist of pumice, glass, plagioclase and quartz, with subordinate vocanic debris, and accessary biotite, cordierite, hypersthene, augite, hornblende, cummingtonite and opaque mineral. Cordierite occurs as euhedral crystals (less than 3mm) or fragments with a thin glass layer on the surface, and is often twinned, and exhibits distinct pleochroism from dark blue to colorless. The unit cell constants are a0=17.138(5)Å, b0=9. 748(2)Å, c0=9.327(7)Å, and V=1558.1(7)Å^3. The distortion index (Δ) is 0.29. The refractive indices are α=1.534(1), γ=1.551(1) and α-γ=0.017. The chemical formula is Na0.04 (Mn0.10 Mg1.48 Fe0.52) Al2.93(Si4.98All.02) O18・nH2O. Cummingtonite usually occurs as fragments (less than 3mm×0.7mm), and which occasionally show polysynthetic twinning. It is pleochroic from brownish green to grayish green.The unit cell constants are a0=9.508(1)Å, b0=18.083(3)Å, c00=5.311(4)Å, β=102.25(1)°, and V=892.4(2)Å^3. The chemical formula is (ÅNa_0.14 Ca0.02) (Ca0.05 Mn0.35 Fe1.60) (Fe0.65 Mg4.17 Ti0.03 Al0.15) (Si7.65 Al0.35) O22 (OH)2. Cordierite and cummingtonite found in the Nodono Formation show almost the same properties as those found in the Joetsu ash and in the Takigasawa ash, apart from the distortion index of cordierite. Based on stratigraphy and mineralogy, it is likely that cordierite and cummingtonite have the same origin as those of the Joetsu ash and the Takigasawa ash. The difference in distortion index suggests that the cordierite from Nodono may have been annealed at lower temperature during redenosition.
