Abstract
Howieite (Hw) was recently found in meta-manganese siliceous rocks intercalated with lawsonite blueschists in the Hakoishi serpentinite mélange of the Kurosegawa belt, Yatsushiro, Kyushu, Japan; the peak pressure and temperature conditions were around 0.7-0.9 GPa and <350 °C, respectively. Hw is closely associated with Mn-rich stilpnomelane (Mn-stp), riebeckite, lawsonite, aegirine, calcite, quartz, and opaque minerals. Both Hw and Mn-stp occur as acicular to fine-grained aggregates and show yellow to pale yellow pleochroism. Electron microprobe analysis suggests that the distribution of Hw is relatively homogeneous in each grain, with an average composition of (Na1.19K0.01)1.21(Fe2+4.87Mn4.07Mg0.71)9.65(Fe3+1.11Al0.89)2.00(Si12.06Ti0.07)12.12O31(OH)13.
Taneyamalite (Tn), a Mn-dominant member of Hw, has been reported from a Mn ore bed in the study area by Aoki et al. (1981). The difference in Mn content between Hw and Tn in the study area may be a consequence of the variety of host rock compositions present. The literature data and this study suggest that chlorite and white mica, which are representative hydrous phases in low-grade metamorphic rocks, are scarce or not present in meta-manganese siliceous rocks containing Hw or Tn. Therefore, we emphasize that Hw-Tn series minerals (along with Mn-stp) act as major water storage in meta-manganese siliceous rocks under low-temperature (<350 °C) conditions.
