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 Mnrich 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 (Na 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.

Mn -bearing lawsonite was discovered in meta -siliceous rocks metamorphosed under the lawsonite -blueschist facies formed at less than 350 °C and 0.8 -1.0 GPa in the Hakoishi serpentinite mélange of the Kurosegawa Belt, Central Kyushu, Japan. The lawsonite deposits were accompanied by those of hematite, braunite (Mn 2+Mn 6 3+SiO 12 ), and quartz, indicating that the rock had metamorphosed under high Oxygen fugacity (f O 2 ) (~ -20 < log f O 2 < -5) at the abovementioned P -T conditions. The Oxygen fugacity led to the conversion of all the iron into ferric compounds and some of the manganese into its trivalent form. As a result, the lawsonite was found to contain a significant amount of (Mn 3+ + Fe