| Abstract |
The minerals are found in the pyrometasomatic magnetite ore deposits of the Sampo mine, Okayama Prefecture, Japan. They occur in a quartz-calcite veinlet, cutting a skarn consisting mainly of manganoa...n hedenbergite, and form foliated or spherulitic aggregates with brownish black in color. Their optical properties, chemical compositions and X-ray powder data indicate that the minerals are similar to the manganoan stilpnomelane described from the Matsuo mine by SHIROZU (1964). Mineralogical properties of a representative specimen(S-1) are as follows; Optical properties: α=l. 602, β=r=l. 672, γ-α=0. 070, 2V(-）≒0.Pleochroism: X=brownish yellow, Y =Z=deep reddish brown. Chemical compositions by EPMA analysis: SiO_2 45. 61, TiO_2 tr., Al_2O_3 2.63, Fe_2O_3 18. 71, FeO 13. 34, MnO 8. 00, CaO 0. 73, MgO 0. 46, Na_2O 0. 06, K_2O 0. 18, H_2O (=total 100. 00-.Σ oxides wt. ％) 9. 45 wt. ％. D_001=12. 70A. As shown in Fig. 3, however, detailed X-ray powder data of the specimens from the Sampo and Obira mines are distinct from those from other localities, such as the Matsuo mine, Kawaguchi, etc. The d_001 value of the former is about 12. 8 A, which is larger than that of the latter (12. 1 A). The specimens with large d。01 value have lower Al_2O_3 and MgO contents than those with small d。01 value. Furthermore, heating behaviours and D. T. A. curves are also described on the two groups. These data suggest that two distinct series exist in stilpnomelane.show more
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