R4: Mineral sciences of the Earth surface

[R4-07] The color change materials and cause of the coloring of the microbial mats on the surface of the calcareous sinter and the color change materials
~Mineral crystallization and oxidation in microbial mats~

*Miki Suzuki¹, Miku Tatuiwa², Terumi Ejima³, Haruki Yamazaki⁴, Hikari Minamisawa⁵, Masaomi Horita⁵, Maki Hamada⁶ (1. Shinshu Univ. Sci., 2. Shinshu Univ. Sci., 3. Shinshu Univ. Sci., GSJ, AIST, 4. Shinshu Univ. Sci., GSJ, AIST, 5. Shinshu Univ. Eng., 6. Kanazawa Univ.)

Keywords:biomats, vivianite, shirahone hot spring

The Microbial mats(biomats) on the calcareous sinter from Shirahone Hot Springs turn purple, red-purple and light blue with the passage of time from the time of collection. The purpose of this study is to clarify the cause of the coloring of the biomats on the surface of the calcareous sinter and the color change process with the time. The study methods are observation and chemical analysis of the purple-colored biomats using field emission electron microprobe analyzer (FE-EPMA) and transmission electron microscope (TEM). The results of progress observation showed that the samples stored in a vacuum changed in color more slowly than those stored in the air. From the analytical results, the purple biomats were contain a small amount of Fe as transition metal element and found to contain fine particles of about 10 nm in diameter containing Fe and P. Based on the constituent elements of the fine particles and the characteristics of color change due to oxidation, the fine particles are presumed to be vivianite. It has been known that the color of vivianite is changed by oxidation in the air (e.g. McCammon and Burns, 1980). Since the color of the sample stored in a vacuum changes more slowly than that stored in the air, it is evident that vivianite particles, formed by Fe²⁺ taken up by bacteria and P, changed its color by oxidation in the air over time. Vivianite is usually colorless and transparent, and changes in color to dark blue, dark patina, indigo blue, blue, and black with the oxidation (e.g. McCommon and Burns, 1980; Anthony et al., 2005), but the samples in the biomats in this study are characterized by purple, red-purple and light blue discoloration. It is considered that the vivianite in the present study is particles of the nano order and the color of the aggregate is different from the characteristic color of the macroscopic crystal.