新鉱物・アマテラス石の発見～日本の国石「ヒスイ」から見つかった新種の鉱物～

2025-08-07 東京大学,山口大学,高輝度光科学研究センター,京都大学,リガク・ホールディングス株式会社

図1. アマテラス石（黒緑色部）を含む鉱物集合体（褐色部はルチル、あんず色部はタウソン石）。画像幅は約2mm。

＜関連情報＞

• https://www.issp.u-tokyo.ac.jp/maincontents/news2.html?pid=27825#
• https://www.issp.u-tokyo.ac.jp/news/wp-content/uploads/2025/08/0805_press_amaterasuite.pdf
• https://www.jstage.jst.go.jp/article/jmps/120/1/120_250420/_article

日本を代表する石、ヒスイ輝石の新鉱物、アマテラス石Sr4Ti6Si4O23(OH)Cl Amaterasuite, Sr4Ti6Si4O23(OH)Cl, a new mineral from jadeitite, a representative stone of Japan

Daisuke NISHIO-HAMANE , Mariko NAGASHIMA, Yuki MORI, Masayuki OHNISHI, Norimasa SHIMOBAYASHI, Takashi MATSUMOTO, Mitsuo TANABE
Journal of Mineralogical and Petrological Sciences  Published:July 18, 2025
DOI:https://doi.org/10.2465/jmps.250420

Abstract

Amaterasuite is a new mineral found in jadeitite, a representative stone of Japan, and was thus named after Amaterasu Omikami, one of the most important goddesses in Japanese mythology, as a tribute to Japanese stone culture. The new mineral was found in the Osayama mountain area, Osa-osakabe, Niimi City, Okayama Prefecture, Japan. Amaterasuite appears as bundles consisting of needle- to plate-shaped crystals as large as 150 µm around rutile. The Mohs hardness is 6. Its tenacity is brittle, and its calculated density is 4.0 g·cm⁻³. Under plane-polarized light, the mineral is pleochroic, changing from blue to brown. The empirical formula, calculated on the basis of 23 O + 2 (OH,Cl) atoms per formula unit, is (Sr_3.32Ba_0.64)_Σ3.96(Ti_5.73Fe_0.16Nb_0.02)_Σ5.91Si_4.15O₂₃(OH)_0.95Cl_1.05; thus, its ideal formula is Sr₄Ti₆Si₄O₂₃(OH)Cl. The unit-cell parameters refined by powder X-ray diffraction using 80 peaks with large d-values are a = 5.85558(2) Å, b = 20.43960(8) Å, c = 33.28240(12) Å, and V = 3983.43(3) ų (Z = 8) in the orthorhombic Fddd space group. The structure of amaterasuite from the metajadeitite area was fully identified as amaterasuite-4O, which is characterized by its dual nature, encapsulating two types (A and B) within a unit cell. The occupancy rates of the A and B types were estimated to be ∼ 85 and ∼ 15%, respectively. The refined site occupancies at the SrA and BaB sites indicate a strong site preference for Sr, similar to the preferential site occupancies in synthetic titanosilicate compounds.