Ehimeite, NaCa2Mg4CrSi6Al2O22(OH)2: The first Cr-dominant amphibole from the Akaishi Mine, Higashi-Akaishi Mountain, Ehime Prefecture, Japan

• NISHIO-HAMANE Daisuke

  The Institute for Solid State Physics, the University of Tokyo

• OHNISHI Masayuki

• MINAKAWA Tetsuo

  Department of Earth Science, Faculty of Science, Ehime University

• YAMAURA Jun-ichi

  The Institute for Solid State Physics, the University of Tokyo

• SAITO Shohei

  Department of Earth Science, Faculty of Science, Ehime University

• KADOTA Ryo

  Department of Earth Science, Faculty of Science, Ehime University

抄録

The first Cr-dominant amphibole, ehimeite, ideally NaCa₂Mg₄CrSi₆Al₂O₂₂(OH)₂, has been found in a chromitite deposit in the Akaishi Mine, Higashi-Akaishi Mountain, Ehime Prefecture, Japan. Ehimeite occurs as prismatic crystals of up to 1.5 cm in length and 0.5 cm in width and is found in association with chromite, kämmererite (Cr-rich clinochlore), Cr-poor clinochlore, phlogopite, and uvarovite. It is transparent, emerald green to pale green in color with pale green streaks, and has a vitreous luster. Optically, it is biaxial positive with α = 1.644(2), β = 1.647(2), γ = 1.659(2), and 2V_calc. = 53°. It has a Mohs’ hardness of 6 and densities of 3.08(3) g/cm³ (measured using heavy liquids) and 3.121 g/cm³ (calculated from powder diffraction data and the empirical formula). The empirical formula is (Na_0.88K_0.07)_Σ0.95(Ca_1.89Na_0.02Mg_0.09)_Σ2.00(Mg_4.03Cr_0.62Al_0.19Fe³⁺_0.07Fe²⁺_0.07Ti_0.03)_Σ5.00(Si_6.14Al_1.86)_Σ8.00O₂₂(OH)₂ on the basis of O = 22 and OH = 2, and ehimeite mainly forms a solid solution, NaCa₂Mg₄(Cr, Al)Si₆Al₂O₂₂(OH)₂, with pargasite. It has a monoclinic unit cell with a = 9.9176(14) Å, b = 18.0009(12) Å, c = 5.2850(7) Å, β = 105.400(7)°, V = 909.6 (17) ų, and Z = 2, and it belongs to the space group C2/m, as refined from powder XRD data. The eight strongest lines in the powder XRD pattern [d (Å), I/I₀, hkl] are (3.370, 58, 150), (2.932, 43, 221), (2.697, 81, 151), (2.585, 50, 061), (2.546, 100, 202), (2.346, 42, 351), (2.156, 35, 261), and (1.514, 55, 263). The crystal structure has been refined to R1 = 0.0488 using single-crystal XRD data. It has been concluded that ehimeite in the Akaishi Mine was formed by the reaction of chromitite and the metamorphic fluid in the retrograde stage of serpentinization during the Sanbagawa metamorphism.

収録刊行物

• Journal of Mineralogical and Petrological Sciences

  Journal of Mineralogical and Petrological Sciences 107 (1), 1-7, 2012

  一般社団法人　日本鉱物科学会

キーワード

• Ehimeite
• New mineral
• Calcic amphibole
• Akaishi Mine
• Higashi-Akaishi Mountain

詳細情報

• CRID: 1390282681521513600
• NII論文ID: 10030122129
• NII書誌ID: AA11460926
• DOI: 10.2465/jmps.110715
• BIBCODE: 2012JMPeS.107....1N
• COI: 1:CAS:528:DC%2BC38XmtlGktbw%3D
• ISSN: 13493825
• ISSN: 13456296
• 本文言語コード: en
• データソース種別:
  • JaLC
  • Crossref
  • CiNii Articles
  • KAKEN
• 抄録ライセンスフラグ: 使用不可