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

摘要

The first Cr-dominant amphibole, ehimeite, ideally NaCa_(2)Mg_(4)CrSi_(6)Al_(2)O_(22)(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, k?mmererite (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/cm~(3) (measured using heavy liquids) and 3.121 g/cm~(3) (calculated from powder diffraction data and the empirical formula). The empirical formula is (Na_(0.88)K_(0.07))_(Σ0.95)(Ca_(1.89)Na_(0.02)Mg_(0.09))_(Σ2.00)(Mg_(4.03)Cr_(0.62)Al_(0.19)Fe~(3+)_(0.07)Fe~(2+)_(0.07)Ti_(0.03))_(Σ5.00)(Si_(6.14)Al_(1.86))_(Σ8.00)O_(22)(OH)_(2) on the basis of O = 22 and OH = 2, and ehimeite mainly forms a solid solution, NaCa_(2)Mg_(4)(Cr, Al)Si_(6)Al_(2)O_(22)(OH)_(2), with pargasite. It has a monoclinic unit cell with a = 9.9176(14) ?, b = 18.0009(12) ?, c = 5.2850(7) ?, β = 105.400(7)°, V = 909.6 (17) ?~(3), and Z = 2, and it belongs to the space group C 2/m , as refined from powder XRD data. The eight strongest lines in the powder XRD pattern [d (?), I /I _(0), 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 R 1 = 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.