The first Cr-dominant amphibole, ehimeite, ideally NaCa
2Mg
4CrSi
6Al
2O
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, 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 2
Vcalc. = 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
0.88K
0.07)
Σ0.95(Ca
1.89Na
0.02Mg
0.09)
Σ2.00(Mg
4.03Cr
0.62Al
0.19Fe
3+0.07Fe
2+0.07Ti
0.03)
Σ5.00(Si
6.14Al
1.86)
Σ8.00O
22(OH)
2 on the basis of O = 22 and OH = 2, and ehimeite mainly forms a solid solution, NaCa
2Mg
4(Cr, Al)Si
6Al
2O
22(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) Å
3, 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.
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