Crystal chemistry of Mn2+-, Sr-rich and REE-bearing piemontite from the Kamisugai mine in the Sambagawa metamorphic belt, Shikoku, Japan

Abstract

The crystal chemistry of piemontite from the Kamisugai mine in the Sambagawa metamorphic belt, Shikoku, Japan was studied using electron microprobe analysis (EMPA) and single-crystal X-ray diffraction methods. The chemical formula derived from EMPA is (Ca1.30Sr0.19La0.03Ce0.05Nd0.02Mn2+0.41)Σ2.00(Al1.70Mn3+0.77Fe3+0.48Mg0.01)Σ2.96Si3.04O12.13(OH)0.87 (Z = 2). This piemontite is characterized by high Sr (2.2-6.2 SrO wt%), Mn (16.2-18.8 Mn2O3 wt%) and REE (0.7-4.0 REE2O3 wt%) contents and it is mainly composed of clinozoisite Ca2Al3Si3O12(OH), piemontite-(Sr) CaSrAl2Mn3+Si3O12(OH), epidote Ca2Al2Fe3+Si3O12(OH) and Mn2+CaAl2Mn3+Si3O12(OH) components. The crystal structure of piemontite [a = 8.8673(2), b = 5.6747(1), c = 10.1594(3) Å, β = 114.71(1)°, space group P21/m] was refined using 1484 unique reflections to R1 = 4.4%. The site occupancies at A1, A2, M1, M2 and M3 are Ca0.59Mn0.41, Ca0.71Sr0.19La0.03Ce0.05Nd0.02, Al0.72Mn0.17Fe0.11, Al1.0, and Al0.11Mn0.55Fe0 .34, respectively. The distribution coefficient KD (= [(Mn + Fe)/Al]M1/[(Mn + Fe)/Al]M3) is 0.048. Although it is known that high Sr contents at A2 seem to promote incorporation of Mn3+ (+ Fe3+) at M1, the KD value does not suggest a stronger transition metal preference at M1 than that of Ca end-member piemontite and epidote. High content of Mn2+ at A1 modifies the arrangement of the coordinating O atoms of A1O9 polyhedra. Thus, the A1 coordination may be described as 6-coordinated. <A2-O> increases with increasing Sr content, and the lengthening of A2-Oi is anisotropic.