Crystal chemistry of Sr-rich piemontite from manganese ore deposit of the Tone mine, Nishisonogi Peninsula, Nagasaki, southwest Japan

The crystal chemistry of Sr-rich piemontite from a layered manganese ore deposit of the Tone mine, Nishisonogi Peninsula, Japan, was studied using methods of electron microprobe analysis, single crystal X-ray structural refinement, ⁵⁷Fe Moessbauer spectroscopy, and X-ray and Time-of-Flight neutron Rietveld analyses to elucidate the intracrystalline distributions of Sr, Mn, and Fe and the general and individual features on the structural changes with Sr contents in piemontite and epidotes. Piemontite in the most piemontite-dominant layer of an ore is [Ca_1.73(15)Sr_0.22(13)]_Σ1.95[Al_1.99(9)Mn_0.68(8)³⁺Fe_0.37(8)³⁺Mg_0.01(0)]_Σ3.05 Si_2.99(1)O₁₂(OH) (Z = 2) in the average chemical formula. A single crystal X-ray structural refinement (R₁ = 2.51% for 2417 unique reflections) resulted in the unit-cell parameters of a = 8.8942(1), b = 5.6540(1), c = 10.1928(2) Å, and β = 115.100(1)°; and the occupancies of Ca_0.711(3)Sr_0.289, Al_0.898(4)(Mn + Fe)_0.102, and (Mn + Fe)_0.949(4)Al_0.051 at the ^XA2, ^VIM1, and ^VIM3 sites, respectively. All Fe in a powdered piemontite sample was Fe³⁺ as indicated by the two Moessbauer doublets (isomer shift = 0.351 and 0.367 mm/s and quadrupole splitting = 2.189 and 1.93 mm/s, respectively) assigned to Fe³⁺ at the M3 site. The neutron Rietveld refinement of the powder sample (R_wp = 2.11%; R_e = 0.88%) resulted in the occupancies of ^M1[Al_0.902(5)Mn_0.098] and ^M3[Mn_0.534(5)Fe_0.267Al_0.20], where ^M3Al was fixed to 0.20Al obtained by X-ray Rietveld refinement (R_wp = 2.95%; R_e = 2.13%). By applying the oxidation state of Fe and the distributions of Al, Fe, and Mn in the M1 and M3 sites in the powder sample, the site occupancies in the piemontite single crystal are constructed as ^A2[Ca_0.711(3)Sr_0.289], ^M1[Al_0.898(4)Mn_0.102³⁺] and ^M3[Mn_0.633³⁺Fe_0.316³⁺Al_0.051]. The A2-O7, -O2', and -O10 distances, 2.303(2), 2.562(1), and 2.592(2) Å, respectively, are longer than those of Ca-piemontites. The mean and distances, 2.050 and 1.923 Å, respectively, are close to the published data of Ca-piemontites and Sr-rich and -bearing piemontites with Mn³⁺ + Fe³⁺ contents in the M3 and M1 sites similar to those of the Tone piemontite. (author)