Crystal chemistry and oxidation state of Fe-rich prehnite from a hydrothermally altered dolerite

Fe-rich prehnite, Ca2(Al,Fe)(AlSi3)O10(OH)2, in a hydrothermally altered dolerite sill from Mitsu, Shimane Peninsula, Japan, was studied using 57Fe Mossbauer spectroscopy and X-ray Rietveld method to determine the oxidation state and distribution of Fe within the prehnite and to clarify its structural properties. Prehnite shows two modes of occurrence: a druse and vein mineral (prehnite I) associated with Fe-rich pumpellyite and laumontite and a replacement of primary plagioclase (prehnite II). The Fe contents of prehnite I and II are 0.33–0.44 and 0.01–0.46 Fe3+ atoms per formula unit, respectively. The Mossbauer spectrum of prehnite II consists of one doublet with isomer shift (IS) = 0.364 mm/s and quadrupole splitting (QS) = 0.284 mm/s assigned to Fe3+ at the octahedral M site. In contrast, the Mossbauer spectrum of prehnite I consists of two doublets assigned to Fe3+ at the M site (IS = 0.369 mm/s and QS = 0.299 mm/s) and Fe2+ at the seven coordinated A site (IS = 1.05 and QS = 2.78 mm/s). According to X-ray Rietveld refinements with Pmna and Pma2 space groups, the fitting with Pma2 gave more reduced reliability factors than those using Pmna for both specimens, implying ordering of Al and Si at the tetrahedral T2 sites. Determined T2–O bond lengths at the Al-rich and Si-rich T2 sites, 1.71–1.72 and 1.62–1.64 A, respectively, also indicate the ordered arrangement of Al and Si at the T2 sites. Refined site occupancies at the A and M sites are represented as A (Ca0.993(9)Fe2 + 0.007) M (Al0.666(6)Fe3 + 0.334) for prehnite I, and A Ca1.0 M (Al0.865(5)Fe3 + 0.135) for prehnite II, respectively. The existence of Fe2+ in the A site filling Ca deficiency in prehnite I is consistent with the result from the Mossbauer analysis.