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 Mössbauer 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 Mössbauer 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 Mössbauer 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 Å, 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 Mössbauer analysis.
KeywordsPrehnite Fe Mössbauer spectroscopy Rietveld method
We thank Dr. Barry Roser for his critical reading of manuscript, Dr. M. Broekmans Editor-in-Chief, and two anonymous reviewers for their constructive comments.
- Akasaka M, Shinno I (1992) Mössbauer spectroscopy and its recent application to silicate mineralogy. J Miner Soc Jpn 21:3–20 (Japanese with English abstract)Google Scholar
- Akizuki M (1987) Al, Si order and the internal texture of prehnite. Can Miner 25: 707–716Google Scholar
- Bertaut EF (2006) Symbols for space groups. In: Hahn T (ed) International tables for crystallography, Vol. A, 5th ed. Chap. 4.3. International Union of Crystallography. Springer Verlag, The Netherlands, pp. 62–76Google Scholar
- Deer WA, Howie RA, Zussman J (2013) An introduction to the rock-forming minerals. 3rd ed. The Mineralogical Society, London. 549pGoogle Scholar
- Izumi F (1993) Rietveld analysis program RIETAN and PREMOS and special applications. In: Young RA (ed) The Rietveld method. Oxford Science Publications, Oxford, pp 236–253Google Scholar
- Kano K, Yoshida F (1985) Geology of the Sakaiminato district. Quadrangle Series, scale 1:50,000, Geological Survey of Japan, 57p. (in Japanese)Google Scholar
- Levien L, Prewitt CT, Weidner DJ (1980) Structure and elastic properties of quartz at pressure. Am Miner 65: 920–930Google Scholar
- Nagashima M, Iwasa K, Akasaka M (2016) Relation between occurrence and chemical compositions of prehnite in hydrothermally altered dolerite from Mitsu, Shimane Peninsula, Japan. Geosci Rep Shimane Univ 34: 1–8 (in Japanese with English abstract)Google Scholar
- Organization Committee of Geological Map of Shimane Prefecture (1997) Geological map of Shimane Prefecture, scale 1:200,000. Department of Geoscience, Shimane UniversityGoogle Scholar
- Papike JJ, Zoltai T (1967) Ordering of tetrahedral aluminum in prehnite. Ca2(Al,Fe+3)[Si3AlO10](OH)2. Am Mineral 52: 974–984Google Scholar
- Peng S-T, Chou K-D, Tang Y-C (1959) The structure of prehnite. Acta Chim Sin 25: 56–63 (in Chinese)Google Scholar
- Young RA (1993) Introduction to the Rietveld method. In: Young RA (ed) The Rietveld method. Oxford Science Publications, Oxford, pp 1–38Google Scholar