www.jgeosci.org Journal of Geosciences, 54 (2009), 373–384 DOI: 10.3190/jgeosci.057 Original paper New data for metakirchheimerite from Jáchymov (St. Joachimsthal), Czech Republic Jakub PlášIl 1* , Jiří ČeJka 2 , Jiří SeJkOra 1 , Jan HlOušek 3 , Viktor GOlIáš 4 1 Department of Mineralogy and Petrology, National museum, Václavské nám. 68, 115 79 Prague 1, Czech Republic; jakub_plasil@nm.cz 2 Natural History Museum, National Museum, Václavské náměstí 68, 115 79 Prague 1, Czech Republic 3 U Roháčových kasáren 24, 100 00 Prague 10, Czech Republic 4 Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic * Corresponding author Metakirchheimerite was found only on a few samples from the Jan Evangelista vein at the “Adit level” of the Svornost shaft, Jáchymov (St. Joachimsthal) ore district, Krušné Hory/Erzgebirge Mountains, Czech Republic. It forms rare crystalline aggregates consisting of ine tabular crystals reaching 0.1 mm in size, which are very brittle, having light pink to light orange colour, with a high vitreous lustre. Metakirchheimerite crystalline aggregates occur on carbonate gangue with disseminated primary minerals: arsenopyrite, uraninite and skutterudite. Studied metakirchheimerite is a (sub-)recent alteration product of the primary minerals, connected to the old open mine adit environment. According to electron microprobe analyses, the empirical formula of metakirchheimerite can be expressed, based on As + P = 2 apfu, as: (Co 0.53 Mg 0.25 Ni 0.08 Zn 0.07 Fe 0.05 Ca 0.03 ) Σ1.01 (UO 2 ) 2.07 [(AsO 4 ) 1.99 (PO 4 ) 0.01 )] Σ2.00 .8H 2 O. Metakirchheimerite from Jáchymov exhibits an extensive cationic substitution trend of the metanováčekite component. The X-ray diffraction pattern of metakirchheimerite is consistent with a triclinic symmetry inferred from the single-crystal study of the synthetic meta- kirchheimerite analogue. Reined unit-cell parameters for the triclinic space group P-1 are a 7.210(4) Å, b 9.771(6) Å, c 13.252(9) Å, α 75.39(4)°, β 83.94(6)°, γ 81.88(6)° with unit-cell volume V 892(1) Å 3 . Both infrared and Raman spectra of metakirchheimerite exhibited stretching and bending vibrations of (UO 2 ) 2+ , (AsO 4 ) 3– , (PO 4 ) 3– and H 2 O groups. The vibration bands of all these units were tentatively assigned. The U–O bond lengths in uranyl and O–H…O hydrogen bond lengths, which were inferred from the spectra, are consistent with published crystal structure data for a synthetic analogue of metakirchheimerite. Keywords: metakirchheimerite, X-ray powder data, chemical composition, vibration spectroscopy, uranyl bond lengths, hydrogen bond lengths Received: 5 October 2009; accepted 18 December 2009; handling editor: M. Novák The online version of this article (doi: 10.3190/jgeosci.057) contains supplementary electronic material. 1. Introduction The importance of understanding uranium mineralogy and chemistry is closely connected with the broad is- sue of spent nuclear fuel (“SNF”) alteration (Finch and Ewing 1992; Wronkiewicz et al. 1992, 1996; Finn et al. 1996; Murakami et al. 1997; Finch et al. 1999; Fayek et al. 2000; Shueneman et al. 2003; Procházka 2007; Procházka et al. 2009), waste management and environ- mental issues (e.g. Catalano et al. 2004, 2006). Hydrated uranyl arsenates of divalent transition metals are more or less abundant minerals of the secondary origin in the su- pergene zones of most uranium deposits (Finch and Mu- rakami 1999). A group of hydrated uranyl arsenates and phosphates, with autunite-type sheet containing divalent cations in their interlayer, is represented by a general for- mula [6] M 2+ (UO 2 ) 2 (TO 4 ) 2 ·nH 2 O (T is P or As), where n is 12, 10 and 8 most frequently. Divalent cations including Co, Ni, Mg, Fe, Mn and Zn occupy six fold-coordinated M 2+ sites (Locock et al. 2004; Locock 2007a, b). This paper presents new mineralogical data on a rare hydrated cobalt uranyl arsenate – metakirchheimerite, found recently in Jáchymov (St. Joachimsthal), Czech Republic. No recent and detailed data for metakirch- heimerite have been published on natural specimens since Walenta’s studies (Walenta 1958, 1964). 2. Previous studies Metakirchheimerite was described as a new mineral by Walenta (1958) from the dump of the Sophia shaft in Wit- tichen, Baden, Germany. On the type locality metakirch- heimerite forms coatings of tetragonal tabular crystals coming in association with metakahlerite, nováčekite, metaheinrichite and erythrite on weathered uraninite.