Aluminium-Bearing Strunzite Derived from Jahnsite at the Hagendorf-Su¨D Pegmatite, Germany

Mineralogical Magazine, October 2012, Vol. 76(5), pp. 1165–1174 Aluminium-bearing strunzite derived from jahnsite at the Hagendorf-Su¨d pegmatite, Germany 1, 1 2 3 I. E. GREY *, C. M. MACRAE ,E.KECK AND W. D. BIRCH 1 CSIRO Process Science and Engineering, Box 312 Clayton South, Victoria 3169, Australia 2 Algunderweg 3, D À 02694 Etzenricht, Germany 3 Geosciences, Museum Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia [Received 15 May 2012; Accepted 23 July 2012; Associate Editor: Giancarlo Della Ventura] ABSTRACT 2+ 3+ Aluminium-bearing strunzite, [Mn0.65Fe0.26Zn0.08Mg0.01] [Fe1.50Al0.50] (PO4)2(OH)2·6H2O, occurs as fibrous aggregates in a crystallographically oriented association with jahnsite on altered zwieselite samples from the phosphate pegmatite at Hagendorf Su¨d, Bavaria, Germany. Synchrotron X-ray data were collected from a 3 mm diameter fibre and refined in space group P1¯ to R1 = 0.054 for 1484 observed reflections. The refinement confirmed the results of chemical analyses which showed that one quarter of the trivalent iron in the strunzite crystals is replaced by aluminium. The paragenesis revealed by scanning electron microscopy, in combination with chemical analyses and a crystal-chemical comparison of the strunzite and jahnsite structures, are consistent with strunzite being formed from jahnsite by selective leaching of (100) metalÀphosphate layers containing large divalent Ca and Mn atoms. KEYWORDS: Al-bearing strunzite, strunzite paragenesis, strunzite structure refinement, Hagendorf Su¨d secondary phosphates. Introduction The feldspar pegmatite at Hagendorf Su¨d, 2+ 3+ STRUNZITE, ideally Mn Fe2 (PO4)2(OH)2·6H2O, Bavaria, contains a large number of secondary is a relatively rare secondary phosphate mineral Al-phosphates including eosphorite, kingsmoun- found in altered granitic pegmatites (Moore, tite, paravauxite, fluellite, morinite, variscite, 1965; Viana and Prado, 2007; Dill et al., 2008). scorzalite, kastningite, nordgauite, whiteite and It is a late-stage product in the supergene wavellite as well as Al-bearing secondary FeÀMn alteration of primary phosphate minerals such as phosphates including keckite, rockbridgeite and triphylite, zwieselite and apatite via intermediate jahnsite (Mu¨cke, 1981; Dill et al., 2008; Grey et phosphates including rockbridgeite, mitridatite, al., 2010; Birch et al., 2011). Strunzite is vivianite, whitmoreite and jahnsite (Mu¨cke, 1981; relatively widespread at Hagendorf Su¨d, and has Keller and Von Knorring, 1989; Dill et al., 2008). been found (by E.K.) between the 45 and 84 m The presence of Al from the breakdown of levels in the Cornelia mine open cut. It is aluminosilicates in contact with solutions rich in particularly common between 60 and 67 m. phosphate is a common feature of secondary Dill et al. (2008) listed ‘‘Al-strunzite’’ as one of pegmatite phosphate alteration (Fransolet, 1980; the phosphate minerals from the Hagendorf Keller and von Knorring, 1989; Dill et al., 2008). pegmatite province, but provided no details of its composition or occurrence. A few occurrences of fibrous Al-bearing strunzite in close association with Al-bearing jahnsite have been found in a * E-mail: [email protected] continuing study of secondary phosphate minerals DOI: 10.1180/minmag.2012.076.5.08 from the Cornelia mine open cut (Grey et al., # 2012 The Mineralogical Society I. E. GREY ET AL. 2010; Birch et al., 2011). We have characterized collected at room temperature using an ADSC both minerals by electron microprobe analysis Quantum 315r detector and monochromatic and refined the structure of the strunzite from radiation with a wavelength of 0.71000 A˚ .Aj single-crystal synchrotron X-ray diffraction data. scan was employed with frame widths of 2º and a We report here the results of these studies counting time per frame of 2 s. The intensity together with an analysis of the crystal-chemical datasets were processed to produce data files relationships between the two minerals, which which were analysed in WinGX (Farrugia, 1999). provides evidence for formation of the strunzite The refinement was made using SHELXL-97 by crystallographically controlled leaching of the (Sheldrick, 2008). jahnsite. The published atom coordinates and metal atom site assignments for strunzite (Fanfani et al., 1978) were used as the initial model for Experimental refinement in space group P1¯. The scattering Hand specimens containing strunzite associated curve for Mn was used for the Mn site, which also with jahnsite on altered zwieselite were collected contains minor Fe, Mg and Zn. Refinement with by one of us (E.K.) from the 67 m level of the isotropic displacement parameters, and with Cornelia mine open cut. The strunzite occurs as refinement of Al/Fe occupancy in the two sheaves of ultrathin white to yellowish fibres; the independent trivalent Fe sites converged to R1 = jahnsite occurs as transparent tabular crystals 0.08. Hydrogen atoms associated with hydroxyl which are commonly corroded. Samples were and water groups were located in difference- mounted in epoxy blocks, polished and carbon- Fourier maps. They were refined with soft coated for scanning electron microscopy and restraints on the OÀH and HÀH distances of electron microprobe (EMP) analyses by wave- OÀH = 0.80(4) A˚ and HÀH = 1.25(8) A˚ . These length dispersive spectrometry on a JEOL JXA restrained OÀH distances are consistent with 8500F Hyperprobe operating at an accelerating distances reported in previous X-ray refinements voltage of 12 kV and a beam current of 11 nA. (e.g. Harte et al., 2005). A common isotropic The beam was defocussed to a 1 mm spot size. displacement parameter was refined for the The analytical standards used were CaSiO3 for hydrogen atoms. The final refinement, incorpor- Ca; AlPO4 for Al, P; MgAl2O4 for Mg; hematite ating anisotropic displacement parameters for all for Fe; MnSiO3 for Mn, ZnS for Zn and albite for non-H atoms, converged to R1 = 0.054 for 1481 Na. The EMP results are given in Table 1. All observed reflections with F >4s(F). Further EMP data were corrected using a j(rz) matrix details of the refinement are summarized in correction (Armstrong, 1988). Table 2. Refined atom coordinates and equivalent A needle-shaped crystal of strunzite, 80 mm isotropic displacement parameters are reported in long and ~3 mm in diameter, was used for X-ray Table 3. Tables of anisotropic displacement data collection on the macromolecular beam line parameters and observed and calculated structure MX2 at the Australian Synchrotron. Data were factors have been deposited with the Principal TABLE 1. Electron microprobe analyses of Al-bearing strunzite and jahnsite from Hagendorf Su¨d. Strunzite (9 analyses) Jahnsite (7 analyses) Range (wt.%) Mean (wt.%) Range (wt.%) Mean (wt.%) CaO 0À0.12 0.05 5.22À6.39 5.48 Na2O 0À0.02 0.01 0.11À0.24 0.15 MnO 9.01À10.90 10.29 14.09À19.25 17.20 Fe2O3 29.19À34.48 31.25 16.87À21.02 18.97 Al2O3 5.39À6.25 5.74 1.71À3.93 2.91 MgO 0À0.09 0.09 0.75À1.17 0.92 ZnO 1.33À1.61 1.39 1.67À2.40 2.07 P2O5 29.90À32.61 31.07 31.69À33.96 32.69 Total 76.60À84.30 79.89 78.07À83.35 80.39 1166 AL-BEARING STRUNZITE TABLE 2. Data collection and structure refinement details for Al-bearing strunzite. Crystal data Unit cell: a, b, c (A˚ ) 10.277(1), 9.826(1), 7.205(1) Unit cell: a, b, g (º) 90.077(4), 98.865(4), 118.442(4) Volume (A˚ 3) 629.8(1) Z 2 Space group P1¯ Calculated density (g cmÀ3) 2.54 Formula from refinement MnFe1.44Al0.56(PO4)2(OH)2·6H2O Formula weight 970.0 Data collection Temperature (K) 293 Wavelength (A˚ ) 0.7100 Crystal size (mm) 0.003 diameter60.08 long Collection mode j scan, 360º, DF = 0.02º Count time per frame (s) 2 2ymax (º) 50 No. of unique reflections 2024 No. of reflections, I >2s(I) 1484 Absorption coefficient (mmÀ1) 3.19 Refinement Data/restraints/parameters 2024/20/235 Goodness-of-fit 0.892 Final R indices (F >4s(F)) R1 =0.054, wR2 = 0.128 Final R indices (all data) R1 =0.078, wR2 = 0.145 Largest diff. peak and hole (e A˚ À3) À0.68, +1.06 Editors and can be downloaded from http:// secondary phosphate assemblages at Hagendorf. www.minersoc.org/pages/e_journals/ According to recent studies, however, keckite can dep_mat_mm.html. be mistaken for jahnsite-(CaMnFe) or jahnsite- (CaMnMn) (Hochleitner and Fehr, 2010; Kampf et al., 2008). Results and discussion The strunzite analyses are similar to those Electron probe microanalysis and paragenesis reported by Frondel (1958), but with one quarter The EMP analyses of the strunzite and associated of the Fe replaced by Al, giving a formula jahnsite crystals are reported in Table 1. Both Mn0.65Fe1.76Al0.50Zn0.08Mg0.01(PO4 ) 2 phases show relatively high compositional homo- (OH)2·6H2O. For comparison, the ideal strunzite geneity between different regions, with no strong formula is MnFe2(PO4)2(OH)2·6H2O, and the inter-element correlations. Weak positive correla- formula from the structure refinement is tions between the Mn, Al and Fe analyses appear MnFe1.44Al0.56(PO4)2(OH)2·6H2O. The analyses to reflect variable analytical totals, possibly due to reported in Table 1 show that strunzite and varying water loss resulting from heating by the jahnsite have similar Fe/Al ratios but the electron beam. The jahnsite is Al-bearing and its amount of these trivalent elements in strunzite is composition is close to that we have previously about double that in jahnsite. We propose that this reported for jahnsite-(CaMnMn) in contact with chemical relationship is due to the strunzite being zwieseliteÀtriplite and with nordgauite in formed from jahnsite by selective leaching of samples from the Cornelia mine open cut (Grey divalent phosphate components, thereby enriching et al., 2010).

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