62nd Annual Meteoritical Society Meeting 5117.pdf

SCHWERTMANNITE AND AWARUITE AS ALTERATION PRODUCTS IN METEORITES. T. P. Pedersen, Dept. of mineralogy, Geological institute, University of Copenhagen, Øster Voldgade 10, Dk-1350 KBH.K.

Schwertmannite: Schwertmannite, a sulphate- bearing analogue of akaganéite, with the actual formula Fe16O16(OH)y(SO4)z*nH2O, where 16 - y = 2z, 2< z<3.5 [1], was encountered as a weathering product in the Ider (Ala- bama) and Cape York iron meteorites (both IIIA medium octahedrites), and in a sample of archeological iron from Sorte Muld, Denmark. Schwertmannite has the $-FeOOH structure (spacegroup P4/m), in which the tunnel sites ac- - commodate SO4-groups [1], instead of Cl -ions as is the case with akaganéite [2]. The distorted ”hydrogen cube” in the tunnel sites, defined in ref. [3], would hardly allow for the presence of a large anionic group such as SO4 in the $- FeOOH structure, unless structurally bonded to the walls of the tunnel sites, through replacement of (OH)-groups from the iron octahedra forming the tunnel walls, by oxygen from the SO4-groups. The schwertmannite is relatively well- Figure 1 Backscattered electron SEM-image of crystallized, compared to the hitherto described material schwertmannite (medium grey) with ”bundle of needles”- morphology, growing into epoxy-filled cavity (black), in a [1,4], with a needle- to bundle-like morphology, or consist- sample of archeological iron (Sorte Muld). Age ing of platelets. Needles and platelets are approximately a approximately 1600 yrs. bp. Scale bar is 200 microns. few microns wide and up to 10 :m long, while the bundles of needles may reach sizes up to 10-20 :m wide and several formed. Awaruite is a realtively well-known alteration hundred microns long. Schwertmannite has the same dull product in serpentinization processes [8,9], in which it grey colour and reflectivity as akaganéite and goethite, when forms through reduction of divalent , in response to viewed in reflected light, but has more pronounced deep red the oxidation of divalent iron (from olivine) to trivalent iron internal reflections than usually encountered in other com- (in ). When Ni-enriched taenite/awaruite is ana- mon iron hydroxides, such as hydrous goethite. Stoichi- lyzed for its Ni- versus Co-content, it is seen that composi- ometrically, it displays substitution of chlorine for sulphate, tions with Ni-content lower than tetrataenite (stoichiometric and perhaps also substitution of water and (OH)-groups for FeNi), have generally lower Co-contents with increasing Ni- sulphate, as the sulphate content of the measured schwert- contents, while phases with Ni-contents above tetrataenite mannite seems to vary slightly. Schwertmannite seems to show an almost linear increase in Co-content with increas- form in analogy to akaganéite, as an early-stage weathering ing Ni-content. The higher cobalt content in awaruite may product of [5], although it is clearly more stable be attributed to cobalt being more siderophile than iron, and not as reactive as akaganéite, probably due to the although less so than nickel. It seems likely that the Ni- structural bonding of the sulphate groups to the iron octahe- enriched phases, and certainly the Co-enriched phases, have

dra of the tunnel walls. formed in response to rising ao2 in the alteration environ- Awaruite: Ni-enriched taenite, ranging in composition ment. up to awaruite (stoichiometric Ni3Fe), was encountered as an alteration product in a series of different investigated References: [1] Bigham, J.M., L. Carlson & E. Murad iron meteorites, namely Cape York and Ider (mentioned (1994). Min. Mag., Vol. 58, p. 641-648. [2] Post, J.E. & above), Sardis (group IA, coarse octahedrite), Wolf Creek V.F. Buchwald (1991). Am. Min., Vol. 76, p. 272-277. [3] (IIIB, medium octahedrite), South Dahna (group IA, coarse von Dreele, R.B. & J.E. Post (1994). Abstr. w. programs - octahedrite), Drum Mountain (IIIA, medium octahedrite), GSA, vol. 26, no.7, p.165-166. [4] Schwertmann, U., J.M. and ”A” and ”B” (two unknown samples of a coarse octa- Bigham & E. Murad (1995). Eur. J. Min., vol. 7, p. hedrite, perhaps belonging to group IA [6]). Awaruite, like 547-552. [5] Buchwald, V.F. & R.S. Clarke (1989). Am. taenite, has space group Pm3m, and an AuCu3-type structure Min., Vol. 74, p. 656-667. [6] Buchwald V.F. (1997), pers. in which Fe at (0,0,0) is twelve-coordinated by Ni at comm. [7] Brandenberger, E. (1939). Schw. Min. und Pet- (½,½,0), which in turn is planar square-coordinated by rogr. Mitt., vol. 19, p. 285-286. [8] Eckstrand, O.R. (1975). Fe[7]. In the investigated samples, Ni-rich taenite and Ec. Geol., vol. 70, p. 183-201. [9] Filippidis, A. (1985). Ec. awaruite occur either with a common taenite morphology (as Geol., vol. 80, p. 1974-1980. lamellae), or with secondary morphologies, such as colloidal bands or coronas around more corrosion resistant phases. The first morphological type may have formed through the depletion in iron/enrichment in nickel of ordinary taenite, while the second morphological type is clearly newly