Geochemical Journal, Vol. 11, pp. 207 to 217, 1977 207 Cape York: The extraordinary mineralogy of an ordinary iron meteorite and its implication for the genesis of III AB irons A. KRACHER,1 G. KURATI and V. F. BUCHWALD2 Division of Mineralogy and Petrology, Natural History Museum, A-1014 Vienna, Austria' and Department of Metallurgy, Technical University of Denmark, DK-2800 Lyngby, Denmark2 (Received July 1, 1977) A study of accessory minerals in the Cape York iron meteorite has been carried out with the electron microprobe. Phases analyzed include chromite, sphalerite, two closely related potassium-bearing sulfides, silica, and native copper. In addition, four phosphate minerals were found: One chemically similar to buchwaldite, but rather rich in iron, one corresponding to terrestrial maricite, and two more, which seem to be new minerals on the basis of microprobe analysis. Evidence from the composition of accessory minerals as well as texture elucidates some details of the fractional crystallization of the group III AB parent melt. A modification of the fractional crystallization model is therefore proposed. We suggest that sulfide nodules have originated as droplets of an immiscible sulfur-rich liquid that continuously segregated from the parent melt during crystallization of the metal. The modified model also explains the Ni/Cr anticorrelation and the high abundance of sulfur in group III AB irons which could not be satisfactorily explained by previous models. INTRODUCTION the compilation of alpha factors by ALBEE and RAY (1970). For other minerals a correction program by The Cape York iron meteorite is the second WEINKE et al. (1974) based on standard ZAF-correc largest recovered iron, and the largest meteorite tion procedures was used. of group III AB. This group, as defined by SCOTTet al. (1973), is the largest chemical group THE CAPE YORK METEORITE of iron meteorites, comprising 157 members, or 33% of all irons (BUCHWALD,1975). In Metallographic descriptions, photomicro the present study interest will be focused on graphs and macrographs, and a detailed discus accessory minerals found in samples of the Cape sion of the large, oriented sulfide inclusions of York fragment Agpalilik (C3W/ 1 through 3), the Cape York meteorite have been presented obtained from the Geological Museum, Copen by BUCHWALD(1975: 410-425). The reported hagen, and jointly examined by the Max-Planck shock-induced alterations of the troilite cor Institut in Mainz and us (preliminary results: respond to peak shock pressures of 130 800 JOCHUMet al., 1975). kbar according to LIPSCHUTZ (1968) and JAIN and LIPSCHUTZ(1969). Apparently, differences ANALYTICAL METHODS in peak shock pressures between different loca tions in a piece as large as the Cape York Microprobe analyses were performed with an ARL (Agpalilik) fragment can be quite large. SEMQ automated microprobe (15 kV acceleration volt The minor element content of the troilite age, 15 nA sample current, except where indicated other is low. Aside from Fe, S, and about 0.13% Cr wise in the text). In order to minimize sample decom all other elements are below 0.02% by weight. position phases rich in volatiles, such as alkali phos A special high-sensitivity microprobe technique phates, were analyzed by scanning (in TV mode) an was used for the determination of manganese in area previously checked for homogeneity. Automatic troilite. Values found range from 180 to 195 procedures that correct on-line for background, dead ppm, which is in good agreement with the results time, and drift were used wherever applicable. Matrix of JOCHUM et al. (1975). effects of oxide minerals were corrected for by apply Accessory phases found to date are listed ing the method of BENCE and ALBEE (1968), and in Table 1. With the exception of carlsbergite, 208 A. KRACHER et al. Table 1. Accessory minerals in the Cape York (III A) iron meteorite Name Formula Reference (1) Phosphates Buchwaldite NaCaPO4 a table 2 I (2) Na4Ca3(Fe, Mg) (P04)4? x table 2 II (maricite ?) NaFePO4 x table 2 III Na4Mn(P04)2 ? x table 3 IV not known x table 3 Sulfides Daubreelite FeCr2S4 b, d (3) Chalcopyrrhotite (Fe, Cu)S b (3) Djerfisherite K3(Fe, Ni, Cu) 13(S, CI) 14±X x table 4 K-bearing sulfide K3(Fe, Ni, CU)13S14±X x table 4 Sphalerite (Zn, Fe)S b, x table 5 Ferroan-alabandite (Mn, Fe)S x see text Other Chromite FeCr204 (4) see text Silica Si02 x see text Copper Cu b, d, x see text Carlsbergite CrN c (1) x = this work, a = OLSEN et al. (1977), b = RAMDOHR(1973), c = BUCHWALDand SCOTT(1971), d = BUCHWALD(1975), (2) = may be ferroan buchwaldite, (3) = not found in present samples, (4) = abundant. which occurs exclusively within the metal phase, trace elements have been found by JOCHUM et all others are concentrated either within the al. (1975) to be enriched near this end. Taking troilite nodules or along their rims. Daubreelite their mass spectrometric data one can infer a is present only in smaller sulfide aggregates. Ga-content of the chromite of about 130ppm, and calculated concentrations of Mn and V are MINERALOGYOF ACCESSORYPHASES in excellent agreement with our microprobe data. Zn, however, is more enriched than can be 1) Chromite Clusters of chromite crystals explained by the presence of chromite alone. ranging from 0.2 to 2 mm in size are frequently observed near one end of troilite nodules, most 2) Phosphates As already reported in pre commonly lined up directly at the rim or parallel liminary form (KRACHER and KURAT, 1975) to it a few millimeters away. Occassionally there are at least four different phosphate similar clusters are also found deep within phases present, none of which has been ob nodules. Shapes vary from perfectly euhedral to served in other meteorites. They tend to occur spherical, at the troilite-metal interface some along or very close to the troilite-metal boundary times taking the form of mushrooms, the thin "stem" protruding into the metal of some of the nodules, but opposite to the , and the chromite-rich end. They form aggregates up rounded "head" into the troilite. to 100µm in size, but are intimately intergrown Chemically all chromite is almost pure with each other, and the size of individual grains stoichiometric FeCr204, the only detectable hardly exceeds 20µm. This small grain size has minor elements being ZnO (average 0. 16 weight up to now made it impossible to obtain X %), MnO (0.58%), and V203 (0.27%). ray data. Essentially the same composition has been Phosphate I is similar to buchwaldite, found by BUNCH and KEIL (1971) for chromites NaCaP04, (OLSEN et al., 1977) except for ap from the Bagdad (III A) and Putnam County preciable contents of FeO and MnO (see Table (IV A) iron meteorites. Chromite of this com 2, first and second columns). Although the position seems to be very common in group former might be just a ferroan variety of the III AB (our unpublished results). latter, the possibility that they are two different The asymmetrical distribution of chromite minerals cannot be excluded, owing to the un along the long axis of the troilite nodules has certainties concerning synthetic NaCaPO4 (KLE been ascribed to the influence of a gravitational MENT and DIHN, 1938; BREDIG, 1942). MOORE field during solidification (BUCHWALD, 1971b). (pers. comm.) has pointed out that Na4Ca3 As a consequence of this distribution, certain Extraordinary mineralogy of the Cape York meteorite 209 Table 2. Averages of microprobe analyses and cation Table 3. Compositional ranges and analyses by (1) of proportions of chemically well characterized chemically poorly characterized phosphates in phosphates. in the Cape York (III A) iron the Cape York (III A) iron meteorite (electron meteorite (in weight %) microprobe analyses, in weight %) I (1) II III (1) IV (1) P205 44.6 44.9 42.3 S03 2.5 3.7 Cr203 0.57 P205 41.5 -44.3 41.1 45.3 -47.8 46.0 FeO 6.9 33.5 CI203 0.62 1.65 3.2 14.2 -15.0 16.1 MnO 1.76 0.06 7.1 A1203 0.2 Mgo 2.39 1.45 FeO 1.24 4.0 3.0 0.52 0.75 CaO 27.1 35.6 0.36 MnO 15.8 -19.2 20.1 1.20 1.85 1.5 Nat 0 17.9 19.5 15.6 ZnO ca. 0.26 K20 0.14 Mgo 0.42 1.54 CaO 0.05 1.49 10.4 -12.0 11.3 Total 100.79 100.16 100.88 Na20 30.9 -35.8 28.8 15.3 -19.8 19.3 K20 0.11 0.20 0.3 6.2 6.9 5.3 Ions on the basis of 16 0 P 3.98 4.00 4.04 (1) OLSEN (pers. comm. ), analyses of phases in ter Cr 0.05 grown with buchwaldite, of which X-ray patterns Fe 0.61 3.16 could not be obtained. Mn 0.16 0.01 0.68 Mg 0.38 0.24 Ca 3.06 4.01 0.04 described below is transsected by numerous very Na 3.66 3.98 3.41 fine veins of native copper, occasionally replaced K 0.02 by nickel-iron. sum M2+ 4.21 4.02 4.12 Although most analyses are not very satis sum M+ 3.68 3.98 3.41 factory either because of intergrown foreign minerals or rapid decomposition under the elec (1) Buchwaldite (OLSEN et al. 1977). tron beam, a general anticorrelation of Na and Mn has been noticed. Since this is highly (Fe, Mg) (P04)4 should constitute a stable phase, implausible to result from solid solution sub possibly isostructural with bredigite, Ca,Mg stitution, phosphate III might in fact be an (Si04)4.