Mineralogical and Geochemical Study of Zhaoping, Xifu and Hami Iron Meteorites

Home , Cohenite, IIG meteorite, Neumann lines, Octahedrite, Plessite

CHINESE ASTRONOMY AND ASTROPHYSICS ELSEVIER Chinese Astronomy and Astrophysics 32 (2008) 293–305

LIN Su1,2 HSU Wei-biao1 1Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 2Graduate School of Chinese Academy of Sciences, Beijing 100039

Abstract The mineralogy and bulk chemical compositions of three iron meteorites (Zhaoping, Xifu and Hami) recently found in China are reported here and are classified on the basis of their bulk chemical compositions. Zhaoping contains 93.4 mg/g Ni, 85.9μg/g Ga, 418 μg/g Ge, 5.24 mg/g Co, 1.94 μg/g Ir, 0.774 μg/g W, and 1.62 μg/g Au and belongs to the low-Ni, low-Au subgroup of IAB. It is a coarse octahedrite and consists of kamacite, taenite, troilite, schreibersite and cohenite. The cohenite has entirely decomposed to graphite and low-Ni kamacite in our samples. Zhaoping contains some inclusions of Mn-free sarcopside which were rarely reported in IAB iron meteorites. Xifu has 74.1 mg/g Ni, 58.8gμg/g Ga, 150μg/g Ge, and 0.913μg/g W. Xifu is a member of group IIICD iron meteorite. Like most of IIICD irons, Xifu is a coarsest octahedrite with kamacite bandwidth larger than 3mm, and contains kamacite, taenite and schreibersite. Carbides and graphite are not found in the sample because of its being het- erogeneous. Hami has 106 mg/g Ni, 5.36 mg/g Co and 0.922μg/g Ir. We did not obtain the Ga and Ge contents in Hami because of their low concentrations and the limited precision of the INAA technique. Hami is an unclassified iron meteorite on the basis of the contents of other trace elements, structure and mineralogy. On mineralogy and structure, Hami resembles Rafruti, another unclassified iron meteorite.

Key words: iron meterites—physical data and processes—astrochemistry

† Supported by National Natural Science Foundation for Distinguished Young Scholars, “One-Hundred- Talent Program” of Chinese Academy of Sciences, and Minor Planet Foundation of China Received 2006–07–04; revised version 2006–10–12 A translation of Acta Astron. Sin. Vol. 48, No. 3, pp. 328–342, 2007 [email protected]

1. INTRODUCTION

Iron meteorites consist mainly of high-density Fe-Ni metal (Ni > 4%) alloys (kamacite and taenite) with minor amounts of sulfides, phosphides, carbides, silicate and phosphate inclusions. The study of iron meteorites not only helps us to understand the composition and structure of the earth’s interior, but also increases our knowledge of melting and differentia- tion processes on planetary bodies. In the early 19th century, scientists began to analyze the chemical composition of iron meteorites [1], and found that some meteorites display similar structures and cooling rates and that their trace elements show special fractional crystallization trends and so on. In order to study iron meteorites systematically and comparatively, scientists made classification. There are three classification schemes for iron meteorites: structural, genetic and chemical classification[2]. The chemical classification is the most widely used one, which groups most iron meteorites into IAB, IC, IIAB, IIC, IID, IIE, IIF, IIG, IIIAB, IIICD, IIIE, IIIF, IVA and IVB group[3−11]. It is believed that iron meteorites of a given chemical group formed on a common parent body. Iron meteorites were thought to have formed by two mechanisms. 11 groups of iron are designated as magmatic groups, which formed by fractional crystallization of molten cores in asteroids and are possibly similar to the core of the earth. Another three groups (IAB, IIICD and IIE) are considered to be nonmagmatic. They originated in small impact melt pools of asteroids[10,12−14].Some iron meteorites structurally and chemically do not fit into any of the above groups, and are defined as ungrouped iron meteorites. Zhaoping, Xifu and Hami iron meteorites are objects recently found in China. In this paper we systematically analyze the petrography, mineralogy and chemical compositions of the three iron meteorites, and make chemical classification.

2. SAMPLES AND ANALYTICAL METHODS

Zhaoping, Xifu and Hami iron meteorites have experienced prolonged terrestrial weathering processes. Their fusion crusts were weathered in various degrees. The most signiﬁcantly corroded iron meteorite is Xifu which was found in Pine Village of Xifu town, Qingdao Municipality (36◦ 18 N, 120◦ 29 E). On the morning of May 18, 2004, Xifu was dug out of the ground from 3 meters below during some construction work. The surface is covered with irregular air vents. Its fusion crust and regmaglypts are not recognized and in some areas the surface had been heavily weathered . It is roughly cone-shaped (130 cm high) and weighs about 3 tons. The perimeter of the largest section is 260 cm. Xifu local government holds the main mass. Zhaoping iron meteorite was discovered in Zhaoping County, Guangdong province (24◦ 14 N, 111◦ 11 E). The egg-shaped mass weighs about 2 tons and is about 120 cm-long, with a maximum diameter of 65 cm. The main mass still resides in Huangyao Village of Zhaoping County. Hami iron meteorite is preserved relatively fresh. It was found in the East-Station of Thirteen-Houses of Hami, Xinjiang province (43◦ 45 N, 92◦ 55 E). The surface of Hami displays well-preserved fusion crust and “thumb prints” . It weighs about 160 kg. Its height is about 30cm and its underside has a maximum length of 42 cm. A private collector holds the main mass. We prepared 5 thick sections (two sections of Zhaoping, two of Hami and one of Xifu) at the Laboratory for Astrochemistry and Planetary Sciences at Purple Mountain Observa- LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 295

Fig. 1 The Xifu iron meteorite. Its fusion crust and regmaglypts are not recognized.

tory, Chinese Academy Sciences. These sections were first etched with a 10% nital solution, and then were observed under reflected light with a Nikon E400 POL optical microscope. Backscattered-electron imaging and quantitative analyses of some small mineral grains were performed with LEO 1530VP field emission scanning electron microscope at Nanjing Insti- tute of Geology and Paleontology. The chemical compositions of most of the minerals were measured with a JEOL JXA-8800M electron microprobe at the State Key Laboratory for Mineral Deposits Research, Nanjing University. The accelerating voltage was 15 keV and the beam current was 15 nA. Both synthetic (NBS) and natural mineral standards were used, and matrix collections were made based on ZAF procedures. The Raman spectra of the minerals were measured with a Renishaw RM2000 micro- Raman spectrometer equipped with a CCD detector in the Department of Earth Sciences of Nanjing University. A microscope was used to focus the excitation laser beam (514.5 nm lines of a Princeton Instruments Ar+ laser). The scanning time was 30 seconds. Twelve elements were determined by instrumental neutron-activation analysis (INAA). Our neutron-activation procedure is similar to that used by Tian et al. [15].Samples and standards were all sawn into roughly 1×6×6 mm pieces and two pieces of each were prepared, one for short and middle irradiations, the other for long irradiation. Table1 gives the bulk chemical composition of Zagora and Caddo County. All the irradiations were conducted at the heavy water research reactor (HWRR) in China Institute of Atomic Energy. Thermal neutron fluxes were 1×1013n.cm−2.s−1 for the short and middle irradiations and 5×1013n.cm−2.s−1 for the long irradiation. The time of the short irradiation was 20 s; the samples were counted two times, at about 7 and 30 minutes after the irradiation; elements of Au, Ca, Cu and Ni were determined in this process. The duration of the middle irradiation was 1000s and that of the long irradiation was 30 minutes; the samples were counted at two times, at about 1 and 3 days after the middle irradiation (As, Au, Co, Cr, Cu, Fe, Ga, Ge, Ir, Re and W) and at 4 and 15 days after the long irradiation (As, Co, Fe and Ni).

Table 1 The bulk chemical compositions of standard samples Cr Co Ni Cu Ga Ge As W Re Ir Au μg/g mg/g mg/g μg/g μg/g μg/g μg/g μg/g ng/g μg/g μg/g Zagora Measured data < 8.3 4.84 nd 264 67.9 218 14.8 0.732 273 2.76 1.72 Literature data 9—146 4.84 253 67.7 239 15.1 0.910 264 2.77 1.61 Caddo County Measured data 44.1 4.95 93.2 nd 97.6 nd 14.2 0.956 nd 2.13 1.61 Literature data 29—343 4.93 93.2 68.7 14.9 1.11 2.55 1.608 296 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305

Fig. 2 The Hami iron meteorite

Literature data are from [23], and are mean values (underlined value is a range), nd means “not determined”. In our analysis, Zagora was used as the standard for the short and middle irradiations and Caddo County, for the long irradiation. All counts were carried out with an HPGe gamma-ray spectrometer (the resolution was 18Kev at 45% detection). Zr foil was used as the neutron ﬂux ratio monitor and high - purity Fe wire, as comparator for K0-NAA.

3. EXPERIMENTAL RESULTS

3.1 Zhaoping The etched surface displays a coarse Widmanstatten structure and numerous Neumann lines (Fig.3a). In two sections, seven minerals were found. Kamacite and taenite are the major phases. Minor phases include schreibersite, troilite, graphite, ferrous phosphate and an unidentified mineral. The Kamacite, α-(Fe, Ni), contains 7.26 wt% Ni. (In some area, the ferrite has only 1.48 wt% Ni, — this is named low-Ni metal in Table 2.) The kamacite lamellae have widths ranging from 0.75 mm to 4.5 mm (average width 1.94 mm). Taenite (γ-(Fe, Ni)) and plessite cover 3-5% by area, mostly as poorly resolvable black taenite wedges, as comb plessite (Fig.3c) and as pearlitic, black and duplex plessite fields (Fig.3d). A typical field will exhibit cloudy taenite edges and an acicular martensitic interior (Fig.3b). Schreibersite is a minor phase, only present as monocrystalline veinlets between grain boundaries and as occasional irregular inclusion in ferrite matrix. Ferrous phosphate was found in one section. It has an idealized empirical formula of Fe3 (PO4)2. (Its chemical composition is listed in Table 3.) The phosphate inclusions are about 1-50μm in size and contain some troilite grains. In some inclusions, we discovered a granular or columnar crystal mineral, which is usually less than 2μm in size (Fig.3e). Several larger crystals were quantitatively analyzed. The P concentration of the mineral is less than half that of the base phosphate and the Fe content is up to 87.42 wt% (Table 3). Its Raman spectrum is displayed in Fig. 4. We also LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 297

Fig. 3 Reflected light photography of Zhaoping, etched (a), (b), (c), (d); backscattered electron images of Zhaoping (e), (f). (a) After etched, the sample surface shows Widmanstatten structure and many Neumann lines, and schreibersite (S) surrounds kamacite lamellae (K). (b) A plessite field with cloudy taenite edges and martensitic interior. (c) Comb plessite. (d) The pointed end of a plessite field with clear taenite rims (T), black B and pearlitic (P) transition zones and duplex interior (D). (e) A phosphate inclusion of Zhaoping in which there are Mn-free sarcopside (Sarc), trolite grains (Tro) and an unidentified mineral (Un). (f) Cohenite has entirely decomposed to nickel-poor ferrite (M) and lamellar graphite (G).

examined mineral grains (both large crystals and smaller than 1μm crystals) with the EDS, and observed inter-grain chemical variations. The mineral grains have diﬀerent contents of P and Fe. Also, for some inclusions, the backscattered-electron images show numerous bright parallel lines cutting through the phosphate grain. They are lamellae of a phase with a higher Fe content than the phosphate. Similar lamellae were previously seen in Bear Creek and Mount Edith [16]. Zhaoping contains 9.34 wt% Ni, 5.24 mg/g Co, 85.9 μg/g Ga, 14.6 μg/g As, 1.62 μg/g Au. The Ge content (418 μg/g) is the highest among the three irons. 3.2 Xifu The Xifu iron meteorite is a coarse octahedrite. Neumann bands cross the kamacite in various orientations. Most of kamacite lamellae have been recrystallized (Fig.5a). The Neumann bands near the surface area are partially bent (Fig.5b). In one section, we found three minerals: kamacite, taenite and schreibersite. The Kamacite covers 97% by area. It contains 5.86 wt% Ni (Table 2). The Schreibersite is only present as grain boundary precipitates and as occasional irregular inclusion. Plessite and taenite occur in small amounts (Fig.5c). The bulk nickel content of Xifu is 7.41 wt%. The abundances of Re and Ir are 298 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305

Table 2 The results of electron microprobe analyses Meteorite Mineral Chemical composition(%) Fe S P Co Ni Mn Cr Cu Total Zhaoping Kamacite 91.25 0.03 0.02 0.65 7.26 0.00 0.00 0.03 99.24 (6) Taenite 75.14 0.01 0.01 0.41 24.11 0.00 0.00 0.00 99.68 (6) Schreibersite 46.55 0.01 16.76 0.16 37.05 0.00 0.00 0.02 100.55 (3) Troilite 60.35 39.21 0.03 0.10 0.21 0.01 0.02 0.00 99.92 (4) Metal(low Ni) 96.69 0.00 0.00 0.32 1.48 0.00 0.02 0.05 98.56 (2) Xifu Kamacite 91.56 0.01 0.15 0.53 5.86 0.02 0.00 0.02 98.33 (3) Taenite 69.87 0.02 0.02 0.17 27.80 0.00 0.00 0.00 97.88 (2) Schreibersite 62.21 0.04 15.97 0.17 21.31 0.00 0.00 0.04 99.74 (4) Hami Kamacite 91.11 0.00 0.02 0.71 6.70 0.02 0.00 0.00 98.56 (2) Taenite 70.02 0.00 0.00 0.32 28.57 0.00 0.02 0.00 98.93 (2)

Element concentrations of mineral are average values. The number in brackets is the number of mineral analyzed.

Table 3 Chemical compositions of phosphate and an unidentiﬁed mineral Mineral Chemical composition(%) Na2OCaOP2O5 MgO TiO2 K2OAl2O3 Cr2O3 MnO FeO Total Phosphate 0.00 0.00 38.24 0.00 0.00 0.00 0.00 0.00 0.00 63.92 102.16 Unidentiﬁed mineral 0.02 0.00 12.95 0.00 0.00 0.00 0.00 0.50 0.07 87.42 100.96

Fig. 4 Raman spectra of the unidentiﬁed mineral in Zhaoping and a terrestrial hematite LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 299 below the detection limits: Re < 110 ng/g, Ir < 0.15 μg/g (Table 4). 3.3 Hami The Hami iron meteorite is an ataxite. The metal is a polycrystalline aggregate of kamacite grains. Taenite is present as irregular blebs of size less than 10μm, mainly located in the kamacite grain boundaries (Fig.5d). No Widmanstatten pattern is recognized and no other minerals are found. Hami has 10.6 wt% Ni and 237gμg/g Cr. The concentrations of Ga, Ge and W are all below the detection limits (Table 4).

Table 4 The results of INAA Cr Co Ni Cu Ga Ge As W Re Ir Au μg/g mg/g mg/g μg/g μg/g μg/g μg/g μg/g ng/g μg/g μg/g Zhaoping 7.22 5.24 93.4 144 85.9 418 14.6 0.774 211 1.94 1.62 Xifu 6.58 5.78 74.1 101 58.8 150 9.41 0.913 < 110 < 0.15 1.19 Hami 237 5.36 106 73.9 < 2 < 10 0.261 < 0.3 439 0.922 0.298

Fig. 5 Reﬂected light photography of Xifu, etched (a, b); backscattered electron images of Xifu (c) and Hami (d). (a) Neumann bands cross the polygonized, recrystallized kamacite lamella, the matrix still displays many decorated subboundaries and four equiaxial kamacite grains meet along secondary grain boundaries. Etched. (b) The Neumann bands near the surface are bent. Etched. (c) Taenite (light) and kamacite (grey) coexist in the plessitic area. ( d) Taenite (light) scattered in kamacite matrix (dark).

4. DISCUSSION

4.1 Chemical Classification Different groups of iron meteorites plot on different areas in the log-log E-Ni diagram (E denotes the elements Ga, Ge and Ir, sometimes Co, Cu, W and Au). The most important 300 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 elements are Ga and Ge. The taxonomic significance of Ga and Ge derives from the fact that they are the most volatile siderophile elements, and that they tend to be strongly differ- entiated between the different groups: both Ga and Ge show a narrow range within similar groups of iron meteorites, and large differences between different groups. Furthermore, since the chemical classification is also based upon the nickel content, which is correlated with the widths of the kamacite bands, there is a general correlation between the structural and chemical classifications. Initially, the chemical classification of irons was based on the concentrations of the elements Ga and Ge[17]. Later, Wasson realized that Ga and Ge couldn’t classify all irons completely, so he determined nickel by atomic-absorption spectrophotometry and analyzed concentrations of Ga, Ge and Ir of all iron meteorites by instrumental neutron-activation analysis to create the chemical classification scheme of Ga-Ni and Ge-Ni. Ir was used to discriminate whether two iron meteorites found within the same area were paired [18−20]. With the development of later techniques, Ga, Ge, Ir, Co, W, Cu and Au can be determined by INAA and RNAA at high precisions. The correlations between the trace elements and nickel and the mineralogy and structures of irons reveal important information about the genesis of the parent-bodies of iron meteorites and the history of the solar system. Recently, Wasson et al. broke with this tradition by choosing Au as the independent parameter instead of Ni. Au has a larger compositional range (resulting from its lower D value, the solid/liquid [24] partition ratio; DAu ≈ 0.4, DNi ≈ 0.9) . As a result, the Au concentration provides a much better estimate of the meteorite’s position within the fractional crystallization sequence of a magmatic group [21−23]. Considering that the experimental error of Ni is lower than that of Au in this work and the relation of nickel content and structure, we still choose Ni as the independent parameter (the X axis) in this paper. 4.2 Classification of Iron Meteorites 4.2.1 Zhaoping Wasson et al. started the investigations of iron meteorites in the 1960s. Up to the present, they have determined the concentrations of Ga, Ge and Ni for more than 500 iron meteorites. Fig.6a and Fig.6b illustrate the ranges of the 13 iron-meteorite groups [25].IIGisanew group, which has only 5 members (Bellsbank, La Primitiva, Tombigbee River, Twannberg and Guanaco). IIG group irons are similar to IIAB in structure and trace element abundance, but the nickel concentrations of IIG group are relatively lower (Table 5). From the Ga(Ge)-Ni diagrams in Figure 6a(b), we find that Zhaoping falls close to the IAB group. To see a better correlation between Zhaoping and IAB irons, we plot six element-Ni diagrams (Ga, Ge, Co, Ir, W and Au) of Zhaoping compared to the INAA data for IAB irons (Fig.7). In 2002, Wasson et al. reanalyzed 12 elements in IAB and closely related iron meteorites. They plotted the data against Au instead of Ni and found that IAB iron meteorites can be subdivided into a main group (MG), and the following subgroups: low-Au and low- Ni (SLL), low-Au and medium-Ni (SLM), low-Au and high-Ni (SHH), high-Au and low-Ni (SHL), and so on. The data used in Figure 7 are adopted from Wasson et al. (2002) [23]. Figure 7 reveals that the concentrations of Ga, Ge, Au, Ni and so on of Zhaoping all fall well within the fields of IAB; it is very similar in chemical composition to the “Udei-Station” LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 301

Fig. 6 The locations of Xifu, Zhaoping and 13 iron - meteorite groups on the log Ga - log Ni and log Ge - log Ni diagrams

Table 5 Properties of iron-meteorite groups Group Bandwidth Structure Ni Ga Ge Ir (mm) (mg/g) (μg/g) (μg/g) (μg/g) IA 1.0 - 3 Om - Ogg 64 - 87 55 - 100 190 - 520 0.6 - 5.5 IB 0.01-1.0 D - Om 87 - 250 11 - 55 25 - 190 0.3 - 2.0 IC < 3 Anom, Og 61 - 68 49 - 55 212 - 247 0.07 - 2.1 IIA > 50 H 53 - 57 57 - 62 170 - 185 2 - 60 IIB 5 - 15 Ogg 57 - 64 46 - 59 107 - 183 0.01 - 0.9 IIC 0.06 - 0.07 Opl 93 - 115 37 - 39 88 - 114 4 - 11 IID 0.4 - 0.8 Of - Om 96 - 113 70 - 83 82 - 98 3.5 - 18 IIE 0.7 - 2 Anom* 75 - 97 21 - 28 62 - 75 1 - 8 IIF 0.05 - 0.21 D - Of 106 -140 9 - 12 99 - 193 0.8 - 23 IIG > 3.3 H - Ogg 41 - 49 33 - 45 37 - 63 0.013 - 0.15 IIIA 0.9-1.3 Om 71-93 17-23 32-47 0.15-20 IIIB 0.6 - 1.3 Om 84 - 105 16 - 21 27 - 46 0.01 - 0.15 IIIC 0.2 - 3 Oﬀ - Ogg 62 - 130 11 - 92 8 - 380 0.07 - 2.1 IIID 0.01 - 0.05 D - Oﬀ 160 - 230 1.5 - 5.2 1.4 - 4.0 0.02 - 0.07 IIIE 1.3 - 1.6 Og 82 - 90 17 - 19 34 - 37 0.05 - 6 IIIF 0.5 - 1.5 Om - Og** 68 - 85 6.3 - 7.2 0.7 - 1.1 0.006 - 7.9 IVA 0.25 - 0.45 Of 74 - 94 1.6 - 2.4 0.09 - 0.14 0.4 - 4 IVB 0.006 - 0.03 D 160 - 180 0.17 - 0.27 0.03 - 0.07 13 - 38 302 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305

Fig. 7 The relation of Zhaoping iron meteorite and the IAB iron meteorites on the log element-log Ni diagrams

grouplets (abbreviated USG in this paper) and closely related to the low-Au and low-Ni subgroup. Zhaoping is a coarse octahedrite which is the most common structure in IAB irons (Table 5). It consists of kamacite, taenite, troilite, schreibersite and graphite. The Ni content of ferrite around graphite is only 1.48%, which is lower than that of “normal” kamacite (Fig.3f). We suggest that cohenite was present. Cohenite is thermodynamically unstable at all temperatures in one atmosphere, so the cohenite in Zhaoping has to be entirely decomposed into graphite lamellae and nickel-poor ferrite because of shocked and cosmic reheating [26,27]. It is a popular phenomenon in IAB irons; for example, in Dungan- non, cohenite was also converted to 5-30 μmgide and very long microcrystalline, cavernous, “arborescent” graphite lamellae in a matrix of nickel-poor ferrite [28]. As described above, pearlitic plessite field is present commonly in Zhaoping, which is a characteristic of IAB group [29]. Besides, there is a phosphate inclusion in Zhaoping. It mainly contains ferrous phosphate, troilite and an unidentified mineral (Fig.3e). Based on the chemical analysis, the phosphate is sarcopside, which is one of the most common phosphate minerals in IIIAB group but is rare in IAB group. Sarcopside, with an idealized empirical formula of (Fe, Mn)3(PO4)2, has a low MnO content. Some irons, such as Avoca IIIAB, contain MnO-free sarcopside [16]. The inclusions are all round or nearly round. They could be trapped liquid drops of Fe-P-S (S content is low). Troilite first crystallized around the surrounding metal, and some of them grew into euhedral crystals. Prior to the build-up of O concentrations to the point where oxidation would be initiated, elements that were concentrated in the trapped liquid, such as Na, Mg, K, Ca, Mn, reacted with S to dissolve in troilite. With LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 303 decreasing temperature and increasing O fugacity, these elements in troilite were oxidized and diffused into adjacent ferrous phosphate that had just been formed [16].Thus,itappears that no troilite, the original source of Mn, was present or no Mn was available from troilite, so the sarcopside in Zhaoping crystallized as pure ferrous orthophosphate. The unidentified mineral in the phosphate inclusion is euhedral, so if it is a primary mineral, it should have crystallized before the sarcopside. However, it shouldn’t be another phosphate based on the chemical composition (12.95 wt% of P2O5 and 87.42 wt% FeO). In Figure 4 we show that the Raman spectrum of the unidentified mineral is similar to that of hematite. The unidentified mineral is identified with five characteristic peaks at 217, 281, 399, 598 and 1311 cm−1 in the stretching region 0 ∼ 1800 cm−1 and hematite with five characteristic peaks at 222, 288, 402, 605 and 1296 cm−1in the same stretching region (the Raman spectrum of the hematite is derived from the Raman spectrum database of Department of Physics, Parma University). Therefore, we suggest that the unidentified mineral is hematite, a product of terrestrial weathering, and the P content we determined is contributed by sarcopside underneath. 4.2.2 Xifu The Ga-Ni and Ge-Ni plots in Figure 6 show that Hami should belong to group IIICD. In Fig.8 we further compare our Xifu data to the IIICD iron data. We note that Xifu falls well within the fields of IIICD on the Ga-Ni, Ge-Ni and W-Ni diagrams; the Au is lower than the IIICD trend, but only slightly; and the Co value plot is ≈1.6× higher than the main trend of IIICD iron. These discrepancies possibly result from the intra-sample heterogeneity. In Xifu, the average Co content of kamacite is 5.3 mg/g and that of taenite is only 1.7 mg/g. Xifu iron meteorite is a coarsest octahedrite; kamacite, taenite and schreibersite are present in our sample, and no troilite is found. These features are very similar to those of IIICD group. Schreibersite is fairly abundant; but troilite is much less common in IIICD than in group IAB. In addition, carbides and graphite are not present, which are common in group IIICD. 4.2.3 Hami The nickel content of Hami is only 10 wt%, which is outside the range of ataxites (Ni ¿ 16 wt %) [30]. The concentrations of Ga (Ga ¡ 2gμg/g), Ge (Ge¡10gμg/g) and Ir (0.992) of Hami are within the ranges of IIICD group (Ni, 62-230 mg/g; Ga, 1.5-92 μg/g; Ge, 1.4-380 μg/g; Ir, 0.02-2.1gμg/g). However, the diagrams of the various elements in Figure 8 show that while Co and Ir of Hami are inside the fields of IIICD irons, the Au content falls obviously below the IIICD trend. We did not find schreibersite, which is one of the most common minerals in IIICD irons. Thus, in this paper we define Hami as an un-classified iron meteorite. Hami resembles Rafruti (an ungrouped iron). Rafruti is also an ataxite. It has 9.43 wt% Ni, 6.0 mg/g Co, 0.16 μg/g Ga, 0.06 μg/g Ge and 0.007 μg/g Ir. It only contains two minerals: kamacite and taenite; the taenite, 8 μm or so in diameter, is dispersed among equiaxial recrystallized kamacite grains. Some people assumed that Rafruti was originally a fine octahedrite which, by the repeated artificial reheating, had been transformed to the present fine – grained ataxite structure [31]. In Hami, the kamacite did not undergo recrystallization or reheating. 304 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305

Fig. 8 The locations of Xifu, Hami iron meteorite and IIICD iron meteorites on the log element-log Ni diagrams

5. CONCLUSIONS

Zhaoping iron is a coarse octahedrite and consists of kamacite, taenite, troilite, schreibersite, cohenite, free-Mn sarcopside and hematite. Cohenite has entirely decomposed to graphite and low - Ni ferrite. Hematite is a product of terrestrial weathering. Based on the bulk chemical composition, Zhaoping should belong to the low-Ni, low-Au subgroup of IAB. Xifu is a coarsest octahedrite and is a member of the group of IIICD iron meteorites. Hami only contains two mineral phases: kamacite and taenite. We deﬁne Hami as an unclassiﬁed iron meteorite on the basis of the contents of trace elements and mineralogy.

ACKNOWLEDGEMENTS We are grateful to Weizhi Tian, Wenlan Zhang and Yongqiang Mao for technical help in the analyses. We thank Jianyun Tan for the preparation of the specimens and we appreciate the various discussions with Sichao Wang and Aicheng Zhang.

References

1 Howard E. Phillosophical Transactions of the Royal Society of London, 1802, 92,168 2 Wang D., Chen.Y., Li.Z. et al., Introduction to Chinese Meteorites (in Chinese), Beijing: Science Press, 1993, p.30 3 Wasson J.T., Geochim Cosmochim Acta, 1967, 31,161 4 Wasson J.T., Geochim Cosmochim Acta, 1969, 33, 859 5 Wasson J.T., Icarus, 1970a, 12, 407 6 Wasson J.T., Kimberlin J. Geochim Cosmochim Acta, 1967, 31, 2065 7 Wasson J.T., Schaudy R. Icarus, 1971, 14, 59 LIN Su et al. / Chinese Astronomy and Astrophysics 32 (2008) 293–305 305

8 Schaudy R., Wasson J.T, Buchwald V.F., Icarus, 1972, 174 9 Scott E.R.D., Wasson J.T, Buchwald V.F., Geochim Cosmochim Acta, 1973, 37, 1957 10 Scott E.R.D., Wasson J.T., Geochim Cosmochim Acta, 1976, 40,103 11 Kracher A, Willis J, Wasson J T. Geochim Cosmochim Acta, 1980, 44, 773 12 Wasson J.T, Wang J. Geochim Cosmochim Acta, 1986, 50, 725 13 Kaare L.R. Physica Scripta, 1989, 39, 410 14 Choi B., Quyang X., Wasson J.T., Geochim Cosmochim Acta, 1995, 59, 593 15 Tian W., Ni B., Wang P, et.al., Fresenius J., Anal. Chem., 1998, 360, 354 16 Olsen E.J, Kracher A., Davis A.M, et.al., Meteor & Planet. Sci., 1999, 34, 285 17 Goldberg E., Uchiyama A., Brown H., Geochim Cosmochim Acta, 1951, 2, 1 18 Wasson J.T., Kimberlin J., Geochim Cosmochim Acta, 1967, 31, 2065 19 Scott E.R.D., Geochim Cosmochim Acta, 1972, 36, 1205 20 Malvin D.J., Wang Daode, Wasson J.T., Geochim Cosmochim Acta, 1984, 48, 785 21 Wasson J.T., Choi B-G, Jerde E.A., et.al., Geochim Cosmochim Acta, 1998, 62, 715 22 Wasson J.T., Richardson J.W., Geochim Cosmochim Acta, 2001, 65, 951 23 Wasson J.T., Kallemeyn G.W., Geochim Cosmochim Acta, 2002, 13, 2445 24 Wasson J.T., Geochim Cosmochim Acta, 1999, 63, 2875 25 Wasson J.T., Meteorites: Their Record of Early Solar-System History, USA: W.H. Freeman, 1985, p.41 26 Buchwald V.F., Handbook of Iron Meteorite. Berkeley: Univ. California Press, 1975, p. 547 27 Buchwald V.F., Phil. Trans. R. Soc. Lond., 1977, A286, 453 28 Buchwald V.F., Handbook of Iron Meteorite. Berkeley: Univ. California Press, 1975, p.548 29 Scott E.R.D., Wasson J.T., Rev. Geophys. Space Phys., 1975, 13, 527 30 Norton O.R., The Cambridge Encyclopedia of Meteorites. Cambridge: Univ. Cambridge Press, 2002, p. 191 31 Buchwald V.F., Handbook of Iron Meteorite Berkeley: Univ. California Press, 1975, p.1004