山东省鄄城陨石中的金属矿物及组构特征

1997年降落在山东省鄄城县的陨石雨,是橄榄石-古铜辉石球粒陨石。该陨石中的金属矿物主要为铁纹石和陨硫铁,其次为镍纹石,金属矿物呈填隙状分布于以橄榄石和古铜辉石为主的硅酸盐矿物粒间及球粒周围。陨石中可见由铁纹石和镍纹石组成的显微蠕虫状连晶,是陨石中金属矿物在降温冷却过程中发生固溶体分离作用而成。陨石中金属矿物的分布特征表明,金属Fe-Ni和硫化物（FeS）应该是星云凝聚不同阶段的产物。陨石中金属矿物的成分和组构特征及陨石中出现的球粒结构、橄榄石的炉条结构等特征表明,该球粒陨石是星云物质快速冷却的产物。     

GRV021512和GRV021931:我国新发现的二块南极橄辉无球粒陨石的岩石学特征

取自南极格罗夫山两块橄辉无球粒陨石 ,GRV0 2 15 12陨石具有典型的橄辉陨石结构 ,由 48.3 %的橄榄石、9.4%的易变辉石和3 8.1%的碳质基质组成。而 GRV0 2 2 93 1则表现为碎裂结构 ,少量斑状橄榄石 (19.1% )和易变辉石 (14 .1% )镶嵌于富碳质基质 (66.3 % )中。两块陨石的粗粒橄榄石和易变辉石的核部成分均匀 ,成分落在橄榄石 -易变辉石橄辉无球粒陨石的富铁亚类之中。所有橄榄石颗粒的边缘和裂隙均具有还原环带。在富碳基质中金刚石与石墨共生呈团块和脉状产出 ,本文就两块陨石的岩石和金刚石的成因进行了讨论。GRV021512和GRV021931:…     

南极 GRV 021788橄辉无球粒陨石的岩石学和矿物学特征

橄辉无球粒陨石是一类特殊的无球粒陨石,既具有高度分异的火成特征,又具有原始球粒陨石的特征。南极格罗夫山GRV021788陨石由橄榄石、辉石以及少量的富碳基质、不透明矿物组成,具有橄辉无球粒陨石典型的岩相学和矿物学特征,包括橄榄石晶体三线共点的接触和富镁还原边等。GRV021788的主要矿物的化学成分为:橄榄石Fa23.2～Fa1.5,易变辉石Fs21.2Wo10.6～Fs18.7Wo10.7。橄榄石颗粒有明显的反环带结构,而易变辉石的反环带则不明显。黑色填隙基质富碳,含金刚石和石墨。岩石学和矿物化学特征表明GRV021788是一橄辉无球粒陨石,属单矿(monomict)Ⅰ类(Fa23.2～Fs20.4)橄辉无球粒陨石。陨石的形成机制与多阶段部分熔融-堆积模式较为一致。     

南极三块（L3）陨石球粒结构类型及其矿物学物征

我国第16次南极考察队回收到6块稀少种类陨石-L3型,GRV99001,GRV99019,GRV99020,GRV99021, GRV99022,GRV99026。本文对其中3块陨石进行研究,研究它们的球粒结构和矿物化学成分．它们虽然都属于非平衡普通球粒陨石(L3),但它们的亚类不同,GRV 99001为L3．4,GRV99026为L3．5,GRV99019为L3．6．它们的球粒结构和球粒内的矿物晶体完整性和矿物组合变化比较大,橄榄石和辉石以高镁为特征．这三块陨石的球粒结构种类比较多,有班状的、炉条状的、扇形的和隐晶质的等．在GRV99001陨石中班状结构的球粒内能见到一个或两个以上完整的单晶橄榄石构成的球粒,也能见到多个细小的或是破碎橄榄石,被包裹在辉石晶体内．而在GRV99019和GRV99026陨石中只能见到多个细小单晶体或是破碎的橄榄石晶体．GRV99001陨石的炉条状结构,好象是由一条带状长石矿物,穿插在单个橄榄石晶体中构成．扇形和伞形结构的球粒,以一个点为中心,向外放射呈扇形．如GRV99019陨石中扇形结构球粒,它们是以辉石为主,陨硫铁充填在低钙辉石缝隙中,形成扇形．另一种是以多个点为中心,如GRV99001陨石,它们是由橄榄石、低钙辉石和长石质的玻璃,构成多个小伞形,形状类似三维立体的球,裂缝中也充填有金属矿物．隐晶质的球粒在GRV99026陨石中有两种,一种是在一厘米等于 100un时呈现隐晶质矿物,而放大到一厘米等于5un时,就可以清楚看到两种低钙辉石矿物,在低钙辉石中还有金属矿物．另一种隐晶质结构球粒由极细小破碎的橄榄石和辉石矿物构成．这三块陨石中的橄榄石和辉石都以高镁为特征．班状结构球粒,在GRV99001陨石中橄榄石的MgO-33．37-51．21,辉石为35．9-36．61;GRV99019陨石中橄榄石23．33-56．58,辉石 21．38-33．07;GRV99026陨石中橄榄石45．91-52．63,辉石34．48-37．35．扇形结构球粒,在GRV99001陨石中橄榄石29．94 -46．22,辉石28．17-30．36;GRV99019陨石中橄榄石29．17-34．38,辉石23．11-27．79．炉条状结构的球粒,在GRV99001 陨石中橄榄石51．84-56．03．隐晶质结构球粒,在GRV99026陨石中辉石20．17-21．54,橄榄石30．84-32．66．由此看出矿物晶体完整性越好镁的含量越高．     

宁强碳质球粒陨石岩石学及化学组成的研究

宁强碳质球粒陨石含有大量聚集的球粒(13.7体积％)和非常低的难熔包体丰度(～2±1体积％),属于异常的CV球粒陨石(氧化亚群)。相对于典型的CV3球粒陨石,宁强具有低的难熔包体和较高橄榄石集合体(8.2体积％)及聚集的球粒丰度,相对于CV3氧化亚群,宁强具有较多的金属Fe-Ni及磁铁矿,高的磁铁矿(3.3体积％)/金属Fe-Ni(0.5体积％)比值(6.6),并有作为主要金属相的镍铁矿(Ni_3Fe),表明宁强球粒陨石类似于CV3氧化亚群。     

吉林陨石中橄榄石和辉石的化学成份特征兼谈平衡球粒陨石中的不平衡性

吉林陨石中的球粒分为正常快速冷却和过冷冷却球粒两种。前一类球粒中的橄榄石和辉石的化学成份相对稳定,代表了平衡球粒陨石的特征。后一类球粒中的橄榄石和辉石不仅在总的化学成份上有别于前者,而且矿物本身成份不稳定,反映了平衡球粒陨石中存在着不平衡性。     

富钙长石-橄榄石型包体与典型富Ca,Al包体的矿物岩石学特征对比研究

本文研究了2个富钙长石-橄榄石型包体和2个富黄长石-尖晶石型和富尖晶石-辉石型包体（分别来自宁强和南极格罗夫山碳质球粒陨石）的矿物岩石学特征,并对它们进行了对比。富钙长石-橄榄石型包体的矿物模式组成具有富橄榄石和缺失黄长石的特征,其可能是球粒和典型难熔包体之间的中间产物,是认识它们之间相互关系的钥匙。矿物岩石学特征表明富黄长石-尖晶石型和富尖晶石-辉石型包体可能是星云直接凝聚的产物,而富钙长石-橄榄石型包体经历过熔融结晶过程。富钙长石-橄榄石型包体的初始物质可能是富Al的球粒或含难熔组分的蠕虫状橄榄石集合体。矿物化学组成对比研究发现,GRV 022459-RI6中的尖晶石具有最富FeO的特征,表明包体的蚀变可能发生在高氧逸度的星云环境。     

清镇顽火辉石球粒陨石的矿物学特征及其成因

清镇陨石以具有高度还原的矿物组合为特征。普通球粒陨石和碳质球粒陨石中的亲石元素在顽火辉石球粒陨石内显示亲铜或亲铁的性质。在该陨石中发现的矿物有陨硫钙石。硫镁矿、钾的硫化物——硫铜钾矿,含Si的铁纹石及Ni的硅化物——硅磷镍矿等。顽火辉石和斜顽火辉石具有低的FeO含量,其FeO/(FeO+MgO)比值范围为0.004—0.01。在未完全熔融的球粒中发现具尘粒状贫Ni金届包裹体的残余橄榄石颗粒。本文首次报道陨石中含Ga的闪锌矿和发现陨石中稀少的碱硅镁柱石。金属-硫化物组合显示再加热和熔融的结构特征,金属发生硫化作用并被硫化物所取代。清镇陨石(EH_3)的化学成分与EH_(4,5)球粒陨石很相似,其La/Sm比值比平均EL高,Co/Ni则比EL低。清镇陨石的形成部位比任何其他球粒陨石群更靠近太阳,可能形成于水星-金星区。     

普通球粒陨石NWA15004岩石矿物学特征及球粒类型研究

普通球粒陨石是目前发现数量最多的陨石,对认识早期太阳星云演化和太阳系物质成分具有重要的意义。Northwest Africa (NWA) 15004是一块非洲西北部新发现的普通球粒陨石。本次研究使用光学显微镜、电子探针以及扫描电镜等分析仪器对该陨石进行详细的岩石学、矿物学及球粒特征研究。结果表明该陨石球粒轮廓较为模糊,基质重结晶明显,橄榄石平均Fa值为25.4 mol%(PMD为2.65%),低钙辉石的平均Fs值21.3 mol%(PMD为3.95%),硅酸盐矿物化学成分较为均一,根据岩相学及矿物学特征将其划分为L5型普通球粒陨石。橄榄石和辉石颗粒发育波状消光和面状破裂,且观察到有熔融囊的出现,表明该陨石受到S3以上的冲击变质作用。球粒的成因和形成的星云环境需要准确的球粒类型划分,球粒按结构类型分类较多,但其化学成分均一,该陨石所有球粒的橄榄石辉石的Mg^#约为74.5,均为Ⅱ型富铁球粒,结合“CIPW标准”计算基质化学成分均为A5型球粒。利用共生单斜辉石和斜方辉石矿物对成分特征计算得到NWA 15004陨石热变质平衡温度为814℃,说明该陨石母体经历了较高程度热变...     

安徽亳县陨石研究

该陨石于1977年lO月20日下午2时30分左右降落于安徽亳县张沃公社吝子门大队,两块陨石分别重5.5公斤与2公斤,其外形分别为锥体和椭球体,表面灰黑色,有0.5mm厚的烧蚀层,新鲜面为浅灰色,均具明显的球粒结构,比重分别为3.426和3.419。 1.矿物学研究初步查明该陨石的透明矿物有镁橄榄石、贵橄榄石、透铁橄揽石、古铜辉石、顽火辉石、斜顽辉石、顽火透辉石、易变辉石、斜长石、正长石、自磷钙矿、石英、黑云母、白云石、方解石、刚玉等;不透明矿物有陨硫铁、铁纹石、镍纹石、铬铁矿、镁铁尖晶石、钛铁矿、镍黄铁矿、石墨、磁铁矿、方铁矿、自然铜和张衡矿等。     

剑阁和恩施球粒陨石

剑阁球粒陨石于1964年10月9日(或19日),陨落于四川省剑阁县杨家村公社民主大队。1964年12月,经四川省地质局实验室涂继尧等同志初步定为石陨石。1966年3月,我所钟大赉和刘秉光两同志,由四川省地质局实验室得到一小块大小为6.2×5×1.8公分,重222克的标本(照片1)。陨石的原形和大小现已无法查清。恩施球粒陨石于1974年12月26日15时30分陨落于湖北省恩施县芭蕉区,地理坐标东经109°51′20″,北纬30°34′40″。1977年9月,笔者赴现场调查时,据直接了解情况的李建华同志介绍,当时在瓦屋公社、乾溪公社和寨湾公社交界处的上空,突然由西北往东南方向发出声响,在70-80里的范围内都能听见。尔后掉下多块陨石。     

Petrology and oxygen isotope compositions of chondrules in E3 chondrites

Chondrules in E3 chondrites differ from those in other chondrite groups. Many contain near-pure endmember enstatite (Fs_<1). Some contain Si-bearing FeNi metal, Cr-bearing troilite, and, in some cases Mg, Mn- and Ca-sulfides. Olivine and more FeO-rich pyroxene grains are present but much less common than in ordinary or carbonaceous chondrite chondrules. In some cases, the FeO-rich grains contain dusty inclusions of metal. The oxygen three-isotope ratios (δ¹⁸O, δ¹⁷O) of olivine and pyroxene in chondrules from E3 chondrites, which are measured using a multi-collection SIMS, show a wide range of values. Most enstatite data plots on the terrestrial fractionation (TF) line near whole rock values and some plot near the ordinary chondrite region on the 3-isotope diagram. Pyroxene with higher FeO contents (∼2-10 wt.% FeO) generally plots on the TF line similar to enstatite, suggesting it formed locally in the EC (enstatite chondrite) region and that oxidation/reduction conditions varied within the E3 chondrite chondrule-forming region. Olivine shows a wide range of correlated δ¹⁸O and δ¹⁷O values and data from two olivine-bearing chondrules form a slope ∼1 mixing line, which is approximately parallel to but distinct from the CCAM (carbonaceous chondrite anhydrous mixing) line. We refer to this as the ECM (enstatite chondrite mixing) line but it also may coincide with a line defined by chondrules from Acfer 094 referred to as the PCM (Primitive Chondrite Mineral) line (Ushikubo et al., 2011). The range of O isotope compositions and mixing behavior in E3 chondrules is similar to that in O and C chondrite groups, indicating similar chondrule-forming processes, solid-gas mixing and possibly similar ¹⁶O-rich precursors solids. However, E3 chondrules formed in a distinct oxygen reservoir.Internal oxygen isotope heterogeneity was found among minerals from some of the chondrules in E3 chondrites suggesting incomplete melting of the chondrules, survival of minerals from previous generations of chondrules, and chondrule recycling. Olivine, possibly a relict grain, in one chondrule has an R chondrite-like oxygen isotope composition and may indicate limited mixing of materials from other reservoirs. Calcium-aluminum-rich inclusions (CAIs) in E3 chondrites have petrologic characteristics and oxygen isotope ratios similar to those in other chondrite groups. However, chondrules from E3 chondrites differ markedly from those in other chondrite groups. From this we conclude that chondrule formation was a local event but CAIs may have all formed in one distinct place and time and were later redistributed to the various chondrule-forming and parent body accretion regions. This also implies that transport mechanisms were less active at the time of and following chondrule formation.     

An exotic kind of cosmic material: graphite-containing xenoliths from the Krymka (LL3.1) chondrite

Seven graphite-containing xenoliths were found in the Krymka (LL3.1) chondrite. The xenoliths have the following chemical and mineralogical characteristics which distinguish them from the Krymka host: (1) low totals in bulk chemical analyses obtained by electron microprobe; (2) high bulk Fe abundances; (3) a uniform recrystallized, chondrule-free texture; (4) the presence of euhedral graphite and carbon-rich material; (5) higher quantities of troilite and metal; (6) a relatively homogeneous composition of silicates; (7) a distinctive composition of metal, chromite and phosphate; (8) isotopically heavy C in graphite compared to both bulk Krymka and graphite in other ordinary chondrites. The xenoliths are mineralogically similar, but not identical, to the Krymka carbonaceous clast K1, which bears graphite microcrystals, organic compounds and mysterite. They resemble carbonaceous chondrites, both chemically and isotopically. The mineralogical, chemical and isotopic data for the graphite-containing fragments suggest that this material represents metamorphosed varieties of a previously unknown type of unequilibrated carbonaceous matter. Most likely, the graphite has a metamorphic origin and was crystallized from C-containing precursor materials through the following transformation sequence: organic compounds → C-rich material → graphite.     

The Rumuruti chondrite group

Since 1994, the Rumuruti (R) chondrites have been recognized as a new, well-established chondrite group differing from carbonaceous, ordinary, and enstatite chondrites. The first R chondrite, Carlisle Lakes, was found in Australia in 1977. Meanwhile, the number has increased to 107 (December, 2010). This group is named after the Rumuruti meteorite, the first and so far the only R chondrite fall. Most of the R chondrites are breccias containing a variety of different clasts embedded in a clastic matrix. Some textural and mineralogical characteristics can be summarized as follows: (a) the chondrule abundance in large fragments and in unbrecciated rocks is ∼35–50 vol%; (b) Ca,Al-rich inclusions are rare; (c) the olivine abundance is typically 65–78 vol%; (d) the mean chondrule diameter is ∼400 μm; (e) in unequilibrated R chondrites, low-Ca pyroxene is dominating, whereas in equilibrated R chondrites it is Ca-rich pyroxene; (f) the typical olivine in a metamorphosed lithology is ∼Fa_38–40; (g) matrix olivine in unequilibrated, type 3 fragments and rocks has much higher Fa (∼45–60 mol%) compared to matrix olivines in type 4–6 lithologies (∼Fa_38–41); (h) spinels have a high TiO₂ of ∼5 wt%; (i) abundant different noble metal-bearing phases (metals, sulfides, tellurides, arsenides) occur. The exception is the metamorphosed, type 5/6 R chondrite La Paz Icefield 04840 which contains hornblende, phlogopite, and Ca-poor pyroxene, the latter phase typically occurring in low-grade metamorphosed R chondrites only.In bulk composition, R chondrites have some affinity to ordinary chondrites: (a) the absence of significant depletions in Mn and Na in R chondrites and ordinary chondrites is an important feature to distinguish these groups from carbonaceous chondrites; (b) total Fe (∼24 wt%) of R chondrites is between those of H and L chondrites (27.1 and 21.6 wt%, respectively); (c) the average CI/Mg-normalized lithophile element abundances are ∼0.95 × CI, which is lower than those for carbonaceous chondrites (≥1.0 × CI) and slightly higher than those for ordinary chondrites (∼0.9 × CI); (d) trace element concentrations such as Zn (∼150 ppm) and Se (∼15 ppm) are much higher than in ordinary chondrites; (e) the whole rock Δ¹⁷O of ∼2.7 for R chondrites is the highest among all meteorite groups, and the mean oxygen isotope composition is δ¹⁷O = 5.36 ± 0.43, δ¹⁸O = 5.07 ± 0.86, Δ¹⁷O = +2.72 ± 0.31; (f) noble gas cosmic ray exposure ages of R chondrites range between ∼0.1 and ∼70 Ma. More than half of the R chondrites analyzed for noble gases contain implanted solar wind and, thus, are regolith breccias. The 43 R chondrites from Northern Africa analyzed so far for noble gases seem to represent at least 16 falls. Although the data base is still scarce, the data hint at a major collision event on the R chondrite parent body between 15 and 25 Ma ago.     

Formation and destruction of magnetite in CO3 chondrites and other chondrite groups

Primitive CO3.00–3.1 chondrites contain ∼2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H₂ and reacts with SiO₂ to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr₂O₃ in a few R4-6 chondrites. Magnetite grains with high Cr₂O₃ behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ∼4 vol.% magnetite with substantial Cr₂O₃ (up to ∼15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.     

Oxygen-isotopic compositions of low-FeO relicts in high-FeO host chondrules in Acfer 094, a type 3.0 carbonaceous chondrite closely related to CM

With one exception, the low-FeO relict olivine grains within high-FeO porphyritic chondrules in the type 3.0 Acfer 094 carbonaceous chondrite have Δ¹⁷O (= δ¹⁷O − 0.52 × δ¹⁸O) values that are substantially more negative than those of the high-FeO olivine host materials. These results are similar to observations made earlier on chondrules in CO3.0 chondrites and are consistent with two independent models: (1) Nebular solids evolved from low-FeO, low-Δ¹⁷O compositions towards high-FeO, more positive Δ¹⁷O compositions; and (2) the range of compositions resulted from the mixing of two independently formed components. The two models predict different trajectories on a Δ¹⁷O vs. log Fe/Mg (olivine) diagram, but our sample set has too few values at intermediate Fe/Mg ratios to yield a definitive answer.Published data showing that Acfer 094 has higher volatile contents than CO chondrites suggest a closer link to CM chondrites. This is consistent with the high modal matrix abundance in Acfer 094 (49 vol.%). Acfer 094 may be an unaltered CM chondrite or an exceptionally matrix-rich CO chondrite. Chondrules in Acfer 094 and in CO and CM carbonaceous chondrites appear to sample the same population. Textural differences between Acfer 094 and CM chondrites are largely attributable to the high degree of hydrothermal alteration that the CM chondrites experienced in an asteroidal setting.     

Mineralogy and texture of Fe-Ni sulfides in CI1 chondrites: Clues to the extent of aqueous alteration on the CI1 parent body

To better understand the role of aqueous alteration on the CI1 parent body, we have analyzed the texture, composition and mineral associations of iron nickel sulfides in four of the five known CI1 chondrites.The most commonly-occurring sulfide present in the CI1 chondrites is the iron-deficient Fe,Ni sulfide pyrrhotite ([Fe,Ni]_1−xS), that has a composition close to that of stoichiometric troilite (FeS). Three of the CI1s (Alais, Ivuna and Tonk) also contain pentlandite ([Fe,Ni]₉S₈), although pentlandite is a rare phase in Ivuna. Cubanite (CuFe₂S₃) was found in both Alais and Ivuna in this study, although it has also been reported in Orgueil (MacDougall and Kerridge, 1977). The pyrrhotite grains in all four chondrites form hexagonal, rectangular or irregular shapes, and show no evidence of Ni or Co zoning. The pyrrhotite grains in Orgueil and Ivuna are, in general, smaller, and show more “corrosions,” or “embayments,” than those in Alais or Tonk.We suggest that the precursor sulfide present in the CI1 chondrites was troilite which, during brecciation and oxidation on the parent body at a temperature of 100°C or less, converted the troilite to magnetite and pyrrhotite with pentlandite inclusions. Subsequently, continued alteration on the parent body removed pentlandite—partially from Alais, Tonk and Ivuna, completely from Orgueil—leaving behind pyrrhotite with spaces (“corrosions”) where the pentlandite had been. Ni derived from the pentlandite was incorporated into ferrihydrite, onto the surface of which the Ni,Na sulfate Ni-bloedite formed.Based on the size and abundant “corrosions” within pyrrhotite grains, combined with observations from other authors, we conclude that Orgueil and Ivuna have undergone a greater degree of alteration than Alais and Tonk. Further work is needed to assess the conditions under which pentlandite would be dissolved preferentially to pyrrhotite, as the study of terrestrial literature indicates that the latter mineral is preferentially removed.     

The unequilibrated enstatite chondrites

The enstatite chondrites formed under highly reducing (and/or sulfidizing) conditions as indicated by their mineral assemblages and compositions, which are sharply different from those of other chondrite groups. Enstatite is the major silicate mineral. Kamacite is Si-bearing and the enstatite chondrites contain a wide variety of monosulfide minerals that are not present in other chondrite groups. The unequilibrated enstatite chondrites are comprised of two groups (EH3 and EL3) and one anomalous member (LEW 87223), which can be distinguished by differences in their mineral assemblages and compositions. EH3 chondrites have >1.8 wt.% Si in their kamacite and contain the monosulfide niningerite (MgS), whereas EL3 chondrites have less than 1.4 wt.% Si in their kamacite and contain the monosulfide alabandite (MnS). The distinct mineralogies, compositions and textures of E3 chondrites make comparisons with ordinary chondrites (OCs) and carbonaceous chondrites (CCs) difficult, however, a range of recrystallization features in the E3s are observed, and some may be as primitive as type 3.1 OCs and CCs. Others, especially the EL3 chondrites, may have been considerably modified by impact processes and their primary textures disturbed. The chondrules in E3 chondrites, although texturally similar to type I pyroxene-rich chondrules, are sharply different from chondrules in other chondrite groups in containing Si-bearing metal, Ca- and Mg–Mn-rich sulfides and silica. This indicates formation in a reduced nebular environment separate from chondrules in other chondrites and possibly different precursor materials. Additionally the oxygen isotope compositions of E3 chondrules indicate formation from a unique oxygen reservoir. Although the abundance, size distribution, and secondary alteration minerals are not always identical, CAIs in E3 chondrites generally have textures, mineral assemblages and compositions similar to those in other groups. These observations indicates that CAIs in O, C and E chondrites all formed in the reservoir under similar conditions, and were redistributed to the different chondrite accretion zones, where the secondary alteration took place. Thus, chondrule formation was a local process for each particular chondrite group, but all CAIs may have formed in the similar nebular environment. Lack of evidence of water (hydrous minerals), and oxygen isotope compositions similar to Earth and Moon suggest formation of the E chondrites in the inner solar system and make them prime candidates as building blocks for the inner planets.     

Microchondrule-bearing clast in the Piancaldoli LL3 meteorite: a new kind of type 3 chondrite and its relevance to the history of chondrules

In the Piancaldoli LL3 chondrite, we found a mm-sized clast containing ~100 chondrules 0.2–64 μm in apparent diameter (much smaller than any previously reported) that are all of the same textural type (radial pyroxene; FS_1–17). This clast, like other type 3 chondrites, has a fine-grained Ferich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si- and Cr-bearing metallic Fe,Ni. However, the very high modal matrix abundance (63 ± 8 vol. %), unique characteristics of the chondrules, and absence of microscopically-observable olivine indicate that the clast is a new kind of type 3 chondrite. Most chondrules have FeO-rich edges, and chondrule size is inversely correlated with chondrule-core FeO concentration (the first reported correlation of chondrule size and composition). Chondrules acquired Fe by diffusion from Fe-rich matrix material during mild metamorphism, possibly before final consolidation of the rock. Microchondrules (those chondrules ? 100 μm in diameter) are also abundant in another new kind of type 3 chondrite clast in the Rio Negro L chondrite regolith breccia. In other type 3 chondrite groups, microchondrule abundance appears to be anticorrelated with mean chondrule size, viz. 0.02–0.04 vol. % in H and CO chondrites and ?0.006 vol. % in L, LL, and CV chondrites.Microchondrules probably formed by the same process that formed normal-sized droplet chondrules: melting of pre-existing dustballs. Because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type, we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs. The sizes of compound chondrules and chondrule craters, which resulted from collisions of similarly-sized chondrules while they were plastic, indicate that size-sorting (of dustballs) occurred before chondrule formation, probably by aerodynamic processes in the nebula. We predict that other kinds of type 3 chondrites exist which contain chondrule abundances, size-ranges and proportions of textural types different from known chondrite groups.     

The Shaw meteorite: History of a chondrite consisting of impact-melted and metamorphic lithologies

The Shaw L-group chondrite consists of three intermingled lithologies. One is light-colored and has a poikilitic texture, consisting of olivine (many skeletal and euhedral) and augite crystals surrounded by larger (up to 1 mm) orthopyroxene grains; plagioclase occurs between orthopyroxene crystals and rare, small (<5 μm) patches of Si-K-rich glass or cryptocrystalline material occurs within the plagioclase. The skeletal olivine crystals contain 0.08–0.16 wt% CaO. Petrofabric measurements show that the c-axes of the olivines are aligned. The light-colored lithology also contains numerous vugs and vesicles: SEM studies reveal euhedral, possibly vapor-deposited, crystals of olivine and pyroxene in the vugs. A second lithologic type is dark-colored, contains remnant chondrules. and has a microgranular texture. Poikilitic orthopyroxene crystals, where present, are smaller (0.1–0.2mm) than they are in the light-colored lithology. Microgranular olivine crystals contain <0.08 wt% CaO: most contain 0.03–0.05 wt% CaO. Vugs are rare and Si-K-rich material is absent. The third lithologic type is gray macroscopically and seems to be intermediate between the other two. It has a well-developed poikilitic texture, but contains neither skeletal olivines (euhedral olivines are rare) nor Si-K-rich material: remnant chondrules are present but less abundant than in the dark lithology. A modal analysis of a 5300 mm² slab shows, contrary to published opinions, that Shaw contains normal L-group chondrite abundances of metal and troilite. However, these phases are distributed irregularly throughout the meteorite. The light colored lithology is nearly devoid of metal and troilite and centimeter-sized metal-troilite globules occur between the three silicate lithologies. Wherever the metal occurs, it consists of nearly homogeneous martensite (13.9 wt% Ni) rimmed by kamacite (7.1 wt% Ni). These data indicate that Shaw is a partly-melted shock-breccia. The light-colored lithology must have been totally melted, as shown by the presence of aligned. CaO-rich, skeletal olivines; Si-K-rich residual material: and vugs and vesicles lined with euhedral crystals of mafic silicates. The dark areas appear to be unmelted target rock of L-group composition. Analysis of the growth of kamacite at the taenite (now martensite) borders indicates a cooling rate of ~ 3 C/10³ yr. or one thousand times faster than most ordinary chondntes. The Shaw impact event probably formed a crater several kilometers in diameter on its meteorite parent body.