10块新疆哈密沙漠陨石的类型及其岩石矿物学特征

沙漠是除南极地区之外的另一陨石样品的主要来源地。我国拥有面积辽阔的沙漠,为陨石的收集提供了有利的地理条件。2013年5月,在首次新疆库姆塔格沙漠陨石考察中,发现并收集到46块陨石样品,确定了3个陨石富集区:Hami、Alatage Mountain(AM)和Kumtag。对其中的10块代表性陨石样品进行了矿物岩石学研究,在此基础上确定了它们的化学群、岩石类型、冲击变质程度和风化程度。10块库姆塔格陨石均为普通球粒陨石,其中1块样品Kumtag014(Fa摩尔百分比为9.1~48.3,相对标准偏差PMD为51.0;Fs摩尔百分比为2.8~30.3,PMD为54.2)属于非平衡型L群。依据橄榄石Fa的PMD将其进一步划分为3.4亚型,属于相当原始的球粒陨石样品。另外9块经历了明显的热变质作用,为5型普通球粒陨石,包括7块L群(Fa22.6~25.9,Fs 17.6~21.4)和2块H群(Fa 18.2~20.8,Fs 16.0~17.8)。这些陨石的冲击变质程度:5块S3型,4块S2型,1块S1型;风化程度:3块W1型,6块W2型,1块W3型。     

南极格罗夫山6块非平衡型普通球粒陨石的矿物-岩石学特征

主要报导了南极格罗夫山新回收的6块非平衡型普通球粒陨石的岩石学和矿物化学特征.它们都保存了原始的矿物-岩石学特征,包括非常清晰的球粒结构、基质不透明、橄榄石和低钙辉石颗粒具有明显的成分环带、以及颗粒之间极不均匀的化学组成等.这6块陨石包括3块高铁群(H3)和3块低铁群(L3)群.根据非平衡型普通球粒陨石中橄榄石成分Fa变化的PMD(相对标准偏差)值与陨石热变质程度的相关性,进一步划分岩石类型亚类:CRV 020016为H3.7型、GRV 020162为H3.5型、GRV 020166为H3.4型、GRV 020106为L3.7型、GRV 020164为L3.7型、GRV 020165为L3.7型.全部6块陨石的冲击变质程度很低,为S1-S2;样品比较新鲜,风化作用划分为W1-W2.     

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。     

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...     

南极格罗夫山陨石GRV020022的岩石矿物学特征

GRV020022是第19次中国南极科学考察队在南极格罗夫山地区蓝冰上发现的一块陨石,为了解其矿物化学成分,对该陨石进行了扫描电镜观察和电子探针分析。结果显示,GRV020022陨石的主要组成矿物为橄榄石、低钙辉石、铁镍金属、陨硫铁、长石等,且矿物成分不均一,如橄榄石Fa _(8.9-25.6)(平均Fa_(18.3))和低钙辉石Fs _(15.9-21.6)(平均Fs _(17.9)),具球粒结构,基质有一定程度的重结晶,岩石类型属于4型。根据GRV020022陨石的橄榄石平均Fa值、低钙辉石平均Fs值、Fe-Ni合金含量及铁纹石中的Co含量(4.8 mg/g),将该陨石划归H群。另外陨石的冲击变质程度和风化等级分别为S3和W1。     

荷叶塘陨石:一个L3型普通球粒陨石的岩石学和地球化学特征

荷叶塘为一块我国降落的原始3型普通球粒陨石,因此具有重要研究意义。本文对荷叶塘陨石光薄片及粉末样品的岩石学、矿物学和全岩组成地球化学特征进行研究,为这块陨石的深入研究提供重要基础数据。研究表明荷叶塘陨石具L3型陨石岩石学特征,具典型的球粒陨石结构,球粒清晰,球粒结构类型多,基质重结晶程度低,组成模式为:球粒80vol%,金属和硫化物含量为5vol%,基质15vol%。矿物化学成分表明,该陨石球粒以Ⅰ型(贫铁型)球粒为主,橄榄石Fa0.41-34.1(PMD=51),低钙辉石Fs1.82-27.2(PMD=88),Wo0.18-3.13(PMD=103),铁纹石中Co含量平均0.62%(PMD=20),矿物成份不均一程度高,橄榄石矿物结晶颗粒内部化学成分变化大,呈正环带分布,与岩浆型结晶顺序一致,球粒与基质及间隙物成分明显不同,表现为不同物质来源。化学成分全岩分析结果显示,荷叶塘陨石亲石、亲铁元素含量均为L型陨石特征。依据以上岩石矿物学和化学组成特征,依照陨石亚分类参数,将其类型划分为L3.4型普通球粒陨石。冲击变质程度S2,风化程度W1。研究结果表明荷叶塘陨石为一块受后期水、热蚀变和风化影响较少的原始类型陨石。组成矿物成分极不均一,在矿物晶体内部,球粒内部及球粒与基质间均有明显变化。     

南极格罗夫山陨石GRV053645的岩石矿物学特征

南极格罗夫山陨石GRV053645是一块非常原始的普通球粒陨石,它很好地保存有太阳星云凝聚和增生的相关信息,对研究太阳星云的起源意义重大。本文描述了这块陨石的岩石矿物学特征,包括球粒结构类型、基质不透明、橄榄石和低钙辉石具有明显的成分环带等。根据Fe-Ni合金含量、球粒平均直径大小和橄榄石平均Fa值,表明这块陨石属于低铁群(L)。利用橄榄石Fa的PMD值与岩石类型亚类的相关性,其岩石类型应为3.4亚类。该陨石冲击变质程度和风化等级都比较低,分别为S2和W1。     

吉林陨石的热变质及冲击变质研究

吉林陨石的岩石学、矿物学、化学组成及球粒结构的研究表明,陨石主要由橄榄石、古铜辉石、铁纹石、陨硫铁、斜长石及少量的铬铁矿、钛铁矿、斜古铜辉石、白磷钙矿及玻璃等近40种矿物组成。橄榄石(Fa18.7％)及古铜辉石(En81.8 Fs15.3 Wo2.9)的成分比较稳定。球粒比较发育,约占20％,但大多球粒的轮廓模糊,有的球粒与基质难以分辨,球粒结构类型多样,组成球粒和基质的矿物成分基本一致。按W.R.范施穆斯(VanSchmus)的分类,吉林陨石应属于H_5球粒陨石。根据同位素地质年龄的测定结果,吉     

石陨石中橄榄石和辉石的微量化学分析

在一般石陨石中，球粒陨石硅酸盐相约占其化学组成的三分之二，而橄榄石(Mg、Fe)，SiO4和斜方辉石(Mg、Fe)SiO3又是组成硅酸盐相的主要矿物。较详细地研究橄榄石和辉石的化学组成对石陨石各化学群的划分及球粒陨石岩石类型的划分都是十分重要的。     

林东陨石的分类与成因探讨

林东陨石发现于我国内蒙古地区,被划分为LL5-6型普通球粒陨石角砾岩。本项工作对林东陨石开展了深入的岩石矿物学分析,提出将其重新划分为表土角砾岩的新观点。林东陨石主要由大至厘米级的角砾、以及微米大小的细粒基质两部分构成。不同角砾之间,表现出明显差异的岩石结构,反映了不同程度的热变质,岩石类型变化范围为4～6型。角砾以岩屑为主,还含有残余球粒和粗粒的矿物碎屑。不同岩石类型角砾的橄榄石Fa值（29.7 mol%～30.5 mol%）、低钙辉石Fs值（24.9 mol%～26.1 mol%）、以及铁纹石的Co含量（2.38%～2.51%）等,表明这些角砾均为低铁低金属的LL化学群,判断其来自同一小行星母体。林东陨石的细粒基质主要由微米大小的矿物碎屑固结而成,颗粒之间有较多的孔隙,整体较为松散。细粒基质的化学组成与岩石角砾中的矿物颗粒相同,应当是后者的机械粉碎产物。据此推测林东陨石的母体是一颗LL群小行星,表面经历了长期的小天体碰撞,形成各种岩屑和微细矿物晶屑,然后固结成林东陨石表土角砾岩。林东陨石的发现为研究小行星表面的演化历史,以及太阳风辐射等太空风化提供了珍贵样品,并为我国小行星探测提供可供对照的对象。     

Trace element composition of silicate minerals in the porphyritic and nonporphyritic chondrules of Elenovka (L5) and Knyahinya (L/Ll5) meteorites

The results of SIMS and EPMA studies on the silicate minerals and bulk compositions (SEM-EDS) of porphyritic and nonporphyritic chondrules from Elenovka and Knyahinya meteorites are reported. The trace element composition of silicate minerals (olivine, low-Са pyroxene) in equilibrated ordinary chondrites (EOC) has not been affected considerably by thermal metamorphism on the chondritic parent bodies. Therefore, equilibrated chondrites can be used for chondrule-forming processes studies. Low-Са pyroxene in nonporphyritic chondrules contains higher REE, Ba, Sr concentrations than that in porphyritic chondrules at similar trace element concentrations in the olivine of chondrules. The data obtained indicate that the formation of non-porphyritic chondrules was triggered by an increase in the cooling rate of chondrules upon the formation of pyroxene, rather than a difference in the initial conditions of chondrule formation. Higher refractory incompatible element (Nb, LREE) concentrations in the olivine of chondrules than those in the olivine of the matrix and contrasting trace element (Zr, Sr, Cr, REE) concentrations in the low-Са pyroxene of the chondrules and the matrix suggest that the matrix and chondrules of the meteorites formed in one reservoir under different physico-chemical conditions (density, redox state, rotation speed, homogeneity, temperature, shocks, electrical discharge, etc.).     

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。     

Relict olivine grains, chondrule recycling, and implications for the chemical, thermal, and mechanical processing of nebular materials

Chondrules and isolated forsterites in five low-subtype ordinary chondrites [NWA 3127 (LL3.1), Sahara 97210 (LL3.2), Wells (LL3.3), Chainpur (LL3.4), and Sahara 98175 (LL3.5)] were studied using petrographic, EMPA, and SIMS techniques to better constrain the origin of chondrules and the olivine grains within them. Our results imply that igneous crystallization, vapor fractionation, redox effects, and open-system behavior were important processes. All olivine grains, including normal, relict, and isolated forsterite grains, show evidence for igneous fractionation under disequilibrium conditions, with olivine crystallizing during rapid cooling (closer to 2000 °C/h than to 100 °C/h). Vapor fractionation is manifested by anti-correlated abundances between refractory elements (Al, Sc, Y, Ti, Ca, V) and volatile elements (Cr, Mn, P, Rb, Fe) in olivine. Redox effects are evidenced in various ways, and imply that Fe, Co, Ni, and P were partitioned more into metal, and V was partitioned more into olivine, under reducing conditions in the most FeO-poor melts. There is no obvious evidence for systematic variations in olivine composition according to meteorite subtype, but shock melting in Sahara 97210 resulted in the injection of glass-derived melt into olivine, resulting in artificially high abundances of Ba, Sr, Na, Ti, and some other incompatible elements in olivine. Terrestrial weathering in a hot desert environment may have mobilized Ba and Sr in some glasses.Our data suggest that chondrules in ordinary chondrites experienced repeated thermal, chemical, and mechanical processing during a “recycling” process over an extended time period, which involved multiple episodes of melting under fluctuating redox and heating conditions, and multiple episodes of chondrule break-up in some cases. Forsterite grains, including normal grains in forsterite-bearing type I chondrules, the cores of isolated forsterites, and relict forsterite in type II chondrules, all crystallized from similar, refractory melts under reducing conditions; relict Mg-olivine and isolated forsterite grains were thus derived from type I chondrules. Olivine in type II chondrules, including normal grains and ferroan overgrowths on relict Mg-olivine, crystallized from more volatile-rich, oxidized, and relatively unfractionated melts. Relict dusty olivine grains in type I chondrules were derived from type II chondrules during incomplete melting episodes involving reduction and some vaporization, with clear (non-dusty) grains in dusty olivine-bearing chondrules crystallizing from the reduced and partly vaporized melts. Melt compositions parental to normal olivine grains in type I and II chondrules are systematically enriched in refractory elements compared to bulk chondrule compositions, implying that chondrules often experienced open-system exchange with more volatile-rich surroundings after some olivine had crystallized, possibly while the chondrules were still partly molten. Type II chondrules could have been derived from type I chondrules by the addition of relatively volatile-rich material, followed by re-melting and little evaporation under oxidizing conditions. In contrast, type I chondrules could have been derived from type II chondrules by re-melting involving more-or-less evaporation under reducing conditions. Chemical, oxygen isotope, and petrographic data are best accommodated by a model in which there were several (>2-3, sometimes ?4-5) melting episodes for most chondrules in ordinary chondrites.     

Chondrules in H3 chondrites: textures,compositions and origins

Major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated (type 3) H-group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types (barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene, fine-grained). Although analyses of individual chondrules scatter widely, the mean composition of each textural type (except barred pyroxene) is rather distinct, as verified by discriminant function analysis. Al₂O₃, TiO₂ and Na₃O are correlated in chondrules, but Al₂O₃ and CaO do not correlate. Compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition.The data suggest that composition played a conspicuous role in producing the observed textures of chondrules, though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important. If compound chondrules formed and joined when they were still molten or plastic, then the data suggest that chondrules of each textural type could have formed together in space or time. The correlation of Al₂O₃ and TiO₂ with Na₂O and not with CaO appears to rule out formation of chondrules by direct equilibrium condensation from the nebula. We conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space. A small percentage of chondrules might have formed by impacts in meteorite parent-body regoliths.     

Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material

Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1-10 μm) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found.Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (>1.5 μm) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts.     

The I-Xe record of alteration in the allende CV chondrite

Complex I-Xe and mineralogical studies have been performed on four heavily-altered Allende fine-grained spinel-rich Ca, Al-rich inclusions (CAIs) and four Allende dark inclusions (DIs) showing various degrees of iron-alkali metasomatic alteration. The CAIs are largely composed of Fe-rich spinel, Al-diopside, and secondary nepheline and sodalite. The DIs consist of chondrules and Allende-like matrix composed of lath-shaped fayalitic olivine, nepheline, sodalite, and Ca, Fe-rich pyroxene ± andradite ± FeNi-sulfide nodules. Chondrule phenocrysts are extensively or completely replaced by fayalitic olivine, nepheline, and sodalite; metal nodules are replaced by FeNi-sulfides, andradite and Ca, Fe-rich pyroxenes. The chondrules and matrices are crosscut by Ca, Fe-rich pyroxene ± FeNi-sulfide ± fayalitic olivine veins. DIs are surrounded by continuous Ca-rich rims composed of andradite, wollastonite, kirschsteinite, and Ca, Fe-rich pyroxenes, whereas the outer portions of the inclusions are depleted in Ca.Three CAIs yield well-defined I-Xe isochrons with ages 3.1 ± 0.2, 3.0 ± 0.2 and 3.7 ± 0.2 Ma younger than the Shallowater internal standard (4566 ± 2 Ma). Similar release profiles suggest the same iodine carrier (most probably sodalite) for all four CAIs. The Allende DIs yield I-Xe ages from 0.8 ± 0.3 to 1.9 ± 0.2 Ma older than Shallowater. Based on the petrographic observations, we infer that the DIs experienced at least two-stage alteration. During an early stage of the alteration, which took place in an asteroidal setting, but not in the current location of the DIs, chondrule silicates were replaced by secondary fayalitic olivine, nepheline, and sodalite. Calcium lost from the chondrules was redeposited as Ca, Fe-rich pyroxene veins and Ca, Fe-rich pyroxene ± andradite nodules in the matrix. The second stage of alteration resulted in mobilization of Ca from the DIs and its re-deposition as Ca-rich rims composed of Ca, Fe-rich pyroxenes, andradite, and wollastonite, around the DIs. We interpret I-Xe ages of the DIs as time of their alteration prior incorporation into Allende. The younger I-Xe ages of the fine-grained spinel-rich CAIs may reflect hydrothermal alteration of the Allende host, which could have occurred contemporaneously with the second stage of alteration of the Allende DIs. The lack of evidence for the disturbance of I-Xe system in the Allende DIs may suggest that fluid responsible for the alteration of the Allende CAIs was in equilibrium with the I- and Xe-bearing phases of the DIs.     

An Fe isotope study of ordinary chondrites

The Fe isotope composition of ordinary chondrites and their constituent chondrules, metal and sulphide grains have been systematically investigated. Bulk chondrites fall within a restricted isotopic range of <0.2‰ δ⁵⁶Fe, and chondrules define a larger range of >1‰ (−0.84‰ to 0.21‰ relative to the IRMM-14 Fe standard). Fe isotope compositions do not vary systematically with the very large differences in total Fe concentration, or oxidation state, of the H, L, and LL chondrite classes. Similarly, the Fe isotope compositions of chondrules do not appear to be determined by the H, L or LL classification of their host chondrite. This may support an origin of the three ordinary chondrite groups from variable accretion of identical Fe-bearing precursors.A close relationship between isotopic composition and redistribution of Fe during metamorphism on ordinary chondrite parent bodies was identified; the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types. The clear link between element redistribution and isotopic composition has implications for many other non-traditional isotope systems (e.g. Mg, Si, Ca, Cr). Isotopic compositions of chondrules may also be determined by their melting history; porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean. Very large chondrules preserve the greatest heterogeneity of Fe isotopes.The mean Fe isotope composition of bulk ordinary chondrites was found to be −0.06‰ (±0.12‰ 2 SD); this is isotopically lighter than the terrestrial mean composition and all other published non-chondritic meteorite suites e.g. lunar and Martian samples, eucrites, pallasites, and irons. Ordinary chondrites, though the most common meteorites found on Earth today, were not the sole building blocks of the terrestrial planets.     

车里雅宾斯克(Chelyabinsk)陨石的岩石矿物特征与冲击变质作用

2013年2月15日,俄罗斯车里雅宾斯克(Chelyabinsk)发生了伴随罕见的空中爆炸的大规模陨石雨事件。本文对3块代表不同冲击变质程度的车里雅宾斯克陨石碎块进行了研究。它们都具有部分熔壳,其中1块仅出现碎裂,1块含有冲击熔融细脉,1块基本由冲击熔融囊和冲击熔脉组成。冲击变质程度最低的样品,代表了该陨石母体小行星的原始岩石矿物学特征:即具有粗粒的岩石结构和均一的矿物化学组成,但仍保留一些残余球粒,表明受到了明显的热变质作用,其岩石类型可划分为5型。铁镁质硅酸盐高的Fe O含量(橄榄石Fa:27.9mol%~28.2mol%,辉石Fs值:23.3mol%~23.7mol%)、以及较低的Fe-Ni金属含量,表明其化学群属于低铁低金属的LL群。我们所分析的样品与前人报导的结果相似,未发现不同岩性的岩屑,表明车里雅宾斯克陨石的原始岩矿特征较为均一。3块陨石碎块中,随着冲击程度的增强,其冲击变质特征依次表现为硅酸盐矿物的破碎、熔长石化更为普遍、陨硫铁与铁镍合金共熔、硅酸盐熔脉的形成、铬铁矿与长石共熔、以及大量熔融囊的发育等。但是,在冲击熔融囊和熔脉中,以及相邻围岩中均未发现高压矿物相。熔脉中的橄榄石晶屑和相邻围岩的橄榄石颗粒表现为化学成分的不均一,在背散射电子图像中呈不同灰度的结构。这与其他强烈冲击变质陨石中橄榄石的林伍德石或瓦茨利石相变相似。该陨石中林伍德石或瓦茨利石的缺失很可能是由于强烈撞击后高温产生的退变质。这也表明车里雅宾斯克陨石的母体小行星可能遭受了非常强烈的撞击事件。     

Mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates

Petrographic, mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from ‘ hot’ deserts and Antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments. Terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of Fe,Ni metal, producing Fe-oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins. Troilite weathers readily in ‘ hot’ desert environments but undergoes very little alteration under Antarctic conditions. Most of the primary porosity of ordinary chondrites has been occluded by the time that ∼ 15 to 25% of the initial Fe⁰ and Fe²⁺ has been oxidised to Fe³⁺ in both environments. Results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products. Dilation of primary pores and brecciation, which has been observed in parts of some meteorites, will only occur if the meteorite is especially metal-rich, or has a low primary porosity. These weathering products are absent from recent falls but have formed in a fall after ∼ 100 yr of museum storage.Cl-bearing akaganéite and hibbingite are common weathering products in Antarctic finds but occur in abundance in only one ‘ hot’ desert meteorite, Daraj 014. The majority of Fe-rich weathering products in meteorites from both environments contain low, but variable concentrations of Si, Mg and Ca. In most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a ∼ 1.0-nm lattice fringe spacing; where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene. In the heavily-weathered Antarctic finds ALHA 78045 and 77002, Si is concentrated in cronstedtite, a Fe-rich phyllosilicate. An unidentified hydrous Si-Fe-Ni-Mg mineral or gel has also partially replaced taenite in ALHA 78045. In addition to Fe-rich weathering products, ‘ hot’ desert meteorites contain sulphates, Ca-carbonate and silica, whereas such minerals are largely absent from Antarctic finds. The abundance of silicate weathering products in Antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments. As preterrestrial cronstedtite is abundant in CM2 carbonaceous chondrites, the Antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites.