The composition,structure, and stability of guinier-preston zones in lunar and terrestrial orthopyroxene

Lunar and terrestrial orthopyroxenes (Mg,Fe,Ca)₂Si₂O₆ contain varying abundances of coherent, Ca-enriched Guinier-Preston (G.P.) zones. G.P. zones 5–6 unit cells thick have been found in one lunar sample whereas all other examples (lunar and terrestrial) are only one unit-cell-thick. Electron diffraction maxima from the larger lunar G.P. zones indicate that d ₁₀₀=18.52 Å whereas, d ₁₀₀=18.2 Å for the host. This increase in the a direction corresponds to an increase in calcium content in the G.P. zones over that of the host of ～25 mol% Ca₂Si₂O₆. Diffraction patterns of the hk0 net from an area containing G.P. zones show extra spots (h=2n+1) not observed in the host orthopyroxene (Pbca), that violate the a-glide of the host. The G.P. zones, therefore, have space group Pbc2₁ if it is assumed that the c-glide of pyroxene is retained and the space group of the G.P. zone is a subgroup of Pbca. The loss of the a-glide in the G.P. zones results in 4 distinct silica chains and 4 distinct cation sites M1A, M1B, M2A, M2B; by symmetry, equivalent M2A or M2B sites are clustered together in only one-half of the unit cell. As one-fourth of the divalent cations in the G.P. zones are calcium, ordering of Ca on M2A or M2B would produce a zone 9 Å thick extended parallel to (100) with the composition of Ca(Mg,Fe)Si₂O₆, but constrained by the host to the structure of orthopyroxene. This zone and the Ca-poor half-unit-cell then constitute an 18 Å thick G.P. zone. Heating experiments of varying duration indicate that the zones become unstable with respect to the host orthopyroxene at ～950°C for Wo_0.6 and ～1,050°C for Wo_2.5. The zones are interpreted in terms of the pyroxene subsolidus as a metastable phase having either a solvus relationship with orthopyroxene or originating as a distinct phase. The size, distribution, composition and structure of G.P. zones may be an important indicator of the low-temperature thermal history of orthopyroxene.     

The influence of multiple diffraction on the space group determination of orthopyroxene,spodumene, low omphacite and pigeonite

Forbidden reflections of some pyroxenes described as having a lower symmetry than Pbca or C2/c have been examined, mainly using the detailed ψ-scanning method of the four-circle automated diffractometer. All reflections violating the systematic absences of Pbca orthopyroxene, C2/c spodumene, P2/n omphacite and P2₁/c pigeonite were found to be due to the Umweganregung process of multiple diffraction; the Umweganregung peaks observed for the ψ azimuth were indexed in terms of a four-circle geometry. Thus, the space groups of orthopyroxene, α spodumene, low omphacite and low pigeonite were confirmed to be Pbca, C2/c, P2/n and P2₁/c, respectively.     

The origin and geological significance of lunar ridges

Lunar ridges are a kind of familiar linear structures developed on the lunar surface. The distribution pattern, formation mechanism and research significance of lunar ridges are discussed in this paper. Single lunar ridges are usually distributed in the form of broken lineation, and, as whole, lunar ridges are trapezoidal or annular in shape around the maria. As to the formation mechanism, only volcanism or tectonism was emphasized in the past, but the two processes are seldom taken into combined consideration. On the basis of detailed analyses, the authors thought that tectonism is a prerequisite for the formation of lunar ridges, while volcanism is the key factor controlling their particular shapes. Finally, the authors pointed out that it is very significant in the study of lunar ridges to link the course of lunar structure evolution with the stress state in the lunar crust.     

The Possible Topologic structure Types of Orthopyroxene with Space Group P21ca

The possible topologic structure types of orthopyroxene with space group P21ca comprise four kinds of tetrahedral chains and four kinds of octahedral sites.all of which are non-equivalent in symmetry,In these structure types,the skew of the octahedral layers has a sequence of --,There are sixteen possible combination forms for the rotation type of tetradral chain.Twelve of them violate Thompson ‘s sparity rule and the remainder constitutes two pairs.In each pair,the two polar forms show a relationship of anti-orientation for their polar a-axes.Thus,there are only two possible different topologic structure types for P21ca-orthopyroxene.The ratios of O-rotated and S-rotated tetrahedral chains for these two structure types are 3:1 and 1:3,respectively,In the view S-rotated tetrahedral chains for these two structure types are 3:1 and 1:3,respectively,In the view of crystallochemical principle,the most likely form is the one with a ratio of 3:1,and its constitutions of two stacks of I-beam,which are non-equivalent both in symmetry and in topology,are and the configurations of the two types of M2 sites are P.P and P.N,respectively,A complementary twinning on(100) would be formed between the anti-oriented structure pairs,and their twin boundary is exactly equivalent to the inversion boundary,Moreover,it is possible that the ordered structure would appear when the atom ratio of Mg:Fe is equal to 3:1 as well as to 1：1。     

The origin of ferroan and magnesian anorthosites in the lunar crust

<正>1 Introduction The widely accepted standard model for the lunar feldspathic crust is:the early Moon was wholly or mostly molten,forming Lunar Magma Ocean(LMO).Olivine and pyroxene crystallized first from that magma ocean and sank     

一块新发现的月球陨石NWA 12279的岩石矿物学、源区和冲击变质作用

NWA 12279为2016年发现的一块斜长岩质月球陨石,由86%的斜长岩和14%的冲击熔融角砾岩组成。斜长岩具嵌晶结构,矿物组成主要为斜长石(70. 6%)、橄榄石(11. 3%)、辉石(10. 0%)、镁铝尖晶石(7. 0%),含少量石英、铬铁矿和钛铁矿;冲击熔融角砾岩具角砾状结构,主要由岩屑(斜长岩、辉长-橄长-斜长岩、微斑熔融角砾岩、辉长岩)、晶屑(橄榄石、辉石、斜长石、尖晶石)、玻屑和基质组成。斜长岩和角砾岩的矿物组成基本一致,主要为斜长石(An_(92. 9～98. 4))、紫苏辉石(Fs_(15. 5～32. 2)Wo_(2. 98～4. 22))、易变辉石(Fs_(27. 9～53. 1)Wo_(7. 19～14. 7))、普通辉石(Fs_(8. 42～38. 9)Wo_(17. 0～44. 1))、橄榄石(Fo_(53. 7～89. 4))、尖晶石[(Mg_(4. 97)Fe_(0. 86))_(5. 83)(Al_(11. 4)Cr_(0. 61))_(12. 0)O_(24)]。通过对陨石的矿物组成、碎屑组成、矿物成分、全岩成分和冲击变质特征进行研究,并和已发现的月球斜长岩进行了对比,认为该陨石可能起源于一个新的富含尖晶石的辉长橄长斜长岩高地。该陨石的斜长岩和角砾岩具有不同的冲击特征,斜长岩区域发育橄榄石面状破裂、斜长石熔长石化及含未熔融的辉石和橄榄石晶屑的冲击熔脉;角砾岩区域发育玻璃质熔脉、冲击熔体及岩石角砾化,这些特征限制了斜长岩区和角砾岩区经历的冲击压力峰值分别约为45 GPa和78 GPa,温度峰值分别约为1 100℃和1890℃,冲击变质阶段为S_5～S_6。     

A lunar rock of deep crustal origin: sample 76535

Lunar sample 76535 is a coarse-grained troctolitic granulite exhibiting a texture indicative of long annealing times. It is composed of homogeneous crystals of plagioclase (58 per cent, An₉₆), olivine (37 per cent, Fo₈₈) and bronzite (4 per cent, En₈₆).Chromian spinel-bronzite-diopside (Wo₄₆En₅₀Fs₄) symplectic intergrowths commonly occur along olivine-plagioclase boundaries and as tiny inclusions within olivine grains. These symplectites apparently formed by a reaction of the type:

$\text{OI + An + Chromite → Opx + Cpx + Al-Mg-chromite}$

. The reaction is related to the experimentally determined reaction

$\text{OI + An = Opx + Cpx + Sp}$

of Kushiro and Yoder (1966). The enstatite content of the diopside coexisting with the bronzite indicates equilibration at about 1000°C. Thermodynamic calculations for 1000°C indicate that the symplectites formed at a minimum pressure of about 0.6 kb. Low alumina contents of the pyroxenes indicate equilibration near this minimum pressure.Clusters of the same assemblage found in the symplectic intergrowths, but containing accessory metal, troilite, Ca-phosphates, baddeleyite, plagioclase and/or K-feldspar occur sporadically throughout the rock. These apparent late stage products crystallized in the low temperature-high pressure region discussed above.Phase relations of co-existing metal phases indicate that the rock cooled at a few tens of degrees/my, corresponding to depths of 10–20 km below the lunar surface, in agreement with the above pressure estimate.We infer that 76535 represents an original cumulate deposited at a depth between about 10 and 30 km. The last liquid crystallized in the relatively high pressure-low temperature field opx + cpx + Al-Mg-chromite. Cooling was extremely slow and accompanied by extensive chemical and textural re-equilibration. Reaction to form the symplectites occurred during the late stages of re-equilibration.     

The solubility of alumina in orthopyroxene coexisting with garnet

The solubility of alumina in complex orthopyroxenes crystallised in equilibrium with garnet has been determined over the pressure-temperature range 8–30 Kb and 800–1250° C. The results are in good agreement with predictions made using the simple thermodynamic model of Wood and Banno (1973). The model has been refined using a combination of the new data on orthopyroxenes of about En₆₀Fs₄₀ (with variable calcium content) and previously published data on more magnesian systems. Given analyses of coexisting Orthopyroxene and garnet in natural assemblages it is possible to calculate a P-T curve for the rock which should, in most cases, be accurate to within 2 or 3 kbar.     

月球质量瘤成因及研究方法综述

月球重力探测一直是国际深空探测的重要目标之一。计算月球重力发现,月球表面存在重力异常区域——质量瘤。通过对质量瘤的特征、成因以及研究方法进行概述总结,认为质量瘤是月幔隆升、高密度物质聚集所致,后期的玄武岩充填可能会增加重力异常,但作用微弱。尽管可以利用月震波分析和月球内部三维密度分布反演新技术,但是基于重力数据和地形资料的研究方法更能高效地对月球的重力场及其特征进行详细研究,高精度的重力场模型可以揭示月球深部构造及层圈形态,进而探索早期月球起源、内部物质演化与运动过程等。该研究方法可为月球和其他类地行星的重力研究提供参考。     

The Luna 20 lithic fragments,and the composition and origin of the lunar highlands

Luna 20 soil 22003,1 (250–500 μ) is similar to Apollo 16 soil 61501,47 (250–500 μ) in terms of the percentage of different types of particles. However, among the lithic fragments, the Apollo 16 sample contains a greater percentage of fragments with more than 70 wt. % modal plagioclase and a significantly greater proportion of KREEP-rich particles. Modal analyses of non-mare lithic fragments in Luna 20 and Apollo 11, 14, 15 and 16 indicate that the KREEP-poor highland regions (the bulk of the lunar terrae), though relatively feldspathic, are compositionally inhomogeneous, ranging in plagioclase content from approximately 35 to 100 wt. %. The average plagioclase content lies in the range 45–70 wt.%. Luna 20 pyroxene analyses cluster in two groups, one more magnesian than the other. The groups persist when pyroxene analyses from KREEP-poor noritic, troctolitic and anorthositic lithic fragments from Apollo 11, 14, 15 and 16 and Luna 20 are included. Olivine compositions mimic these pyroxene groups.Within each pyroxene group Cr₂O₃ and TiO₂ decrease as $\text{Fe}\text{(Fe + Mg)}$ increases, suggesting a relationship by fractional crystallization. The two groups suggest that at least two magma compositions were involved. To account for these observations we envisage a Moon-wide magma system in which initial accretionary heterogeneities were imperfectly erased by diffusion and convection. During the cooling of this magma system fractional crystallization was effected by the flotation of plagioclase and sinking of pyroxene, olivine and perhaps ilmenite. The endproduct was an upper layer enriched in plagioclase and a lower layer enriched in mafic silicates. KREEP-rich rocks, which are predominantly noritic in major element composition, may be mechanical mixtures of KREEP-poor norite and material residual after fractional crystallization of the surface magma system.     

The spin-forbidden absorption spectrum of Fe2+ in orthopyroxene

In this article, we have established the energy matrices of the strong-field terms SΓ of the d ⁴ (d ⁶) electron configuration in a crystal-field with C _2v symmetry. When we used this procedure to calculate the spin-forbidden absorption bands of Fe²⁺ in the M(2) site in orthopyroxene, we substituted the single-electron crystal-field energy levels (determined by the experimental results of the spinallowed spectrum) into the energy matrices instead of the single-electron crystal-field matrix elements. Thus, by means of only one parameter B (C=4B), most of the spin-forbidden bands of Fe²⁺ have been determined. Furthermore, when a similar treatment was made of the M(1) site of O _h symmetry, the entire spin-forbidden spectrum of Fe²⁺ in orthopyroxenes could be semiquantitatively explained. This shows that the method is particularly useful for the calculation of spin-forbidden spectra of complexes with the d ⁴ (d ⁶) configuration in a low-symmetry site.     

On the space group of threadgoldite

Summary It is shown that the crystal structure of the autunite group mineral threadgoldite, Al(UO₂)₂(PO₄)₂(OH)·8H₂O, recently determined byPiret et al. (1979) in the acentric space groupCc, can be better described and refined in the centrosymmetric space groupC2/c. The bond distances thus obtained are more reasonable than the previous ones.

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Zur Raumgruppe des Minerals Threadgoldit

Zusammenfassung Es wird gezeigt, daß die Kristallstruktur des Uranglimmer-Minerals Threadgoldit, Al(UO₂)₂(PO₄)₂(OH)·8H₂O, die kürzlich vonPiret et al. (1979) unter Verwendung der nichtzentrosymmetrischen RaumgruppeCc bestimmt wurde, besser in der zentrosymmetrischen RaumgruppeC2/c zu beschreiben und verfeinern ist. Die so erhaltenen Bindungslängen sind kristallchemisch plausibler als die zuvor gefundenen Werte.

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With 1 Figure     

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths

Experiments dissolving orthopyroxene (En₉₃) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f _O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO₂ diffuses outward from the boundary layer whereas CaO and Al₂O₃ diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na₂O and K₂O is strongly affected by f _O2. Under reducing conditions Na₂O and K₂O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K₂O and to a lesser extent Na₂O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO₂ contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas. Received: 30 August 1996 / Accepted: 15 April 1998     

Reactions involving hydration of orthopyroxene in anorthosite-gabbro

Hydration of orthopyroxene (En₆₀) in contact with plagioclase (An₃₆) leads to oronas of the type orthopyroxene-cummingtonite-edenite + quartz-edenitic hornblende. The compositions of the minerals involved indicate that kyanite needles in the adjacent plagioclases form as part of the reaction: orthopyroxene (En₆₀) + plagioclase (An₃₆) + H₂O→ cummingtonite + edenite/edenitic hornblende + plagioclase (< An₃₆) + kyanite + quartz. The reaction proceedings consumption first of the orthopyroxene and later of the cummingtonite to produce a corona structure consisting of a core of intergrown edenite and quartz surrounded by edenitic hornblende. Corona evolution of this type is controlled by diffusion.     

Electron microscopical study of garnet exsolution in orthopyroxene

By means of electron-optical methods the structure and orientation of lamella-like garnet exsolutions in an orthopyroxene from the eclogite of Gilsberg, Saxony Granulite Complex, are investigated. It can be shown that besides the macroscopic and microscopic submicroscopic exsolution lamellae also exist; they have widths up to nearly 5 nm. The intergrowth conditions are such shat (100)_op is parallel to {100}_gr and [001]_op parallel to 〈110〉_gr. The lamellae are coherent, but sometimes extended dislocation arrangements are found in microscopic lamellae which are caused by a deformation. These investigations point to formation of the rock as a result of a solid state reaction under high pressure conditions with subsequent diffusive-dissociative exsolution of garnet from primary pyroxene.     

The crystal chemistry of lithium and Fe3+ in synthetic orthopyroxene

Lithian ferrian enstatite with Li₂O = 1.39 wt% and Fe₂O₃ 7.54 wt% was synthesised in the (MgO–Li₂O–FeO–SiO₂–H₂O) system at P = 0.3 GPa, T = 1,000°C, fO₂ = +2 Pbca, and a = 18.2113(7), b = 8.8172(3), c = 5.2050(2) Å, V = 835.79(9) Å³. The composition of the orthopyroxene was determined combining EMP, LA-ICP-MS and single-crystal XRD analysis, yielding the unit formula ^M2(Mg_0.59Fe _0.21 ²⁺ Li_0.20) ^M1(Mg_0.74Fe _0.20 ³⁺ Fe _0.06 ²⁺ ) Si₂O₆. Structure refinements done on crystals obtained from synthesis runs with variable Mg-content show that the orthopyroxene is virtually constant in composition and hence in structure, whereas coexisting clinopyroxenes occurring both as individual grains or thin rims around the orthopyroxene crystals have variable amounts of Li, Fe³⁺ and Mg contents. Structure refinement shows that Li is ordered at the M2 site and Fe³⁺ is ordered at the M1 site of the orthopyroxene, whereas Mg (and Fe²⁺) distributes over both octahedral sites. The main geometrical variations observed for Li-rich samples are actually due to the presence of Fe³⁺, which affects significantly the geometry of the M1 site; changes in the geometry of the M2 site due to the lower coordination of Li are likely to affect both the degree and the kinetics of the non-convergent Fe²⁺-Mg ordering process in octahedral sites.     

Ferric iron in orthopyroxene: a Mössbauer spectroscopic study

Ferric iron solid solution in synthetic orthopyroxene has been studied along the joins MgSiO₃-Al₂O₃ · Fe₂O₃ and MgSiO₃-Fe₂O₃. The partially reduced synthetic orthopyroxenes showed that major incorporation of ferric iron can only occur together with a concomitant incorporation of Al. Maximum solid solution of ferric iron along the join MgSiO₃-Fe₂O₃ was found to be only 0.63 wt% Fe₂O₃ at 1000°C and 2 kb total pressure. From the observed Mössbauer parameters octahedral ferric iron can be assigned to the MI position in orthopyroxene. Incorporation of Fe³⁺ and/or Al will increase the disorder of Fe²⁺ and Mg between the M1 and M2 sites, which is also observed in a ferric iron-containing aluminous orthopyroxene of metamorphic origin. In the assemblage orthopyroxene + sillimanite + quartz the ferric iron content of orthopyroxene is directly related to oxygen fugacity.     

Electron petrography of exsolution in an enstatite-rich orthopyroxene

Lattice defects along (100) planes in an orthopyroxene, En_92.8-Fs_7.0-Wo_0.2, have been observed in the transmission electron microscope. From an analysis of the electron diffraction contrast at these defects it is proposed that the defects are regions of iron and possibly calcium enrichment. The defects are completely coherent with the lattice of the matrix and appear to be metastable or transition zones. They do not have any direct relationship with the spinodal decomposition proposed for the exsolution textures in some of the lunar pyroxenes of Apollo 11 and 12. Possible cationic ordering within these zones is discussed and the effects of such ordering on magnetic properties are considered.     

月球地质年代学研究方法及月面历史划分

介绍了确定月球地层单元相对年龄和绝对年龄的方法。建立相对年龄的方法主要有4种：地层叠置法、撞击坑大小频率分布统计法、撞击坑形态法和月壤成熟度法;研究绝对年龄的方法有两种：样品的同位素测年法和月球成坑计年法。回顾了现用月球层序划分的形成及发展过程,在此基础上,提出了改善月球年代划分的建议,分别用冥月宙、古月宙和新月宙表示月球内动力地质作用为主的时期、内外动力地质作用共同作用的时期和外动力地质作用为主的时期。推荐以南极艾肯盆地的形成为界线,将前酒海纪划分为前艾肯纪和艾肯纪,分别表示月球内动力地质作用为主的演化期和内外动力地质作用并重的演化初期。这种改进后的“三宙六纪”的月球年代划分既可以形成逻辑上更符合月球动力学演化过程的月球年代划分,同时又有助于开展在月球背面的地质研究。