Chemical etching of amphiboles and pyroxenes

Chemical etching of defect structures in pyroxenes and amphiboles has been investigated. Many features, such as very thin (～1 micrometer) exsolution lamellae not observed by transmission optical microscopy of thin sections, have been observed in reflected light after chemical etching of cleaved or polished crystal surfaces. A new feature in the microstructure of pyroxenes, not previously reported in the literature, has been revealed by one of the etchants.     

The lamellar structure in moonstone and anorthoclase from Korea

Moonstone and anorthoclase from Korea were investigated using electron microscope. Lamellae of moonstone were in the range of 1,200 to 3,000 Å, and anorthoclase have fine lamellae ranging from 20 to 900 Å. The outer part of every specimen has exsolved more perfectly, and the lamellae are coarse. The inner part is in the process of exsolution, and the lamellae are extremely fine. The lamellae are parallel to (¯801), and the streations on diffraction spots are oriented normal to the lamellae.     

Coupled CaAl-NaSi diffusion in plagioclase feldspar: Experiments and applications to cooling rate speedometry

The rate of CaAl-NaSi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400° to 1000°C in calcic plagioclase (An_80?81) by homogenizing coherent exsolution lamellae. The dependence of the average interdiffusion coefficient on temperature is given by the expression: $\text{D}\text{?}\text{= 10.99 (}\text{cm}{}^2\text{/sec) exp}\text{(?123.4(}\text{kcal/mol}\text{)/RT), (T}$ in °K). This value is for diffusion perpendicular to the (03 1?) interface of the lamellae. CaAl-NaSi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400° to 1200°C and possibly 10 orders of magnitude slower at subsolidus temperatures.The large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions (e.g., the Skaergaard), which retain delicate Ca, Na compositional zoning profiles on the micron scale, but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks. CaAl-NaSi diffusion is slow, the closure temperature is high (within the solidus-liquidus interval), and Ca-Na zoning is preserved. Oxygen diffusion is faster, the closure temperature is lower (350°-400°C) and the feldspars exchange oxygen with the low-temperature hydrothermal fluids.The complex micron-scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase. Cooling histories typical of large mafic intrusions (e.g. the Stillwater) are slow, begin at high initial temperatures (1200°C) and result in homogenization of oscillatory zones on the scale of 10 microns. The oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature (1000°C) limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion.     

Microtextures of opaque inclusions: their use as indicators for hiatuses during garnet porphyroblast growth

Within an analysed garnet porphyroblast, opaque inclusions imaged with the backscatter facility of a scanning electron microscope show different microtextures depending on their position within the porphyroblast. Three different zones can been distinguished: Zone 1 contains a Ti‐rich magnetite that has decomposed to a symplectite of fine and narrowly spaced exsolution lamellae of ilmenite and magnetite. Zone 2 shows a Ti‐rich magnetite symplectite with thicker and more widely spaced exsolution lamellae of ilmenite and magnetite. Within zone 3, Ti‐rich magnetite symplectite has totally been replaced by recrystallized magnetite crystals bordered by a thin ilmenite rim. Similar microtextures within ulvöspinel‐rich magnetite have elsewhere shown to be the result of an increase in oxidation and rate of diffusion. During metamorphism of metapelites, such an increase can be reasonably envisaged because of dehydration reactions progressing during rising temperatures, and this has occurred during the overgrowth of the three different microtextures by the garnet porphyroblast. Because the microtextures are homogeneous within the three different zones, it is deduced that the oxidation reaction rate of the opaque inclusions was substantially lower than the garnet growth rate. As a consequence, hiatuses in the garnet growth history must have occurred between the evolution from one microtexture to the next. A comparison between the inclusion trail geometry and the microtextural zone boundaries shows a perfect coincidence between these and the sites where inclusion trails become strongly deflected and truncated. This correlation confirms that, in the studied case, sharp microstructural boundaries (as truncation zones or deflection zones) coincide with growth hiatuses. The study therefore highlights the potential use of opaque inclusions to confirm or reject the occurrence of growth hiatuses within garnet porphyroblasts, especially in cases where discontinuities in the inclusion trail patterns are otherwise arbitrarily associated with growth hiatuses.     

Luna 20 pyroxenes: exsolution and phase transformation as indicators of petrologic history

Pyroxenes from our sample of Luna 20 soil are predominantly orthopyroxene with subordinate pigeonite. The orthopyroxenes are chromium-rich bronzites and contain submicroscopic lamellae of augite in a twinned orientation exsolved on (100). These lamellae have a composition close to the diopside-hedenbergite join. Asymmetric diffuse streaks parallel to $\text{a}{}^1$ indicate stacking faults parallel to (100) and possibly very thin (10–20 Å) lamellae of clinobronzite parallel to (100). Pigeonite crystals are very complex crystallographically and chemically, with optically visible (001) augite exsolution lamellae and two sets of chromite exsolution lamellae. In addition, there are submicroscopic (100) augite lamellae and a second generation of clinohypersthene lamellae which appear to have exsolved from the (001) augite lamellae. The clinohypersthene host, which has a large number of stacking faults parallel to (100), has partially inverted to hypersthene of the same composition. The hypersthene occurs as very fine lamellae (less than 1000 Å) parallel to the (100) plane of the clinohypersthene. X_D^Fe-Mg values for five host-lamellae pairs in pigeonite K-4 indicate a significant amount of subsolidus readjustment. We tentatively conclude that many of the bronzite and pigeonite crystals were derived from rocks crystallized from a high level magma chamber in the lunar highland crust.     

Twinning and exsolution in an antiperthite

An antiperthite feldspar (composition of the main part An_27.2 Ab_69.2Or_3.6) has been studied by x-rays and transmission electron microscopy. Complex twinning and exsolution on very fine scale are described for the first time for this compositional range. Evidence is given for a distinct intermediate region between the plagioclase and the potash feldspar. The formation of the crystal probably involves partial replacement, at least two step exsolution, and transformation of monoclinic plagioclase to triclinic plagioclase.     

The Discovery of Phlogopite Exsolution Lamellae in Garnets of Eclogite Inclusions, Liaoning Province

In No. 50 kimberlite pipe of Fuxian County, Liaoning Province, an eclogite inclusion(nodule), which is extremely rare in kimberlites, was discovered and phlogopite exsolutionlamellae were found in garnets of the inclusion. Microscopic, TEM and energy spectral observa-tions and studies confirmed that these lamellae are phlogopite. They are colourless and acicularin section, generally 0.5-5μm in width and 10-100μm in length. Nevertheless, fine lamellae,0.05-0.1μm wide and 1-2μm long, are also well developed. Along [111] of the garnet, three setsof phlogopite lamellae show oriented arrangement approximately at angles of 60°-70°, indi-cating that these lamellae might be the product of exsolution from garnet as a result ofpressure-release when eclogite ascended from the relatively deep level to the relatively shallowlevel of the mantle. Tiny acicular exsolution minerals (or inclusions) are commonly found ingarnet and pyroxene in eclogite inclusions of kimberlites all over the world and it has been re-ported that the identified exsolution minerals include pyroxene and rutile. This is the first timethat phlogopite exsolution lamillae were found in eclogite inclusions in the world.     

Compositional relations of pyroxenes in a ferrogabbro from Beaver Bay intrusion,Minnesota

Four different types of pyroxene found in a Beaver Bay ferrogabbro were analysed by microprobe. The crystallization sequence of pyroxene is augite-ferroaugite with exsolution lamellae of Ca-poor clinopyroxene → ferropigeonite with exsolution lamellae of augite → ferrohypersthene without exsolution lamellae+augite and ferroaugite without exsolution lamellae. The core of augite-ferroaugite with exsolution lamellae is cumulus pyroxene, whereas others, including the margin of augite-ferroaugite with lamellae, are interpreted to have crystallized from the intercumulus liquid. The sequence of crystallization of minerals from intercumulus liquid is different from that of minerals which have accumulated successively to the bottom and which were related to the fractionation of the whole layered series. The difference may be attributed to the different oxidation state of crystallization.     

Crystallographic investigation of the huttenlocher exsolution at high temperature

Three plagioclases in the composition range An_66–70 from the different geological environments were selected for investigation of the Huttenlocher exsolution by Laue and Weissenberg methods with monochromatic X-ray radiation. In order to find the characteristics of the Huttenlocher exsolution at high temperatures heating experiments were carried out. The reversible phase transition between body centered plagioclase and high plagioclase in the anorthite-rich composition of the plagioclase and the irreversible transition from intermediate plagioclase to high plagioclase in the anorthite-poor composition were observed.     

海南文昌二辉橄榄岩中辉石出溶结构的结晶学取向分析

本文用电子背散射衍射技术(electron backscatter diffraction,EBSD)测试了海南文昌玄武岩中二辉橄榄岩包体中的辉石主晶与其出溶片晶的结晶学取向关系。结合电子探针成分测试,得出:单斜辉石(透辉石)主晶中出溶了两组不同方向的片晶,一组为斜方辉石(顽火辉石-易变辉石)片晶,另一组为单斜辉石(普通辉石)片晶。由于出溶片晶在EBSD测试切面上体现为以线状体,因此需要找到一种方法将线状体所代表的片晶的晶面符号推算出来。本文介绍了一种利用吴氏网进行坐标系旋转的"晶带相交法",该方法可以作图推算出溶片晶的晶面符号。根据"晶带相交法"得出,斜方辉石(顽火辉石-易变辉石)出溶片晶为(100),单斜辉石(普通辉石)出溶片晶为~(401)。根据前人的研究资料,出溶片晶~(401)可能指示最小出溶压力为9.5~12.5GPa。出溶片晶的结晶学取向涉及到主晶与出溶体的晶体结构匹配关系,并且与出溶温度-压力有关,因此出溶片晶的结晶学取向分析具有晶体化学理论意义和反映地质温压过程的实际意义。这种"晶带相交法"可以推广应用于其他矿物出溶结构的结晶学取向研究中。     

Lamellar exsolution systems in clinopyroxene

Exsolution systems in synthetic pyroxenes were studied by transmission electron microscopy. An iron free sample En₈₀Wo₂₀ was prepared by devitrifying glass at 1300°C. Samples with bulk composition En₅₀Fs₃₀Wo₂₀ and En₃₅Fs₃₈Wo₂₇ were given various but well-defined heat treatments. The exsolution systems observed cannot unambiguously be related to the heat treatment. Periodic lamellar exsolution was observed parallel to (001) and (100) with sharp satellite reflections in the diffraction diagram. In more complex exsolution systems coarse (100) lamellae were found together with fine lamellae parallel to (001) and (100). An unusual phenomenon occurs at a (100) twin boundary where both individuals display exsolution lamellae parallel to (001). Pigeonite lamellae in one twin meet augite lamellae of the other individual at the twin boundary and vice-versa. The precise matching is achieved by a change in width near the boundary. Smoothly curved phase boundaries are developed in the obtuse angle of crosshatched (100) and (001) pigeonite lamellae in augite, whereas the boundaries in the acute angle are straight with sharp edges. This is consistent with elastic energy constraints.     

攀枝花钒钛磁铁矿中纳-微米尖晶石的出溶序次及其成因机制

钛磁铁矿内部尖晶石出溶体的成分组成和形成机制对估算磁铁矿固溶体的成分以计算铁钛氧化物的氧逸度-温度具有重要意义.为了探究攀枝花钒钛磁铁矿中尖晶石的成因及形成机制,运用岩相观察和各种微区原位观测手段,系统研究了各类尖晶石的矿物学特征.尖晶石的粒度在纳微米之间,有3种类型:第1种为钛磁铁矿晶界处不规则的大颗粒尖晶石,Mg#为60~70;第2种为钛磁铁矿颗粒内部的粒状尖晶石,Mg#为71~77;第3种为沿钛磁铁矿（100）方向定向分布的尖晶石片晶,Mg#为75~77.3类尖晶石分别与磁铁矿主晶具有相同的取向关系:{111}_Mag//{111}_Spl,{110}_Mag//{110}_Spl和{100}_Mag//{100}_Spl.3类尖晶石均是磁铁矿主晶的出溶体,钛磁铁矿晶界处及其内部的粒状尖晶石的形成与某些晶体缺陷关系密切,是在降温过程中较早出溶的产物,尖晶石片晶在两者之后以旋节分解出溶形成.      

Exsolution and coarsening in iron-free clinopyroxene during isothermal annealing

Exsolution and coarsening in Fe-free clinopyroxene of composition En_45.1Di_54.9 was studied at temperatures of 1300, 1200, and 1100°C, and annealing times between 10 min and 4320 h. Based on the wavelength of the exsolution lamellae, the exsolution process can be divided into exsolution sensu stricto and coarsening. During exsolution sensu stricto, the average wavelength of the “001” and “100” lamellae remains constant in contrast to the subsequent coarsening process. A progressive development of the microstructure is observed before coarsening which includes formation of island-like regions of dark contrast without phase separation, occurrence of pigeonite and diopside domains predominantly elongated along (100), formation of “100” exsolution lamellae, dissolution of the “100” lamellae, and predominant occurrence of “001” exsolution lamellae. All observations are in accordance with the exsolution mechanism of spinodal decomposition. After termination of the exsolution process, coarsening of the exsolution lamellae is observed. The “001” lamellae coarsen according to the rate law

     

Coherent elastic energies of exsolution boundaries in peristerite and böggild intergrowths

Elastic energy calculations based upon the coherent model of Willaime and Brown [Acta Cryst. A30, 316–331 (1974)] have been carried out for some exsolution textures in peristerite and Böggild intergrowths. For peristerites it is demonstrated that substitution of K for Na moves the orientation of the exsolution lamellae from (08 \(\overline {\text{1}} \) ) to about ( \(\overline {\text{1}} \) , 21, \(\overline {\text{2}} \) ) in agreement with observations. An electron microscope study of exsolution textures in labradorite and andesine plagioclases has been carried out and information about small structural differences in these lamellar structures has been obtained from elastic energy calculations.     

Exsolution of Ca-clinopyroxene from orthopyroxene aided by deformation

Monoclinic calcium-poor shear-transformation lamellae and calcium-rich exsolution lamellae occur parallel to (100) in orthopyroxene. The formation of both structures from an orthopyroxene host involves a shear on (100) parallel to [001], with additional cation exchange in the exsolution case. The shear transformation involves a macroscopic simple shear angle of 13.3° (shear strain of 0.236) and produces a specific a-axis orientation with respect to the sense of shear; we have found that this orientation dominates in exsolution lamellae in kinked orthopyroxene, where the sense of shear is known. In undeformed orthopyroxene, there is generally no preferred sense of orientation of the monoclinic a axes. We advance a specific model for exsolution involving nucleation and growth by shear transformation combined with cation exchange, thus circumventing the classical nucleation barrier and permitting exsolution at lower solute supersaturations.     

The Stannern trend eucrites: Contamination of main group eucritic magmas by crustal partial melts

We report on the petrology of a new eucrite belonging to the Stannern trend and discuss the origin of this trend. The eucrite Northwest Africa 4523 (NWA 4523) is an equilibrated eucrite consisting of dark clasts embedded in a fine-grained crystallized matrix. Two types of clasts have been observed: medium-grained ophitic/subophitic clasts, and very fine-grained clasts. Despite textural differences, the clasts display the same mineralogy, in particular the same kind of pyroxenes with pigeonitic cores containing sparse exsolution lamellae, and augitic rims, zoned plagioclases and the occurrence of K-feldspar. The major and trace element abundances of a large medium-grained clast are very similar to Stannern or Bouvante.The Stannern trend eucrites are characterized by high incompatible trace element abundances. Their trace element patterns normalized to a representative Main Group eucrite, exhibit significant Eu, Sr and Be negative anomalies. In this paper, we show that contamination of Main Group eucritic magmas by melts derived by partial melting of the asteroid’s crust can successfully explain both the high incompatible trace elements concentrations and the distinctive Eu, Sr, Be anomalies shown by the Stannern trend eucrites. This model is in agreement with the view that Stannern and some Main Group-Nuevo Laredo trend eucrites have been contemporaneously erupted, and with the probable assumption that Stannern trend eucrites formed rather late in the history of the 4-Vesta’s crust.     

The microstructures of perthitic alkali feldspars revealed by hydrofluoric acid etching

Etching of alkali feldspar cleavage fragments with hydrofluoric acid vapor, followed by study of the surfaces by scanning electron microscopy (SEM), is a simple and rapid technique for characterizing the microtextures of crypto- and microperthites. This technique has a number of advantages over conventional transmission electron microscopy (TEM) including ease of sample preparation and the large areas of crystals which can be imaged. Alkali feldspars studied by the method can yield important information on the cooling history of igneous and metamorphic rocks, fluid-feldspar interactions and the morphology and microstructures of albite exsolution lamellae. Some of these applications are illustrated by examples of etched crypto- and micro-perthites from the Klokken layered syenite, south Greenland and the Shap granite, north-west England.     

Economic minerals of the Kovdor baddeleyite-apatite-magnetite deposit,Russia: mineralogy,spatial distribution and ore processing optimization

The comprehensive petrographical, petrochemical and mineralogical study of the Kovdor magnetite-apatite-baddeleyite deposit in the phoscorite–carbonatite complex (Murmansk Region, Russia) revealed a spatial distribution of grain size and chemical composition of three economically extractable minerals — magnetite, apatite, and baddeleyite, showing that zonal distribution of mineral properties mimics both concentric and vertical zonation of the carbonatite-phoscorite pipe.The marginal zone of the pipe consists of (apatite)-forsterite phoscorite carrying fine grains of Ti–Mn–Si–rich magnetite with ilmenite exsolution lamellae, fine grains of Fe–Mg-rich apatite and finest grains of baddeleyite, enriched in Mg, Fe, Si and Mn. The intermediate zone accommodates carbonate-free magnetite-rich phoscorites that carry medium to coarse grains of Mg–Al-rich magnetite with exsolution inclusions of spinel, medium-grained pure apatite and baddeleyite. The axial zone hosts carbonate-rich phoscorites and phoscorite-related carbonatites bearing medium-grained Ti–V–Ca-rich magnetite with exsolution inclusions of geikielite–ilmenite, fine grains of Ba–Sr–Ln-rich apatite and comparatively large grains of baddeleyite, enriched in Hf, Ta, Nb and Sc. The collected data enable us to predict such important mineralogical characteristics of the multicomponent ore as chemical composition and grain size of economic and associated minerals, presence of contaminating inclusions, etc. We have identified potential areas of maximum concentration of such by-products as scandium, niobium and hafnium in baddeleyite and REEs in apatite.     

中国大陆科学钻探主孔540-600m榴辉岩中赤铁矿-钛铁矿固溶体出溶结构的特征及对榴辉岩折返动力学过程的意义

在中国大陆科学钻探主孔540-600m榴辉岩中集中发育了一种具特殊出溶结构的不透明金属矿物——具高度精细共生结构的赤铁矿-钛铁矿固溶体。在这段榴辉岩中，这种固溶体少部分以细小圆粒状包裹体分布于单斜辉石和石榴石内部，大部分则呈海绵陨铁状充填于单斜辉石和石榴石空隙之间。出溶分多个不同的期次，相对粗粒的赤铁矿质钛铁矿和钛铁矿质赤铁矿出溶片晶中又包含有更细粒的出溶片晶，在次一级的赤铁矿质钛铁矿域出溶钛铁矿质赤铁矿，同时在钛铁矿质赤铁矿域又出溶赤铁矿质钛铁矿，出溶片的大小可以从毫米级一直变化到纳米级，以至于很难测到纯的端元组分。出溶片在单个颗粒中按照平行（001）的方向定向排列，但在整个样品中则没有规律。出溶片中钛铁矿的摩尔百分数从23．80变化到89．84，反映了从不具明显磁性的赤铁矿和钛铁矿端元组分向具有强磁性的出溶体演化的过程。这样的出溶过程需要一个缓慢降温的环境，说明在超高压地体快速折返的整个过程中可能存在一个近等温降压或滞留在高温状态下（近等温等压）的阶段。这种固溶体在矿物的磁性特征方面十分特别：（1）可以包含异常强且稳定的天然剩磁；（2）所保存的剩磁即使在强的退磁磁场中也不会消退；及（3）能引起区域上显著的磁异常。因此，这种固溶体可能是导致CCSD主孔540～600m段榴辉岩较高磁化率、区域高度航磁异常及高度V、Ti异常的主要因素。赤铁矿-钛铁矿固溶体出溶结构在540～600m岩性段集中分布的现象与这一岩性段在各方面均有异常表现的奇特现象相吻合，结合前人的研究，认为赤铁矿-钛铁矿固溶体的成因与导致这段岩性段各种异常的成因有着直接的联系。赤铁矿-钛铁矿固溶体出溶结构的出溶成分和所具磁性强烈依赖于榴辉岩温压条件的变化特征，对它的继续深入研究，将会为精确限定苏鲁超高压变质带变质岩石形成中的温压变化，解释CCSD主孔附近区域的磁性异常及钒钛矿的勘探等问题起到重要的指示作用。     

羌塘地区榴辉岩中矿物出溶体及其成因机制探讨

西藏羌塘地区榴辉岩石榴子石和磷灰石中均发育矿物出溶体,结合电子探针和扫描电镜分析及能谱分析,确定了石榴子石中的出溶体有金红石、石英和角闪石,指示其可能经历了高压/超高压变质作用。本研究在磷灰石中发现了金红石出溶,并认为其可能是在高压/超高压条件下元素相互置换的结果,置换方式为Ti~(4+)(Fe~(2+)、Si~(4+))=2Ca~(2+),这些出溶体的形成为榴辉岩曾经历高压/超高压变质作用提供了矿物学证据。