Formation mechanism of pigeonite lamellae in skaergaard augite

Fine textures of exsolution lamellae and interface boundaries between augite and pigeonite in augite crystals from Skaergaard ferrogabbro 4430 have been studied by high resolution electron microscopy and X-ray methods. Thick pigeonite lamellae have higher densities of (100) stacking faults than thin lamellae. The displacement vector of the faults has been determined as 5/6c from the measured density of faults and the relative rotation of the augite and pigeonite lattices. The augite and pigeonite lattices are apparently coherent, and no growth ledges were observed at the interfaces. The stacking faults are often combined with the antiphase boundary of pigeonite resulting in a total displacement vector of 1/2(a+b)+5/6c. The observation of thick and thin pigeonite lamellae indicated that the thickening of (001) pigeonite lamellae was controlled by coherency strains accumulated at the interfaces between augite and pigeonite.     

An analytical electron microscopic study of a pyroxene-amphibole intergrowth

Samples of a garnet granulite from the mafic border units of the Lake Chatuge, Georgia alpine peridotite body were found to contain lamellar intergrowths of a pargastic amphibole in augite having the typical appearance of an exsolution feature. Single crystal X-ray diffraction, optical, electron microprobe and conventional and analytical electron microscopic studies have provided data limiting the compositions and structures of the coexisting phases. Individual lamellae of both materials are from 0.5 to 2.0 m in width with the lamellar interface parallel to {0 1 0}. The formulae of the minerals, as determined by a combination of electron microprobe and analytical electron microscopy, are (Na_0.1Ca_1.0Mg_0.6Fe³⁺ _0.3)(Si_1.8Al_0.2)O₆ for the pyroxene and Na_0.7Ca_1.9(Mg_2.1Fe²⁺ _1.4Fe³⁺ _0.5Ti_0.1Cr_0.1Al_0.8)(Si_5.9Al_2.1) O₂₂(OH)₂ for the amphibole. Several other studies have described intergrowths similar to those observed in this work, in general favoring exsolution as the formation mechanism for the intergrowths. In the Lake Chatuge samples however, replacement of pyroxene by amphibole is in part indicated by continuous gradation of amphibole lamellae into amphiboles rimming the clinopyroxenes.Contribution No. 368 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan     

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.     

The Subsolidus History of the Artfj?llet Gabbro: a TEM Study of Olivine, Augite and Orthopyroxene

A transmission electron microscope study of the microstructuresof olivine, augite and ortho-pyroxene in the Artfj?llet gabbroshows that the gabbro has been affected by two phases of deformation,exsolution and alteration. During the first phase, deformationand annealing of olivine led to the formation of elongated subgrains,with (100) subgrain boundaries consisting of regularly spacededge dislocations with [100] Burgers vectors. Localized strongerdeformation resulted in the development of mosaic subgrainswith (100) and (001) boundaries. Exsolution produced blebs and(100) lamellae of orthopyroxene in augite. Two types of blebsoccur, referred to as symplectitic augite and blebby augite.Symplectitic augite formed through discontinuous precipitation.The blebs in blebby augite are considered to have nucleatedat subgrain boundaries or dislocations in the augite. Blebs,melon pips and (100) lamellae of augite formed in orthopyroxene.These blebs and melon pips are thought to be due to nucleationat dislocations or to thickening of (100) lamellae at dislocations.Brown hornblende formed as blebs in augite and to a lesser extentorthopyroxene. This first event took place while the gabbrowas at a temperature of c. 900-1000 ?C and is thought to berelated to D_2b, a regional flattening and thrusting event. Renewed exsolution in the pyroxenes occurred later, probablybefore the second subsolidus phase, -but its timing is poorlyconstrained. "100" lamellae of low-Ca clinopyroxene formed inaugite at an estimated temperature of c. 600 ?C. Thin (100)lamellae and Guinier-Preston zones were developed in orthopyroxene. During the second phase, deformation resulted mainly in cracks,along which hydrous fluids entered the gabbro, causing a variabledegree of metamorphism at a temperature of c. 500-550 ?C. Inolivine irregular dislocations of mixed edge-screw characterformed. Cr-spinel with an unknown phase, or magnetite with diopsideprecipitated in the olivine. Oxide minerals formed in the pyroxenesand brown hornblende: Cr-spinel in Cr-rich augite; rutile andilmenite in other augite, orthopyroxene and brown hornblende.The formation of these oxide minerals is thought to be due toexsolution of cations such as Ti and Cr, which these mineralsdo not tolerate in their structures at low temperatures, combinedwith oxidation in the case of magnetite in olivine. Alterationresulted in thin amphibole lamellae parallel to (010) in augite.This second subsolidus phase is correlated with the D₃ regionaldeformation phase and the concomitant retrograde metamorphism.     

Exsolution of Ca-amphibole from glaucophane and the miscibility gap between sodic and calcic amphiboles

Transmission and analytical electron microscopy (TEM/AEM) of glaucophane from glaucophane + Ca-amphibole-bearing blueschist and eclogite from two Vermont localities (Ecologite Brook and Tillotson Peak) and one California locality (Cazadero) has revealed the first evidence from exsolution for the miscibility gap between sodie and calcic amphiboles. The Tillotson Peak samples and the Cazadero samples contain coarsegrained glaucophane—actinolite pairs, while the Eclogite Brook samples contain coexisting glaucophane and actinolitic hornblende. Ca-rich glaucophanes contain abundant fine-scale lamellae of Ca-rich amphibole. These lamellae are usually oriented near (100) and . High-resolution TEM (HRTEM) images show them to be coherent. The exsolution lamellae are so narrow, beyond the resolution of AEM, that their true Ca contents are obscured by analytical contributions from the surrounding host. The AEM data suggest that the lamellae are either winchite or actinolite, depending on the true Ca concentration. In most cases, the exsolution lamellae have very curved interfaces and show variable orientation. This is attributed to the close similarity of unit-cell parameters for the two amphiboles. Three-dimensional optimal phase-boundary calculations using EPLAG (Fleet 1982) show that the observed 100 and orientations are consistent with the minimization of area strain between the two lattices along the interface. Some samples show evidence for incipient exsolution in the form of homogeneously distributed, fine-scale precipitates. These results suggest that Ca-rich glaucophanes from other glaucophane—actinolite assemblages may be exsolved at the TEM scale. The coexisting amphiboles from Eclogite Brook also have been studied using the electron microprobe (EMP). The compositional gap defined by the Eclogite Brook pairs is consistent with previously reported results, but shows a wider break along the glaucophane—actinolite pseudobinary join, suggesting very limited solid solution up to temper-atures of about 500–550°C. The glaucophanes are relatively poor in Ca, except for one anomalous grain containing Ca-amphibole lamellae. Ferric iron, estimated by normalization to fixed cation numbers, is strongly partitioned into the actinolitic hornblende and the glaucophanes are very poor in the riebeckite component.     

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。     

TEM study of the transformation of augite to sodic pyroxene in eclogitized ferrogabbro

Transmission and analytical electron microscopy has been used to study relicts of augite that occur in various stages of transformation to sodic pyroxene. The augite relicts are characterized by a hatching produced by two sets of former 001 exsolution lamellae that possess high dislocation densities and were altered completely to sodic pyroxene, even where the augite matrix is still fresh. With further alteration, the sodic pyroxene in these 001 lamellae recrystallized and grew into the augite matrix, resulting in irregular lamellae that consist of subgrains having low dislocation densities. Needles and thin (100) lamellae of sodic pyroxene developed on the 001 lamellae. Alteration of the augite matrix proceeded by growth of areas with defects (dislocations, stacking faults). All sodic pyroxenes in these microstructures have the same orientation as the precursor augite, indicating a topotactic reaction mechanism. The reactions occurred at roughly constant Si and mainly involved replacement of Ca and Mg by Na and Al. Dislocations may have played a prominent role in the transformation by acting as diffusion pathways and by migrating into untransformed augite, leaving sodic pyroxene in their wake. At the grain boundaries of the augite, discrete grains of sodic pyroxene formed without any fixed orientation relation with the augite, consistent with a non-topotactic reaction. The predominance of the topotactic reaction inside the augite over the non-topotactic grain-boundary reaction is attributed to the scarcity of fluids during eclogite metamorphism.     

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.     

Sigmoidal exsolution by internal shear stress in pyroxenes from chondritic meteorites

Pyroxenes of pigeonitic and augitic bulk compositions in H3–4 chondritic meteorites commonly exhibit sigmoidal precipitates, rather than the elsewhere common lamellar associations. Most often, submicrometric sigmoids with calcic clinopyroxene composition occur within clinoenstatite; more rarely, clinoenstatite sigmoids occur within calcic clinopyroxene. The sigmoids appear as 001 terminated lamellae, with terminations rotated in opposite directions towards the 100 orientation. Pre-exsolution pigeonite and augite formed at temperatures higher than 980 °C, whereas sigmoidal exsolution occurred between 990 and 830 °C. Local anomalous lattice parameters determined by electron diffraction suggest that lattice parameters are most strained where the exsolution texture is most poorly defined. Shear strain occurs during exsolution due to mismatching lattice parameters and variable angles. In response to shear stress, the lamellae relax and assume sigmoidal strained morphologies. Sigmoidal exsolution is strongly controlled by (100) orthoenstatite stacking faults that possibly trigger exsolution.     

Exsolution in augite from the Skaergaard Intrusion

The exsolution phenomena of augite from Ferrogabbro 4430 of the Skaergaard Intrusion were examined in detail by single crystal X-ray diffraction and heating experiments to study the stepwise exsolution process. In the augite crystals, five different phases were detected: pigeonite (001), pigeonite (100), orthopyroxene (a), orthopyroxene (p) and a small amount of clinoamphibole. The two different pigeonites nearly share the corresponding (001) and (100) planes with the host. Orthopyroxene (a) and orthopyroxene (p) have (100) in common with the host and with exsolved pigeonite (001), respectively. Clinoamphibole was observed in the form of rather weak reflections in many crystals. It has (010) in common with the host.A large number of augite crystals exhibited a pigeonite (001) phase with curved, rotated reflections and diffuse streaks along the a* direction in (h0l) precession photographs. It appears that these streaks are related to orthopyroxene (p). Orthopyroxene (p) seems to be crystallized from pigeonite (001) by nucleation at (100) stacking fault planes (inverted pigeonite). Pigeonite (100) may be formed at growth ledges between augite host and exsolved orthopyroxene (a) at a later stage of exsolution to stabilize the boundaries.From the X-ray diffraction profiles and the results of the heating experiments, a possible exsolution sequence is suggested. Clinoamphibole appears to be a product of alteration at the latest stage of the exsolution process. It seems to be related to particular conditions of partial water pressure.     

Electron Petrography of Exsolution Textures in an Iron-Rich Clinopyroxene

Ion-thinned samples of an iron-rich augite in an inninmoritepitchstone from Ardnamurchan, Scotland, have been examined bytransmission electron microscopy at 100 and 1000 kV. Eutecticcrystallization of augite and pigeonite occurred and subsequentlysemi-coherent irregularly-shaped regions and coherent (001)lamellae of pigeonite were exsolved in the augite. Nucleationoccurred heterogeneously at grain boundaries and subsequentgrowth of the lamellae proceeded by the migration of ledgesalong the (001) interface. The analytical electron microscope EMMA-4 has been used to analysethe exsolved pigeonite lamellae. Their composition was foundto be similar to that of the co-existing primary pigeonite. It is concluded that the augite cooled through the solvus slowlyenough for heterogeneous nucleation and growth to occur whereaspyroxenes which cool rapidly unmix by the mechanism of spinodaldecomposition.     

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.     

Geothermometry of exsolved augites from the Laramie Anorthosite Complex,Wyoming

Exsolved augite pyroxenes from the ferromonzonite border facies of the ferrosyenite in the Laramie Anorthosite Complex have been studied with the transmission electron microscope and the electron microprobe to determine their exsolution histories. The Lindsley and Andersen (1983) geothermometer gives initial crystallization temperatures of 1000° C for the bulk augite crystal (Wo₃₂ En₂₂ Fs₄₆). Exsolved lamellae are predominantly pigeonites with very low calcium contents (Wo_1–3 En_23–24 Fs_71–74) and have formation temperatures estimated to be in the range of 600 to 975° C. The uniform compositions of lamellae and hosts, despite the range in lamellar size and orientation, suggest that either 1) the ferromonzonite experienced an extended plateau in cooling or a reheating event at 600 to 650° C or 2) the pyroxenes recorded a blocking temperature. Two-feldspar geothermometry on exsolved feldspars also records 600° C and suggests that these low temperatures are not blocking temperatures.     

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。     

Clinopyroxenes from mantle-related xenocrysts in alkaline basalts from Hannuoba (China): augite–pigeonite exsolutions and their thermal significance

Optically homogeneous augite xenocrysts, closely associated with spinel–peridotite nodules, occur in alkali basalts from Hannuoba (Hebei province, China). They were studied by electron and X-ray diffraction to define the occurrence and significance of pigeonite exsolution microtextures. Sub-calcic augite (Wo₃₄) exsolved into En_62–62Fs_25–21Wo_13–17 pigeonite and En_46–45Fs_14–14Wo_40–42 augite, as revealed by TEM through diffuse coarser (001) lamellae (100–300 Å) and only incipient (100) thinner ones (<70 Å). C2/c augite and P2₁/c pigeonite lattices, measured by CCD-XRD, relate through a(Aug)?a(Pgt), b(Aug)?b(Pgt), c(Aug)≠c(Pgt) [5.278(1) vs 5.189(1)Å] and β(Aug)≠β(Pgt) [106.55(1) vs 108.55(2)°]. Cell and site volumes strongly support the hypothesis that the augite xenocrysts crystallised at mantle depth from alkaline melts. After the augite xenocrysts entered the magma, (001) lamellae first formed by spinodal decomposition at a T_min of about 1,100 °C, and coarsened during very rapid transport to the surface; in a later phase, possibly on cooling, incipient (100) lamellae then formed.     

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.     

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

     

Subcalcic diopsides from kimberlites: Chemistry,exsolution microstructures,and thermal history

Twenty-six subcalcic diopside megacrysts (Ca/(Ca+ Mg)) = 0.280–0.349, containing approximately 10 mol% jadeite, from 15 kimberlite bodies in South Africa, Botswana, Tanzania, and Lesotho, have been characterized by electron microprobe analysis, X-ray-precession photography, and transmission electron microscopy. Significant exsolution of pigeonite was observed only in those samples for which Ca/(Ca+Mg)0.320. The exsolution microstructure consists of coherent (001) lamellae with wavelengths ranging from 20 to 31 nm and compositional differences between the hosts and lamellae ranging from 10 to 30 mol% wollastonite. These observations suggest that the exsolution reaction mechanism was spinodal decomposition and that the megacrysts have been quenched at various stages of completion of the decomposition process.Annealing experiments in evacuated SiO₂ glass tubes at 1,150° C for 128 hours failed to homogenize microstructure, whereas, at 5 kbar and 1,150° C for only 7.25 hours, the two lattices were homogenized. This pressure effect suggests that spinodal decomposition in the kimberlitic subcalcic diopside megacrysts can only occur at depths less than 15 km; the cause of the effect may be the jadeite component in the pyroxene. Apparent quench temperatures for the exsolution process in the megacrysts range from 1,250° C to 990° C, suggesting that decomposition must have commenced at temperatures of more than 1,000° C.These P–T limits lead to the conclusion that, in those kimberlites where spinodal decomposition has occurred in subcalcic diopside megacrysts, such decomposition occurred at shallow levels (<15 km) and, at the present erosion level, temperatures must have been greater than 1,000° C.     

Solidus and subsolidus compositional relationships of some coexisting Skaergaard pyroxenes

Electron-microprobe analyses of coexisting Ca-rich and Ca-poor pyroxenes from rocks of the Skaergaard intrusion indicate that their compositional relationships are controlled by two types of tie-line in the pyroxene quadrilateral. Solidus tie-lines join bulk compositions of pairs of pyroxenes that crystallized contemporaneously from a melt at equilibrium. Subsolidus tie-lines join the compositions of lamellae and host materials in pyroxene exsolution intergrowths. The solidus tie-line for a pair of pyroxenes in a specimen and their subsolidus tie-lines do not coincide and the subsolidus tie-line for inverted pigeonite is further from the hedenbergite-ferrosilite join of the quadrilateral than that for augite.The orientation of solidus tie-lines within the pyroxene quadrilateral indicates that during the simultaneous crystallization of two pyroxenes from the Skaergaard magma there was similar partitioning of Mg and Fe in the two phases relative to the melt. The relationship of the subsolidus tie-lines of a pair of coexisting pyroxenes to their solidus tie-line indicates that during the formation of exsolution intergrowths, changes in the composition of the pyroxene matrix involved primarily a change in its CaMg+Fe ratio while those of the lamellae involved both a change in their CaMg+Fe ratio and in their MgFe ratio. The MgFe ratio of the augite lamellae in inverted pigeonite progressively increased with cooling while that of the Ca-poor lamellae in augite progressively decreased with cooling.     

Symplectitic augite: evidence for discontinuous precipitation as an exsolution mechanism in Ca-rich clinopyroxene

Abstract Part of the augite in the Artfjället gabbro consists of symplectitic intergrowths between augite and blebs or lamellae of orthopyroxene. Mineral compositions are consistent with formation of these symplectites by exsolution of orthopyroxene from magmatic augite at a temperature of ca . 900–1000°C. The microstructures indicate that the exsolution mechanism is discontinuous precipitation, whereby the boundary of an augite grain sweeps through a neighbouring augite, leaving the symplectite in its wake. The formation of this symplectitic augite is catalysed by the presence of an intergranular water-rich fluid phase, which promotes grain boundary mobility.