Albite Lamellae Research Articles

Low–pressure granulite–facies metamorphism in the Southern Chinese Altay orogenic belt, NW China: P–T estimates, U–Pb ages and tectonic implications

The Chinese Altay orogenic belt formed in the Paleozoic is an important part of the Central Asian Orogenic Belt (CAOB), accompanying with remarkable metamorphism. Its tectono–metamorphic evolution history is still a controversial topic. In this contribution, we investigate the petrography, metamorphic peak P–T conditions and zircon geochronology of the newly discovered garnet–absent metapelitic and felsic gneisses at Wuqiagou area in the southern Chinese Altay. They are characterized by an assemblage of orthopyroxene + biotite + plagioclase + quartz + Fe–Ti oxides ± cordierite ± K–feldspar, in which most K–feldspar grains have transformed into perthites with thin albite lamellae. The Wuqiagou garnet–absent gneisses experienced pervasive partial melting, as testified by microstructures such as mineral pseudomorphs after melt films or pockets. Phase diagram modelling constrains the peak P–T conditions to 3.5–5.5 kbar and 800–900 °C, with possible geothermal gradients of 45–75 °C/km, indicative of a prominent low–pressure granulite–facies metamorphic event in the southern Chinese Altay. SIMS zircon U–Pb dating results show a weighted mean age of 255.8 ± 1.8 Ma. This age is interpreted to represent the timing of this low–pressure granulite facies metamorphism, which is highly coeval with the timing of their hosted mafic granulite lenses (∼255 Ma) and nearby mantle–derived mafic intrusions (∼257 Ma). Based on available petrological, geochemical and chronological data, we propose that the Permian low–pressure granulite–facies metamorphic event in the southern Chinese Altay was likely associated with the intrusions of deep–derived mafic magma at a relatively shallow crustal level (12–18 km) in a post–orogenic extensional setting, with a possible link with the Tarim mantle plume activity.

Scapolite and analcime: Monitors of magmatic fluid metasomatism in a major shear zone

Scapolite and analcime are uncommon alteration minerals in syenite. Here we describe a syenite body in a fault zone that hosts these minerals and we elucidate the unusual chemical conditions that led to their formation. The Clarke Head syenite is part of a regional late Devonian–early Carboniferous back-arc A-type granitoid suite in the Cobequid Highlands of Nova Scotia, Canadian Appalachians. The syenite contains magmatic rutile and analcime. Magmatic K-feldspar has been largely replaced by scapolite of marialite composition, which shows a temporal transition from more Cl rich to more carbonate rich varieties. Scapolite veins cut both the syenite and associated gabbro and diorite. Analcime I occurs as a late stage interstitial magmatic mineral. Analcime II forms by alteration of albite lamellae in perthite replaced by scapolite, analcime III vugs that cross-cut scapolite and analcime IV occurs in late veins cross-cutting earlier analcime phases and scapolite. The regional A-type granite plutonism was Na- and halogen rich, with widespread late-magmatic albitization and F-related mobility of REE minerals. However, only at Clarke Head is there extreme Na- and Cl-rich metasomatism, indicated by scapolite and Cl-rich hastingsite (with >1 wt% Cl content). Cl-rich fluids were derived from regional halite evaporites at faulted basin margins. The main circulating fluids were likely of magmatic origin, based on the δ18O of both replacement and vein scapolite. As the NaCl supply was reduced, scapolite became more Ca-rich. Halogen-rich fluids also mobilized Ti and Zr, which were reprecipitated in hydrothermal rutile and zircon as the halogen activity diminished. Scapolite replacement of K-feldspar released large amounts of K+ into the circulating fluids, which converted ferromagnesian silicates in fault zones to secondary biotite.

Geochemistry and growth morphology of alkali feldspar crystals from an IAB iron meteorite – insight into possible hypotheses of their crystallization

Alkali feldspar crystals have been recognized in the troilite-graphite nodules of the Morasko IAB iron meteorite. Their chemical, microtextural and structural properties were studied using electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), transmission electron microscopy (TEM) and Raman spectroscopy. The feldspars occur as perthitic or antiperthitic intergrowths, whereas the albite lamellae are perfectly twinned. The structural properties reveal intergrown phases with fairly disordered patterns. The electron microprobe analyses demonstrate that the intergrown phases are mainly rich in sodium or potassium, resulting in compositions that are close to those of albite or orthoclase. The compositions, calculated on the basis of a segmented perthite-antiperthite image, showed that the Or-to-Ab proportions in the homogenized crystals were almost 0.3:0.7, thus indicating that the anorthoclase crystallized under high-temperature conditions. Two hypotheses of crystal formation could account for these characteristics: crystallization from a melt or from a metasomatic solution. Relics with evidence of metasomatic replacement of former minerals were not found. Accordingly, this work focuses on arguments that support the other hypothesis. Large ion lithophile elements (LILEs, e.g., Ba, Sr, Rb, LREE, Pb, and Ga) were used to track the origin of the crystals. Their concentrations indicate crystallization from a parent melt strongly depleted in LILEs. Alkali feldspar is commonly a product of a highly differentiated melt. However, highly differentiated melts are typically enriched in LILEs, which here is not the case. The melt that crystallized the feldspar cannot be related to impact-induced partial melting of the chondritic material alone. The derived melt probably was contaminated by silica-rich target material during interaction between the IAB projectile and the target material and was accompanied by metal and sulphide melts that were both immiscible with a silicate melt.

Authigenic albite formation due to water–rock interactions — Case study: Magnus oilfield (UK, Northern North Sea)

It is the aim of this contribution to test whether organic–inorganic interactions could induce the formation of authigenic albite. This concept and related results are being compared with modelling scenarios which are purely based on inorganic geochemical reactions. In order to unravel the pathway of authigenic albite formation, this paper presents results of a multidisciplinary study from imaging, geochemistry, mineralogy, and hydrogeochemical modelling. The Jurassic reservoir sandstones of the Magnus oilfield (UK, North Sea) were chosen as a test site.Albite occurs with 4–18wt.% in the Magnus sandstones and its contents vary with depth. However, albite contents increase with increasing K-feldspar contents and decreasing grain size. It occurs in three forms: (1) as lamellae in perthite, (2) as overgrowth on/in corroded feldspar, and, (3) as cloudy replacing albite patches in K-feldspar. The albite overgrowth has the highest chemical purity (100% albite) whilst albite lamellae and replacing albite patches are slightly less pure (containing 1–4% anorthite). Albite appears non-altered, and has a euhedral morphology and dull cathodoluminescence. It commonly co-occurs with corroded K-feldspar grains.The precipitation of diagenetic albite in the Magnus sandstones is attributed to deep burial 80Ma ago and may have continued until today at temperatures between 90–120°C. The results of hydrogeochemical modelling offer two possible pathways for the authigenic albite formation: (1) Dissolution of unstable minerals (such as kaolinite and chalcedony) coupled to reduction of ferric iron minerals by products generated during oil generation, migration and degradation; (2) Dissolution of non-end member feldspar, such as K-feldspar with 10% albite, coupled to illite formation can account for trace amounts of albite due to an elevated Na+/K+ activity ratio in the pore water.

Eight-phase alkali feldspars: low-temperature cryptoperthite, peristerite and multiple replacement reactions in the Klokken intrusion

Eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the Klokken intrusion. The processes leading to the formation of the first four phases have been described previously. The feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition, between 750 and 600 °C, depending on bulk composition, to give fully coherent, strain-controlled braid cryptoperthites with sub-μm periodicities. Below ~500 °C, in the microcline field, these underwent a process of partial mutual replacement in a deuteric fluid, producing coarse (up to mm scale), turbid, incoherent patch perthites. We here describe exsolution and replacement processes that occurred after patch perthite formation. Both Or- and Ab-rich patches underwent a new phase of coherent exsolution by volume diffusion. Or-rich patches began to exsolve albite lamellae by coherent nucleation in the range 460–340 °C, depending on patch composition, leading to film perthite with ≤1 μm periodicities. Below ~300 °C, misfit dislocation loops formed, which were subsequently enlarged to nanotunnels. Ab-rich patches (bulk composition ~Ab91Or1An8), in one sample, exsolved giving peristerite, with one strong modulation with a periodicity of ~17 nm and a pervasive tweed microtexture. The Ab-rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at ~450 °C. This is the first time peristerite has been imaged using TEM within any perthite, and the first time peristerite has been found in a relatively rapidly cooled geological environment. The lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion. All the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity. We term this material ultra porous late feldspar (UPLF). It is composed predominantly of regions of microporous very Or-rich feldspar (mean Ab2.5Or97.4An0.1) associated with very pure porous albite (Ab97.0Or1.6An1.4) implying replacement below 170–90 °C, depending on degree of order. In TEM, UPLF has complex, irregular diffraction contrast similar to that previously associated with low-temperature albitization and diagenetic overgrowths. Replacement by UPLF seems to have been piecemeal in character. Ghost-like textural pseudomorphs of both braid and film parents occur. Formation of patch perthite, film perthite and peristerite occurred 104–105 year after emplacement, but there are no microtextural constraints on the age of UPLF formation.

Time–temperature evolution of microtextures and contained fluids in a plutonic alkali feldspar during heating

Microtextural changes brought about by heating alkali feldspar crystals from the Shap granite, northern England, at atmospheric pressure, have been studied using transmission and scanning electron microscopy. A typical unheated phenocryst from Shap is composed of about 70 vol% of tweed orthoclase with strain-controlled coherent or semicoherent micro- and crypto-perthitic albite lamellae, with maximum lamellar thicknesses 700°C, and after >48 h at 700°C, all such regions were essentially compositionally homogeneous, as indicated by X-ray analyses at fine scale in the transmission electron microscope. Changes in lamellar thickness with time at different T point to an activation energy of ~350 kJmol−1. A lamella which homogenised after 6,800 h at 600°C, therefore, would have required only 0.6 s to do so in the melting interval at 1,100°C. Subgrains in patch perthite homogenised more slowly than coherent lamellae and chemical gradients in patches persisted for >5,000 h at 700°C. Homogenisation T is in agreement with experimentally determined solvi for coherent ordered intergrowths, when a 50–100°C increase in T for An1 is applied. Homogenisation of lamellae appears to proceed in an unexpected manner: two smooth interfaces, microstructurally sharp, advance from the original interfaces toward the mid-line of each twinned, semicoherent lamella. In places, the homogenisation interfaces have shapes reflecting the local arrangements of nanotunnels or pull-aparts. Analyses confirm that the change in alkali composition is also relatively sharp at these interfaces. Si–Al disordering is far slower than alkali homogenisation so that tweed texture in orthoclase, tartan twinning in irregular microcline, and Albite twins in albite lamellae and patches persisted in all our experiments, including 5,478 h at 700°C, 148 h at 1,000°C and 5 h at 1,100°C, even though the ensemble in each case was chemically homogeneous. Nanotunnels and pull-aparts were modified after only 50 min at 500°C following the simulated 40Ar/39Ar step-heating schedule. New features called ‘slots’ developed away from albite lamellae, often with planar traces linking slots to the closest lamella. Slot arrays were often aligned along ghost-like regions of diffraction contrast which may mark the original edges of lamellae. We suggest that the slot arrays result from healing of pull-aparts containing fluid. At 700°C and above, the dominant defects were subspherical ‘bubbles’, which evolved from slots or from regions of deuteric coarsening. The small degree of partial melting observed after 5 h at 1,100°C was often in the vicinity of bubbles. Larger micropores, which formed at subgrain boundaries in patch perthite during deuteric coarsening, retain their shape up to the melting point, as do the subgrain boundaries themselves. It is clear that modification of defects providing potential fast pathways for diffusion in granitic alkali feldspars begins below 500°C and that defect character progressively changes up to, and beyond, the onset of melting.

P-T-X-controlled element transport through granulite-facies ternary feldspar from Lofoten, Norway

Fluid transport on the grain-scale controls many rock properties and governs chemical exchange. Charnockites from Lofoten indicate fluid penetration into ternary alkali feldspars controlled by their microtextures. In a process of fluid infiltration at granulite-facies conditions (∼600°C and 8–11 kbar), tiny pyroxenes enclosed in alkali feldspar reacted to amphiboles, which are always spatially connected to perthitic albite. Investigation of these microtextures by TEM imaging of Focused Ion Beam (FIB) prepared foils revealed that pyroxenes in contact with albite lamellae show dissolution features. An amorphous Fe- and Cl-bearing material interpreted to be a residuum of the percolating fluid was found within albite lamellae. Textures and mineral compositions indicate that a Cl-rich aqueous fluid attacked the lamellae, which then provided pathways for further fluid flow. A correspondence was found between feldspar compositions, their microtexture and their degree of alteration as a result of their permeability for fluids at specific temperatures. Hence, in addition to pressure and temperature, small variations of feldspar composition can strongly influence the fluid permeability of feldspathic rocks under lower crustal conditions.

Nanotunnels and pull-aparts: Defects of exsolution lamellae in alkali feldspars

We have studied defects associated with flat, lens-shaped perthitic albite lamellae in alkali feldspars using SEM and TEM. In orthoclase phenocrysts from the Shap granite, Cumbria, NW England, bulk composition Or 70.20 Ab 29.05 An 0.85 , no dislocations were found even in optically “fresh” parts of grains. Instead, dissolution inferred to be localized on edge-dislocation loops has created tiny “nanotunnels” typically T effect of the ~1% An in the feldspars is taken into account for commencement of coherent exsolution. The orientation of the Pericline twins was fixed shortly after coherent exsolution began. Previous work has indicated that edge dislocations would start to form during cooling at ≤400 °C, so that nanotunnels form at still lower T . Dry heating experiments were carried out to establish the stability of the defects and the homogenization behavior of the exsolution lamellae. Na ↔ K exchange is rapid on heating above the coherent solvus and chemical homogenization of lamellae is complete after 24 h at 700 °C. In contrast, nanotunnels persist for >148 h at 1000 °C and >5748 h at 700 °C. Below the coherent solvus, exsolution lamellae thin on heating, leaving nanotunnels stranded in the orthoclase matrix. Microtextures related to Si-Al ordering patterns in the framework, such as Albite twins, are not eliminated, forming ghost-like lamellar strain patterns in chemically homogeneous feldspar. The presence of nanotunnels in optically “fresh” alkali feldspars shows that not only granites but also granulite-facies rocks have been pervasively affected by fluids at low T . Both nanotunnels and pull-aparts have important implications for feldspar reactivity in the upper crust, for 18 O exchange, and for transport of 40 Ar both in nature and in laboratory step-heating.

Dislocation formation and albitization in alkali feldspars from the Shap Granite

Other| June 01, 1997 Dislocation formation and albitization in alkali feldspars from the Shap Granite Martin R. Lee; Martin R. Lee University of Edinburgh, Department of Geology and Geophysics, Edinburgh, United Kingdom Search for other works by this author on: GSW Google Scholar Ian Parsons Ian Parsons Search for other works by this author on: GSW Google Scholar Author and Article Information Martin R. Lee University of Edinburgh, Department of Geology and Geophysics, Edinburgh, United Kingdom Ian Parsons Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X Copyright © 1981 by the Mineralogical Society of America American Mineralogist (1997) 82 (5-6): 557–570. https://doi.org/10.2138/am-1997-5-616 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Martin R. Lee, Ian Parsons; Dislocation formation and albitization in alkali feldspars from the Shap Granite. American Mineralogist 1997;; 82 (5-6): 557–570. doi: https://doi.org/10.2138/am-1997-5-616 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Orthoclase-rich alkali feldspars in the Lower Devonian Shap granite, northwest England, contain two generations of albite-rich feldspar. These have partially replaced earlier exsolution microtextures consisting of albite lamellae (coarse semicoherent albite films and fine coherent albite platelets) in tweed orthoclase. The earlier generation of replacive albite-rich feldspar (~Ab90An9OrI) occurs together with orthoclase-rich feldspar (~Ab10Or90) in veins that crosscut exsolution microtextures throughout grain interiors. This episode of recrystallization was mediated by magmatic fluids at ~410 °C (estimated from two-feldspar geothermometry) and was driven by stored elastic strain energy, which was relatively homogeneously distributed throughout the microtextures. The later generation of replacive albite-rich feldspar, which is restricted to grain margins and is compositionally pure (Ab>99), was produced by magmatic-hydrothermal fluids at ~370 °C. This generation of albite-rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semi-coherent albite films. Marked differences in controls of the localization of the two generations of replacive albite-rich feldspar by pre-existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid-feldspar interaction and associated albitization but before the second phase. This relation indicates that edge dislocations formed between 410 and 370 °C. These observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Differential stress control on the growth and orientation of flame perthite: A palaeostress-direction indicator

Albite flames result from the replacement of K-feldspar by albite through alkali exchange. In flame perthites from deformed granites within the Grenville Front Tectonic Zone, replacement is driven by retrograde breakdown of plagioclase and hydration of K-feldspar. Within the strain fabric of the host rock, albite flames preferentially form subparallel to the inferred maximum compression direction and also develop at high-stress points such as grain-to-grain contacts. Flame tips are generally parallel to the ‘normal’ perthite crystallographic direction i.e. the Murchison plane, which is the orientation of the plane of minimum crystallographic lattice misfit between the albite and K-feldspar. We propose that the morphology of an albite flame is controlled by the coherent propagation of its tip into the K-feldspar crystal lattice, and that the orientation of the flames with respect to the rock fabric is controlled by the differential stress imposed during metamorphism. This type of replacement requires dry conditions in which there would be inefficient dissolution-reprecipitation along completely incoherent interfaces (normal replacement), and increased strength of feldspar. Under these conditions coherency is maintained to avoid a very large kinetic barrier. The differences in lattice misfit along the Murchison plane between K-feldspar and albite requires either high elastic strain energy (if the fit is coherent), or lattice dislocation energy (if the fit is semi-coherent). Such energy might be expected to inhibit the replacement growth of albite lamellae in a host feldspar. However, compression parallel to the Murchison plane reduces the lattice misfit along the compression direction. This results (1) in an increase in the Helmholtz free energy of the K-feldspar, and (2) in a reduction in the Helmholtz energy of the albite lamellae or of the semi-coherent interface. Thus, albite replacement allows a decrease in net free energy without the destruction of the alumino-silicate framework and is therefore favoured under conditions of high differential stress. Since flame growth is controlled by the imposed differential stress and not by strain in the host rock, flame perthite has potential as a palaeostress-direction indicator.

Microtextural controls of weathering of perthitic alkali feldspars

The relationship between the microtexture and dissolution behaviour of fresh, HF acid-etched and naturally weathered alkali feldspar phenocrysts from the Lower Devonian Shap granite has been investigated by SEM and TEM. A novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface. Further electron microscope work suggests that Shap phenocrysts are representative of the alkali feldspar in the protolith of many soils. Fresh and unweathered Shap feldspars have a complex microtexture, comprising areas of pristine cryptoperthite and lamellar microperthite cross-cut by volumes of microporous altered feldspar or “patch perthites.” Cryptoperthites are made up of <75 nm wide albite exsolution lamellae (platelets) in tweed orthoclase, whereas lamellar microperthites contain >75 nm wide albite films. The platelets are coherent, but albite films have numerous edge dislocations along their interface with orthoclase; (001) and (010) cleavage surfaces intersect ∼2–3 edge dislocations/μm 2. In three dimensions, these edge dislocations form an orthogonal net in the “Murchison plane” of easy fracture, close to (601). Patch perthites are irregular, semicoherent to incoherent intergrowths of albite and irregular microcline subgrains, with ∼0.65-0.70 sub-μm to μm-sized pores/μm 2. Microporous patch perthites form by dissolution-reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime. Dissolution of Shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture, especially by dislocations and exsolution lamellae. The core and strain energy associated with dislocation outcrops on (001) and (010) cleavage surfaces promotes rapid dissolution at those sites and formation of nn-sized etch pits after <30 s of laboratory etching with HF acid vapour. With progressive HF etching, crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite. Naturally weathered feldspars were collected from the glacial erratic boulders, fine gravels surrounding exposed granite surfaces, and from peat soil overlying the granite. During natural weathering, etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate ≥15 μm below the cleavage surface, forming an interconnecting, ladder-like grid of submicrometer wide channels in the Murchison plane. Coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly, but faster than orthoclase. This is probably because the albite lamellae have significant elastic coherency strain, but this is much less, per unit volume of albite, than the core and strain energy associated with edge dislocations. Patch perthites etch in HF vapour and weather rapidly in nature to produce a honeycomb-like texture of interconnecting nanometer- to micrometer-sized pits, which nucleate at preexisting micropores or incoherent subgrain boundaries. The size and density of etch pits on microperthite surfaces, which is determined by rates of growth and coalescence, may be a useful progress variable for natural and experimental dissolution. All alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale. These microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains, the controls on absolute dissolution rates and why they commonly change over time, the nonlinear variation of dissolution rate with grain size, the ratio of alkali ions released into solution, and disparities between laboratory dissolution rates and those observed in the field.

Low-temperature coherent exsolution in alkali feldspars from high-grade metamorphic rocks of Sri Lanka

In the alkali feldspars of the amphibolite- and granulite-facies rocks of Sri Lanka, a late-stage, final exsolution event is observed which produced film lamellae and fine-scale spindles. These were investigated by optical, microprobe, single-crystal, transmission electron microscopy and atomic resolution microscopy techniques. The lamellae and spindles exsolved below the coherent solvus at temperatures as low as 300 to 350° C. Precession photographs and ARM micrographs show that the intergrowth is perfectly coherent. In sections ‖ (010) the rhombic section of the Pericline twins corresponds to analbite or high albite. The albite lamellae and spindles nucleated and grew at low temperatures in a metastable disordered structural state within a tweed-orthoclase matrix and became periodically twinned analbite or high albite, which subsequently developed only a slight increase in Al, Si order. The relationship between twin periodicity and lamellar width, predicted for coherent intergrowths by Willaime and Gandais (1972), is obeyed. In Or-rich grains, in which coherent exsolution is the only exsolution event, the film lamellae tend to be restricted to the rim, the fine-scale spindles to the centre of the grains. The films nucleated heterogeneously at grain boundaries and grew towards the grain centres. Fine-scale spindles probably nucleated homogeneously in the interior part of grains. Heterogeneous nucleation and coherent growth are not mutually exclusive.

Glide twinning and pseudotwinning in peristerite: twin morphology and propagation

Optically visible Albite glide “twins” in a peristerite (∼An9Or1.6), identified from their tapering shape and relationship to grain boundaries, were studied by transmission electron microscopy. Near the tips in sections ⊥a, the microstructure consists of small (∼400 nm long) lensshaped Albite twins centred exclusively on the oligoclase lamellae. The lenses extend partly outwards into the two adjacent low albite lamellae and induce strong inhomogeneous strain. Where the lenses are closer together, they form, depending on the sense of shear, nearly linear left or right-stepping en echelon arrays, with overlap of the strain fields. Slightly farther in from the tip, the twin domains coalesce to form continuous pinch-and-swell lamellae, being always thicker in the oligoclase. Because of Si,Al order, only elastic glide pseudotwins are possible in low albite. In oligoclase glide pseudotwins may be mechanically stable (metastable relative to Si,Al order) and may deviate only slightly from true twins. Pseudotwins develop first in the oligoclase, propagate dynamically by jumping across the intervening albite lamellae, extend lengthways and thicken sideways and finally coalesce. They are stabilized by diffusion-controlled inversion of Si,Al order giving rise to true twins described in a companion paper.

Dynamic recrystallization and associated exsolution in perthites: Evidence of deep crustal thrusting

Mylonitic gneisses from Parry Sound, Ontario, are intensely deformed and occur within a major deep crustal shear zone developed during thrusting of granulite facies rocks (800°C, 900–1100 MPa) northward onto an amphibolite facies terrane. We have examined, by optical and electron microscopy, microstructures in perthites from these mylonites that preserve the deformation‐recrystallization cycle which operated during high‐ductility deformation of the crystals. The exsolution and recrystallization microstructures are intimately associated. Exsolution in the perthites occurred in two stages. A calcic plagioclase (oligoclase) exsolved first and subsequently coarsened and deformed during the single‐crystal ductility phase of shear zone deformation. A progressive increase in size and boundary incoherence of oligoclase domains during deformation led to recrystallization of the host perthite crystals as discrete orthoclase and oligoclase grains. The principal mode of recrystallization was subgrain rotation, with a component of grain growth that followed the formation of mobile high‐angle boundaries. Misorientation of subgrains to form grains at the recrystallization front is abrupt and occurred over one or two subgrain diameters. The second exsolution stage postdated perthite deformation and saw formation of fine albite lamellae in both host and recrystallized orthoclase. Temperatures inferred from the exsolution textures for independently determined confining pressures indicate that this deformation occurred under at least granulite facies conditions. The perthites thus record deformation from crustal depths of the order of 34–42 km. Continental megathrusts, such as the Parry Sound shear zone, provide mechanisms for both the generation of large strains observed in otherwise nonductile minerals and the preservation of ensuing microstructures.

Optical and electron microscope investigation of temperature-dependent microstructures in anorthoclase

Optical and transmission electron microscopy has been used to study the cross-hatched microstructure (associated with the intersection of albite and pericline twin lamellae) that is characteristic of b*c* sections of anorthoclase. With the aid of a specimen-heating stage fitted to each microscope, the changes that occur in the microstructure on heating and subsequent cooling have been observed over the temperature range from ~20 to 950 °C. Observations of unheated crystals indicate that the cross-hatched microstructure consists of a chessboard pattern of triclinic areas (diagonal pairs of which are essentially albite-twin related) and of optically monoclinic areas that consist of domains of fine-scale albite-twin lamellae and domains of pericline-twin lamellae. The four-spot diffraction pattern characteristic of M twinning is observed only from volumes of specimen containing both types of domain. The complex twin interactions at the domain boundaries are described in detail. The changes in optical contrast of the twins when the crystals are heated are shown to be due to changes in the orientation of the refractive index ellipsoid; the disappearance of twins does not necessarily indicate a change to monoclinic symmetry. The observations also indicate that the relative stability of albite and pericline twins is a function of temperature, the pericline twin being more stable than the albite twin at high temperatures near the monoclinic–triclinic transformation and the opposite at room temperature. Evidence is also given that the twin microstructure that develops at the transformation is strongly influenced by external stress.

Polysynthetic twin width distributions in anorthoclase

The widths of just under 7000 Albite and Pericline lamellae were measured in a suite of 20 polysynthetically twinned anorthoclase megacrysts collected at two sites in New Mexico. The majority of the twins were formed during the inversion of the anorthoclase crystals from their high-temperature monoclinic modification to their present triclinic form. Textural evidence suggests that a few of the lamellae postdate the transformation and are mechanical in origin. Twin density is exceedingly inhomogeneous on all scales and the related black and white lamella sets are unequally developed within even a single crystal. This type of inhomogeneity is probably a common feature of all inversion twinning since it arises as a natural consequence of the complex and rapidly changing conditions within a crystal undergoing transformation.

Albite of Secondary Origin in Charny Sandstones, QueBec: A Re-evaluation

ABSTRACT The contested origin of pink, untwinned sodic plagioclase grains in the Cambrian Charny sandstones of the northern Appalachians near Quebec, first studied by Middleton and discussed by Lajoie. has been re-evaluated by detailed petrographic, X-ray and electron microprobe analyses. The majority of the grains exhibit a perthire-like lamellar structure; the coexisting feldspars are An8-12 (host) and An0 (lamellae). We contend that these represent antiperthitic feldspars derived from Grenville anorthositic massifs and transformed in situ, during diagenesis. The K-feldspar lamellae were efficiently albitized during earlier diagenesis, whereas the original andesine host composition failed, even at higher grade conditions, to reach the equilibrium composition and degree f Si-Al order of albite. This is a reflection of the profound changes required in the proportion of tetrahedrally coordinated cations in the plagioclase and the low solubilities of these constituents in the pore fluids. Twinned grey sodic plagioclase, also An8-12 but free of albite lamellae, may represent albitized detritus from the ubiquitous grey gneisses that surround the Grenville anorthositic complexes.

Braid perthite in nepheline syenite pegmatite, Langesundsfjorden, Oslo Region (Norway)

Three occurrences of braid perthite (lamellae parallel to (110) and ( 1 10 )) and micro braid perthite (lamellae parallel to ( 861 ) and ( 861 )) from Permian nepheline syenite pegmatites in monzonite (larvikite) of the Oslo Graben are described. All the crystals studied show a zoned arrangement with either alternating mmacro- and microperthitic bands parallel to (010) or with microperthitic cores surrounded by macroperthite and a marginal zone of plagioclase. Both types of perthite are mesoperthitic and composed of maximum microcline and low albite. Chemical analyses suggest that the alkali feldspar is stoichiometric, while precession exposures imply unusual cell parameters in the host K-feldspar phase. The macroperthite seems to have crystallized at the expense of the microperthite. The braided texture of exsolved albite lamellae may be due to some kind of strain causing cracking along the prism and pyramidal planes.

An electron-microscope study of peristerite plagioclases

SummaryPlagioclase feldspars in the peristerite range An2-An17 have been investigated by transmission electron-microscopy and electron diffraction. All except the more anorthite-rich specimens were found to be unmixed into albite and oligoclase lamellae, between a few hundred and several thousand Å thick and approximately parallel to . A discussion is given of the part played by these lamellae when optical schiller is exhibited; and the effect of heat treatment on the lamellar structure and optical schiller is described.

A note on albite twinning in plagioclase felspars

SummaryPrevious work by Donnay on the basis of the geometrical theory of twinning related the width of albite twin lamellae to the composition of the plagioclase. The curve derived by Donnay has been recalculated for both high- and low-temperature felspars from X-ray data; the two curves differ markedly in the region between 0 and 20 % An. Some experimental evidence in support of the deductions that can be made is given.