Simple Twinning Research Articles

Microstructural interpretation of the garnet-sillimanite-bearing granite gneiss of Ikare area, southwestern Nigeria

This study presents microstructures from the granite gneiss of the Basement complex of southwestern Nigeria. The granite gneiss of Ikare area comprises of the mineral assemblage; garnet, biotite, K-feldspar, quartz, plagioclase, sillimanite and opaque minerals. K-feldspar porphyroblasts have cuspate interstitial quartz grains, also quartz grains are included in K-feldspar. Most K-feldspar grains are highly strained and in some cases are fractured. Prismatic and fibrolite-type sillimanite are both present. Biotite is closely associated with fibrolitetype sillimanite. Sillimanite was observed to mimic the foliation produced by biotite. K-feldspar is the dominant mineral and could have formed by the dehydration reaction of biotite + plagioclase + quartz + sillimanite → garnet + K-feldspar + melt. Microscopic structures such as strained feldspar grains, micro-fractures in Kfeldspar, simple twinning in K-feldspar, myrmekite, mimicking textures, microperthitic intergrowth and cuspate volumes of quartz surrounding K-feldspar are indicative of melt-bearing and solid-state deformation of regional metamorphic rocks.

English

The Keban syenitic intrusive is composed of porphyric syenite, monzonitic-syenite and monzonite. These rocks were emplaced in a shallow level and display a phaneritic texture with euhedral K-feldspar megacrystals (Kfms) with simple twinning and growth structures. These Kfms contain mafic and felsic inclusions. The fact that the inclusions are usually placed along the longer axis of the K-feldspar crystals and their marginal zones indicates that the amount of melt of the magma was still high towards the end of the cooling. The petrographic properties indicate that the Kfms were formed as a result of multi–phase crystallization and continued to grow during alkali feldspar phenocrystals crystallization period. The contact of the megacrystals with the matrix, their simple twinning and their relationships with the inclusions, indicate that the megacrystals were formed in a subsolidus environment and in the liquid as individual (not in situ) crystals and in a dynamic magma. The major element content indicates that megacrystals are orthoclase. Major element whole-rock geochemistry data reveal a high-K, metaluminous character, with a low aluminum saturation index (ASI) value. These rocks are rich in Rb, Ba, Cs, Th and light rare earth element (LREE), but are poor in heavy rare earth element (HREE), MgO (0.52%wt) and Ni (<20 ppm).   Key words: Keban syenitic intrusive, K-feldspar megacrystal, mineral chemistry, syenite-monzonite porphyry, Eastern Turkey.

How late are K-feldspar megacrysts in granites?

Various petrologists have suggested that K-feldspar megacrysts grow in granites that are extensively crystallized, even at subsolidus conditions. However, experimental evidence indicates that, though K-feldspar nucleates relatively late in the crystallization history, abundant liquid is available for development of large crystals. A great deal of evidence, involving many different factors, favours a magmatic/phenocrystic origin for K-feldspar megacrysts in granites, namely simple twinning, oscillatory zoning, euhedral plagioclase inclusions, and concentric, crystallographically controlled arrangements of inclusions. In addition, abundant evidence has been presented of (1) mechanical accumulation of K-feldspar megacrysts in granites, (2) alignment of megacrysts and megacryst concentrations in magmatic flow foliations, (3) involvement of megacrysts in zones of magma mixing in granite plutons, and (4) occurrence of megacrysts in some volcanic rocks, implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation. Detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas. Irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased, the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains.

Igneous origin of K-feldspar megacrysts in deformed granite of the Papoose Flat Pluton, California, USA

Many points of evidence, especially igneous microstructures and structures resulting from solid-state deformation, indicate that K-feldspar megacrysts in deformed granites of the Papoose Flat pluton are residual phenocrysts, not porphyroblasts. Evidence of an igneous origin includes features such as crystal shapes, simple twinning, zonally arranged euhedral plagioclase inclusions, oscillatory compositional zoning, and local occurrence in microgranitoid enclaves. Evidence of solid-state deformation of the megacrysts (which is consistent with their existence prior to the mylonitic deformation) includes marginal recrystallization and neocrystallization, microcline twinning, marginal replacement by myrmekite, and recrystallized/neocrystallized “tails”. Evidence of porphyroblastic growth, such as overgrown inclusion trails, is absent. This appears to be the situation in most felsic augen gneisses and mylonites.

Transmission electron microscopy observations of ferroelectric domains in the tetragonal Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 ceramics modified by bismuth and zinc

Ferroelectric domains of the tetragonal (Pb0.985Bi0.01)(Ni1/4Zn1/12Nb2/3)0.2(ZrσTi1−σ)0.8O3 (σ=0.30 and 0.40) ceramics were investigated by transmission electron microscopy and electron diffraction. It was observed that the 90° macrodomains displayed the shape of simple lamella twinning, and that the δ-fringe contrast was frequently observed within these configurations of 90° domains. The 180° domain walls with zigzag structure between 90° domain walls were also observed. The formation sequences of the 90° and 180° domain structures were discussed in terms of energy minimization during the phase transition from cubic to tetragonal ferroelectric phase on cooling.

Ferroelectric domain structures in tetragonal bismuth- and zinc-modified Pb(Ni⅓Nb⅔)O3-PbZrO3-PbTiO3 ceramics observed by TEM

Ferroelectric domain structures in tetragonal (Pb0.985Bi0.01)(Ni¼Zn(1/12) Nb⅔)0.2(ZrσTi1-σ)0.8O3 ceramics (σ = 0.30 and 0.40) were investigated by transmission electron microscopy. It was observed that the 90° macrodomains displayed the shape of simple lamella twinning, and that the σ-fringe contrast was frequently observed within these configurations of 90° domains. 180° domains with zigzag structure between 90° domain boundaries were observed. The formation of 90° and 180° domain structures is discussed in terms of energy minimization during the transformation from the cubic phase to the tetragonal phase on cooling.

Mantled feldspar from the Noring granite, Peninsular Malaysia: petrography, chemistry and petrogenesis

Mantle feldspar found in the Noring granite, have been investigated in terms of their petrography and geochemistry. Petrographically the mantle feldspar consists of whitish plagioclase rim which mantle the pinkish K-feldspar core. The K-feldspar core consists of a single crystal or several crystals usually showing simple twinning. Plagioclase rim is generally composed of numerous euhedral to subhedral plagioclases with size usually less than 1 mm. In places the plagioclase rim consists of a single crystal about 2 mm thick. :irormation of the mantled feldspar can be explained in two stages, (1) magma mixing and (2) late stage replacement. The plagioclase rim probably precipitated from a different magma type to the Noring magma probably of andesitic composition. The K-feldspar that has crystallised in the Noring magma act as substrates for the growth of plagioclase, producing the mantled texture. The aggregates of individuals crystals that have floated together in the magma attached to the K-feldspar as in the synneusis model of Vance (1969). Once this nucleation takes place there is relatively the straight forward process of crystal growth yielding the mantled texture (Hibbard, 1981). Small irregular outline plagioclase inclusions occur in the K-feldspar core being most abundant near the contact with plagioclase rim suggesting that the K-feldspar core replacing and enclosing relicts of plagioclase rim at the very late stage.

Ferroelectric domain structures of polycrystalline PbTiO3 thin films prepared by MOCVD

Ferroelectric domain structures of polycrystalline PbTiO3 thin films were investigated by using transmission electron microscopy. The grain size of the prepared films ranged from 300 Å to 2000 Å. It was observed that 90° domains displayed the shape of simple lamella twinning, with neighboring grains connected across grain boundaries; even at very small grains such as 300 Å, domain structures were found. In some 90° domain boundaries, defects such as dislocations were found. These configurations of 90° domains are associated with stress relief due to the very large tetragonality of PbTiO3 thin films which was estimated to be about 1.057; this value was nearly constant independent of grain size. 180° domains showed 90°-domain-dependent structures with irregular morphologies. The structure of 180° domains was discussed in terms of energy minimization during phase transitions.

K-feldspar augen in felsic gneisses and mylonites—deformed phenocrysts or porphyroblasts?

Abstract A review of mesostructural and microstructural evidence favours the hypothesis that K-feldspar augen and megacrysts in felsic gneisses and mylonites are generally, if not always, residual phenocrysts, rather than porphyroblasts that grew either during or after the deformation. In most examples, the augen appear to represent variably deformed megacrysts in former granitoids, as they commonly have similar distributions and shapes of inclusions and many of them show simple twinning; megacrysts in granitoids show abundant evidence of a phenocryst origin. Some augen may represent phenocrysts that grew in migmatite melt leucosomes and later were deformed, and others may have been phenocrysts in felsic volcanic or pyroclastic rocks, although megacrysts are uncommon in these rock-types. A porphyroblastic origin is opposed by the typical zonal distribution of inclusions and the common evidence of plastic deformation and partial recrystallization of the augen. The use of K-feldspar megacrysts as indicators of K metasomatism or granitization is of doubtful validity.

X-ray crystallographic studies of the alanine-specific racemase from Bacillus stearothermophilus. Overproduction, crystallization, and preliminary characterization.

To facilitate large-scale purification and crystallographic study, we have subcloned the gene for the alanine racemase of Bacillus stearothermophilus from pICR401 (Inagaki, K., Tanizawa, K., Badet, B., Walsh, C. T., Tanaka, H., and Soda, K. (1986) Biochemistry 25, 3268-3274) and overproduced the enzyme in Escherichia coli W3110 lacIq using the tac promoter of PKK223-3. This system yields alanine racemase as 6% of the bacterial cytosolic protein. Purification by a modification of the procedure of Inagake et al. yielded 75 mg of homogeneous alanine racemase from 30 g of cells (wet weight). Large, well-formed crystals of alanine racemase have been grown from polyethylene glycol 8000 using vapor diffusion. These crystals have unit cell dimensions a = 85.3 A, b = 110.0 A, and c = 89.9 A. The crystals belong to space group P2(1), with beta fortuitously equal to 90 degrees within experimental error; however, they are frequently twinned by second order pseudomerohedry with twin fraction (the ratio of the volume of the smaller twin domain to the total volume of the crystal) ranging from about 0 to 0.5. Fortunately, for crystals with low twin fraction, computational methods have been developed for the analysis and correction of simple twinning (Fisher, R. G., and Sweet, R. M. (1980) Acta Crystallogr. A36, 755-760). The crystals contain two alpha 2 dimers of alanine racemase in the asymmetric unit. We have identified several potentially useful heavy atom derivatives in low resolution screening experiments and are proceeding with high resolution data collection.

A model for the structure of metallic glasses based on chemical twinning

A model for transition-metal-metalloid glasses has been constructed and refined by energy minimization with the use of a Lennard-Jones 6-12 potential. The model is based on the concept that domains of positionally correlated atoms exist in these glasses on a scale of 1-2 nm. Within each domain, positional order is governed by rules appropriate to a single type of structural operation (simple chemical twinning in this case) acting in a prescribed direction. (The structure of compositionally equivalent crystalline phases may be represented by similar rules acting over essentially infinite domains.) Positions of atoms in the interface between domains are defined by two (or more) operations appropriate to each of the bounding domains so that the structure has a high degree of homogeneity. Computed properties are compared with experimental structural data for a number of typical amorphous transition-metal-metalloid alloys. Agreement between experimental and calculated partial pair-correlation functions and structure factors is good, supporting the notion of extensive local and medium-range positional order in these alloys. Moreover, the discrepancies that exist between model data and experimental results suggest that the model is less ordered than an actual glass. This model provides some insight into the nature of local geometrical distortions of structural units, coupling between positional and compositional fluctuations and the relative stability of glassy and microcrystalline alloys.

Deformation twinning in calcite, dolomite, and other rhombohedral carbonates

Transmission electron microscope (tem) observations of single and multiple twins in calcite and dolomite are presented, and the results are analysed by means of selected area diffraction and trace analysis. Simple twinning in rhodochrosite and kutnahorite is also analysed. It is shown that the ordered carbonates, such as dolomite, have a common twinning plane {01\(\bar 1\)2} and this appears to be their only mode of deformation twinning. The carbonates with higher symmetry, such as calcite, have {01\(\bar 1\)8} as the primary twinning plane but calcite itself has other twinning mechanisms, of which the most important is illustrated. Crossing and stopping twins are also discussed. It is shown that twinning in calcite, which occurs predominantly at low temperatures, is characterized by the generation of large numbers of glide dislocations.

Determination of minerals in platinum concentrates from the Transvaal by X-ray methods

Concentrates from the platiniferous norites of the Bushveld, Transvaal, are not completely soluble in aqua regia. The insoluble portion consists of steel-grey fragments first analysed chemically by R. A. Cooper and considered by him to be a new platinum mineral represented by the formula Pt(As,S)2. The name cooperite was proposed for the new mineral by F. Wartenweiller, and after further work Cooper decided that the arsenic found in the early analysis was due to the presence of sperrylite, and he changed the formula to PtS2. H. Schneiderhöhn observed simple twinning and, less frequently, polysynthetie lamellae on polished sections of mineral grains from the same deposits, and he suggested that cooperite is probably orthorhombic and isomorphous with marcasite. The latest account of the new mineral has been published by H. R. Adam who gave several analyses of cooperite from the Rustenburg and Potgietersrust districts and concluded that the ‘mineral is PtS2with a small amount of excess metal (platinum, palladium, and nickel) present in solid solution’.