Twin Composition Plane Research Articles

Diffusion-controlled and replacement microtextures in alkali feldspars from two pegmatites: Perth, Ontario and Keystone, South Dakota

AbstractMacro- and micro-perthitic microclines from pegmatites from Perth, Ontario (Wards catalogue 46 E 0510) and Keystone, South Dakota (Wards 46 E 5125) have been studied using light and electron microscopy. A sample of the type perthite from Perth, Ontario (Hunterian Museum, Glasgow, M2361)was compared using light microscopy. It differs in bulk composition and microtexture from the Wards sample. The Perth sample from Wards is a mesoperthite, with sub-periodic ∼mm-thick albite veins near (100), with irregular surfaces. The microcline has regular tartan twins and formed fromorthoclase by a continuous process. The Keystone sample is a microperthite, with non-periodic albite veins mainly in {110}. Irregular tartan twins, volumes of irregular microcline and subgrains suggest that the microcline formed by dissolution–reprecipitation. Microcline in both samplescontains semicoherent cryptoperthitic albite films that formed after the development of tartan twins. The bulk compositions of these intergrowths imply exsolution below ∼400°C. Diffusion parameters imply sustained heating for between 0.11 My at 400°C, 1.5 GPa and 8.4 My at 300°C,1 GPa. Unrealistic times are required at 200°C. Subsequently, the crystals reacted with a fluid leading to replacive growth of the vein perthites. Unusually, Albite twin composition planes in replacive subgrains have sub-periodic dislocations, formed by coalescence of advancing growthtwins. Processes that might lead to periodic, replacive intergrowths are discussed. The Perth and Keystone feldspars have been used for experimental work on dissolution during weathering and on anomalous thermoluminescence fading. Their microtextures make them unsuitable for obtaining propertiesthat can be extrapolated to feldspars in general.

Twin defects in thick stoichiometric lithium tantalate crystals prepared by a vapor transport equilibration method

The twins were observed and investigated in vapor transport equilibration (VTE) treated lithium tantalate crystals by burying congruent lithium tantalate crystals (CLT) in a Li-rich polycrystalline powder. Twins and their etched patterns were observed under an optical polarizing microscope, and the geometry of the twins was discussed. Twin composition planes were the 011¯2 planes. The cause of twinning was analyzed and verified by experiment. The results indicate that the emergence of twins is due to sintering stress, which arises from sintered Li-rich polycrystalline powders at high temperature. 3.2mm thick stoichiometric lithium tantalate (SLT) crystals without twins were obtained by setting corundum crucibles over the top of the crystals to make crystals free from the sintering stress. In addition, cracks were observed at the intersection of twin bands, and the stress caused by the dislocation pile-up was considered to be the reason for the formation of cracks.

Non-180° domains formation mechanism in LiTaO 3 grains of an Al 2O 3/LiTaO 3 composite

High density (>99.5%) Al 2O 3/LiTaO 3 (ALT) composite was obtained by hot pressing at 1300 °C/25 MPa in nitrogen atmosphere. The microstructure, which included a domain structure in LiTaO 3 grains, was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Most of the domains observed in LiTaO 3 grains of the Al 2O 3/LiTaO 3 (ALT) composites belong to a non-180° type domain, which cannot exist in LiTaO 3 single crystals. This non-symmetry domain microstructure belongs to ferroelectric twinning with { 1 ¯ 0 1 2 } twin composition plane. The non-180° domains formation mechanism was proposed.

Structure and chemistry of (111) twin boundaries in MgAl2O4 spinel crystals from Mogok

The atomic scale structure and chemistry of (111) twins in MgAl2O4 spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections $$ [\ifmmode\expandafter\bar\else\expandafter\=\fi{1}10] $$ and $$ [11\ifmmode\expandafter\bar\else\expandafter\=\fi{2}]. $$ Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy (EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg2+ ions are replaced with Be2+ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl2O4 (chrysoberyl) and BeMg3Al8O16 (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary.

Phenocryst fragments in rhyolitic lavas and lava domes

Although rhyolitic lavas and lava domes are characterised by evenly porphyritic textures, not all the phenocrysts are whole euhedra. We undertook image analysis of 46 rhyolitic lava and lava dome samples to determine the abundance and shape of quartz and feldspar phenocryst fragments. Phenocryst fragments were identified in nearly all samples. On average, fragments amount to ∼5% of the total phenocryst population, or ∼0.5 modal%. The abundance of fragments in lavas and lava domes is not related to the groundmass texture (whether vesicular, flow banded, massive, glassy or crystalline), nor to distance from source. Fragments are, however, more abundant in samples with higher phenocryst contents. The phenocryst fragments in rhyolitic lavas and lava domes are mainly medium to large (0.5–3.5 mm), almost euhedral crystals with only a small portion removed, or chunky, equant, subhedral fragments, and occur in near-jigsaw-fit or clast-rotated pairs or groups. The fragments probably formed in response to decompression of large melt inclusions. Shear during laminar flow then dismembered the phenocrysts; continued laminar shear separated and rotated the fragments. Fractures probably formed preferentially along weaknesses in the phenocrysts, such as zones of melt inclusions, cleavage planes and twin composition planes. Rare splintery fragments are also present, especially within devitrified domains. Splinters are attributed to comminution of solid lava adjacent to fractures that were later healed. For comparison, we measured crystal abundance in a further 12 rhyolite samples that include block and ash flow deposits and ignimbrite. Phenocryst fragments within clasts in the block and ash flow samples showed similar shapes and abundances to those fragments within the lava and lava domes. Crystal fragments are much more abundant in ignimbrite (exceeding 67% of the crystal population) however, and dominated by small, equant, anhedral chunks or splinters. The larger crystals in the ignimbrite are subrounded. The phenocrysts within ignimbrite pumice lapilli are also more intensely fractured than those in lavas and lava domes. Thus, in deformed and altered volcanic successions, data on crystal fragment abundance and shape can help discriminate lavas from pyroclastic facies.

Allabogdanite, (Fe,Ni)2P, a new mineral from the Onello meteorite: The occurrence and crystal structure

Allabogdanite, (Fe,Ni) 2 P, is a new mineral from the Onello iron meteorite (Ni-rich ataxite). It occurs as thin lamellar crystals disseminated in plessite. Associated minerals are nickelphosphide, schreibersite, awaruite, and graphite. Crystals of the mineral, up to 0.4 × 0.1 × 0.01 mm, are flattened on (001) with dominant {001} faces, and other faces that are probably {110} and {100}. Mirror twinning resembling that of gypsum is common, with possible twin composition plane {110}. Crystals are light straw-yellow with bright metallic luster. Polished (001) sections look silvery-white against an epoxy background. In reflected light in air, the mineral has a creamy color, with distinct anisotropy from light to dark creamy tint. No bireflectance was observed. R 1 /R2 (λ, nm) in air: 48.4/37.2(440), 46.7/36.8(460), 47.0/37.6(480), 47.5/38.1(500), 47.6/38.8(520), 48.2/39.2(540), 49.0/39.9(560), 49.6/40.7(580), 50.1/41.6(600), 50.5/41.9(620), 51.9/43.0(640), 52.3/44.3(660), 53.3/45.0(680), 54.4/46.2(700). No cleavage or parting was observed. Moh’s hardness is 5–6; the mineral is very brittle, and its calculated density 7.10 g/cm 3 . Its chemical composition (determined by microprobe methods, average of nine analyses) is: Fe 57.7, Ni 20.7, Co 1.4, P 20.4, Total 100.2 wt%, corresponding to (Fe 1.51 Ni 0.50 Co 0.03 ) 2.04 P 0.96 (three atoms per formula unit). Crystal structure: R 1 = 0.040 for 138 unique observed (| F o|≥ 4σ F ) reflections. Orthorhombic, Pnma , unit-cell parameters refined from powder data: a = 5.748(2), b = 3.548(1), c = 6.661(2) A, V = 135.8(1), A 3 , Z = 4; unit-cell parameters refined from single-crystal data: a = 5.792(7), b = 3.564(4), c = 6.691(8) A, and V = 138.1(3) A 3 . Strongest reflections in the X-ray powder diffraction pattern are [ d in A, ( I ) ( hkl )]: 2.238(100)(112), 2.120(80)(211), 2.073(70)(103), 1.884(50)(013), 1.843(40)(301), 1.788(40)(113), 1.774(40)(020). The mineral is named for Alla Bogdanova, Geological Institute, Kola Science Centre of Russian Academy of Sciences.

Twin structure in Yb:YAl3(BO3)4crystal

Twin structure is a defect in the self-frequency-doubling laser crystal material Yb:YAl3(BO3)4(YbYAB). Based on atomic force microscopy (AFM) observations, a check-like twin structure in YbYAB crystals is found. This kind of twin structure has three twin composition planes: (10\bar{1}1), (0\bar{1}11) and (\bar{1}101). These three twin composition planes result in a series of twin boundaries in two directions, forming a check-like pattern on each face. From the orientation of the chemical etching pits on each side of the twin boundary, it is found that the twin element is `twin plane ⊥yaxis', alternatively indicated as `twin plane {11\bar{2}0}'. The two single crystals composing the twin are a right form and a left form. This kind of twin is similar to the Brazil twin found in quartz.

New M3O5-Anatase Intergrowth Structures Formed during Low-Temperature Oxidation of Anosovite

Oxidation of anosovite at temperatures ≤300°C produces new phases of general composition MnO2n−1, based on unit cell intergrowth of M3O5 and anatase structural elements. The structural principles on which the intergrowth series are based were established by analysis of high-resolution transmission electron microscope images. The structure for the dominant intergrowth phase, with composition M6O11, was refined using the Rietveld method applied to powder X-ray data. This phase has monoclinic symmetry and space group C2/m, with a=9.946(2), b=3.744(2), c=20.994(3) Å, and β=93.30(1)°. Its structure comprises planar intergrowth of M3O5 compositional blocks with the pseudobrookite-type structure and MO2 compositional blocks with the anatase-type structure, parallel to (001)≡(001)M3O5≡(101)an. Alternatively, the structure can be considered as chemically twinned anatase with (101)an as the twin composition plane. The anatase blocks are alternately 2-octahedra and 3-octahedra wide along [001]. Local regions corresponding to compositions M5O9 (twinned anatase blocks are 2-octahedra wide) and M7O13 (3-octahedra wide anatase blocks) were also observed in TEM images. The results have relevance to the digestion of high-titania slags (anosovite-rich) in the sulfate-route titania pigment process because high sulfuric acid solubility is maintained after the low-temperature treatment, while the preoxidation of trivalent titanium avoids acid consumption by solution oxidation.

Transmission electron microscopy of muscovite alteration of tourmaline

High-resolution transmission electron microscopy reveals dense Brazil-twin boundaries on the unit-cell scale in length-slow chalcedony. The twins can be considered as the result of stacking of left- and right-handed quartz with a (101) twin composition plane. Although most twin sequences on the unit-cell scale are nonperiodic, moganite-type domains result where they are periodic. It is proposed that the twins formed during rapid crystallization rather than as transformation products of a precursor phase such as moganite. The twin boundaries are energetically less stable than twin-free areas and may indicate non-equilibrium crystallization at a high supersaturation of aqueous SiO 2 species in the parent fluid.

Intergrowth defects and dielectric response of BaTiO3 nano-particles

Characteristic differences are observed for the dielectric response and microsturctures of BaTiO3 nanoscale fine powders prepared using sol gel (SG) and steric acid gel (SAG) methods. The former exhibit a critical size below which there is no paraelectric/ferroelectric phase transition whereas BaTiO3 prepared via the SAG route remained cubic for all conditions of preparation. The SAG preparations always showed chemical intergrowth defects and (111) multiple twinning. The density of such defects tended to increase as the particle size decreased. The SG preparations were single phase and became tetragonal above some critical size (> 130 nm). Some SG BaTiO3 showed (111) twinning. The electron optical image analysis of nanocrystalline BaTiO3 particles by computer simulation and image-matching techniques has identified the intergrowth phase as polytypic variants of the perovskite structure of BaTiO3; i.e., mixtures of cubic and hexagonal BaTiO3. These defects are stoichiometric and were found for SAG BaTiO3 particles of all sizes. Image analysis was used to determine that the nanoscale multiple twins involve a pure mirror symmetry operation with BaO atoms forming the twin composition plane. The relationship between the observed nanostructures and defects and the dielectric properties is discussed.

Surface characters of twin domains and exsolution lamellae in a feldspar crystal: Atomic force microscopy study

Atomic force microscopy (AFM) of a (001) cleavage surface of feldspar crystal reveals exsolution lamellae, wave-like (001) surfaces of the Na-rich lamellae, and surface steps with heights about 0.6 nm and 0.3 nm. The wave-like shape of (001) surfaces of albite twin domains results from surface relaxation of periodic twin domains. Both twin boundary and twin composition planes are parallel to (010). AFM images with 3-dimensional information show surface height difference between Na-rich and K-rich lamellae in some areas, which indicates that the boundaries are semi-coherent along the c axis.

A proposed mechanism for the growth of chalcedony

The structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica. Although the paragenesis of chalcedony has provoked marked disagreement among researchers, a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures (<100° C). These conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short-chain linear polymers via bridging silica monomers. This assembly occurs through a spiral growth mechanism activated by a screw dislocation withb=n/2 [110], wheren is an integer. The proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale, such as: (1) the direction of fiber elongation along [110] rather than [001]; (2) the periodic twisting of chalcedony fibers about [110]; (3) the high density of Brazil twin composition planes; (4) the common intergrowth of moganite within chalcedony.

Non-classical twinning of alumina precipitates in rutile

Abstract Electron diffraction and conventional transmission electron microscopy results are presented which reveal that alumina-doped rutiles (Ti,Al)O1·9961 contain alumina precipitates about 0·5 μm in diameter after application of a [111]R† compressive stress at 1323K. The precipitates grow with bilateral heart-shaped symmetry about the plane of maximum resolved shear stress (111)R. They exhibit a non-typical twin composition plane (1216)α which is parallel to (110)R and which bisects the precipitates parallel to the invariant twin plane, thus minimizing long-range elastic strains.

Argon-loss by alkali feldspars

Ar-loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the K-Ar method, and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins. We have done a 40Ar- 39Ar step-heating study of a suite of alkali feldspars from the Klokken syenite intrusion (U-Pb age 1166 ± 4 Ma) which have been fully characterized by transmission electron microscopy. All are low albite-low microcline intergrowths, but some are non-turbid, ‘strain-controlled’ cryptoperthites, while others are turbid, coarse microperthites. Geological and textural evidence suggests that the turbidity developed by fluid-rock interactions during the initial cooling of the pluton, in the stability field of low microcline (<500°C), 10 5 a after crystallization. The finest cryptoperthites have numerous fully coherent perthite lamellar boundaries (lamellar periodicity ~ 40 nm) and twin composition planes. The have retained radiogenic 40Ar and yield total degassing ages as high as 1125 ± 16 Ma and maximum ages on the age spectra of 1162 ± 16 Ma. The coarse turbid microperthites, with patchy intergrowths and twins on the mm-scale, gave total degassing ages as low as 650 Ma (maximum age 750 Ma). Argon leakage occurs least in feldspars with the most lamellar interfaces, but in proportion to the degree of turbidity, which is thought to be caused by large numbers of micropores. Strain-controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids. These retain argon and can give reliable K-Ar ages in old rocks. Saddle-shaped age spectra appear to result from the preferential degassing, at 1000°C, of part of the feldspar structure which has lost more of its argon in nature. The laboratory degassing does not occur in a manner analogous to that which has occurred naturally.

Zoned ternary feldspars in the Klokken intrusion: exsolution microtextures and mechanisms

The microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the Klokken intrusion, described in the preceding paper, were determined by TEM and their origin and evolution deduced. The feldspars normally have a plagioclase core and an alkali feldspar rim; cores become smaller and rims larger and the An content of both decrease with distance from the contact of the intrusion. The following microtextural sequence was observed. The inner plagioclase cores are homogeneous oligoclase-andesine with Albite growth twins only, but are crypto-antiperthitic towards the outer core. At first small platelets of low sanidine a few nanometres thick and up to ∼10 nm long occur sporadically only on Albite-twin composition planes. With further increase in bulk Or they are homogeneously distributed in the plagioclase. Thicker, through-going plates in platelet-free areas are found, which induce Albite twins in the surrounding plagioclase. The microtextures in the rims are regular cryptomesoperthitic, with (¯601) lenses or lamellae, depending on the bulk Or-content, of low sanidine in Albite-twinned low oligoclase-andesine. Albite and Pericline twins in plagioclase in an M-twin relationship, together with lenticular low sanidine, were found in only one small area. The overall diffraction symmetry of the mesoperthites is monoclinic, showing that exsolution started in a monoclinic feldspar, whereas that of the antiperthites is triclinic. The intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres. The cryptomesoperthites are very regular where Or-rich and probably arose by spinodal decomposition. The platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere. Near the intermediate zone they coarsened to give larger plates which induced Albite-twins in the plagioclase. Because of the zoning, microtextures that were initiated in areas of given composition, can propagate laterally into zones of different composition. A diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly-cooled rocks.

Twinning and crystal slip in black monoclinic ZnP2

MonoclinicβZnP2(black ZnP2) single crystals, synthesized in the presence of excess P, contain regions of polysynthetic lamellar-twinned structure, with dislocations and stacking fault-like features. The twin law is a* twin axis, (100) composition plane: (100) is also the slip plane. The twin composition plane migrates across (100) lattice fringes. In the revisedβZnP2crystal structure, Zn(1) and P(4) positions are related across a (100) twin composition plane at x = 0.84 by two twin operations, with axes throughis also a possible slip plane, with three partial dislocations, ½[001], ½[001], and ½[001]. A third possible twin operation relates Zn(2) and P(1) positions across a (100) twin plane atx= 0.5, with twin axis throughAll twin and slip operations result in very little distortion in nearest- and nextnearest-neighbor coordination geometries. Twin and stacking fault mistakes may be facilitated by approach of the monoclinic cell parameter ratioc[α sin(β—π/2)] to 4 (which yields a pseudo-orthorhombic unit cell) and by Zn1-xP2nonstoichiometry.

Apparent re-entrant corner effect upon the morphologies of twinned crystals; A case study of quartz twinned according to Japanese twin law

It has been demonstrated on quartz crystals twinned according to the Japanese twin law that the characteristic flattened morphology of twinned crystals and their larger development than single crystals are not due to the re-entrant corner effect in the strict sense, but due to the concentration of screw dislocation along the twin composition plane. The efficiency of dislocations varies depending on the faces appearing at the re-entrant twin junction.

Chemical twinning and ferroelectric domains in LiTaO3

Abstract Ferroelectric twinning, with {0112} and {0114} twin composition planes, has been induced in lithium tantalate by partial exchange of Li+ by Ag+. This process leads to new types of ferroelectric domains, which involve rotations of the polarization vector by 66°, 104°, 75.4° and 104.6°. The domain walls are chemical in origin and very much more stable than the well-known 180°-ferroelectric domains in pure LiTaO3. The structure of the domain walls is clarified by a crystallographic analysis of the possible structures and interpretation of high-resolution electron microscope images and selected-area diffraction patterns. Non-polar boundaries, or stacking faults, and recurrent twin dislocations, or ‘swinging superlattice structures’, were also observed and analysed.

Inversion Twin in Zinc Oxide Crystal

ZnO needle-shaped crystals are grown by the chemical reaction of ZnF2 with water vapor. Their polarity and inversion twins are observed by X-ray and etching techniques. The crystals consist of the hexagonal core part and the surrounding part. Initially, the core part grows in the [0001] direction, i.e., the growth occurs on the Zn-surface, and the surrounding part is formed by the lateral growth of the core part. The observation for some crystals suggests that the lateral growth starts from the six edges of the hexagonal core crystals. The inversion twins are found only for the surrounding part and observed frequently on {101̄0} and {112̄0} composition planes. It may be concluded that fault energies for these twin composition planes are lower than those for the basal planes.

Unusual Twinning in Graphite

ACCORDING to the Kennedy interpretation1 of twinning in graphite, the twin boundary can be regarded as the result of insertion or removal of a full hexagon width on alternate planes in the hexagonal lattice. This means that the twins are regions of the crystal which are misoriented in relation to the matrix by an angle of tilt, tan−1 a/c, a and c being the unit cell dimensions, equal respectively to the width of a hexagon and the distance between alternate basal planes. This interpretation is corroborated by the recent work of Freise and Kelly2, there being no doubt that the twin composition plane is {112caron;1} with respect to the morphological axes (conventional notation). In Kennedy's paper, theoretical reasons are adduced which favour the existence of {112caron;1} rather than {11caron;01} twins, which result from the insertion or removal of a hexagonal length, 2a/√3, into every other plane. It is also stated that “No cases of the observation of a {11caron;01} twin composition plane appear to have been reported, although it may be possible to produce such twins in thin flakes of very perfect graphite”. This statement forms the basis of this communication.