Polysynthetic Twinning Research Articles

Hydrous species in feldspars: A reassessment based on FTIR and SIMS

Recent interest in hydrogen incorporation in feldspars has been driven by the potential of this common mineral species to record magmatic water contents. Accurate measurement of H concentrations in feldspars by Fourier transform infrared (FTIR) spectroscopy is hampered by the need to collect polarized spectra in three mutually perpendicular directions, which can be impractical for crystals characterized by small dimensions, polysynthetic twinning, and/or chemical zoning. SIMS is an attractive alternative to FTIR, offering high spatial resolution, high precision, and the feasibility of attaining low detection limits. In this study we compare FTIR and SIMS data for 19 feldspars, including plagioclase, anorthoclase, sanidine, microcline, and orthoclase. We present adjustments to previously published FTIR data on some of these samples. Our new SIMS and FTIR data are well correlated and we demonstrate the feasibility of quantitatively measuring H concentrations as low as 1–2 ppmw H_2O using SIMS. Combination of the new data together with re-evaluation of the NMR calibration of Johnson and Rossman (2003) indicates that the IR absorption coefficients for hydrous species in feldspar increase with decreasing frequency of their O-H absorptions, in accord with theory. We derive new molar integral IR absorption coefficients (I) for feldspars with the following hydrous species as defined by Johnson and Rossman (2003): Type I and II H_2O (microcline and orthoclase): I = 120 470 ± 11 360 L·mol^(−1) H_2O cm^(−2) Type IIb OH (sanidine): I = 150 000 ± 15 000 L·mol^(−1) H_2O cm^(−2) Type IIa OH (plagioclase and anorthoclase): I = 202 600 ± 20 260 L·mol^(−1) H_2O cm^(−2) These absorption coefficients depend on critical assumptions with regards to SIMS matrix effects. If accurate, one important implication is that the H concentrations of plagioclase crystals estimated in the literature are too high by up to a factor of two, requiring revision of previously estimated plagioclase-melt H partitioning coefficients.

Twinning of Polymer Crystals Suppressed by Entropy

We propose an entropic argument as partial explanation of the observed scarcity of twinned structures in crystalline samples of synthetic organic polymeric materials. Polymeric molecules possess a much larger number of conformational degrees of freedom than low molecular weight substances. The preferred conformations of polymer chains in the bulk of a single crystal are often incompatible with the conformations imposed by the symmetry of a growth twin, both at the composition surfaces and in the twin axis. We calculate the differences in conformational entropy between chains in single crystals and chains in twinned crystals, and find that the reduction in chain conformational entropy in the twin is sufficient to make the single crystal the stable thermodynamic phase. The formation of cyclic twins in molecular dynamics simulations of chains of hard spheres must thus be attributed to kinetic factors. In more realistic polymers this entropic contribution to the free energy can be canceled or dominated by nonbonded and torsional energetics.

Effects of Inorganic Anions on the Morphology of Sodium Oxalate Crystallized from Highly Alkaline Solutions

The crystallization of sodium oxalate (Na2C2O4) crystals was studied in the presence of different inorganic anions of the Bayer liquor in the alumina industry, including OH–, CI–, Al(OH)4–, CO32–, and SO42–. The morphology of the obtained Na2C2O4 crystals depends on the type and concentration of anion species under similar oxalate supersaturation. The presence of divalent anions (CO32–, SO42–) has a greater influence on the morphological variability, including cyclic twins, spherulites, and double-leaf structures, through the branching growth process. The specific surface areas of Na2C2O4 crystals grown in the presence of divalent anions (CO32–, SO42–) are more than 2 times higher than those in the presence of monovalent anions (OH–, CI–, Al(OH)4–). X-ray photoelectron spectra results indicate that the inorganic anions (CI–, Al(OH)4–, CO32–, SO42–) are adsorbed onto the surface of Na2C2O4 crystals, thereby inducing internal stress (defects) of crystals and modifying the crystal morphology.

Understanding the Formation of Pentagonal Cyclic Twinned Crystal from the Solvent Dependent Assembly of Au Nanocrystals into Their Colloidal Crystals

Due to the presence of high-density twinned-defects and diffuse elastic strain, pentagonal cyclic twinning (PCT) structures may have more fascinating properties than the corresponding single crystalline structure. Although there are a lot of reports concerning the PCT nanocrystals of many monometals and bimetal alloys, the established growth mechanisms of PCT structures are still not straightforward and usually inconsistent with each other. In this Letter, using dodecanethiol-capped Au nanocrystals (NCs) as building blocks, taking self-assembly of Au NCs into their colloidal crystals as the crystallization model, we found the competition of crystal cohesive energy and surface free energy plays an important role in the formation of PCT colloidal crystal. By rationally selecting the solvent to tailor the crystal cohesive energy, the structure of the colloidal crystals can be tuned. PCT structures are more likely to form when the interaction between building blocks is weak, while the single crystal structure is formed when the interaction is strong. The results demonstrate that the thermodynamic factors are the origin of pentagonal cyclic twinning.

An experimental study of cyclic plastic deformation of extruded ZK60 magnesium alloy under uniaxial loading at room temperature

To experimentally study the role of the twinning/detwinning process in cyclic plastic deformation of Mg alloys, both strain-controlled and stress-controlled cyclic tension–compression experiments were conducted using extruded ZK60 magnesium (Mg) alloy in ambient air. The fully revered strain-controlled experiments had strain amplitudes varying from 0.25% to 4%. Three types of cyclic deformation are identified in light of the involvement of twinning/detwinning. The stress-controlled experiments reveal the cyclic ratcheting behavior of the material. Due to a strong basal texture of the material, no sustainable twinning/detwinning deformation occurs when the stress level is below the twinning or detwinning yield stress. As a result, ratcheting deformation under certain loading conditions is dominated by dislocation slip and is similar to that of other metallic materials. Cyclic twinning and detwinning in a stress-controlled experiment result in significant cyclic hardening. Under stress-controlled loading condition when ratcheting deformation occurs, the ratcheting rate decays with increasing number of loading cycles following a power law fashion regardless of the deformation mechanisms. Strain ratcheting is discussed for deformation involving the twinning/detwinning process and the experimental observations are explained in light of the micromechanisms.

Correlating Growth Habit of Boron-Rich Low-Dimensional Materials with Defect Structures by Electron Microscopy

Boron carbide and boron suboxide low-dimensional materials with α-rhombohedral symmetry, here simply referred to as boron-rich nanomaterials, exhibit a variety of growth habits, including rodlike fibers, slablike platelets, and some intermediary structures, as confirmed by scanning electron microscopy (SEM) observation. The defect structures of these variants have been thoroughly characterized by advanced transmission electron microscopy techniques, which reveal the prevalence of two basic contact twinning: parallel twinning and cyclic twinning. The growth habits and defect structures of boron-rich materials are found to be strongly correlated. This has been summarized by a growth habit map, which allows us to establish a quick strategy for the determination of defect structures by simple SEM shots in boron-rich materials, alleviating ceramists from the need and complexity in the identification of multitwinned structures.

Shock metamorphism of Elephant Moraine A79001: Implications for olivine–ringwoodite transformation and the complex thermal history of heavily shocked Martian meteorites

Lithology A of Martian meteorite Elephant Moraine (EET) A79001 contains fragments entrained within a 100μm-thick shear-induced shock vein. These fragments, the shock vein matrix and walls of olivine along the vein, as well as shock deformation and transformation in rock-forming minerals in the bulk rock, were investigated using scanning electron microscopy, the electron microprobe and Raman spectroscopy. The presence of ringwoodite, the spinel-structured high-pressure (Mg,Fe)2SiO4 polymorph, has been confirmed in EETA79001 for the first time. Ringwoodite occurs within and around the shock vein, exhibiting granular and lamellar textures. In both textures ringwoodite consists of ∼500nm size distinct grains. Ringwoodite lamellae are 115nm to 1.3μm wide. Planar fractures in olivine provided sites for heterogeneous nucleation of ringwoodite. Analyses performed on the largest grains (⩾1μm) show that ringwoodite is consistently higher in iron (Fa27.4–32.4) relative to surrounding olivine (Fa25.1–267.7), implying that there was Fe–Mg exchange during their transformation, and therefore their growth was diffusion-controlled. In the shock environment, diffusion takes place dynamically, i.e., with concurrent deformation and grain size reduction. This results in enhanced diffusion rates (⩾10−8m2/s) over nm – μm distances.Shock deformation in host rock minerals including strong mosaicism, pervasive fracturing, polysynthetic twinning (pyroxene only), extensive shock melting, local transformation of olivine to ringwoodite, and complete transformation of plagioclase to maskelynite in the bulk rock, indicate that EETA79001 was strongly shocked. The short shock duration (0.01s) combined with a complex thermal history, resulted in crystallization of the 100μm thick shock vein in EETA79001 during the pressure release, and partial back-transformation of ringwoodite to olivine. Based on the pressure stabilities of clinopyroxene+ringwoodite, crystallization at the shock vein margin began at ∼18GPa. Olivine and clinopyroxene crystallized at <14GPa closer to the shock vein center. These represent a minimum limit to the shock pressure loading experienced by EETA79001.

Cyclic twinning and internal defects of boron-rich nanowires revealed by three-dimensional electron diffraction mapping

Structural characterization of individual nanosized boron-rich nanowires has been carried out through analysing the three-dimensional (3D) electron diffraction intensity distribution. Not only can the cyclic twinning structure of these nanowires be easily determined, the new approach also reveals the heterogeneous strain relaxation within the intact nanowire, through the accurate determination of the orientation relationship between the constituent crystallites. The quantitative analysis of the fine structure in the 3D diffraction dataset indicates that this may be related to the distribution of defects such as stacking faults, microtwins and dislocations. It is envisaged that the non-destructive nature of this approach could open the way for the in situ study of the structural evolution of complex nanomaterials and polycrystalline materials in general.

Correlating planar microstructures in shocked zircon from the Vredefort Dome at multiple scales: Crystallographic modeling, external and internal imaging, and EBSD structural analysis

Microstructural and geochronological analysis of shocked zircon has greatly advanced understanding the formation and evolution of impact structures. However, fundamental aspects of shock-produced planar microstructures in zircon remain poorly known, such as their deformation mechanisms, crystallographic orientations, and how planar microstructures visible at the grain scale by scanning electron microscopy correlate to microstructures visible at sub-micrometer scales by transmission electron microscopy and electron backscatter diffraction (EBSD). To unify observations of planar microstructures in zircon made at different scales into a consistent framework, we integrate the results of: (1) three-dimensional crystallographic modeling of planar microstructure orientations, with (2) 360° external prism backscattered electron imaging at the grain scale, and (3) polished section cathodoluminescence and EBSD analysis at the sub-micrometer scale for a suite of detrital shocked zircons eroded from the Vredefort Dome in South Africa. Our combined approach resulted in the documentation of seven planar microstructure orientations that can be correlated from grain to sub-micrometer scales of observation: (010), (100), (112), (11̄2), (1̄12), (1̄1̄2), and (011). All orientations of planar microstructures exhibit minor variations in style, however all are considered to be fractures; no amorphous ZrSiO4 lamellae were identified. We therefore favor the usage of “planar fracture” (PF) over “planar deformation feature” (PDF) for describing the observed planar microstructures in zircon based broadly on the nomenclature developed for shocked quartz. Some {112} PFs visible at the grain scale contain impact microtwins detectable by EBSD, and are the first report of polysynthetic twinning in zircon. The microtwins consist of parallel sets of thin lamellae of zircon oriented 65° about <110> and occur in multiple crosscutting {112} orientations within single grains. Curviplanar fractures and injected melt are additional impact-related microstructures associated with PF formation. Crosscutting relations of shock microstructures reveal the following chronology: (1) Early development of c-axis parallel PFs in (010) and (100) orientations; (2) the development of up to four {112} PFs, including some with microtwins; (3) the development of curviplanar fractures and the injection of impact derived melt; (4) the development of (011) PFs associated with compressional deformation; and (5) grain-scale non-discrete crystal plastic deformation. Experimental constraints for the onset of PFs, together with the absence of reidite, suggest formation conditions from 20 to 40 GPa for all of the planar microstructures described here.

Micromorphology and internal structure of apographitic impact diamonds: SEM and TEM study

Micromorphology, internal structure and phase composition of perfect apographitic crystals of impact diamond — paramorphoses were investigated using goniometry, SEM and TEM. X-ray and electron diffraction analyses of the paramorphoses evidence that they are all crystalline and textured. They consist of up to 50vol.% lonsdaleite and diamond each with minor graphite. The paramorphoses mimic the morphology of the original graphite crystals. On the (0001) planes of the paramorphoses striations caused by twinning as well as bulges on (112¯0) planes together with various micro- and nano-features of growth and etching are observed. Twin striations on (0001) planes of the paramorphoses create triangular-shaped ledges. The walls of the ledges correspond to {112¯1} and {1.12¯.15} planes. The growth and etch figures are strictly oriented along (112¯0). Etch pits with trigonal or hexagonal symmetry are characteristic for diamond whereas rectangular shaped etch pits and growth plates are typical for lonsdaleite. Different types of etch and growth figures allow – at least qualitatively – discrimination between diamond and lonsdaleite. The presence of lonsdaleite and diamond already can be expected from the morphological characteristics observed in SEM images. The orientation of the growth figures and etch pits on the (0001) planes (surfaces) with respect to the hexagonal contours of the paramorphoses suggests that the (0001) planes of the precursor graphite were oriented parallel to (101¯0) planes of lonsdaleite and to (111) planes of diamond. The orientation relationship of the three phases establishing the paramorphoses are confirmed by electron diffraction data from TEM foils cut oriented from the paramorphoses. The internal structure of the paramorphoses indicates that shock compression initiated polysynthetic twinning of the precursor graphite followed by the phase transformation to lonsdaleite and/or diamond. The twinning plane in graphite is (112¯1), and twinning occurs by a rotation of the graphite layers at 20°48′ relative to the basal plane. These twin-related bands in the paramorphoses are predominantly composed of lonsdaleite (0.3–1.0μm wide) and those composed of diamond are 1.5–2.5μm wide. The observed bulges on the (0001) planes of the paramorphoses and the orientation relationship of graphite, lonsdaleite and diamond structures suggest a martensitic transformation of graphite to lonsdaleite and/or diamond.

Annealing-Induced {011}-Specific Cyclic Twins in Tetragonal Zirconia Nanoparticles

Zirconia (ZrO2) nanocrystals with average size of 4 nm are fabricated by oxidation of the nonstoichiometric ZrC0.6 with ordered carbon vacancies at 450 °C under atmosphere. The nanocrystals are pre...

High temperature deformation behaviors of polysynthetically twinned (PST) Ti–47Al–2Cr–2Nb alloy

The characteristics of deformation and fracture of polysynthetically twinned (PST) Ti–47Al–2Cr–2Nb alloy at 800°C had been identified by scanning and transmission electron microscopy. The tensile properties of the fully laminated Ti–47Al–2Cr–2Nb showed anisotropy for three different orientations (N, P and I orientation) at elevated temperature. Some differences in fracture modes between three oriented-lamellae at 800°C had been observed, which were mainly due to the normal stress and shear stress. Also, it was found that the α2-lamellae decomposed and the γ-lamellae coarsened during high temperature deformation, but similar tendency was more obvious in hard orientation. At last, the deformation modes had been identified by transmission electron microscopy. For the hard orientation loaded PST Ti–47Al–2Cr–2Nb samples (N and P orientation) all variants of γ-lamellae deformed only by hard dislocation slip and hard twinning. While the γ-lamellae deformed more profusely for soft orientation loaded PST Ti–47Al–2Cr–2Nb sample (I orientation). It was observed that a majority of γ-variant lamellae deformed by the soft modes of parallel twinning and by soft ordinary dislocation glide and hard modes of twinning as well as hard dislocation activity had been observed for the remainder of the variants of γ-lamellae in the soft orientation loaded PST-TiAl.

Microstructure control and mechanical properties of directionally solidified TiAl–Nb alloys

The double directional solidification (DS) technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys. Polysynthetically twinned (PST) crystals with lamellar boundaries parallel to or inclined 45° to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface. The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions. Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase. Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys, the tensile plastic elongations of the alloys are only close to 2%.

LEAD-ANTIMONY SULFOSALTS FROM TUSCANY (ITALY). XII. BOSCARDINITE, TlPb4(Sb7As2) 9S18, A NEW MINERAL SPECIES FROM THE MONTE ARSICCIO MINE: OCCURRENCE AND CRYSTAL STRUCTURE

The new mineral species boscardinite was discovered in the barite – pyrite – iron oxides deposit of Monte Arsiccio, near Sant’Anna di Stazzema, in the Apuan Alps, Tuscany, Italy. It forms a millimetric compact mass in a quartz vein embedded in dolomitic rocks. Other associated sulfides are stibnite and zinkenite. Boscardinite is metallic grey. Under the ore microscope, it is white; pleochroism is not discernible. Anisotropism is distinct, with an ubiquitous polysynthetic twinning; rotation tints are in shades of grey. Minimum and maximum reflectance data for COM wavelengths [λ (nm), R air (%)] are: 470: 33.8/39.3, 546: 32.1/38.0, 589: 31.2/36.9, 650: 29.7/35.3. The hardness was not measured owing to the scarcity of the available material. Electron-microprobe analyses of two samples gave (wt. %, result mean of five analyses): 1) sample 4977: Ag 1.48(4), Tl 9.72(26), Pb 23.36(20), Sb 35.25(60), As 5.78(10), S 22.14(45), Se 0.04(1), total 97.77(90); 2) sample 4989: Ag 1.37(7), Tl 8.96(19), Pb 25.74(20), Sb 33.46(32), As 6.54(8), S 22.08(29), Se 0.01(1), total 98.16(63). On the basis of ∑ Me = 14 apfu , they lead to the formulae Ag 0.36 Tl 1.23 Pb 2.92 (Sb 7.50 As 2.00 ) ∑9.50 S 17.88 Se 0.01 and Ag 0.33 Tl 1.13 Pb 3.20 (Sb 7.09 As 2.25 ) ∑9.34 S 17.76 , respectively. A single-crystal X-ray study of boscardinite indicates triclinic symmetry, space group P 1, with a 8.0929(4), b 8.7610(5), c 22.4971(11) A, α 90.868(4), β 97.247(4), γ 90.793(4)°, V 1582.0(2) A 3 , Z = 2. The d values (A) of the main powder-diffraction lines, corresponding to multiple hkl indices, are (relative intensity visually estimated): 3.705 (ms), 3.540 (ms), 3.479 (m), 3.085 (m), 2.977 (ms), 2.824 (vs), 2.707 (s), 2.324 (ms), and 2.176 (ms). Boscardinite is the Tl–Sb homeotype of baumhauerite; its crystal structure has been solved by X-ray single-crystal study on the basis of 4319 observed reflections with a final R 1 = 0.045. It can be described in the sartorite homologous series as formed by the 1:1 alternation of sartorite-type and dufrenoysite-type layers. The simplified structural formula is based on 18 sulfur atoms and can ideally be written as TlPb 4 (Sb 7 As 2 ) ∑9 S 18 . The name boscardinite honors Matteo Boscardin for his contribution to knowledge of the regional mineralogy of Italy.

Crystallographic relationships in the crossed lamellar microstructure of the shell of the gastropod Conus marmoreus

The crossed lamellar microstructure of mollusk shells shows a very complex hierarchical architecture constituted of long rod-shaped aragonite crystals stacked parallel to each other inside each first order lamella, which are almost perpendicular to the ones contained in parallel neighboring lamellae. To better understand the construction and properties of the crossed lamellar microstructure we have performed a detailed study to determine the crystallographic characteristics and their evolution during shell growth using scanning electron microscopy, transmission electron microscopy and X-ray diffraction texture analysis. The arrangement of crystals is rationalized by a set of twin law relationships between aragonite crystals. Specifically, the aragonite rods, or third order lamellae within each first order lamella, internally consist of polysynthetic twins bounded by {1 1 0} mirror planes. In turn, the polysynthetically twinned aragonite crystals also show a constant crystallographic orientation with respect to aragonite crystals in adjacent first order lamellae. It can be seen as another twin law in which crystals from adjacent lamellae are bounded by (1 1 0) planes but with their c-axes rotated within this plane by 30°. Thus there are two sets of twin laws that relate crystal units at lower (third order lamellae) and higher (first order lamellae) length scales. These hierarchical relationships play a crucial role in the construction, organization and properties of this complex microstructure. The later orientational relationships have never been described in geological aragonite and are only found in biogenic materials with a crossed lamellar microstructure. Their occurrence is probably determined by the presence of shell organic components which regulate crystal growth and may favor unusual crystallographic relationships.

In Situ TEM Investigations on Thermoelectric Bi2Te3/Sb2Te3 Multilayers

AbstractIn this work, the impact of heat treatment on the real structure of Bi2Te3/Sb2Te3 multilayers is investigated. The material was heated in situ in the transmission electron microscope (TEM) and ex situ inside a furnace after preparing these layers with the so‐called nanoalloying deposition technique via molecular beam epitaxy (MBE) equipment. The samples were prepared as a lamella for TEM studies using focused ion beam technique. EDX elemental mapping and high angle annular dark field mode‐STEM were performed to monitor changes of the morphology and interdiffusion phenomena after heating up to 250 °C. A grain growth started during heating and the chemical layer structure was smeared out partly but remained in several grains and was found to be adjusted parallel to a major lattice plane in a crystallite. High resolution TEM shows polysynthetic twinning in a number of crystals.

Analysis of the plastic anisotropy and pre-yielding of (γ/α 2)-phase titanium aluminide microstructures by crystal plasticity simulation

The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α 2-Ti 3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively.

Deformation mechanisms in micron-sized PST TiAl compression samples: Experiment and model

Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel γ-TiAl and, fewer, α 2-Ti 3Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti–50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 μm × 9 μm. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings.

Hydroxylchondrodite Mg5(SiO4)2(OH)2: A new mineral of the humite group and its crystal structure

Hydroxylchondrodite was found in the Perovski� tovaya Kop’ on the western slope of the Chuvashskie Mountains in the Zlatoust region of the Southern Urals (Russia). It is located in the contact zone between magnesium skarn and host marble being intergrown in aggregates composed of calcite and cli� nochlore with dependent amounts of dolomite and tremolite. Among the other minerals are diopside, garnet of the andradite–grossular row, magnetite, per� ovskite, hydroxylclinohumite, and titanite. Hydroxyl� chonrodite occurs as coarsely formed tabular or lens� like individuals with sizes up to 0.5 × 1.5 × 1.5 cm and their intergrowths up to 2 cm. Thin polysynthetic twinning by {001} is observed under the microscope; the thickness of twin lamellas is usually ~1 µm, rarely up to 5 µm. The mineral is semitransparent, reddish

Alexandrite and colour-change chrysoberyl from the Lake Manyara alexandrite-emerald deposit in northern Tanzania

Mineralogical and gemmological properties of alexandrite from the Mayoka deposit, Lake Manyara mining area, Tanzania, are described. The alexandrite crystals are located in a phlogopite- bearing schist which is part of a metamorphic-metasomatic association of mafi c or ultramafi c rocks in contact with pegmatitic intrusions. Emeralds are associated with the pegmatites. Single crystals, contact twins, penetration twins and cyclic twins (trillings) are all present in this area and are developed in two major habits, one tabular parallel to the a pinacoid and the other columnar along the a-axis. Microscopic features of faceted and rough alexandrites are characterized by internal growth structures refl ecting the external morphology of the samples. Common mineral inclusions are phlogopite, apatite and zircon. Associated with series of channels parallel to the a-axis three groups of elongated or tabular negative crystals oriented in a plane parallel to the a pinacoid form milky white zones in some samples. The complexity of alexandrite colours is clarifi ed to some extent by correlation of colorimetric parameters with trace element contents and with the orientation of the alexandrites. The designation of samples as alexandrites and a possible distinction from colour-change chrysoberyl is discussed. Alexandrite and green chrysoberyl from Lake Manyara is compared with samples from other occurrences in phlogopite-bearing schist such as the 'classical' alexandrite deposits in the Urals, Russia, and from Novello, Zimbabwe.