We studied V-shaped twins of chrysoberyl (BeAl2O4) from Rio das Pratinhas pegmatites near Arataca in the Bahia state of Brazil. The local structure of the twin boundaries was determined using powder X-ray diffraction analysis (XRD), transmission electron microscopy (TEM) methods, and density functional theory (DFT) calculations. To provide the most reliable model for DFT and HRTEM simulations the structure of chrysoberyl was first refined in the orthorhombic space group 62 (Pmnb) with unit-cell parameters: a = 5.4825(1) Å, b = 9.4163(2) Å, and c = 4.4308(1) Å, with 0.5 at% of Fe3+ present on the Al(2) sites, suggesting an average composition of BeAl1.99Fe0.01O4. TEM study of V-shaped twins showed that the twin boundary lies in the (130) planes, and the angle measured between the crystal domains related by mirror twin operation is ∼59.5°. Rigid structural model of (130) twin boundary in chrysoberyl was refined by DFT calculations, using a pseudo-potential method. The twin boundaries show local enrichment with Ti. Bulk chrysoberyl contains numerous nanosized TiO2 precipitates with a distorted rutile structure, following the orientation relationship of [001]Ch{120}Ch||[010]R{103}R. The increase of Ti at the twin boundaries and the formation of rutile-type TiO2 precipitates in bulk chrysoberyl suggest a transient Ti-exsolution that took place after the twin formation. Nanostructural analysis of rutile exsolutions has shown that the c-axis of rutile is oriented in such a way that the channels along this direction are providing shortest pathways for cation diffusion that allow lateral growth of the precipitates after their nucleation as long as Ti4+ ions continue to segregate from the hosting chrysoberyl matrix to form rutile. On further cooling, elastic accommodation of the rutile structure was probably driven by a temperature dependent contraction of chrysoberyl lattice parameters, which helps to understand the dynamics of geochemical processes during the crystallization of chrysoberyl and rutile exsolution.
COBISS.SI-ID: 28468775
Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of large single hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the (010)R{101}R||(001)H{110}H orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact (010)R{401}R||(001)H{170}H. The intergrowth is dictated by the formation of {170}H|{401}R equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile–rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60° and 120°, (2) {101} twins with incidence angles of 114.44° and their complementaries at 65.56°, (3) {301} twins at 54.44° with complementaries at 125.56° and (4) low-angle tilt boundaries at 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile–rutile interfaces were confirmed in Mwinilunga samples. Using a HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian–ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier–Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form {301}R|{030}H equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian– ilmenite crystals was close to Ilm67Hem33, typical for Fe–Ti rich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at ~900 °C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian–ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO2.
COBISS.SI-ID: 28374567
A comparative study of carbonate samples from the carbonatite-like dyke of the Madenska River complex at the Kriva Lakavica section, samples of calcite skarns from the Damjan Fe-ore deposit and the Sasa Pb-Zn ore deposit, and samples of marbles from the Pohorje Mountains was performed to provide critical evidence for magmatic or sedimentary origin of this carbonatite-like dyke. We found that carbonatite-like dyke is not a normal carbonatite, but instead represents melted carbonates, probably associated with an unexposed, deep-seated, causative magmatic body. This dyke has a fluidal texture and carries xenoliths of ultramafic rocks that can be up to 35 cm in size. Its isotopic composition plots between primary unaltered carbonatites and marine carbonates, and ranges between 13.79‰ and 18.89‰ for δ18OVSMOW and between –1.22‰ and 1.31‰ for δ13CVPDB. These values are significantly lower than those observed in carbonatites analyzed during this study and range between 6.53‰ and 8.10‰ for δ18OVSMOW and between –5.82‰ and –4.32‰ for δ13CVPDB, which is the primary isotope signature of most magmatic carbonatites. Similarly high δ18O and δ13C values were found in skarns of the Damjan Fe deposit close to the Madanska River complex and in Sasa Pb-Zn deposit, as well as in high-grade regional metamorphic calcitic marbles of the Pohorje massive. The emplacement levels of the carbonatite-like dyke, due to several tectonic processes, are uncertain. The type of country rocks (sedimentary carbonates, ultramafic, mafic, and granitic rocks), hydrothermal alternation, and metasomatic and regional metamorphic processes seem to be the most important parameters that affect the O and C isotopic patterns found in the Kriva Lakavica carbonatite-like dyke and in the investigated samples.
COBISS.SI-ID: 28988711