Rosa Anna Fregola

Crystallographic and spectroscopic characterization of a natural Zn-rich spinel approaching the endmember gahnite (ZnAl2O4) composition

Abstract The crystal chemistry of a natural, gem-quality, blue-grey Zn-rich spinel crystal from Jemaa, Kaduna State, Nigeria, was studied using electron microprobe, single-crystal X-ray diffraction, optical absorption and Raman spectroscopies. The composition of the crystal approaches the gahnite endmember (ZnAl2O4), ~94 mol.%, with the remainder being dominated by a hercynite component (FeAl2O4). The unit-cell dimension is 8.0850(3) Å and the tetrahedral and octahedral bond distances are determined as T-O 1.9485(6) Å and M-O 1.9137(3) Å. Crystal chemical analysis resulted in the empirical structural formula T(Zn0.94Fe0.032+Al0.03)M(Al1.96Fe0.032+Fe0.013+)O4, which shows Zn and Al almost fully ordered in the tetrahedrally and octahedrally coordinated T and M sites, respectively. Raman spectra obtained using the excitation of the blue 473.1 nm line of a Nd:YAG laser display three of the five Raman-active modes predicted for the general oxide spinel group of minerals. These are the Eg mode at 420.6 cm-1 and the T2g modes at 510 cm-1 and 661 cm-1, due to vibrations in the AlO6 octahedra. Optical absorption spectra recorded in the UV/VIS-NIR-MIR range 2000-29000 cm-1 show a dominant absorption band at ~5000 cm-1 which is caused by spin-allowed electronic d-d transitions in Fe2+ located at the T sites. The blue-grey hue exhibited by the sample is mainly due to spin-forbidden electronic transitions in TFe2+ and to MFe2+ ↔ MFe3+ intervalence charge transfer, and the poor saturation of the colour is due to the small concentration of Fe2+ and Fe3+.

Chemical control of 3T stacking order in a Li-poor biotite mica

Abstract The microstructural features of a biotite (Mg-rich annite) crystal from dacite rocks of Džep, Serbia, were studied by X-ray diffraction topography (XRDT) and high-resolution and analytical transmission electron microscopy (HRTEM and ATEM, respectively). The average chemical composition, obtained by electron microprobe analysis (EMPA) and secondary-ion mass spectrometry (SIMS) data, is (K0.87Na0.05Ca0.01)Σ=0.93(Fe2+1.36Mg1.25Ti0.22Al0.14Mn0.03Li0.01)Σ=3.01(Si2.84Al)Σ=4.00O10[(OH)1.53O0.35F0.10Cl0.02]Σ=2.00. Inhomogeneous regions from either different polytypes or twins were mapped by XRDT, and these images were used to guide sampling for TEM analysis. Three stacking arrangements, each belonging to subfamily-A of mica polytypes, were identified by selected area electron diffraction (SAED) patterns and HRTEM images: (1) dominant semi-random 1Mr-n(120°), including an occasional repetition of long-period inhomogeneous stacking sequences belonging to the 3T structural series; (2) highly faulted 2M1, sometimes with short-range-ordered long-period inhomogeneous sequences based on the 2M1 structural series; and (3) perfectly ordered 3T stacking repeating over long distances (several micrometers). ATEM data revealed that the microchemical composition of 3T differs from that of the host matrix. The 3T is enriched in interlayer cations (K and Na), and depleted in Si and octahedral cations. No evidence of Li was detected. Compositional control on the stability of 3T stacking is suggested.

Cation ordering over short-range and long-range scales in the MgAl2O4-CuAl2O4 series

Abstract A multi-analytical approach using electron microprobe analysis, X-ray structural refinement, and optical absorption spectroscopy was applied to characterize short-range and long-range structures of synthetic spinel single crystals along the MgAl2O4-CuAl2O4 solid-solution series. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Cu2+, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a significant degree of inversion, i.e., occurrence of Al at T counterbalanced by occurrence of divalent cations at M, which increases slightly from 0.24 to 0.29 for the highest Cu2+ contents. Short-range information derived from optical spectra suggests that the local TCu2+-O distances remain constant at increasing Cu2+ content, whereas local MCu2+-O distances are ca. 0.02 Å shorter in Cu-poor MgAl2O4 spinels as compared to MCu2+-O distances in end-member CuAl2O4. The observed splitting of an absorption band, at ca. 7000 cm-1 , caused by electron transitions in TCu2+ as well as the anomalous broadness of an absorption band, at ca. 13 500 cm-1, caused by electron transitions in MCu2+ indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd3m symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a MCu2+-O distance of 2.080 Å and a TCu2+-O of 1.960 Å. The very limited variations in the unit-cell parameter a from 8.079 to 8.087 Å are mainly related to the disordering of Al. Because of the very similar size of Cu2+ and Mg at the T and M sites, the spinel structure responds to the Cu2+ → Mg substitution by increasing cation disordering in such a manner that mean M-O distances remain constant and the mean T-O distances decrease slightly. This results in increasing length of shared octahedral edges and thereby increase of the octahedral cation-cation repulsion. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure is demonstrated to be valid also in the MgAl2O4-CuAl2O4 solid-solution series.

A 94-layer long-period mica polytype: A TEM study

Abstract A 94-layer long-period mica polytype was studied by transmission electron microscopy (TEM). This is the longest periodicity found up to now in micas (c ≈ 95.9 nm). It was observed in a fragment of a Mg-rich annite (biotite) crystal from dacite rocks of Džep, Serbia. The crystal region containing it extends about 800 nm along 1/c*. One-dimensional lattice fringe images obtained by bright-field (BF) illumination allowed identification of the very long-period polytype. The latter was characterized by selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). It is an inhomogeneous polytype belonging to the subfamily-A, based on the 2M1 structural series. Its 94-layer stacking sequence is: [(22̄)3202̄(22̄)32̄2(22̄)22̄2(22̄)32̄2(22̄)202̄(22̄)2̄2(22̄)32̄2(22̄)22̄2(22̄)42̄2(22̄)22̄2(22̄)32̄2(22̄)42̄2(22̄)22̄2]. The fringe contrasts of the BF images were correlated to the stacking sequence determined from HR images. The correlation verified that the same sequence occurred eight consecutive times. Analytical electron microscopy (AEM) revealed that the chemical composition of the 94-layer mica polytype is similar to that previously observed in randomly stacked and faulted areas of the same crystal. No remarkable chemical variation occurs between the 94-layer polytype and its adjacent crystal regions, the latter containing non-periodic stacking faults.

X-ray diffraction topographic study of twinning and growth of natural spinels

Two spinel twinned crystals coming from Pegu (Myanmar) have been studied by means of X-ray topographic methods, optical microscopy and electron microprobe analysis. Topographs, taken with conventional and synchrotron sources, showed the presence of a twin lamella in both samples as well as the presence of cross-twinning in one of them. In particular, white beam synchrotron radiation source topography allowed to detect the twin individuals, to identify and to locate the twin planes, and to determine spatial relationships and mutual orientations of the twin components. By combining optical and topographic observations, an unusual morphology, flattened crosswise the twin planes bounding the twin lamella, has been outlined in both samples and related to the growth mechanism. Microchemical analyses showed that the twin individuals in each sample have nearly identical compositions, whereas the samples differ mainly for the Mg and Zn contents. These observations suggest that they grew in similar and chemically closed micro-environments. The results, taken as a whole, indicate that twinning and cross-twinning occurred since nucleation of the samples, and they mark similar growth conditions.

XRDT Study of Structural Defects of 6H-SiC Crystals

X-Ray Diffraction Topography (XRDT) and Optical Microscopy (OM) are adopted to study extended structural defects in 6H-SiC bulky crystals. Topographs are taken by means of White Beam Synchrotron Radiation Source (WB-SRS-XRDT) and by means of monochromatic radiation (MoKα1) with conventional source (Lang method). All studied samples are characterised by the presence of linear defects, dislocations and microchannels, uniformly distributed in the crystal. Such defects draw a net of independent systems of parallel lines, with different orientation and different contrast widths. Micro-channels are parallel to the c axis, whereas dislocations are perpendicular or nearly parallel to the c axis. The last are unit screw dislocations. It has been concluded that the growth mechanism is driven by screw dislocations and that channels results from the coalescence of parallel dislocations.

Optical absorption spectroscopy study of the causes for color variations in natural Fe-bearing gahnite: Insights from iron valency and site distribution data

Abstract Four gahnite single crystals with variable colors from pale blue to green have been studied by a multi-analytical approach with the aim to evaluate existing assignments of optical absorption bands. Combined information from electron microprobe analyses, Mössbauer spectroscopy, IR-spectroscopy, single-crystal X‑ray structure refinements, and optical absorption spectroscopy confirms the conclusions of earlier studies that the absorption bands recorded in the visible spectral region up to ~540 nm (above ~18 500 cm-1) are related to electronic d-d transitions in tetrahedrally coordinated Fe2+. It also demonstrates that a set of absorption bands between ~550-625 nm (~16 000-18 200 cm-1) are caused by spin-allowed and spin-forbidden d-d transitions in tetrahedrally coordinated Co2+. Two absorption bands at higher wavelengths (~680 and ~800 nm, i.e., ~14 700 and ~12 500 cm-1) are assigned to electronic transitions in exchange coupled VIFe3+-IVFe2+ pairs and a band at ~950 nm (~10 500 cm-1) is assigned to a spin-allowed electronic transition in VIFe2+. Low-Fe gahnite crystals owe their blue color to traces of cobalt at concentration levels in the order of 200 ppm and less, while the green color of gahnite crystals with higher Fe-contents is due to a combination of electronic ligand-metal transitions causing strong UV-absorption and electronic transitions in exchange coupled Fe1-Fe3+ cation pairs that absorb in the red region of the visible spectrum. A detailed characterization of samples that includes cation site occupancy and iron valency data is demonstrated to be crucial for interpreting optical absorption spectra. Also electronic transitions in trace element chromophores below the detection limit of electron microprobe may participate to light absorption. All this information contribute to the comprehension of the causes of crystal color of minerals, gemstones, and ceramic pigments

Crystal chemistry of spinels in the system MgAl2O4-MgV2O4-Mg2VO4

Abstract Eight spinel single-crystal samples belonging to the spinel sensu stricto-magnesiocoulsonite series (MgAl2O4-MgV2O4) were synthesized and crystal-chemically characterized by X-ray diffraction, electron microprobe and optical absorption spectroscopy. Site populations show that the tetrahedrally coordinated site (T) is populated by Mg and minor Al for the spinel sensu stricto compositions, and only by Mg for the magnesiocoulsonite compositions, while the octahedrally coordinated site (M) is populated by Al, V3+, minor Mg, and very minor amounts of V4+. The latter occurs in appreciable amounts in the Al-free magnesium vanadate spinel, T ( Mg ) M ( Mg 0.26 V 1.48 3+ V 0.26 4+ ) O 4

Cross-twinning in a natural spinel from Sri Lanka

^{\rm{T}}{\left( {{\rm{Mg}}} \right)^{\rm{M}}}\left( {{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.26}}}}{\rm{V}}_{{\rm{1}}{\rm{.48}}}^{{\rm{3 + }}}{\rm{V}}_{{\rm{0}}{\rm{.26}}}^{{\rm{4 + }}}} \right){{\rm{O}}_{\rm{4}}}

A first report on anion vacancies in a defect MgAl 2 O 4 natural spinel

, showing the presence of the inverse spinel VMg2O4. The studied samples are characterized by substitution of Al3+ for V3+ and (Mg2++V4+) for 2V3+ described in the system MgAl2O4-MgV2O4-VMg2O4. The present data in conjunction with data from the literature provide a basis for quantitative analyses of two solid-solution series MgAl2O4-MgV23+O4