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 asT–O 1.9485(6) Å andM–O 1.9137(3) Å. Crystal chemical analysis resulted in the empirical structural formula^T(Zn_0.94Fe²⁺_0.03Al_0.03)^M(Al_1.96Fe²⁺_0.03Fe³⁺_0.01)O₄, which shows Zn and Al almost fully ordered in the tetrahedrally and octahedrally coordinatedTandMsites, 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 E_gmode at 420.6 cm^–1and the T_2gmodes at 510 cm^–1and 661 cm^–1, due to vibrations in the AlO₆octahedra. Optical absorption spectra recorded in the UV/VIS-NIR-MIR range 2000 29000 cm^–1show a dominant absorption band at ∼5000 cm^–1which is caused by spin-allowed electronicd–dtransitions in Fe²⁺located at theTsites. 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⁺.

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 Fd3̄m 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.

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_0.26V³⁺_1.48V⁴⁺_0.26)O₄, 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 and MgV₂³⁺O₄-V⁴⁺Mg₂O₄. Unit-cell parameter increases with increasing V³⁺ along the series MgAl₂O₄-MgV₂O₄ (8.085–8.432 Å), but only slightly increases with increasing V³⁺ along the series VMg₂O₄-MgV₂O₄ (8.386–8.432 Å). Although a solid solution could be expected between the MgAl₂O₄ and VMg₂O₄ end-members, no evidence was found. Amounts of V⁴⁺ are nearly insignificant in all synthetic Al-bearing vanadate spinels, but are appreciable in Al-free vanadate spinel. An interesting observation of the present study is that despite the observed complete solid-solution along the MgAl₂O₄-MgV₂O₄ and MgV₂O₄-VMg₂O₄ series, the spinel structure seems to be unable to stabilize V⁴⁺ in any intermediate members on the MgAl₂O₄-Mg₂VO₄ join even at high oxygen fugacities. This behavior indicates that the accommodation of specific V-valences can be strongly influenced by crystal-structural constraints, and any evaluation of oxygen fugacities during mineral formation based exclusively on V cation valence distributions in spinel should be treated with caution. The present study underlines that the V valency distribution in spinels is not exclusively reflecting oxygen fugacities, but also depends on activities and solubilities of all chemical components in the crystallization environment.

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 Fe²⁺-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.

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 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 1M_r-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.