Ulf Hålenius

Structural relaxation around Cr3+ and the red-green color change in the spinel (sensu stricto)-magnesiochromite (MgAl2O4-MgCr2O4) and gahnite-zincochromite (ZnAl2O4-ZnCr2O4) solid-solution series

Abstract Optical absorption spectra of flux-grown single crystals in the spinel s.s.-magnesiochromite and gahnite-zincochromite solid solutions were recorded with the aim of exploring variations in local Cr-O bond distance as a function of composition. With increasing Cr contents, the crystals vary in color from pale red to intensely red to dark greenish. These variations are reflected in the optical spectra by the position and intensity of the two spin-allowed electronic d-d transitions in six-coordinated Cr3+ at ~18 000 (ν1) and 25 000 cm-1 (ν2). From the shift of the ν1 band position, a decrease in crystal field splitting, 10Dq, for six-coordinated Cr3+ with increasing Cr contents is evident in both solid-solution series. Based on published Cr-O bond distances for the CrO6 polyhedra in magnesiochromite and zincochromite of 1.995 and 1.991 Å, respectively, and applying the ligand field relationships, local Cr-O bond distances in gahnite and spinel with Cr contents at trace levels are determined to be 1.974(2) and 1.960(3) Å, respectively. These local Cr-O distances result in relaxation parameters (ε) equal to 0.69(2) and 0.60(3) for Cr-O bonds in the Mg(Al1-xCrx)2O4 and Zn(Al1-xCrx)2O4 series, respectively. However, the presently obtained Racah B values indicate increasing Cr-O bond covalency with increasing Cr3+ contents. This suggests that color changes and accompanying 10Dq variations may be due to variations in Cr-O bond covalency along the two solid-solution series, without or with very minor local Cr-O bond distance variation. Consequently, the ε values obtained from the present optical absorption spectra should be regarded as minimum values.

Thermodynamics of mixing and ordering in pyrope–grossular solid solution

Abstract Static lattice energy calculations have been combined with cluster expansion formalism to predict pairwise ordering interactions in the pyrope-grossular solid solution. The ordering interactions, the Js, have been then used to calculate the activity-composition relations over a wide temperature range with the help of the Cluster Variation Method. It is shown that short-range ordering in the system is driven by size mismatch. The prediction of the right signs and magnitudes of the ordering interaction energies requires separation of the mixing enthalpy into the configuration-dependent (chemical) and the configuration-independent (elastic) components. The study predicts the existence of a miscibility gap below 500ºC.

Recommended nomenclature for the sapphirine and surinamite groups (sapphirine supergroup)

Abstract Minerals isostructural with sapphirine-1A, sapphirine-2M, and surinamite are closely related chain silicates that pose nomenclature problems because of the large number of sites and potential constituents, including several (Be, B, As, Sb) that are rare or absent in other chain silicates. Our recommended nomenclature for the sapphirine group (formerly aenigmatite group) makes extensive use of precedent, but applies the rules to all known natural compositions, with flexibility to allow for yet undiscovered compositions such as those reported in synthetic materials. These minerals are part of a polysomatic series composed of pyroxene or pyroxene-like and spinel modules, and thus we recommend that the sapphirine supergroup should encompass the polysomatic series. The first level in the classification is based on polysome, i.e. each group within the supergroup corresponds to a single polysome. At the second level, the sapphirine group is divided into subgroups according to the occupancy of the two largest M sites, namely, sapphirine (Mg), aenigmatite (Na), and rhönite (Ca). Classification at the third level is based on the occupancy of the smallest M site with most shared edges, M7, at which the dominant cation is most often Ti (aenigmatite, rhönite, makarochkinite), Fe3+ (wilkinsonite, dorrite, høgtuvaite) or Al (sapphirine, khmaralite); much less common is Cr (krinovite) and Sb (welshite). At the fourth level, the two most polymerized T sites are considered together, e.g. ordering of Be at these sites distinguishes høgtuvaite, makarochkinite and khmaralite. Classification at the fifth level is based on XMg = Mg/(Mg + Fe2+) at the M sites (excluding the twolargest and M7). In principle, this criterion could be expanded to include other divalent cations at these sites, e.g. Mn. To date, most minerals have been found to be either Mg-dominant (XMg > 0.5), or Fe2+-dominant (XMg < 0.5), at these M sites. However, XMg ranges from 1.00 to 0.03 in material described as rhönite, i.e. there are two species present, one Mg-dominant, the other Fe2+-dominant. Three other potentially new species are a Mg-dominant analogue of wilkinsonite, rhönite in the Allende meteorite, which is distinguished from rhönite and dorrite in that Mg rather than Ti or Fe3+ is dominant at M7, and an Al-dominant analogue of sapphirine, in which Al > Si at the two most polymerized T sites vs. Al < Si in sapphirine. Further splitting of the supergroup based on occupancies other than those specified above is not recommended.

Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Abstract Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with quartz, Cr-V-bearing tremolite and muscovite-celadonitechromphyllite- roscoelite, diopside-kosmochlor-natalyite, Cr-bearing goldmanite, escolaite-karelianite, dravite-oxy-vanadium-dravite, V-bearing titanite and rutile, ilmenite, oxyvanite-berdesinskiite, shreyerite, plagioclase, scapolite, zircon, pyrite, and an unnamed oxide of V, Cr, Ti, U, and Nb. Crystals are emerald green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-chromium-dravite has a Mohs hardness of approximately 7½, and a calculated density of 3.3 g/cm3. In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: ω = 1.767(5), ε = 1.710(5). Vanadio-oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1260(2), c = 7.3759(1) Å, V = 1661.11(4) Å3, Z = 3. Crystal chemistry analysis resulted in the empirical structural formula: X(Na0.89K0.06□0.05) Y(V3+2.77Mg0.17Fe3+0.06) Z(Cr3+ 1.85Al1.59V3+ 0.78Mg1.78) T[(Si5.95Al0.05)O18] B(BO3)3V(OH2.91O0.09) W(O0.86F0.14). The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoKa X-radiation. Ideally, vanadio-oxy-chromiumdravite is related to oxy-chromium-dravite and oxy-vanadium-dravite by the homovalent substitution V3+ ↔ Cr3+. Tourmaline with chemical compositions classified as vanadio-oxy-chromium-dravite can be either Cr3+-dominant or V3+-dominant as a result of the compositional boundaries along the solid solution between Cr3+ and V3+ that are determined at Y+Z(V5Cr2), corresponding to NaY(V3)Z(V2Cr2Mg2) Si6O18(BO3)3(OH)3O, and Y+Z(V1.5Cr5.5), corresponding to NaY(V1.5Cr1.5)Z(Cr4Mg2)Si6O18(BO3)3(OH)3O.

A first record of strong structural relaxation of TO4 tetrahedra in a spinel solid solution

Abstract Optical absorption spectroscopy and X-ray structural refinements of seven different spinel single crystals on the (Mg1-xMnx)Al2O4 solid solution (x = 0.02-1.00) evidences exceptionally strong relaxation (ε = 0.83) of IVMn2+-O bonds. Our single-crystal structure refinements demonstrate that the ideal IVMn2+-O bond distance in fully ordered galaxite (MnAl2O4) should be 2.050 Å, which is 0.014 Å longer than previously suggested, and that structural parameters are mainly affected by the variations occurring at the TO4 tetrahedron. The very strong structural relaxation observed around the T site may be explained by the fact that the TO4 polyhedra of the spinel structure share only corners with neighboring MO6 octahedra and are fully isolated from neighboring TO4 tetrahedra. This provides structural flexibility around the T site and allows for considerable local T-O bond distance variations.

Structural refinement and crystal chemistry of Mn-doped spinel: A case for tetrahedrally coordinated Mn3+ in an oxygen-based structure

Abstract Spinel single crystals of four compositions along the MgAl2O4-MgMn2O4 join, with Mn3+ up to 0.25 apfu, were synthesized by use of a flux-growth method. The crystals were analyzed by electron microprobe, X-ray single-crystal diffraction, and optical absorption spectroscopy. Results revealed that Mg contents vary from 0.90 to 0.99 apfu, Mn2+ ≤ 0.11 apfu, Mn3+ varies from zero to 0.25 apfu, and Al-contents from 1.75 to 1.99 apfu. The unit-cell parameter increases linearly from 8.0883(3) to 8.1413(4) Å with increasing Mn3+ content. The crystals show moderately disordered cation distributions, with i = 0.23(1), and different distribution trends have been observed: the Mg content is constant at the T site and is replaced by Mn2+ at the M-site; Al decreases while Mn3+ increases in T- and M-sites. Mn3+ shows a preference for the T site, and a specific bond distance was refined, IVMn3+-O = 1.88(1) Å. Unpolarized room-temperature single-crystal spectra reveal two relatively broad absorption features at ca. 23 000 and 10 800 cm-1, which are assigned to spin-allowed d-d transitions in Mn3+ located at octahedral and tetrahedral sites, respectively. The bond valence approach shows that the bonds are strained in the tetrahedron indicating underbonding in T, whereas the bonds are unstrained in the octahedron. To reduce the M-M repulsion, the steric effect is driven by the movement of the oxygen atoms, which improves the shielding effect around the M-site, thus increasing the distortion of the structure relative to the CCP. As a result the tetrahedron undergoes an isotropic expansion, which constrains the structure to incorporate larger cations such as Mn3+ rather than smaller cations like Al at the T-site. This behavior, which is in disagreement with predictions based on crystal field energy considerations, illustrates the greater importance of steric factors on the cation distribution in spinels.

Galaxite, MnAl2O4, a spectroscopic standard for tetrahedrally coordinated Mn2+ in oxygen-based mineral structures

Abstract Chemical analyses, crystal structure refinement, cation order determination, and single-crystal optical absorption spectrum of synthetic galaxite are presented. New optical absorption spectra of natural Mn-bearing willemite, rhodochrosite, Mn-rich forsterite, and tephroite are reported for comparative purposes. The structure of a synthetic galaxite end-member is characterized by a relatively large unit-cell edge, a0 = 8.2104(3) Å, a u-parameter equal to 0.26588(7), a T-O distance of 2.0034(6) Å, and an M-O distance of 1.9310(5) Å. Mn2+ is strongly ordered at the tetrahedral T-site as demonstrated by the refined structural formula T(Mn2+0.90Al0.10)M(Mn2+0.10Al1.90)O4. The optical absorption spectrum of galaxite in the range 300.800 nm shows a set of five relatively sharp bands at 20 300, 22 250, 23 390, 25 970, and 27 780 cm-1 marking spin-forbidden transitions in Mn2+ at the tetrahedral site. The molar absorption coeficient of the field-independent 6A1(S) → 4Eg4A1g(G) absorption band at 23 390 cm-1 equals 1.90 L/(mol·cm), which is approximately an order of magnitude higher than for corresponding bands in spectra of compounds that contain isolated Mn2+-centered octahedra. The calculated crystal field splitting, 10Dq, for Mn2+ at the T-site in galaxite equals 5290 cm-1. This compares well with derived 10Dq-values of 5860 and 5510 cm-1 for Mn2+ at the tetrahedral T1- and T2-site in Mn-bearing willemite. In agreement with theory, the 10Dq for Mn2+ in MnO4 tetrahedra is ca. 30% smaller than corresponding values in MnO6 octahedra. The lower Racah B-parameters of the spectroscopic data indicate that the degree of covalency of Mn2+-O bonds is higher in tetrahedra than in octahedra.

Blue spinel crystals in the MgAl2O4-CoAl2O4 series: Part II. Cation ordering over short-range and long-range scales

Abstract Optical absorption spectroscopy and X-ray structural refinements were used to characterize short-range and long-range structures of 10 gem-quality, blue spinel single crystals synthesized on the (Mg1-xCox) Al2O4 solid solution (x = 0.07-1.00). The site distributions of Mg, Co2+, and Al show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Co2+, with a marked preference of Co2+ for tetrahedral coordination with respect to Mg, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a certain degree of inversion, i.e., occurrence of Al at T counterbalanced by the occurrence of divalent cations at M, which decreases from 0.24 to 0.13 with increasing Co2+ content. Short-range information based on the crystal field splitting parameter Dq derived from single-crystal optical spectra suggests that the local Co2+-O bond length at the T-site may increase marginally at increasing Co2+ content. An almost constant value for the Racah B-parameter, also derived from optical spectra, for tetrahedrally coordinated Co2+ suggests that any influence of substitutional second nearest neighbor cations on the ionicity of Co2+-O bonds at the T-site is very small. Long-range information shows that variations in the unit-cell parameter from 8.084 to 8.105 Å along the solid-solution series are mainly related to the ordering of Al at the M site as a result of the replacement of Mg by Co2+. Therefore, the spinel structure responds to the chemical variation by ordering of Al in such a manner that M-O remains almost constant and T-O increases. In this way, the lengths of shared octahedral edges are reduced and the destabilization effect due to the increased octahedral cation-cation repulsion is minimized. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure has also been shown to be valid in the MgAl2O4-CoAl2O4 solid-solution series.

Crystal chemistry of the MgAl2O4-MgMn2O4-MnMn2O4 system: Analysis of structural distortion in spinel and hausmannite-type structures

Abstract Single crystals of spinel and hausmannite having seven different compositions in the MgAl2O4- MgMn2O4-MnMn2O4 system were synthesized and structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, tetrahedral and octahedral bond lengths increase with increasing substitutions of Mn2+ for Mg and Mn3+ for Al, respectively. A transition from cubic to tetragonal symmetry occurs at a critical concentration of Mn3+ > 1.4 atoms per formula unit as a result of the Jahn-Teller distortion around octahedrally coordinated Mn3+. The present data in conjunction with data from the literature provide a basis for quantitative analyses of the cation polyhedral-distortion parameters and their variations in spinel- and hausmannite-type structures (Fd3̄m and I41/amd, respectively). In contrast to the linear correlation between (octahedral quadratic elongation) and σ2M (octahedral bond-angle variance) observed for many silicates and isomorphic structures, these two distortion parameters are not correlated in multiple oxides with spinel- and hausmannite-type structures. By using a model of multiple linear regression, it is demonstrated that varies as a function of both σ2M and ΔM (octahedral bond-length distortion). The degree of octahedral distortion is significant in the spinel structures and is in fact comparable with that calculated for the hausmannite-type structures. The degree of octahedral distortion is related to steric effects in both spinel- and hausmannite-type structures, whereas the electronic effects caused by Mn3+ account for the transition from cubic to tetragonal symmetry.

Gut contents and feeding in the Cambrian arthropod Naraoia

Abstract Element and diffraction analyses show that the gut contents in the Lower Cambrian arthropod Naraoia from Chengjiang, China, consist of genuine sediment. Such content in the gut is consistent with deposit-feeding in various marine worm groups. Deposit-feeding was probably the basic method of feeding in species of Naraoia, but it does not rule out occasional scavenging and even occasional capture of small prey. Less developed intestinal diverticula in N. longicaudata indicate that this species scanned the substrate surface for more nutritious food.