Stanislav Matsyuk

Nyerereite and nahcolite inclusions in diamond: evidence for lower-mantle carbonatitic magmas

Abstract Nyerereite and nahcolite have been identified as micro- and nano-inclusions in diamond from the Juina area, Brazil. Alongside them are Sr- and Ba-bearing calcite minerals from the periclase-wüstite series, wollastonite II (high), Ca-rich garnet, spinels, olivine, phlogopite and apatite. Minerals of the periclase-wüstite series belong to two separate groups: wüstite and Mg-wüstite with Mg# = 1.9-15.3, and Fepericlase and periclase with Mg# = 84.9-92.1. Wollastonite-II (high, with Ca:Si = 0.992) has a triclinic structure. Two types of spinel were distinguished among mineral inclusions in diamond: zoned magnesioferrite (with Mg# varying from 13.5-90.8, core to rim) and Fe spinel (magnetite). Olivine (Mg# = 93.6), intergrown with nyerereite, forms an elongate, lath-shaped crystal and most likely represents a retrograde transformation of ringwoodite or wadsleyite. All inclusions are composed of poly-mineralic solid mineral phases. Together with previously found halides, sulphates and other mineral inclusions in diamond from Juina, they form a carbonatitic-type mineral paragenesis in diamond which may have originated in the lower mantle and/or transition zone. Wüstite inclusions with Mg# = 1.9-3.4, according to experimental data, may have formed in the lowermost mantle. The source for the observed carbonatitic-type mineral association in diamond is lower-mantle natrocarbonatitic magma. This magma may represent a juvenile mantle melt, or be the result of low-degree partial melting of deeply-subducted carbonated oceanic crust. This magma was rich in volatiles, such as Cl, F and H, which played an important role in the formation of diamond.

Hydroxyl in omphacites and omphacitic clinopyroxenes of upper mantle to lower crustal origin beneath the Siberian platform

Abstract A series of clinopyroxenes from the lower crust and upper mantle beneath the Siberian platform was investigated by Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The IR spectra of all our samples exhibit three groups of absorption bands at (1) 3445−3465, (2) 3500-3540, and (3) 3600-3624 cm-1. Using synchrotron IR radiation, which utilizes a spot size of only 5 × 5 μm, we realized that the intensities of the absorption bands, mostly those of group 3, showed extreme variation within one crystal. TEM as well as polarized and high-pressure IR spectroscopy indicated that the OH groups that cause the bands of group 3 were not intrinsic but due to nm-sized inclusions of sheet silicates. The intensities and peak positions of the bands of group 2 correlate with the amount of tetrahedral Al3+ indicating that the charge-deficient substitution of Al for Si is responsible for the bands of group 2. The intensities and peak positions of the bands of group 1 correlate with the concentration of vacancies at M2 indicating that the cation vacancies at M2 control the incorporation of hydroxyl responsible for the bands of group 1. The bands of groups 1 and 2 are caused by the same type of OH dipole, however, occurring in different structural environments. The concentration of the structurally bound water of the omphacitic clinopyroxene is in the range from 31 to 514 ppm H2O (by weight). Surprisingly, the lowest concentration was found in clinopyroxene, that comes from the highest pressure region, i.e., the diamond-bearing eclogite xenoliths of the Mir kimberlite pipe. The highest values were obtained in omphacites of the lower-pressure grospydites of the Zagadochnaya kimberlite pipe and in omphacitic clinopyroxene of the lower pressure granulites of the Udachnaya kimberlite pipe, respectively. The low water content of clinopyroxene from the high-pressure region seems to be controlled by low water activity during crystallization. However, hydrogen loss during the uplift cannot be ruled out.

Local mean chromium–oxygen distances in Cr3+-centered octahedra of natural grossular-uvarovite garnet solid solutions from electronic absorption spectra

Abstract The crystal field parameter 10Dq[cm⁻¹] of octahedral chromium obtained as the energy of the 4A2g → 4T2g band of Cr3+ in the electronic absorption spectra, EAS, of a series of 13 natural grossular-uvarovite garnets with compositions close to the binary join Ca3(Al1–xCrx)2Si3O12, depend on the chromium fraction xCr of the solid solutions as 10Dq[cm⁻¹] = (–515.51 · xCr) + 16579 (r = 0.984) The 10Dq[cm⁻¹]-values were evaluated in terms of local mean octahedral distances, R̅(Cr–O), [Å]local with the result that R̅ (Cr–O) local = 0.01262 · xCr + 1.9812 (r = 0.982) (a) Equation (a) is in very good agreement with mean 〈Al/Cr—O〉; distances of “individual” octahedra R̅i calculated for the case XCr(individual) = 1.00 from the distance relations obtained in X-ray structure refinements of low symmetry chromian garnets (Wildner and Andrut, 2001). The proves again, that the EAS-method to derive local 3dN-ion–oxygen distances is reliable. R̅ (Cr–O) local = ƒ(xCr) and the corresponding R̅i-function deviate strongly from the Vegard line of the crystal averaged function R̅(Cr–O)average = ƒ(xCr), the “virtual crystal model”. This indicates significant structural relaxation around Cr3+[6] in the garnet solid solutions studied. The relaxation coefficient calculated from the data, ∊lim xCr→0 = 0.82, is closer to ∊ = 1.0 for full relaxation, i.e. the “hard sphere model”, than to ∊ = 0.0 for absent relaxation in the “virtual crystal” model. This is proposedly related to the uptake of strain around octahedral chromium, in the large (CaO8) polyhedra interconnected with the octahedra of the grossular-uvarovite garnets.

Optical absorption study of natural garnets of almandine-skiagite composition showing intervalence Fe2+ + Fe3+ → Fe3+ + Fe2+ charge-transfer transition

Abstract A broad (FWHM ≈ 7300 cm-1) intense band at ~21 700 cm-1 in the optical absorption spectra of natural Fe2+, Fe3+-rich garnets is attributed to electronic intervalence charge-transfer transitions (IVCT), VIIIFe2+ + VIFe3+ → VIIIFe3+ + VIFe2+. In Fe3+, Fe2+-bearing garnets of predominantly almandine compositions, this band causes yellowish tinges in addition to the pink color, typical of pure Fe3+-free almandines. In garnets from deeper-seated mafic granulites from kimberlite pipes in Siberia with high skiagite (Fe32+Fe23+Si3O12) contents, IVCT causes intense brownish-yellow colors. The relatively high energy of the band (~21 700 cm-1) compared to diverse minerals showing IVCT between Fe2+ and Fe3+ in adjacent octahedral sites, is attributed to the charge-transfer transition taking place between Fe2+ and Fe3+ in non-equivalent, dodecahedral and octahedral sites of the garnet structure. Band intensity is directly correlated with the product of Fe2+ and Fe3+ as measured by Mössbauer spectroscopy. The energy of the IVCT band is nearly independent of temperature, whereas its intensity decreases slightly with increasing temperature. Pressure induces a weak shift of the band to lower energies, Δν/ΔP ≈ -75 cm-1/GPa, but intensity of the bands remains practically unchanged. Such temperature and pressure dependencies are quite different from those in other minerals showing IVCT between Fe2+ and Fe3+ in equivalent octahedral positions of structure.

The crystal chemistry of the humite minerals: Fe2+-Ti4+ charge transfer and structural allocation of Ti4+ in chondrodite and clinohumite

Single crystals of the humite-group minerals, with general crystal chemical formula \(n{\cdot}[M_{2}SiO_{4}]{\cdot}[M_{1-x}Ti_{x}(F,OH)_{2-2x}O_{2x}]\) where M is predominantly Mg eventually substituted by Fe 2+ , are studied by electron microprobe analysis, X-ray diffraction and polarised electronic absorption spectroscopy, in the present paper two chondrodites (n = 2) and four clinohumites (n = 4). The aim was to elucidate colour and pleochroism of such minerals and to evaluate their local crystal chemical properties, esp. the structural allocation of Ti 4+ . The dominating features of all spectra are: (i) a slightly polarised absorption edge in the UV at energies > 35000 cm −1 ; (ii) a strong and broad band at 23300 cm −1 with band widths near 6000 cm −1 and strongly polarised with E ‖ X in all specimen of the two minerals; and (iii) a complex low energy band system in the NIR (11600 cm −1 in E ‖ X , 9500 cm −1 in E ‖ Z and 7700 cm −1 in E ‖ Y ) which corresponds in all details to the dd-band system in olivines caused by Fe 2+ in (M1) and (M2). Such spectral properties explain the observed colour and pleochroism, X golden yellow to orange, Y and Z light yellow to almost colourless. The band properties of (ii) are typical of excitation of metal-metal charge transfer, MM-CT, the band energy is consistent with that expected for MM-CT in Fe 2+ Ti 4+ pairs at a distance near 3.2 A. The analysis of the relation between optical and crystallographic vectors in the minerals studied suggests that the FeTi-CT interaction occurs predominantly along the polyhedral units M2 5 M3M3M2 5 in the structures of both chondrodite and clinohumite as it is the case for the iron-rich clinohumite of Platonov et al. (2001). The evaluation of all information obtained suggests that Ti 4+ is allocated in the M3 positions of the low-titanium chrondrodites and clinohumites studied.

The crystal chemistry of the humite minerals : spectroscopic studies and structure refinement of an unusual iron-rich clinohumite

Abstract An orange-red mineral with strong pleochroism from magnesian skarns of the Salminski rapakivi massif which was originally described as an olivine,is identified as an unusually iron-rich clinohumite.The composition of the mineral is,as obtained by combining data of electron microprobe analyses,Mössbauer analyses and unconstrained structure refinements: [Mg5.2(Fe3.422+Mn0.07Ni0.01Zn0.04Ca0.01Fe0.113+ Ti0.044+)Σ=3.70] × [Si4.03O16[F1.21(OH,O)0.79]Σ=2.00]. Thus,the Fe-end member fraction is XFe = 0.42, i.e. unusually high when compared with those of the usual magnesian clinohumites, e.g. = 0.06 in the crystal studied structurally by Robinson et al.(1973). Fe is the sumof octahedral,Mg-substituting cations which could - due to the similarity of their scattering curves and the very low fractions except for that of iron - not be discriminated in the structure refinement. Such refinements yield the following Fe-fractionations, fFe, between the different octahedral sites of the structure: fFe(M1c) =0.17, fFe(M1n) = 0.33, fFe(M26) = 0.32, fFe(M25) = 0.13, fFe(M3) = 0.05 (ΣFe = 1.00). The analysis of the Mössbauer spectra yields a fractionation pattern very close to that obtained by XRD. The Fe-fractioning in the ferrous clinohumite with XFe = 0.42 is nearly the same as that in magnesian clinohumite with XFe = 0.06, such similarities being obviously related to the fact that a slightly distorted hexagonally close anion package forms the basis of all the humite mineral structures. The structural changes on iron incorporation pertain predominantely changes in the individual and mean M–O distances and O–M–O angles. Polyhedral distortions Δoct and σoct2 increase most drastically, compared to magnesian clinohumite with XFe = 0.06, in the M26 octahedra. The polarized electronic absorption spectra are dominated by two features: an intense, broad band centered at 22800 cm-1 strongly polarized E ∥ X (nα ∢ b ca. 10°), which was identified as Fe2+(M25)-Ti4+(M3) charge-transfer transition in the M25M3M3M25 octahedral chain fragments of the structure. The strong polarization of this band together with the position of the UV absorption edge,causes the pleochroismof the mineral.The second prominent feature in the polarized single crystal spectra is a complex band system in the NIR which is assigned to dd-transitions of Fe2+(M1c + M1n), bands at 10800 and 7600 cm-1, and of Fe2+(M26), bands at 9400 and 8800 cm-1. Such transitions are in analogy with the behaviour of Fe2+ in the M1 and M2 sites of the olivine structure.