L. Secco

Comparative compressibility and structural behavior of spinel MgAl2O4 at high pressures: The independency on the degree of cation order

Abstract The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, V0, the bulk modulus, KT0, and its first pressure derivative, K′, were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) Å3, KT0 = 193(1) GPa, K′ = 5.6(3) for the ordered sample and V0 = 528.39(2) Å3, KT0 = 192(1) GPa, K′ = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the MgAl2O4 spinel.

High-pressure phase transformation in LiFeGe2O6 pyroxene

Abstract A synthetic pyroxene with composition LiFeGe2O6 and space group P21/c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a, c, β, and unit-cell volume. The transition is marked by the disappearance of b-type reflections (h + k = odd) forbidden in a C-centered lattice. The volume bulk modulus of the P21/c phase is estimated to be 110 GPa as compared to 147 GPa of the C2/c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P21/c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C2/c phase shows only minor deformations with pressure (~1.9%) due to its already strong kinking (~130°). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms.

Crystal structure refinement of 13 olivines in the forsterite-fayalite series from volcanic rocks and ultramafic nodules

The results of the present paper confirm the overall dependence of the geometric parameters of olivines on the Mg/(Fe+Mn) ratio. M1 and M2 volumes linearly decrease with increasing forsterite (Fo) content but at different rates. The difference between the volumes of the octahedral sites decreases gradually fromFo 92% toFo 50% and then remains virtually constant down toFo 0.2%.

Dalnegroite, Tl5-xPb2x(As,Sb)21xS34, a new thallium sulphosalt from Lengenbach quarry, Binntal, Switzerland

Abstract Dalnegroite, ideally Tl4Pb2(As12Sb8)Σ20S34, is a new mineral from Lengenbach, Binntal, Switzerland. It occurs as anhedral to subhedral grains up to 200 μm across, closely associated with realgar, pyrite, Sb-rich seligmanite in a gangue of dolomite. Dalnegroite is opaque with a submetallic lustre and shows a brownish-red streak. It is brittle; the Vickers hardness (VHN25) is 87 kg mm-2 (range: 69-101) (Mohs hardness ~3-3½). In reflected light, dalnegroite is highly bireflectant and weakly pleochroic, from white to a slightly greenish-grey. In cross-polarized light, it is highly anisotropic with bluish to green rotation tints and red internal reflections. According to chemical and X-ray diffraction data, dalnegroite appears to be isotypic with chabournéite, Tl5-xPb2x(Sb,As)21-xS34. It is triclinic, probable space group P1, with a = 16.217(7) Å, b = 42.544(9) Å, c = 8.557(4) Å, α = 95.72(4)°, β = 90.25(4)°, γ = 96.78(4)°, V = 5832(4) Å3, Z = 4. The nine strongest powder-diffraction lines [d (Å) (I/I0) (hkl)] are: 3.927 (100) (2̅10 0); 3.775 (45) (22̅2); 3.685 (45) (4̅60); 3.620 (50) (440); 3.124 (50) (2̅8̅2); 2.929 (60) (42̅2); 2.850 (70) (4̅42); 2.579 (45) (0 1̅4̅ 2); 2.097 (60) (024). The mean of 11 electron microprobe analyses gave elemental concentrations as follows: Pb 10.09(1) wt.%, Tl 20.36(1), Sb 23.95(1), As 21.33(8), S 26.16(8), totalling 101.95 wt.%, corresponding to Tl4.15Pb2.03(As11.86Sb8.20)S34. The new mineral is named for Alberto Dal Negro, Professor in Mineralogy and Crystallography at the University of Padova since 1976.

Intracrystalline relationships in olivine, orthopyroxene, clinopyroxene and spinel from a suite of spinel lherzolite xenoliths from Mt. Noorat, Victoria, Australia.

A detailed crystal chemical study of coexisting olivine, orthopyroxene, clinopyroxene and spinel from selected Victorian (Australia) lherzolite suites was carried out by means of single crystal x-ray diffraction and electron probe microanalysis to obtain actual site occupancies. The aim of this study was primarily to characterise the intracrystalline configurations and related cation ordering on sites in major mantle constituents. The results demonstrate that cation ordering on sites is subject to distinctive crystallographic controls which depend on the petrological evolution of the suite. Mg-Fe2+ ordering in M1–M2 pyroxene sites depends on variations of the smaller cations, mainly Alvi, Ti4+, Fe3+, and related configurations of M 1. Pressuresensitive Alvi is crucial to Fe2+, the more ordered clinopyroxene showing high Alvi configurations which tend to exclude the larger bivalent cations and yield small polyhedral volumes for M 1, M 2, T sites and the unit cell. Conversely, the coexisting orthopyroxene, characterised by lower Alvi configuration and higher M 1 and unit cell volumes, is relatively more disordered. Olivine is consistent with the coexisting clinopyroxene, the more disordered crystals coexisting with more disordered clinopyroxene, while Al-Mg order in the coexisting spinel shows the reverse relationship. Estimated temperatures of apparent equilibration based on current geothermometers are not considered realistic. Assumptions of ideal cation mixing on sites in pyroxene and spinel are not supported.

New accurate elastic parameters for the forsterite-fayalite solid solution

Abstract Three natural olivines with Fo80Fa20, Fo71Fa29, and Fo62Fa38 compositions were investigated in situ at high pressure by single-crystal X‑ray diffraction using a diamond-anvil cell up to -8 GPa at room temperature. The bulk modulus, KT0, and its first pressure derivative, K′, do not show any significant variation among the compositions investigated and, using the data on a further sample with Fo92Fa8 composition recently investigated in the same laboratory and using the same experimental technique, we obtain, for the first time, a single equation of state for the entire Fo92Fa8-Fo62Fa38 compositional range. The equation has the following coefficients: KT0 = 124.7(9) GPa and K′ = 5.3(3) and can be used for thermodynamic calculations involving the most common mantle olivine compositions.

Crystal chemistry of a clinopyroxene series in ultrafemic xenoliths from North-Eastern Brazil

A series of clinopyroxenes (Cpx) in peridotitic spinel nodules from Rio Grande do Norte, North-Eastern Brazil, was investigated by X-ray diffraction and electron microprobe analyses and compared with an analogous series from spinel peridotite nodules from Mt. Leura, Victoria, Australia. The examined Cpx series is mainly characterized by increasing Mg/(Mg+Fe2+) ratio (i.e. increase of refractory character of the host nodules), by the substitutions of Alvi vs Mg in the M1 site and Na vs (Ca+Mg) in the M2 site, whereas Aliv in the T site remains substantially constant. The strong Na depletion in the Brazilian Cpx causes a different charge balance from that in the Australian Cpx, in which Na remains quite constant. This feature, associated with the differences in polyhedral and cell volumes which are significantly higher in the Brazilian Cpx than in those of the Australian Cpx, suggests that the Brazilian Cpx crystal chemistry may be related to a lower pressure regime, compared with that of the Australian Cpx series.

Crystal chemistry of Cr3+-V3+-rich clinopyroxenes

Abstract Eleven clinopyroxenes from the Sludyanka Crystalline Complex in Russia belonging to the ternary join NaVSi2O6-NaCrSi2O6-CaMgSi2O6 (natalyite-kosmochlor-diopside) were studied by means of X-ray single crystal diffractometry and electron probe microanalysis. The crystal chemical data show that the T site is almost completely occupied by Si, so that the Na (V3+,Cr3+) → Ca Mg substitution mechanism ensures charge balance. Changes in M1 site geometry are explained by the aggregate ionic radius, and are influenced by Mg occupancy and V3+/(V3+ + Cr3+) ratio. The M2 site geometry depends both on Na content and on the (V3+,Cr3+) → Mg substitution in M1 site. Changes in M2-O3c1 bond length are mainly related to Na content, whereas the longest M2-O3c2 bond lengths are significantly affected by the V3+/(V3+ + Cr3+) ratio of the M1 site. The T site geometry is affected by chemical and geometrical variations at the M1 and M2 sites, principally the M1 site occupancy.

The crystal structure of dalnegroite, Tl5–xPb2x(As,Sb)21–xS34: a masterpiece of structural complexity

Abstract The crystal structure of the rare mineral dalnegroite, Tl5−xPb2x(As,Sb)21−xS34 with x ≈ 1, was determined for a crystal from Lengenbach, Binn Valley, Switzerland. The structure is triclinic, space group P1, with a = 16.218(3), b = 42.546(7), c = 8.558(1) Å, α = 95.70(4), β = 90.18(3), γ = 96.38(4)º, V = 5838.9(9) Å3, Z = 4. Refinement of an isotropic model led to an R1 index of 0.0536 for 22226 observed reflections and 980 parameters, and R1 = 0.0590 for all 25266 independent reflections. Although dalnegroite cannot be considered a layered compound, its structure can be usefully described as a regular alternation of two kinds of layers stacked along the b axis, with four layers in the unit cell: (1) a layer 7.8 Å thick, at y ≈ 0.15 and 0.65, can be considered as derived from the SnS archetype; (2) a layer 13.6 Å thick, at y ≈ 0.35 and 0.85, derived from the PbS archetype. Different chemical compositions, such as Tl:Pb and Sb:As ratios, for different samples belonging to the chabournéite-dalnegroite family could play a central role in controlling different degrees of order, leading to different superstructures.

Crystal-chemistry of high-pressure clinopyroxene from spinel lherzolite nodules Mts. Leura and Noorat Suites, Victoria, Australia

SummaryThe clinopyroxene suites from lherzolite nodules from Mts. Leura and Noorat (Victoria, Australia) have been investigated by X-ray diffraction and electron probe microanalyses (Dal Negro et al., 1984; Cundari et al., 1986).The evolution of the host nodule is shown by general depletion of AlIv, AVI, Ti4+, Fe2+ and enrichment of Si, Mgm,, Fe3+, Cr3+, Ca, while Na depletion occurs only in the clinopyroxenes from Mt. Noorat. Different mechanisms of cation substitution are thus involved in the two clinopyroxene suites, suggesting different total pressures of equilibration in the stability field of spinel.Modifications involving the M1 octahedron are mainly ascribed to variable amounts of trivalent cations, the volume of the M 1 site increasing with decreasing content of trivalent cations in each suite. The differences in M 1 site configuration between the Mt. Leura and Mt. Noorat clinopyroxene suites are ascribed to the different amounts of A1v1 and FeM,.The volume of the T tetrahedral site is generally related to A1Iv in each suite. An increase in T site volume from Mt. Leura to Mt. Noorat clinopyroxenes was found however, due to lengthening of the T-Obrg bond lengths, for similar AIIv contents. The volume of the M2 site, generally related to Ca content, was generally higher in the Noorat clinopyroxenes for similar Ca (and Ca + Na) contents, due to the longer M2-O3C1 bond length, strongly related to Na content. Cell volume is linearly correlated to M 1 volume in each suite, but is definitely higher in the Noorat clinopyroxenes for similar M 1 volume. All the structural data show that the total pressure of equilibration in the Noorat clinopyroxenes was lower than that in the Leura clinopyroxenes, as suggested by chemical data.ZusammenfassungDie Klinopyroxenparagenesen aus Lherzolitheinschlüssen von Mt. Leura und Mt. Noorat (Victoria, Australien) wurden mittels Röntgendiffraktometrie und Elektronenmikrosonde untersucht (Dal Negro et al., 1984; Cundari et al., 1986). Die Lherzolitheinschlüsse zeigen generell eine Abnahme von AlIV, AlVI, Ti4+, FeMI2+ und eine Zunahme von Si, Mgm,, Fe3+, Cr3+ und Ca. Eine Abnahme von Na tritt nur in Klinopyroxenen von Mt. Noorat auf. Verschiedene Substitutions-Mechanismen der Kationen weisen auf verschiedene Equilibrationsdrucke im Stabilitätsbereich der Spinelle hin und sind daher in den beiden Klinopyroxenserien zu berücksichtigen. Modifikationen in der oktaedrischen M1-Position wurden vor allem dem wechselnden Anteil an dreiwertigen Kationen zugeschrieben, wobei in jeder der beiden Serien das Volumen der Ml-Position mit abnehmenden Gehalten dreiwertiger Kationen zunimmt. Die unterschiedliche Konfiguration der M1-Position der Klinopyroxene von Mt. Leura und Mt. Noorat wird mit unterschiedlichen Gehalten an AlVI und FeMI2+, in Zusammenhang gebracht.Das Volumen der tetraedrischen T-Position ist normalerweise mit den Gehalten an AllIV in jeder Serie verknüpft. Ein Vergleich der Klinopyroxene von Mt. Leura und Mt. Noorat zeigte jedoch, daß bei gleichen Gehalten an AlVI das Volumen der tetraedrischen Position infolge einer Aufwertung der T-Obrg.-Bindungen zunimmt. Das Volumen der M2-Position, üblicherweise mit dem Ca-Gehalt korreliert, ist in den Klinopyroxenen von Mt. Noorat bei ähnlichen Ca (und Ca f Na) Gehalten infolge größerer M2--O3C1-Abstände größer. Sie stehen also mit den Na-Gehalten in Verbindung.Das Volumen der Elementarzelle korreliert mit dem der M1-Position in jeder Serie; es ist aber in den Klinopyroxenen von Mt. Noorat deutlich höher. Die Ergebnisse der Strukturuntersuchungen zeigen-wie auch die chemischen Daten-, daß die genannten Equilibrationsdrucke für die Klinopyroxene von Mt. Noorat niedriger waren als für die von Mt. Leura.