A. Dal Negro

Residual electron density at theM2 site inC2/c clinopyroxenes: Relationships with bulk chemistry and sub-solidus evolution

The systematic study of both natural and synthetic clinopyroxenes often indicates the presence in the difference Fourier map of a maximum of residual density of up to 0.8 electrons, here labelledM2′, close to theM2 site along the diad axis, defining a square pyramid co-ordination polyhedron. To investigate the nature and the crystalchemical implications of this feature, a limited but representative set of clinopyroxenes of volcanic, metamorphic and synthetic origin has been investigated by X-ray structure refinement (at 0.7 and 0.4 Å resolution), by microprobe analysis and by transmission electron microscopy. The most important results are: a) at increasing resolution, the height of theM2′ peak increases while its co-ordinates move towardM2; b) as (Ca + Na) content approaches 1.0 atom per formula unit,M2′ vanishes; c)M2′ has been found in clinopyroxenes which show differing incipient exsolution microstructures, from spinodal decomposition to non-periodic fluctuations, as well as in homogeneous specimens. The presence ofM2′ is interpreted in terms of the simultaneous coexistence in the crystals of two different structural models, approximately diopside and clinoenstatite. An accurate evaluation of the totalM2 + M2′ site occupancy is strongly suggested in XREF work, particularly when thermodynamic and kinetic considerations have to be obtained with accurate determinations of site occupancy factors as a starting point.

Crystal chemistry and evolution of the clinopyroxene in a suite of high pressure ultramafic nodules from the Newer Volcanics of Victoria, Australia

A clinopyroxene suite from lherzolite inclusions associated with the Victorian (Australia) “Newer Volcanics” has been investigated with the aim of understanding the clinopyroxene crystal-chemical response to increasing temperature (e.g. a melting model and/or crystallization processes prevailing at high pressure).The M1 clinopyroxene polyhedron dominates the intracrystalline physical-chemical variations, essentially given by the triple substitution AlVIFeM12Ti4+⇌Cr3+ Fe3+MgM12+corresponding to an increase in the volume of M1 with increasing Mg/Mg+Fe2+ (mg) for the clinopyroxene. A relative Ca2+ increase in M2 ensures the necessary charge balance. However, Na+ occupancy of M2 persists to the highest mg values, i.e. maximum thermal stability, where the volume of M2 is the largest due to FeM22+depletion. The variations of M1 and M2 volumes are greater than, and opposite to, the variations in the volume of T (tetrahedron) by factors of ca. 3 and 1.5, respectively. Inclusions with relatively low clinopyroxene content (Mt. Porndon specimens) show distinct intracrystalline variations, essentially reflecting lower AlVI, i.e. higher volume of M1, and implying a lower pressure regime compared to clinopyroxene-rich analogues (Mt. Leura specimens). The intracrystalline relationships of the Mt. Porndon clinopyroxene suggest that the host peridotite inclusions survived larger degree of mantle melting at shallower depths relative to the Leura analogues.

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.

Crystal chemistry of Ca-rich pyroxenes from undersaturated to oversaturated trachytic rocks, and their relationships with pyroxenes from basalts

Abstract A study of 23 Ca-rich pyroxenes from silica-undersaturated and -oversaturated rocks, shows that all the pyroxenes from trachytes are characterized by a relatively large M1 site, related to its high Fe 2+ occupancy. The volume of the M1 site is inversely related to that of the tetrahedral site. The high Si content of the pyroxene from trachytes causes significant increase in the longest M2-O3C2,D2 distances. The most striking difference between the pyroxenes from trachytes and those from basalts is found in the volumes of the M1 and T sites, and in the degree of distortion of the M2 polyhedron (ΔM2). The relationship between the β angle and ΔM2 provides an estimate of: (1) the degree of alkalinity in trachytic melts, and (2) indications on the nature of their possible parental “basaltic” magmas.

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.

Distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks

A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe2+ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Aliv to balance the effect of increasing (Fe3++Ti4++Alvi+Cr3+)M1.The Fe2+/Fe3+ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe2+ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe2+ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.

Kinetics of cation ordering in natural Mg(Al,Cr3+)2O4 spinels

Abstract The kinetics of cation ordering (quench method) in two natural Mg(Al2-yCry)O4 spinels (y~ 0.03-0.06 and 0.24), highly ordered in terms of Mg-Al, were studied by means of X-ray single-crystal diffraction. The equilibrium distribution of Mg and Al between tetrahedral and octahedral sites was investigated at 650 °C (in disordering and ordering) and at 850 °C (in ordering), through several time-steps to monitor the rate of cation distribution before equilibrium was achieved. The cation distributions for both disordering and ordering experiments were obtained by measuring the oxygen positional parameter u, which is correlated to the inversion parameter x (Al in T site), and then to the composition of the samples. The Mueller kinetic model, satisfactorily applied to the experimental data, allowed the calculation of the kinetic ordering constants K, linearly related to temperature by means of Arrhenius equations. The kinetics of ordering processes are influenced by Cr content. The equilibrium for both the isotherms at 650 and 850 °C was reached at different elapsed times by the low- and high-Cr spinels: the time for the low-Cr sample was, in both the ordering experiments, about double that of the high- Cr sample. Consequently, the activation energy (186 and 175 kJ/mol for low- and high-Cr samples, respectively) for the intracrystalline Mg-Al ordering decreases with Cr increase.

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.

Influence of magma composition and oxygen fugacity on the crystal structure of C2/c clinopyroxenes from a basalt-pantellerite suite

The crystallochemical variations of clinopyroxene in response to changes in fO2 and melt composition have been determined for a basalt-pantellerite suite (Boseti Complex, Main Ethiopian Rift) by crystal structure refinement and microprobe analysis. The pyroxene evolutionary trend has both a “Ca-minimum” and late iron enrichment. During crystallization from basalts to trachytes, clinopyroxene geometry depends mainly on the relationships between T and M2 sites; for example, high SiO2 activity in the magma causes high Si occupancy in T site, which in turn requires low Ca occupancy in M2 site in order to fulfill the local charge balance requirements. In contrast, clinopyroxene crystallized from acid melts is characterized by high Fe2+ (M1) content and therefore by a very large M1 site. Longer 〈M1-O1〉 and M1-O2 bond lengths require shorter T-O1 and T-O2 bond lengths and high Si occupancy in T site. It is concluded that the “Ca-minimum” in the clinopyroxene structure is regarded as the lowest value at which the charge balance requirements are satisfied in a C2/c clinopyroxene structure.

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.