Reinvestigation and application of olivine-quartz-orthopyroxene barometry

Experiments in a piston-cylinder apparatus have been carried out at 700–1050°C, 10–16 kbar to determine the stability of ferrosilite (FeSiO₃) relative to fayalite + quartz. Reaction reversals within 0.1-kbar intervals locate the equilibrium at 10.5, 11.0, 11.5, 12.0, 12.6, 13.3, 14.1 and 14.8 kbar at 700, 750, 800, 850, 900, 950, 1000, and 1050°C, respectively, reflecting the intercept with the α-β quartz transition at about 880°C. The tight reversals severely constrain the reaction slope, providing a basis for limited extrapolation and calculations. However, the lack of accurate activity and cation-distribution data for orthopyroxene and olivine generates substantial uncertainties when considering the effects of large proportions of additional components such as MgO. Experiments and calculations indicate that additional components dramatically extend the pyroxene stability field and that pressures that have been inferred from ferrosilite-rich pyroxenes in natural assemblages are 1–3 kbar too high.     

Basaltic vitrophyre 15597: An undifferentiated melt sample

Apollo 15 sample 15597, from the rim of Hadley Rille, is a pyroxene vitrophyre consisting primarily of acicular pyroxene phenocrysts and glass matrix. The pyroxene centers are a Ca-poor and Mg-rich pigeonite of Wo₄En₇₀Fs₂₆ which zones outward to Wo₁₅En₅₂Fs₃₃. The latter is in contact with an epitaxial augite overgrowth of Wo₃₀En₄₀Fs₃₀ which itself zones to a Mg-poor but still Ca-rich ferroaugite of Wo₃₂En₁₀Fs₅₈. Toward the rims, Al₂O₃ and TiO₂ contents increase to a maximum of 13.8 and 3.5 wt% respectively. These zoning trends are attributed to pyroxene crystallization in the absence of co-crystallization of plagioclase or a Ti-rich phase. Grains of chromite and native nickeliron are disseminated throughout the rock. The chromites have the highest Cr₂O₃ and V₂O₃ contents (～ 55and1.8wt%) of any reported lunar spinel, and appear to have formed for the most part before pyroxene crystallization began.Textural evidence, including the vitrophyric nature of the sample itself, the unusual compositions of the pigeonite centers (high Mg and low Ca) and chromites (high Cr), and the extreme chemical zonation of the pyroxenes all give strong evidence that this rock arrived at the lunar surface in an essentially entirely liquid state, and that eruption was followed by metastable pigeonite nucleation, rapid metastable growth and continued metastable pyroxene nucleation, and final solidification. It thus may represent one of the best examples of a mare basalt completely unaffected by local differentiation.     

Compositional heterogeneity of distal tephra deposits from the 1912 eruption of Novarupta, Alaska

Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An₂₅ plagioclase, Fs₄₂ orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An₄₅ plagioclase and Fs₃₅, orthopyroxene.Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.     

Petrogenesis of basalts from the project FAMOUS area: experimental study from 0 to 15 kbars

Tholeiitic basalt glasses from the FAMOUS area of the Mid-Atlantic Ridge are among the most primitive basaltic liquids reported from the ocean basins. One of the more primitive of these[Mg/(Mg+Fe²⁺) = 0.68;Ni= 232ppm;TiO₂ = 0.61] glasses (572-1-1) was selected for an experimental investigation. This study found olivine to be the liquidus phase from 1 atm to 10.5 kbar where it is replaced by clinopyroxene. The sequence of appearance of phases at 1 atm pressure is olivine (1268°C), plagioclase (1235°C) and clinopyroxene (1135°C). The sample is multiply saturated at 10.5 kbar with olivine (Fo₈₈), clinopyroxene (Wo₃₂En₆₀Fs₉), and orthopyroxene (Wo₅En₈₃Fs₁₂). From the 1-atm data we have measured (FeO/MgO) olivine/(FeO*/MgO) liquid (K′_D) for olivine-melt pairs equilibrated at 12 temperatures in the range 1268–1205°C.K′_D varies from 0.30 at 1205°C to 0.27 at 1268°C. Analysis of high-pressure olivine melt pairs indicates a systematic increase inK′_D with pressure.Evaluation of the 1-atm experiments reveals that fractionation of olivine followed by olivine + plagioclase can generate much of the variation in major element chemistry observed in the FAMOUS basalt glasses. However, it cannot account for the entire spectrum of glass compositions — particularly with respect to TiO₂ and Na₂O. The variations in these components are such as to require different primary liquids.Comparison of clinopyroxene microphenocrysts/xenocrysts found in oceanic tholeiites with experimental clinopyroxenes reveal that the majority of those in the tholeiites may have crystallized from the magma at pressures greater than ～ 10 kbar and are not accidental xenocrysts. Clinopyroxene fractionation at high pressures may be a viable mechanism for fractionating basaltic magmas.The major and minor element mineral/meltK′_d's from our experiments have been used to model the source region residual mineralogy for given percentages of partial melting. These data suggest that ～20% partial melting of a lherzolite source containing 0–10% clinopyroxene can generate the major and minor element concentrations in the parental magmas of the Project FAMOUS basalt glasses.     

Pyroxenes in Serra de Magé: cooling history in comparison with Moama and Moore County

Single-crystal X-ray, optical, and microprobe study of pyroxenes in the Serra de Magéfeldspar cumulate eucrite indicate complex exsolution features from a slow cooling history. Two pyroxenes now exist: “low” orthohypersthene ( P2₁ca) as host ( ～82 vol.%) and augite ( C2/c) in four distinct habits. This pyroxene pair yields an apparent “equilibration” temperature of ～900°. These relations are typical for orthopyroxene of both the Stillwater and Kintoki-San types, indicating an original pigeonite pyroxene with a bulk composition En₅₁Fs₃₉Wo₁₀. Variations in augite-hypersthene textural relationships suggest variable initial compositions from about Wo₈ to Wo₁₁. The bulk composition is intermediate to those of initial pigeonites in Moama and Moore County but the augite-hypersthene tie line is longer suggesting a slower cooling history. Our examinations of all three meteorites show that Serra de Magéaugite lamellae are as thick or thicker than those in the other meteorites, contrary to the measurement of Miyamoto and Takeda. The compositional data, textural relations, and existence of P2₁ca hypersthene suggest at least a comparable if not slower cooling history for Serra de Magé.     

High-pressure phase transformations in the system of MgSiO3-FeSiO3

Two synthetic pyroxenes (FeSiO₃, MgSiO₃) and five natural pyroxenes with compositions of about Fs₈₀En₂₀, Fs₆₀En₄₀, Fs₅₀En₅₀, Fs₄₀En₆₀, and Fs₂₀En₈₀ have been subjected to pressures up to250 ± 50kbars at a temperature of about1500 ± 200°C in a diamond anvil cell heated by an infrared laser beam. After quenching and unloading X-ray data analysis indicates that (1) those with Mg less than 50% undergo the following reactions: 2(Mg,Fe)SiO₃ (pyroxene) → (Mg,Fe)₂SiO₄ (spinel) + SiO₂ (stishovite) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO₂ (stishovite) with increase of pressure, and (2) those with Mg higher than 60%, undergo the following reactions: 2(Mg,Fe)SiO₃ (pyroxene) → (Mg,Fe)₂SiO₄ (spinel) + SiO₂ (stishovite) → 2(Mg,Fe)SiO₃ (hexagonal phase) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO₂ (stishovite) with increase of pressure.     

Cation distribution in low-calcium pyroxenes: Dependence on temperature and calcium content and the thermal history of lunar and terrestrial pigeonites

Four pyroxenes with compositions En₄₈Fs₄₈Wo₄, En_47·5Fs_47·5Wo₅, En₄₅Fs₄₅Wo₁₀ and En₄₀Fs₄₀Wo₂₀, synthesized at 1200°C at atmospheric pressure, were heat-treated at 500, 600, 700, and 800°C for various lengths of time. These pyroxenes are variously ordered with respect to Fe²⁺ and Mg²⁺ without unmixing. The Fe²⁺-Mg²⁺ distribution over the two nonequivalent sites M1 and M2, determined through Mössbauer spectroscopy, is found to be a function of both temperature and concentration of Ca²⁺ at the M2 site. The preference of Fe²⁺ for the M2 site increases with decreasing temperature and increasing Ca²⁺. These data can be used to determine cation equilibration temperatures of lunar and terrestrial pigeonites. The lunar pigeonites usually indicate equilibration temperatures of 700–860°C, except the pigeonite from rock 14053, which may have been subjected to shock heating due to meteoritic impact.     

Calorimetric study of the stability of high pressure phases in the systems CoOSiO2 and “FeO”SiO2, and calculation of phase diagrams in MOSiO2 systems

High temperature calorimetric measurements of the enthalpies of solution in molten if2 PbO · B₂O₃ of α- and γ-Fe₂SiO₄ and α-, β-, and γ-Co₂SiO₄ permit the calculation of phase relations at high pressure and temperature. The reported triple point involving α-, β-, and γ-Co₂SiO₄ is confirmed to represent stable equilibrium. The curvature in the α?β phase boundary in Co₂SiO₄ and of an α?γ boundary in Fe₂SiO₄ at high temperature is explained in part by the effects of compressibility and thermal expansion, but better agreement with the observed phase diagram is obtained when one considers the effect of small amounts of cation disorder in the spinel and/or modified spinel phases. The calculated ΔH⁰ and ΔS⁰ values for the α?β, α?γ, and β?γ transitions show that enthalpy and en changes both vary strongly in the series Mg, Fe, Co, and Ni, and are of equal importance in determining the stability relations. The disproportionation of Fe₂SiO₄ and Co₂SiO₄ spinel to rocksalt plus stishovite is calculated to occur in the 170–190 kbar region; cation disorder and/or changes in wüstite stoichiometry can affect the P?T slope. The calorimetric data for CoSiO₃ and FeSiO₃ are in good agreement with the observed phase boundary for pyroxene formation from olivine and quartz. The decomposition of pyroxene to spinel and stishovite at pressures near the coesite-stishovite transition is predicted in both iron and cobalt systems. The use of calorimetric data, obtained from small samples of high pressure phases, is very useful in predicting equilibrium phase diagrams in the 50–300 kbar range.     

Mineralogy of the Ibitira eucrite and comparison with other eucrites and lunar samples

Ibitira meteorite is interpreted as a strongly metamorphosed, unbrecciated, vesicular eucrite with a primary variolitic and secondary hornfelsic texture dominated by 60% pyroxene (bulk composition En₃₇Fs₄₈Wo₁₅ exsolved into lamellae several micrometers wide of augite En₃₂Fs₂₇Wo₄₁ and pigeonite En₄₀Fs₅₆Wo₄) and 30% plagioclase An₉₄ (mosaic extinction and variable structural state). Minor phases are 5% tridymite plates one-quarter occupied by plagioclase (An₉₄) inclusions; several percent intergrowths of ilmenite and Ti-chromite with trace kamacite Fe₉₉Co_0.5Ni_0.2 and narrow olivine (Fa₈₃) rims; one grain of low-Ti-chromite enclosed in tridymite; trace troilite with kamacite Fe₉₈Co_1.0Ni_0.9. Euhedral ilmenite, Ti-chromite, plagioclase and merrillite in vesicles indicate vapor deposition. These properties can be explained by a series of processes including at least the following: (1) igneous crystallization under pressure low enough to allow vesiculation, (2) prolonged metamorphism, perhaps associated with vapor deposition and reduction, to produce the coarse exsolution of the pyroxene and the coarse ilmenite-chromite intergrowths, (3) strong shock which affected the plagioclase and tridymite but not the pyroxene, (4) sufficient annealing to allow recrystallization of the plagioclase and tridymite, and partial conversion to the low structural state of the former.     

High-pressure phase equilibria in a garnet lherzolite,with special reference to Mg2+Fe2+ partitioning among constituent minerals

Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg²⁺Fe²⁺, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg₂SiO₄Fe₂SiO₄. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe²⁺/(Mg + Fe²⁺) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe²⁺/(Mg + Fe²⁺) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe²⁺ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg²⁺Fe²⁺ partition relation among coexisting minerals, an increase of about 1% in the Fe₂SiO₄ component in (Mg,Fe)₂SiO₄ modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.     

Experimental investigation of CaMg exchange between olivine,orthopyroxene, and clinopyroxene: potential for geobarometry

The distribution of Ca and Mg among coexisting olivine, clinopyroxene and orthopyroxene has been studied in a piston-cylinder apparatus in the temperature range 1100–1300°C and pressure range 9–41 kbar. Ca in olivine decreases with increasing pressure and decreasing temperature. The pressure effect is the result of Ca going into the higher-coordination M2 site in clinopyroxene as pressure is increased. For the CaMg exchange reaction between olivine and clinopyroxene, ΔV°=0.249J bar^?1 mole^?1; this is sufficient for pressure estimates accurate to ±3kbar if temperatures of equilibration are independently known. CaMg exchange between olivine and orthopyroxene is not sufficiently pressure dependent to be used as a geobarometer.Application of the olivine-clinopyroxene geobarometer to coarse garnet lherzolites from southern Africa gives P-T results consistent with a continental geotherm. For spinel lherzolites from southwestern United States, the geotherm appears to be displaced to higher temperatures indicating oceanic affinities. Application of the geobarometer to natural systems requires assumptions about activity relationships in clinopyroxene which should be checked by experiment.     

Experimental investigation of reaction coronas on olivine in Apollo 14 high-grade breccias

Reaction coronas of pyroxene ± ilmenite occur around clasts of olivine in Apollo 14 high-grade metamorphic breccias. In experiments of several months duration, there was no evidence of corona formation at 1000°C, but at 1050°, withfO₂ at or above Ilm-Ru-Fe and below Fe-Fe₁?x O, incipient coronas formed around Fo_50–70 in synthetic 14311 matrix. In addition, withfO₂ controlled by Ilm-Ru-Fe at 1050°C, the olivines reduced to Fo₆₈, En₆₉ + Fe. Reduction of olivine under these conditions is inconsistent with the calculated stability relations and is attributed to uncertainties in the activity coefficient for olivine or pyroxene. The experiments also suggest that vesicularity in the Apollo 14 high-grade breccias may correlate with the amount of glassy material in their unmetamorphosed precursors. The metamorphic event is attributed to burial in a hot ejecta blanket, such as that of the Imbrium event.     

High-pressure decompositions in cobalt and nickel silicates

High-pressure stability relations in cobalt and nickel silicates have been studied over the pressure range 130–330 kbar employing a double-staged split-sphere-type high-pressure apparatus.γ-Co₂SiO₄ and γ-Ni₂SiO₄ decompose directly into their constituent oxide mixtures (rocksalt plus stishovite) 175 kbar and 280 kbar, respectively. The result that γ-Ni₂SiO₄ has a wider stability field in pressure than γ-Co₂SiO₄, is consistent with simple crystal-field theory.The experimental precision is high enough to show that the decomposition boundary of γ-Co₂SiO₄ has a positive slope (dP/dT > 0) and a preliminary determination of the boundary curve is P(kbar) = 0.065 T (°C) + 110.No positive evidence for the existence of high-pressure forms of CoSiO₃ and NiSiO₃ has been obtained in these quenching experiments, and they finally decompose into constituent oxide mixtures as in the cases of orthosilicates.     

Iron-magnesium order-disorder in an orthopyroxene crystal from the Johnstown meteorite

Equilibrium reversals of Fe²⁺Mg distribution between the M1 and M2 sites of an orthopyroxene from the Johnstown meteorite were achieved at several temperatures between 700 and 1000°C. One single crystal was used for the whole thermal treatment and for collecting all the X-ray data after quenching. The intracrystalline ion exchange for the bulk chemical composition: (Mg_1.453Fe_0.441Cr_0.024Ca_0.054Mn_0.015Fe_0.005Ti_0.003Al_0.005)(Si_1.960Al_0.040)O₆ is given by: ln K_D = −3027(±39)/T(K) + 0.872(±0.013)> where K_D is the distribution coefficient for the reaction: Fe_M2²⁺ + Mg_M1 = Mg_M2 + Fe_M1²⁺.A transmission electron microscopy (TEM) study of part of the crystal showed the presence of very thin augite lamellae and Guinier-Preston zones indicating a relatively rapid cooling of the host rock at temperatures close to 1000°C. The new temperature scale yields a relatively high quenching temperature of 379 (±8)°C for the pyroxene which appears consistent with a rapid cooling (estimated few degrees per hundred years) of a magmatic cumulate excavated by an impact on its parental body.     

Estimates of possible diffusional effects in trace element separations

Igneous material dredged from the Rio Grande rise, South Atlantic Ocean, includes basaltic rocks, some having mafic nodules and megacrysts, and volcanic breccias composed largely of basaltic fragments. These samples represent the only volcanic rocks recovered from this aseismic rise. Bulk compositions show alkalic basalt, trachybasalt, and trachyandesite; the rock types are similar to those of nearby Tristan da Cunha, Gough, and the Walvis ridge. Microprobe analyses show basaltic groundmass to have olivine, Fo₈₅, pyroxene, Fs₁₃Wo₄₆, feldspar, An₇₁, plus interstitial alkali feldspar. Mafic nodules and megacrysts have olivine, Fo_86–90 and pyroxene Fs_6–7.5Wo_45–46; Al₂O₃ 2.5–4 wt.%.The Rio Grande rise rocks have compositional characteristics of an alkalic basaltic suite, and not of mid-ocean ridge tholeiite. Based on mineral compositions, nodules and megacrysts in basalt are interpreted as cognate inclusions. Because oceanic alkalic basaltic rocks are almost invariably associated with islands and seamounts, the Rio Grande rise probably represents a series of alkalic-basalt islands that formed and eventually subsided during rifting of the South Atlantic; the dredged volcanic breccias are probably slump deposits from those volcanoes. This interpretation lends support to the Rio Grande rise having formed at a hot spot, but the possibility of alkalic rocks having formed along fracture zones should not be discounted.     

Differential weathering of feldspar and pyroxene in an arctic-alpine environment

The relative relief of adjacent plagioclase and pyroxene minerals was accurately measured on a recently-exhumed outcrop of gabbroic rocks in Leirdalen, southern Norway. Above the level of the former soil surface the feldspars protrude above the pyroxenes whereas below that level the pyroxenes are higher. Differential relief declines with increasing depth of burial down to 80 cm. On exposed surfaces the mean loss of pyroxene relative to feldspar is 103 cm³ m^?2 of rock surface. With burial down to 50 cm in an arctic-alpine Brown Soil the mean loss of feldspar relative to pyroxene is 179 cm³ m^?2. These figures represent minimum amounts of weathering over about 9000 years. The results confirm the importance of chemical weathering in arctic-alpine environments and the techniques may provide useful physical measures of degree of weathering for use in rate studies and relative-dating.     

Compression and phase behavior of solid CO2 to half a megabar

CO₂ has been investigated up to 514 kbar at23 ± 2°C by both optical and in situ X-ray diffraction studies using a diamond-anvil pressure cell. CO₂ solidifies in an unknown structure in the pressure range 5 to 23 kbar, and transforms to ordinary dry-ice structure above 23 kbar at room temperature. Isothermal compression data for dry ice have been obtained above about 24 kbar. These appear to be the first data at room temperature known in the literature. The data fitted to the Birch equation of state yieldK₀ = 29.3 ± 1.0kbar andK₀^′ = 7.8 assuming the volume of the hypothetical dry ice at zero-pressure and room temperature is 31.4 ± 0.2 cm³/mole. The isothermal bulk modulus(K₀) thus derived is consistent with the compression data and compressibilities for dry ice obtained at low temperatures using dilatometry and ultrasonic techniques, respectively, reported in the literature. By comparing shock-wave data for relevant materials, it is suggested that CO₂ is not likely to transform to one of the crystalline forms of SiO₂ which is otherwise expected from empirical grounds, but may instead decompose into C (diamond) + O₂, at high pressures.     

Elasticity of polycrystalline stishovite

Ultrasonic data for the velocities of SiO₂-stishovite have been determined as a function of pressure to 10 kbar at room temperature for polycrystalline specimens hot-pressed at pressures P = 120kbar and temperatures T = 900°C. These cylindrical specimens are 2 mm in diameter and 0.9–1.4 mm long and have a grain size less than 10 μm. Compressional and shear wave velocities were measured both parallel and perpendicular to the axis of pressing and were found to be isotropic at 10 kbar with ν_p = 11.0 ± 0.2km/sec andν_s = 6.9 ± 0.3km/sec; this shear velocit is substantially higher than that of Mizutani et al. (1972) perhaps due to the presence of crack orientations in their specimen which affected ν_s but not ν_p. The Murnaghan P-V trajectories calculated from the ultrasonic data [bulk modulus K_s = 2.5 ± 0.3Mbar and assuming (?K_s/?P)_T = 6 ± 2] are consistent with recent hydrostatic compression data and with the shock wave compression data above 600 kbar. The combined evidence from the data of the ultrasonic and hydrostatic compression techniques suggests that the most probable value of the bulk modulus of stishovite at zero pressure is close to the upper limit of the uncertainty of our ultrasonically determined value, K₀ = 2.7?2.8Mbar. Elasticity data for rutile-type oxides are not compatible with normal K_s-V₀ systematics perhaps due to the neglect of non-central forces in the lattice model. These new stishovite data would make it impossible to satisfy the elasticity-density data of the lower mantle using an oxide mixture with either olivine or pyroxene stoichiometry.     

High-pressure phase transformations in the system CaSiO3-Al2O3

Phase assemblages for five selected compositions in the system CaSiO₃-Al₂O₃ have been investigated in the pressure range 100–300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al₂O₃. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl₂O₄-SiO₂. Garnet solid solutions are stable between about 10 and 25 mole% Al₂O₃. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO₃ composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier.