The CaGeO3–Ca3Fe2Ge3O12 garnet join: an experimental study

Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the join^VIIICa₃^VI(CaGe)^IVGe₃O₁₂–^VIIICa₃^VIFe₂^IVGe₃O₁₂ at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO₃ end-member consist of tetragonal garnet with a small amount of triclinic CaGe₂O₅. Samples with nominal compositions between X_Fe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with X_Fe>1.2 (X_Fe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mössbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe³⁺ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe²⁺ at the octahedral sites of the garnet structure. The relative abundance of Fe²⁺ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal ^VIIICa₃^VI(CaGe)^IVGe₃O₃ garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into ^VIIIMg₃^VI(MgSi)^IVSi₃O₃ majorite.     

Crystal chemistry of wadsleyite II and water in the Earth’s interior

Wadsleyite II is a variably hydrous magnesium-iron silicate phase similar to spinelloid IV and a potential host for H in the Transition Zone of the Earths mantle. Two separate samples of wadsleyite II synthesized at 17.5 GPa and 1400°C and at 18 GPa and 1350°C have been characterized by electron microprobe, single-crystal X-ray diffraction, visible, IR, Raman, and Mössbauer spectroscopies, and transmission electron microscopy including electron energy-loss spectroscopy. The two samples have the following chemical formulae: Mg_1.71Fe_0.18Al_0.01H_0.33Si_0.96O₄ and Mg_1.60Fe_0.22Al_0.01 H_0.44Si_0.97O₄. Mössbauer spectroscopy and electron energy loss spectroscopy (EELS) indicate that about half of the iron present is ferric. Refinement of the structures shows them to be essentially the same as spinelloid IV. Calculated X-ray powder diffraction patterns show only subtle differences between wadsleyite and wadsleyite II. The hydration mechanism appears to be protonation of the non-silicate oxygen (O2) and possibly the oxygens surrounding the partially vacant tetrahedral site Si2, charge-balanced by cation vacancies in Si2, M5 and M6. The unit cell volume of this phase and its synthesis conditions indicate that it may be an intermediate phase occurring between the fields of wadsleyite and ringwoodite, if sufficient trivalent cations are available. The unit cell parameters have been refined at pressures up to 10.6 GPa by single-crystal X-ray diffraction in the diamond anvil cell. The refined bulk modulus for the sample containing 2.8 wt% H₂O is 145.6 ± 2.8 GPa with a K of 6.1 ± 0.7. Similar to wadsleyite and ringwoodite, hydration has a large effect on the bulk modulus. The presence of this phase in the mantle could serve to obscure the seismic expression of the phase boundary between wadsleyite and ringwoodite near 525 km. The large apparent effect of hydration on bulk modulus is consistent with hydration having a larger effect on seismic velocities than temperature in the Transition Zone.     

Petrographic investigation of shatter cone melt films recovered from MEMIN impact experiments in sandstone and iSALE modeling of their formation boundary conditions

Shatter cones are diagnostic for the recognition of meteorite impact craters. They are unambiguously identifiable in the field and the only macroscopic shock deformation feature. However, the physical boundary conditions and exact formation mechanism(s) are still a subject of debate. Melt films found on shatter cone surfaces may allow the constraint of pressure–temperature conditions during or immediately after their formation. Within the framework of the MEMIN research group, we recovered 24 shatter cone fragments from the ejecta of hypervelocity impact experiments. Here, we focus on silicate melt films (now quenched to glass) found on shatter cone surfaces formed in experiments with 20–80 cm sized sandstone targets, impacted by aluminum and iron meteorite projectiles of 5 and 12 mm diameter at velocities of 7.0 and 4.6 km s⁻¹, respectively. The recovered shatter cone fragments vary in size from 1.2 to 9.3 mm. They show slightly curved, striated surfaces, and conical geometries with apical angles of 36°–52°. The fragments were recovered from experiments with peak pressures ranging from 46 to 86 GPa, and emanated from a zone within 0.38 crater radii. Based on iSale modeling and petrographic investigations, the shatter coned material experienced low bulk shock pressures of 0.5–5 GPa, whereas deformation shows a steep increase toward the shatter cone surface leading to localized melting of the rock, resulting in both vesicular as well as polished melt textures visible under the SEM. Subjacent to the melt films are zones of fragmentation and brittle shear, indicating movement away from the shatter cone apex of the rock that surrounds the cone. Smearing and extension of the melt film indicates subsequent movement in opposite direction to the comminuted and brecciated shear zone. We believe the documented shear textures and the adjacent smooth melt films can be related to frictional melting, whereas the overlying highly vesiculated melt layer could indicate rapid pressure release. From the observation of melting and mixing of quartz, phyllosilicates, and rutile in this overlying texture, we infer high, but very localized postshock temperatures exceeding 2000 °C. The melted upper part of the shatter cone surface cross-cuts the fragmented lower section, and is accompanied by PDFs developed in quartz parallel to the {112} plane. Based on the overprinting textures and documented shock effects, we hypothesize shatter cones start to form during shock loading and remain an active fracture surface until pressure release during unloading and infer that shatter cone surfaces are mixed mode I/II fracture surfaces.     

Iron deficiency in pyrrhotite of suevites from the Chesapeake Bay impact crater,USA—A consequence of shock metamorphism?

Abstract– Pyrrhotite from suevite of the 35 Ma Chesapeake Bay impact structure (CBIS) shows a shock metamorphism and we report on several mineralogical and magnetic features. Pyrrhotite shows strong brittle deformation with a high density of stacking faults, twinning parallel to the hexagonal (001) planes and average fault distances in the order of 10 nm. Although the determination of a superstructure was not possible due to the lattice defects, the reflections of the NiAs subcell, which is typical of all pyrrhotite modifications, were clearly detected. This phase is ferrimagnetic with a Curie temperature (T_C) between 350 and 365 °C, and suevite with this phase does not show the 34 K transition. The most peculiar feature is the low metal/sulfur ratio of 0.81, which indicates a distinctly higher vacancy concentration than for 4C pyrrhotite and a composition close to smythite (Fe₉S₁₁). This phase carries a stable natural remanent magnetization and is relatively hard magnetic. Steep inclinations of the natural remanent magnetization vector, however, suggest that this phase has been remagnetized by the drilling process. A possible explanation is the magnetic domain size of faultless areas of about 10 nm in diameter, which is at the lower limit of the single domain size near the threshold, below which superparamagnetic behavior occurs. The low thermal stability of this phase excludes postshock heating above 300 °C for the suevite of the CBIS. Our results imply that the iron‐deficient pyrrhotite is produced by shock metamorphism, although an iron loss due to shock has never been reported before for pyrrhotite.     

Aluminum incorporation in α-PbO2 type TiO2 at pressures up to 20 GPa

Aluminum incorporation into the high pressure polymorph of TiO₂ with the structure of α-PbO₂ has been studied from 10 to 20 GPa and 1300 °C by XRD, high-resolution ²⁷Al MAS-NMR and TEM. Al-doped α-PbO₂ type TiO₂ can be recovered at atmospheric pressure. Al₂O₃ solubility in α-PbO₂ type TiO₂ increases with increasing the synthesis pressure. The α-PbO₂ type TiO₂ polymorph is able to incorporate up to 35 wt.% Al₂O₃ at 13.6 GPa and 1300 °C, being the substitution of Ti⁴⁺ by Al³⁺ on normal octahedral sites the mechanism of solubility. The transition to the higher pressure TiO₂ polymorph with the ZrO₂ baddeleyite structure, Akaogiite, has not been observed in the quenched samples at room pressure. The microstructure of the recovered sample synthesized at 16 GPa and 1300 °C points to the existence of a non-quenchable aluminum titanium oxide phase at these conditions.     

A new oxygen-deficient perovskite phase Ca(Fe<Subscript>0.4</Subscript>Si<Subscript>0.6</Subscript>)O<Subscript>2.8</Subscript> and phase relations along the join CaSiO<Subscript>3</Subscript>–CaFeO<Subscript>2.5</Subscript> at transition zone conditions

A new oxygen-deficient perovskite with the composition Ca(Fe_0.4Si_0.6)O_2.8 has been synthesised at high-pressure and -temperature conditions relevant to the Earths transition zone using a multianvil apparatus. In contrast to pure CaSiO₃ perovskite, this new phase is quenchable under ambient conditions. The diffraction pattern revealed strong intensities for pseudocubic reflections, but the true lattice is C-centred monoclinic with a=9.2486 Å, b=5.2596 Å, c=21.890 Å and =97.94°. This lattice is only slightly distorted from rhombohedral symmetry. Electron-diffraction and high-resolution TEM images show that a well-ordered ten-layer superstructure is developed along the monoclinic c* direction, which corresponds to the pseudocubic [111] direction. This unique type of superstructure likely consists of an oxygen-deficient double layer with tetrahedrally coordinated silicon, alternating with eight octahedral layers of perovskite structure, which are one half each occupied by silicon and iron as indicated by Mössbauer and Si K electron energy loss spectroscopy. The maximum iron solubility in CaSiO₃ perovskite is determined at 16 GPa to be 4 at% on the silicon site and it increases significantly above 20 GPa. The phase relations have been analysed along the join CaSiO₃–CaFeO_2.5, which revealed that no further defect perovskites are stable. An analogous phase exists in the aluminous system, with Ca(Al_0.4Si_0.6)O_2.8 stoichiometry and diffraction patterns similar to that of Ca(Fe_0.4Si_0.6)O_2.8. In addition, we discovered another defect perovskite with Ca(Al_0.5Si_0.5)O_2.75 stoichiometry and an eight-layer superstructure most likely consisting of a tetrahedral double layer alternating with six octahedral layers. The potential occurrence of all three defect perovskites in the Earths interior is discussed.     

Microstructural defects in experimentally shocked diopside: A TEM characterization

Transmission electron microscopy was used for characterizing the defect microstructure induced by shock experiments in a single crystal of diopside. The shock-induced defects found in the crystal can be divided in four distinct types:

1. A high density and pervasive distribution of dislocations in glide configuration (glide systems (100)[0
2. Mechanical twin lamellae, mostly parallel to (100), the (001) twin lamellae are less abundant. li]3)
3. Heterogeneously distributed tiny molten zones (3 to 20 μm size) which, after cooling, appear as a glass with a chemical composition very close to the one of the original diopside.

The present TEM study reveals that the defect micro-structure in shocked diopside consists of a large variety of shock-induced defects. Especially, the amorphous PDFs which were never observed in statically deformed diopside seem to be an important characteristic micro-structural defects in shocked silicate minerals. Although the presence of amorphous PDFs is not yet confirmed for naturally shocked clinopyroxene, we strongly suggest that these features can serve as a diagnostic tool for recognizing impact phenomena on all planetary bodies of our solar system.     

Twinned calcite as an indicator of high differential stresses and low shock pressure conditions during impact cratering

Shock-related calcite twins are characterized in calcite-bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre-impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (<7 GPa). Calcite, however, shows a high density (>1/μm) of twins with widths <100 nm. Different types of twins (e-, f-, and r-twins) crosscutting each other can occur in one grain. Interaction of r- and f-twins results in a-type domains characterized by a misorientation relative to the host with a misorientation angle of 35°–40° and a misorientation axis parallel to an a-axis. Such a-type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer.