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1. New minerals in type A inclusions from Allende and clues to processes in the early solar system: Paqueite, Ca 3 TiSi 2 (Al,Ti,Si) 3 O 14 , and burnettite, CaVAlSiO 6

   https://doi.org/10.1111/maps.13826  

   Ma, Chi ; Beckett, John R. ; Tissot, François L. H. ; Rossman, George R. ( May 2022 , Meteoritics & Planetary Science)

   Paqueite (Ca₃TiSi₂[Al,Ti,Si]₃O₁₄; IMA 2013‐053) and burnettite (CaVAlSiO₆; IMA 2013‐054) are new refractory minerals, occurring as euhedral to subhedral crystals within aluminous melilite in A‐WP1, a type A Ca‐Al‐rich inclusion, andCGft‐12, a compact type A (CTA) from the Allende CV3 carbonaceous chondrite. Type paqueite from A‐WP1 has an empirical formula of (Ca_2.91Na_0.11)Ti⁴⁺Si₂(Al_1.64Ti⁴⁺_0.90Si_0.24V³⁺_0.12Sc_0.07Mg_0.03)O₁₄, with a trigonal structure in space groupP321 and cell parametersa = 7.943 Å,c = 4.930 Å, V = 269.37 ų, andZ = 1. Paqueite’s general formula is Ca₃TiSi₂(Al,Ti,Si)₃O₁₄and the endmember formula is Ca₃TiSi₂(Al₂Ti)O₁₄. Type burnettite fromCGft‐12has an empirical formula of Ca_1.01(V³⁺_0.56Al_0.25Mg_0.18)(Si_1.19Al_0.81)O₆. It assumes a diopside‐typeC2/cstructure witha = 9.80 Å,b = 8.85 Å,c = 5.36 Å, β = 105.6°,V = 447.7 ų, andZ = 4. Burnettite’s general formula is Ca(V,Al,Mg)AlSiO₆and the endmember formula is CaVAlSiO₆. Paqueite and burnettite likely originated as condensates, but the observed grains may have crystallized from local V‐rich melts produced during a later thermal event. ForCGft‐12, the compositions of paqueite, clinopyroxene, and perovskite suggest that type As drew from two distinct populations of grains. Hibonite grains drew from multiple populations, but these were well mixed and not equilibrated prior to incorporation into type A host melilite.

    

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2. Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO 3 chondrites

   https://doi.org/10.1111/maps.13282  

   Simon, Steven B. ; Krot, Alexander N. ; Nagashima, Kazuhide ( April 2019 , Meteoritics & Planetary Science)

   The distribution of the short‐lived radionuclide²⁶Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions inCO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearingCAIs in the Dominion Range (DOM) 08006 (CO3.0) andDOM08004 (CO3.1) chondrites. All minerals inDOM08006CAIs as well as hibonite, spinel, and pyroxene inDOM08004 are uniformly¹⁶O‐rich (Δ¹⁷O = −25 to −20‰) but grossite and melilite inDOM08004CAIs are not; Δ¹⁷O of grossite and melilite range from ~ −11 to ~0‰ and from ~ −23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial²⁶Al/²⁷Al ratios (²⁶Al/²⁷Al)₀is seen, with four having (²⁶Al/²⁷Al)₀≤1.1 × 10⁻⁵and six having (²⁶Al/²⁷Al)₀≥3.7 × 10⁻⁵. Five of the²⁶Al‐richCAIs have (²⁶Al/²⁷Al)₀within error of 4.5 × 10⁻⁵; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10⁻⁵given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the²⁶Al‐poorCAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in theDOM08006CAIs, as well as spinel, hibonite, and Al‐diopside in theDOM08004CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where theCOgrossite‐bearingCAIs originated. Oxygen isotopic heterogeneity inCAIs fromDOM08004 resulted from exchange between the initially¹⁶O‐rich (Δ¹⁷O ~−24‰) melilite and grossite and¹⁶O‐poor (Δ¹⁷O ~0‰) fluid during hydrothermal alteration on theCOchondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic²⁶Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ¹⁷O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on theCOparent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected mostCAIs in CO ≥3.1 chondrites.

    

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3. Enhanced spontaneous polarization in double perovskite Bi 2 FeCrO 6 films

   https://doi.org/10.1111/jace.16386  

   Meng, Dehuan ; Xiao, Yunsheng ; He, Hongcai ; Liao, Yulong ; Zhang, Huaiwu ; Zhai, Junyi ; Chen, Zuhuang ; Martin, Lane W. ; Bai, Feiming ( March 2019 , Journal of the American Ceramic Society)

   We report the pulsed‐laser deposition of epitaxial double‐perovskite Bi₂FeCrO₆(BFCO) films on the (001)‐, (110), and (111)‐oriented single‐crystal SrTiO₃substrates. All of the BFCO films with various orientations show theandsuperlattice‐diffraction peaks. The intensity ratios between the‐superlattice and the main 111‐diffraction peak can be tailored by simply adjusting the laser repetition rate and substrate temperature, reaching up to 4.4%. However, both optical absorption spectra and magnetic measurements evidence that the strong superlattice peaks are not correlated with theB‐site Fe³⁺/Cr³⁺cation ordering. Instead, the epitaxial (111)‐oriented Bi₂FeCrO₆films show an enhanced remanent polarization of 92 μC/cm²at 10 K, much larger than the predicted values by density‐functional theory calculations. Positive‐up‐negative‐down (PUND) measurements with a time interval of 10 μs further support these observations. Therefore, our experimental results reveal that the strong superlattice peaks may come fromA‐ orB‐site cation shifts along the pseudo‐cubic [111] direction, which further enhance the ferroelectric polarization of the BFCO thin films.

    

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4. Polymetamorphism in high‐ T metamorphic rocks: An example from the central Appalachians

   https://doi.org/10.1111/jmg.12509  

   Broadwell, Kirkland S. ; Caddick, Mark J. ; Beard, James S. ( October 2019 , Journal of Metamorphic Geology)

   Contact metamorphism associated with mafic intrusives is one of several mechanisms that has been invoked to produce extensive high‐temperature (HT) metamorphism and associated partial melting of the crust. Indisputable evidence for polymetamorphism in these settings can be difficult to decipher because both melt loss and retrogression (i.e. rehydration) can erase or obscure the records of earlier HT metamorphism by modifying HT mineral parageneses and compositions. Here, a combination of detailed field and petrographical observations, inverse mineral thermometry, and thermodynamic forward modelling is used to delineate the polymetamorphic history of migmatites from the Smith River Allochthon (SRA) in the central Appalachians. Bulk rock geochemical data suggest that some metapelitic samples lost a significant amount of melt during interpreted contact metamorphism with the Rich Acres gabbro, resulting in a residual bulk composition (<50 wt% SiO₂, ~30 wt% Al₂O₃). Garnet cores (Grt1) in SiO₂‐depleted samples are interpreted to grow during this HT contact metamorphism, with Fe‐Ti oxide thermometry on spinel inclusions in Grt1, cordierite–garnet thermometry, and thermodynamic forward modelling constraining peakP–Tconditions during contact heating of the migmatites to ~800ºC and ∼0.5 GPa. This is associated with an inferred peak assemblage prior to melt loss of crd+kfs+pl+grt+bt+spl (mag+usp+hc)+ilm+sil+qtz+melt. Garnet in SiO₂‐depleted samples has a distinct high‐Ca rim (Grt2), which appears to record a younger metamorphic event. A combination of substantial melt loss and later rehydration appears to be a major control on the ability of SiO₂‐depleted samples to faithfully record evidence for this polymetamorphism. The tectonic implications of this younger metamorphic event are not entirely clear, but it appears to record renewed burial and heating of the SRA sometime after the Taconic orogeny, which may be related to either the neo‐Acadian or Alleghanian orogenies.

    

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5. Rare‐earth Metal Substitution in Calcium Germanides with the Tetragonal Cr 5 B 3 Type Structure

   https://doi.org/10.1002/zaac.202200183  

   Suen, Nian‐Tzu ; Bobev, Svilen ( July 2022 , Zeitschrift für anorganische und allgemeine Chemie)

   Calcium germanides with two mid‐late rare‐earth metals, Ca_5−xGd_xGe₃and Ca_5−xTb_xGe₃(x≈0.1−0.2), have been synthesized and structurally characterized. Additionally, a lanthanum‐rich germanide with calcium substitutions, La_5−xCa_xGe₃(x≈0.5) has also been identified. The three structures have been established from single‐crystal X‐ray diffraction methods and confirmed to crystallize with the Cr₅B₃‐type in the tetragonal space groupI4/mcm(no. 140;Z=4; Pearson symboltI32), where part of the germanium atoms are interconnected into Ge₂‐dimers, formally [Ge₂]⁶⁻. Rare‐earth metal and calcium atoms are arranged in distorted trigonal prisms, square‐antiprisms and cubes, centered by Ge or rare‐earth/calcium metal atoms. These studies show that the amount of trivalent rare‐earth metal atoms substituting divalent calcium atoms is in direct correlation with the lengths of the Ge−Ge bond within the Ge₂‐dimers, with distance varying between 2.58 Å in Ca_5−xGd_xGe₃and 2.75 Å in La_5−xCa_xGe₃. Such an elongation of the Ge−Ge bond is consistent with the notion that the parent Ca₅Ge₃Zintl phase (e. g. (Ca²⁺)₅[Ge₂]⁶⁻[Ge⁴⁻]) is being driven out of the ideal valence electron count and further reduced. In this context, this work demonstrates the ability of the germanides with the Cr₅B₃structure type to accommodate substitutions and wider valence electron count while maintaining their global structural integrity.

    

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