Search

Louisfuchsite (IMA 2022-024), with an end-member formula Ca2(Mg4Ti2)(Al4Si2)O20, is a new refractory mineral identified in a Ca-Al-rich inclusion (CAI) from the NWA 4964 CK3.8 carbonaceous chondrite. Louisfuchsite occurs with spinel, perovskite, grossmanite, plus secondary rutile, titanite, and ilmenite in three regions in the CAI. The mean chemical composition of type louisfuchsite by electron probe microanalysis is (wt%) Al2O3 25.48, SiO2 18.40, MgO 17.92, TiO2 15.36, Ti2O3 3.13, CaO 14.92, FeO 3.30, V2O3 0.67, Cr2O3 0.08, total 99.26, giving rise to an empirical formula of C a 2.00 (M g 3.44 T i 1.49 4 + F e 0.36 T i 0.34 3 + A l 0.24 V 0.07 3 + C a 0.06 C r 0.01 ) Σ 6.01 (A l 3.63 S i 2.37 ) Σ 6.00 O 20 . Louisfuchsite has the P 1 rhönite structure with a = 10.37(1) Å, b = 10.76(1) Å, c = 8.90(1) Å, α = 106.0(1)°, β = 96.0(1)°, γ= 124.7(1)°, V = 741(2) Å3, and Z = 2, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.44 g/cm3. Louisfuchsite is a new refractory phase from the solar nebula, crystallized from an 16O-rich (Δ17O ~ –24 ± 2‰) refractory melt with the initial 26Al/27Al ratio of (5.09 ± 0.58) × 10–5 under reduced conditions. The mineral name is in honor of Louis Fuchs (1915–1991), a mineralogist at Argonne National Laboratory, for his many contributions to mineralogical research on meteorites. [ABSTRACT FROM AUTHOR]

Isolated corundum grains and corundum ± Mg‐deltalumite [(Al,Mg)(Al,◻)2O4] ± hibonite assemblages were investigated in the CH3.0 metal‐rich carbonaceous chondrite Sayh al Uhaymir (SaU) 290. Although very refractory inclusions containing abundant Zr‐ and Sc‐rich oxides and silicates, hibonite, grossite, or perovskite have been previously described in CH chondrites, this is the first discovery of corundum and Mg‐deltalumite in CHs and the first discovery of Mg‐deltalumite in nature. Magnesium‐deltalumite can be indexed by the Fd3m spinel‐type structure and gives a perfect fit to the synthetic Al‐rich spinel cells. Corundum‐Mg‐deltalumite grains, 5–20 μm in size, are occasionally rimmed by a thin layer of hibonite replacing corundum. Some corundum grains contain tiny inclusions of ultrarefractory Zr,Sc‐rich minerals and platinum‐group element (PGE) nuggets. All corundum, hibonite, and Mg‐deltalumite grains studied have 16O‐rich compositions (average Δ17O ± 2SD = −22 ± 3‰). Two corundum grains show evidence for significant mass‐dependent fractionation of oxygen isotopes: Δ18O ~ +34‰ and ~ +19‰. We suggest that the SaU 290 corundum‐rich objects were formed by evaporation and/or condensation in a hot nebular region close to the proto‐sun where the ambient temperature was close to the condensation temperature of corundum. A corundum grain with tiny inclusions of Zr‐ and Sc‐rich phases and PGE metal nuggets recorded formation temperatures higher than the condensation temperature of corundum. Two corundum‐rich objects with highly fractionated oxygen isotopes must have crystallized from a melt that experienced evaporation. Corundum grains corroded by hibonite recorded gas–solid interaction in this region during its cooling. The Mg‐deltalumite ± corundum ± hibonite objects were formed by rapid crystallization of high‐temperature (>2000°C) refractory melts. The lack of minerals with condensation temperatures below those of corundum and hibonite in the SaU 290 corundum‐rich objects suggests that after formation, these objects were rapidly removed from the hot nebular region by disk wind and/or by turbulent diffusion and disk spreading. [ABSTRACT FROM AUTHOR]

We report on the primary and secondary mineralogies of three coarse‐grained igneous calcium‐aluminum‐rich inclusions (CAIs) (Compact Type A [CTA], Type B [B], and forsterite‐bearing type B [FoB]) from the Northwest Africa (NWA) 5343 (CK3.7) and NWA 4964 (CK3.8) carbonaceous chondrites, compare them with the mineralogy of igneous CAIs from the Allende (CV3.6) chondrite, and discuss the nature of the alteration processes that affected the CK and CV CAIs. The primary mineralogy and mineral chemistry of the CK3 CAIs studied are similar to those from Allende; however, primary melilite and anorthite are nearly completely absent. Although the secondary minerals identified in CK CAIs (Al‐diopside, andradite, Cl‐apatite, clintonite, forsterite, ferroan olivine, Fe,Ni‐sulfides, grossular, ilmenite, magnetite, plagioclase, spinel, titanite, and wadalite) occur also in the Allende CAIs, there are several important differences: (i) In addition to melilite and anorthite, which are nearly completely replaced by secondary minerals, the alteration of CK CAIs also affected high‐Ti pyroxenes (fassaite and grossmanite) characterized by high Ti3+/Ti4+ ratio and spinel. These pyroxenes are corroded and crosscut by veins of Fe‐ and Ti‐bearing grossular, Fe‐bearing Al,Ti‐diopside, titanite, and ilmenite. Spinel is corroded by Fe‐bearing Al‐diopside and grossular. (ii) The secondary mineral assemblages of grossular + monticellite and grossular + wollastonite, commonly observed in the Allende CAIs, are absent; the Fe‐bearing grossular + Fe‐bearing Al‐diopside ± Fe,Mg‐spinel, Fe‐bearing grossular + Fe,Mg‐olivine ± Fe,Mg‐spinel, and Ca,Na‐plagioclase + Fe‐bearing Al‐diopside + Fe‐bearing grossular assemblages are present instead. These mineral assemblages are often crosscut by veins of Fe‐bearing Al‐diopside, Fe,Mg‐olivine, Fe,Mg‐spinel, and Ca,Na‐plagioclase. The coarse‐grained secondary grossular and Al‐diopside often show multilayered chemical zoning with distinct compositional boundaries between the layers; the abundances of Fe and Ti typically increase toward the grain edges. (iv) Sodium‐rich secondary minerals, nepheline and sodalite, commonly observed in the peripheral portions of the Allende CAIs, are absent; Ca,Na‐plagioclase is present instead. We conclude that coarse‐grained igneous CAIs from CK3.7–3.8 s and Allende experienced an open‐system multistage metasomatic alteration in the presence of an aqueous solution–infiltration metasomatism. This process resulted in localized mobilization of all major rock‐forming elements: Si, Ca, Al, Ti, Mg, Fe, Mn, Na, K, and Cl. The metasomatic alteration of CK CAIs is more advanced and occurred under higher temperature and higher oxygen fugacity than that of the Allende CAIs. [ABSTRACT FROM AUTHOR]

The CB (Bencubbin‐like) metal‐rich carbonaceous chondrites are subdivided into the CBa and CBb subgroups. The CBa chondrites are composed predominantly of ~cm‐sized skeletal olivine chondrules and unzoned Fe,Ni‐metal ± troilite nodules. The CBb chondrites are finer grained than the CBas and consist of chemically zoned and unzoned Fe,Ni‐metal grains, Fe,Ni‐metal ± troilite nodules, cryptocrystalline and skeletal olivine chondrules, and rare refractory inclusions. Both subgroups contain exceptionally rare porphyritic chondrules and no interchondrule fine‐grained matrix, and are interpreted as the products of a gas–melt impact plume formed by a high‐velocity collision between differentiated planetesimals about 4562 Ma. The anomalous metal‐rich carbonaceous chondrites, Fountain Hills and Sierra Gorda 013 (SG 013), have bulk oxygen isotopic compositions similar to those of other CBs but contain coarse‐grained igneous clasts/porphyritic chondrule‐like objects composed of olivine, low‐Ca‐pyroxene, and minor plagioclase and high‐Ca pyroxene as well as barred olivine and skeletal olivine chondrules. Cryptocrystalline chondrules, zoned Fe,Ni‐metal grains, and interchondrule fine‐grained matrix are absent. In SG 013, Fe,Ni‐metal (~80 vol%) occurs as several mm‐sized nodules; magnesiochromite (Mg‐chromite) is accessory; daubréelite and schreibersite are minor; troilite is absent. In Fountain Hills, Fe,Ni‐metal (~25 vol%) is dispersed between chondrules and silicate clasts; chromite and sulfides are absent. In addition to a dominant chondritic lithology, SG 013 contains a chondrule‐free lithology composed of Fe,Ni‐metal nodules (~25 vol%), coarse‐grained olivine and low‐Ca pyroxene, interstitial high‐Ca pyroxene and anorthitic plagioclase, and Mg‐chromite. Here, we report on oxygen isotopic compositions of olivine, low‐Ca pyroxene, and ±Mg‐chromite in Fountain Hills and both lithologies of SG 013 measured in situ using an ion microprobe. Oxygen isotope compositions of olivine, low‐Ca pyroxene, and Mg‐chromite in these meteorites are similar to those of magnesian non‐porphyritic chondrules in CBa and CBb chondrites: on a three‐isotope oxygen diagram (δ17O vs. δ18O), they plot close to a slope‐1 (primitive chondrule mineral) line and have a very narrow range of Δ17O (=δ17O–0.52 × δ18O) values, −2.5 ± 0.9‰ (avr ± 2SD). No isotopically distinct relict grains have been identified in porphyritic chondrule‐like objects. We suggest that magnesian non‐porphyritic (barred olivine, skeletal olivine, cryptocrystalline) chondrules in the CBas, CBbs, and porphyritic chondrule‐like objects in SG 013 and Fountain Hills formed in different zones of the CB impact plume characterized by variable pressure, temperature, cooling rates, and redox conditions. The achondritic lithology in SG 013 represents fragments of one of the colliding bodies and therefore one of the CB chondrule precursors. Fountain Hills was subsequently modified by impact melting; Fe,Ni‐metal and sulfides were partially lost during this process. [ABSTRACT FROM AUTHOR]

The mineral wadalite (ideal and simplified formula: [Ca.sub.6][Al.sub.5][Si.sub.2][O.sub.16][Cl.sub.3]) has been discovered for the first time in a meteorite, specifically in coarse-grained, igneous type B calcium-aluminum-rich inclusions (CAIs) from the CV carbonaceous chondrite Allende. We report the results of electron microprobe, scanning electron microscopy, and transmission electron microscopy analyses of wadalite-bearing assemblages in the Allende CAIs and propose that wadalite formed by metamorphic reaction between akermanitic melilite and anorthite, likely mediated by chlorine-bearing fluids. Petrographic relationships support the likelihood of multistage alterations by fluids of different chemistries interspersed or coinciding with thermal metamorphic episodes on the Allende parent asteroid. Fluid involvement in metamorphism of Allende CAIs implies that these objects experienced open-system alteration after accretion into the CV chondrite parent asteroid, which may have resulted in disturbances of their oxygen- and magnesium-isotope systematics. Keywords: Wadalite, Allende, type B CAIs, aqueous alteration, fluid-mediated metamorphism DOI: 10.2138/am.2010.3296

Author(s): Alexander N. Krot (corresponding author) [1]; Yuri Amelin [2]; Patrick Cassen [3]; Anders Meibom [4] Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed [...]

Author(s): Alexander N. Krot (corresponding author) [1]; Hisayoshi Yurimoto [2]; Ian D. Hutcheon [3]; Glenn J. MacPherson [4] Chondrules and Ca-Al-rich inclusions (CAIs) are high-temperature components of meteorites that formed [...]

Secondary Ca, Fe-rich minerals (CFM; diopside-hedenbergite, andradite, wollastonite, kirschteinite) are widespread in the Allende carbonaceous chondrite. About 90 vol% of the total CaO content of the Allende matrix is concentrated in CFM. The conditions and environment (solar nebular or asteroidal) of this alteration are still matters of controversial scientific discussion. Here we present evidence for late-stage, high-temperature processes recorded in Ca, Fe-rich rims around Allende dark inclusions 3529 and IV-1 studied using scanning (SEM) and transmission electron microscopy (TEM), and electron probe microanalysis. The rims show a multilayered structure, with the outermost layer intergrown with the matrix olivines and chondrule fragments of the Allende host, indicative of in situ formation. The central portion of the rim around IV--1 contains several wollastonite polytypes (a polysynthetically twinned polytype of pseudowollastonite, wollastonite-2M, and wollastonite-1T) and an intergrowth of hedenbergite-PM (primitive monoclinic) and augite-hedenbergitess. These findings require temperatures above 1000 [degrees]C and fast cooling rates after formation of the central part of the rim. The close occurrences of three different polymorphs of wollastonitc suggest that this process was highly localized and may have resulted from shock metamorphism.