Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.208c&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 208
Other
1
Scientific paper
An unusual coarse-grained Ca-Al-rich inclusion (CAI) was discovered in the Leoville CV3 meteorite. The inclusion has a smooth shape, is flattened, and measures ~4 mm in maximum dimension. This compact CAI has a predominantly igneous texture but shows the effects of solid-state recrystallization. It displays multiple deformation features, as is common in other Leoville samples (Caillet et al., 1992). Some large crystals are twisted or oriented parallel to the lineation of the rock or show undulatory extinction and kink-banding. The inclusion consists mainly of melilite, but a striking feature is the occurrence of large wollastonite crystals forming areas up to 400 micrometers long. Such large wollastonites have not previously been reported from a CAI core. Other primary phases are perovskite (up to 120 micrometers across) and few Ti-V-rich fassaite crystals. Sparse idiomorphic crystals (10 micrometers across) of a Ti-rich mineral phase are poikilitically enclosed within melilite. Surprisingly, spinels are absent in this CAI. In transmitted light, the pyroxenes are an intense green color, and perovskite appears reddish pink to light violet. Wollastonite shows a pronounced blue luminescent color under the electron beam. This unique white inclusion is surrounded by relatively simple and regular rimming sequence consisting, from the inside outward, of spinel+perovskite, Al-rich melilite, Al-diopside, and olivine layers. Almost no alteration products, such as grossular or sodalite, were encountered around the CAI. However, two tiny anorthite crystals are partly replaced by nepheline. Calcite fills cracks perpendicular to the long axis of the CAI. However, some calcite patches (apparently replacing wollastonite) appear to have preexisted the faulting event(s). The texture indicates melilite was the first mineral to crystallize. Compositions range from Ak(sub)2 at the rim to Angstrom k(sub)40 in the interior with a mean value of Ak(sub)24. In most minerals, Na2O is below the detection limit but it reaches 0.15 wt% in Ak-rich melilite. FeO (<0.05 wt%) reaches its maximum value, 0.5 wt%, exclusively in brecciated parts intermixed with matrix. Perovskite contains <1 wt% (Al2O3 + V2O3 + SiO2 + Nb2O5 + Ce2O5 + ThO2) and includes possible pyrochlore. Fassaite crystals contain high TiO2 (up to 21 wt%) and Al2O3 (up to 24 wt%), and also V2O3 (up to 4%). Ti^3+ represents about 83% (average value) of the total Ti (calculations by the method of Dowty and Clark, 1973), which is high compared to Ti^3+ in pyroxenes from type-A or even type-B inclusions. The large Ti^3+ excess relative to Ti^4+ indicates highly reducing conditions (Beckett and Grossman, 1986) when the "white angel" formed. Another Ti-rich mineral is similar in composition to the Al-Ca-Ti-phase found in Essebi by El Goresy et al. (1984) and contains 26.6 to 28.2 wt% TiO2 and probably crystallized late from the CAI. The bulk composition of the inclusion as determined by broad-beam microprobe methods is (in wt%): SiO2 ~ 29.59, Al2O3 ~ 24.65, CaO ~ 39.94, TiO2 ~ 2.02, MgO ~ 3.73, FeO ~ 0.1, MnO+Cr2O3 < 0.03, Na2O+K2O < 0.05. Unusually high Ca and Ti and low Al contents together with the textures and mineral chemistry suggest a complex history for this anomalous type-A CAI. The "white angel" presumably represents an ancient condensate from an Al-depleted solar nebula reservoir that was subsequently melted under reducing conditions. References Caillet C., MacPherson G.J. and Zinner E.K. (1992) submitted to Geochim. Cosmochim. Acta. Beckett J.R. and Grossman L. (1986) Lunar and Planet. Sci. (abstract) 17, 36-37. Dowty E. and Clark J.R. (1973) Amer. Mineral. 58, 230-242. El Goresy A., Palme H., Yabuki H., Nagel K., Herrwerth I. and Ramdohr P. (1984) Geochim. Cosmochim. Acta. 48, 2283-2298.
Buseck Peter R.
Caillet L. V. C.
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