Allende

The Allende meteorite fell on February 8, 1969, in the state of Chihuahua, Mexico, in a spectacular event witnessed by thousands of observers. With a total recovered mass of approximately 2 tonnes, it represents one of the most important meteoritic specimens ever studied. It belongs to the class of carbonaceous chondrites, primordial rocks that formed in the solar nebula before the assembly of the terrestrial planets.

What makes Allende extraordinary is the abundant presence of CAIs, millimeter-sized crystals of perovskite, melilite, spinel, and anortite that condensed from the hottest regions of the primitive nebula. These inclusions are isotopically anomalous, containing ratios of isotopes of magnesium, aluminum, and other elements that testify to the incorporation of material from stellar supernovae. The matrix consists of olivine, pyroxene, and altered clay minerals, along with chondrules (silicate spheres) and carbonaceous components. Studies on Allende have revolutionized cosmogony, providing direct evidence of stellar nucleosynthesis and the absolute chronology of solar formation.

Classification: CV3 (carbonaceous chondrite Vigarano-type, aqueous alteration grade 3). Primary mineralogical composition: olivine (Fa15-35, variable composition), low-Ca pyroxene (Fs5-15Wo2-5), high-Ca pyroxene (Wo45-50), plagioclase (An70-95), magnetite, troilite, nickel-iron. CAIs: perovskite (CaTiO3), melilite (Ca2(Mg,Al)(Si,Al)2O7), spinel (MgAl2O4), anortite (CaAl2Si2O8), hibonite (CaAl12O19). Density: 3.3-3.5 g/cm³. Refractive index: variable by phase (olivine: n=1.63-1.70; pyroxene: n=1.69-1.73). The CAI fraction constitutes approximately 15-20% of total mass. Pb-Pb radiometric dating on CAIs: 4567.3 ± 0.2 Ma (absolute age of nebular condensation). Anomalous isotopic ratios of 26Mg/24Mg and 27Al/26Al indicate decay of 26Al (T1/2 = 717 ka) and incorporation of nucleosynthesis material. Volatile composition: ~3-5% organic carbon, amorphous carbon compounds, soluble organic matter. Aqueous alteration: phyllosilicates (serpentine, talc), carbonates (calcite, dolomite), sulfates. Thin sections show porphyritic texture with chondrules of 0.1-3 mm. Raman spectroscopy: D and G bands of organic matter, characteristic peaks of olivine and pyroxene.