Chi Ma*, John R. Beckett and George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.

Monipite (IMA 2007-033), MoNiP, is a new phosphide mineral that occurs as one 1 × 2 μm crystal in a Type B1 Ca-Al-rich inclusion (CAI) ACM-2 from the Allende CV3 carbonaceous chondrite. It has an empirical formula of (Mo_0.84Fe_0.06Co_0.04Rh_0.03)(Ni_0.89Ru_0.09)P, and a P6̄ 2m Fe₂P type structure with a = 5.861, c = 3.704 Å, V = 110.19 ų, and Z = 3. The calculated density using our measured composition is 8.27 g/cm³, making monipite the densest known mineral phosphide. Monipite probably either crystallized from an immiscible P-rich melt that had exsolved from an Fe-Ni-enriched alloy melt that formed during melting of the host CAI or it exsolved from a solidified alloy. Most of the original phosphide in the type occurrence was later altered to apatite and Mo-oxides, leaving only a small residual grain. Monipite occurs within an opaque assemblage included in melilite that contains kamiokite (Fe₂Mo₃O₈), tugarinovite (MoO₂), and a Nb-rich oxide [(Nb,V,Fe)O₂], none of which has previously been reported in meteorites, together with apatite, awaruite (Ni₂Fe), and vanadian magnetite.

Reference
Ma C, Beckett JR and Rossman GR (2014) Monipite, MoNiP, a new phosphide mineral in a Ca-Al-rich inclusion from the Allende meteorite. American Mineralogist 99:198-205.
[doi:10.2138/am.2014.4512]
Copyright: The Mineralogical Society of America

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Alex R. Howe and Roman R. Rafikov

Department of Astrophysical Sciences, Princeton University, Ivy Lane, Princeton, NJ 08540, USA

The Oort cloud remains one of the most poorly explored regions of the solar system. We propose that its properties can be constrained by studying a population of dust grains produced in collisions of comets in the outer solar system. We explore the dynamics of μm-sized grains outside the heliosphere (beyond ~250 AU), which are predominantly affected by the magnetic field of the interstellar medium (ISM) flow past the Sun. We derive analytic models for the production and motion of small particles as a function of their birth location in the cloud and calculate the particle flux and velocity distribution in the inner solar system. These models are verified by direct numerical simulations. We show that grains originating in the Oort cloud have a unique distribution of arrival directions, which should easily distinguish them from both interplanetary and interstellar dust populations. We also demonstrate that the distribution of particle arrival velocities is uniquely determined by the mass distribution and dust production rate in the cloud. Cometary collisions within the cloud produce a flux of μm-sized grains in the inner solar system of up to several m⁻² yr⁻¹. The next generation dust detectors may be sensitive enough to detect and constrain this dust population, which will illuminate the Oort cloud’s properties. We also show that the recently detected mysterious population of large (μm-sized) unbound particles, which seems to arrive with the ISM flow, is unlikely to be generated by collisions of comets in the Oort cloud.

Reference
Howe AR and Rafikov RR (2014) Probing Oort Cloud and Local Interstellar Medium Properties via Dust Produced in Cometary Collisions. The Astrophysical Journal 781:52.
[doi:10.1088/0004-637X/781/1/52]

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