NWA 1827: A Rare Meso-C, perhaps from asteroid 16 Psyche

“NWA 1827 very rare Meso-c. Largest slice out of main mass. Huge. Green crystals. Matrix is metallic. Similar to Pallasite but more silicated and less metal. Only 8 meso-c in existence. And of the 8, 6 are paired with this. So actually 3 known type c mesos. Youxi and NWA 1878 are not paired.” — Michael Farmer

From various books on them: “Mesosiderites are arguably the most enigmatic group of differentiated meteorites. Mesosiderites are breccias composed of roughly equal proportions of Fe–Ni metal and silicates. Unlike pallasites, where the silicates are consistent with a deep mantle origin, the silicates in the mesosiderites are basaltic, gabbroic, and pyroxenitic. Metal compositions of mesosiderites are almost uniform, suggesting that the metal was molten when mixed with the silicates. Many innovative models have attempted to explain the mixture of crustal materials with core metal. Models include impacts of molten planetesimals onto the surface of a large differentiated asteroid and breakup and reassembly of a large differentiated asteroid with a still molten core.

Another unusual characteristic of mesosiderites is their very slow metallographic cooling rates of < 1 degree per mission years! These cooling rates are the slowest for any natural geological material, suggesting that the mesosiderite parent body must have been large.

Mesosiderites have young Ar–Ar ages of ~3.95 Ga. The young Ar–Ar ages have been attributed to extended cooling within a large asteroid or impact resetting.

The iron metal and troilite of mesosiderites are presumed to represent core materials of an asteroid. Mixing of this with crustal silicates requires an unusual formation process. Some have suggested that a naked molten core (a core with the silicate crust and mantle largely stripped off) impacted a differentiated asteroid at low velocity. Others have suggested that an impact disrupted the differentiated, mesosiderite parent body, which reaccreted. This process mixed materials from different portions of the parent body, with mesosiderites representing a location where the core and crust were mixed together.”

Some analysis of Asteroid 16 Psyche, the largest metallic asteroid (M-type), suggests the possible origin. “Psyche is widely considered to be the collisionally-exposed core of an ∼500-km diameter differentiated parent body, which was similar to asteroid 4 Vesta. However, there is no dynamical family associated with Psyche nor are there spectroscopic data for the existence of the mantle or crustal material from the parent body. The usual explanation for the missing material requires that the Psyche parent body was collisionally-disrupted early in solar system history, followed by collisional grinding. We think it more likely that Psyche has possibly been shattered by impacts but not catastrophically disrupted. In this case, it would be a plausible candidate parent body for the mesosiderites. “

Found in the Atlas Mountains of Morocco in 2003. 31.5 cm x 21 cm x 4 mm. 733g slice. A mesosiderite is a brecciated meteorite containing subequal silicate and metallic components; the silicates are dominantly igneous rock fragments. (Met Bull)

From NakhlaDog: “I did a bit of looking into the differences in Meso Types A, B, C with Dr Ted Bunch at NAU and DR. Alex Ruzicka with Cascadia Meteorite Lab The difference lies in pyroxene differences:

TYPE A plagioclase>Ca-pyroxene> orthopyroxene
TYPE B clinopyroxene>plagioclase>orthopyroxene
TYPE C abundant orthopyroxene with traces of Ca-pyroxene and plagioclase

and the implication suggests a NEAR-SURFACE ORIGIN in Type A toward A DEEPER ORIGIN FOR Type B and C (going deeper) though the differences are not yet fully studied.

NWA 1827 and a few paired stones represent the only available Type C Mesosiderite available in the world

Much went to science, much was lost in preparation. If you think you’ve seen them all then look again, a busy brown, green, yellow crystalline matrix, and a whole lot of metal.”