Pyroxene and ungrouped pallasites
Pallasites that fit neither main group nor Eagle Station, among them the pyroxene-bearing pair Vermillion and Yamato 8451.
- Classification
- Stony-iron meteorites › Pallasites › Pyroxene and ungrouped pallasites
- In our catalogue
- 5 meteorites
- Heaviest
- Vermillion, 34.36 kg
A few pallasites have their own chemistry and oxygen isotopes. The best-known pair, Vermillion and Yamato 8451, contain pyroxene as well as olivine, and are called the pyroxene pallasite grouplet. Others are recorded as ungrouped or anomalous, each perhaps from its own parent body.
The pyroxene pallasites
The pyroxene pallasite grouplet (PPX) has two members: Vermillion, found in Kansas in 1991 (about 34 kg), and Yamato 8451, found in Antarctica in 1984. Besides olivine, they contain a few per cent of orthopyroxene, which is almost absent in other pallasites, along with phosphates. Their metal shows a fine Widmanstätten pattern. Olivine and pyroxene compositions, metal chemistry and oxygen isotopes are distinct from the main group and from the Eagle Station grouplet, so they sample a third pallasite parent body.
Some researchers argue the two are not closely related enough to form a true grouplet, and treat each as ungrouped. Either way, they add at least one more melted body to the list.
Ungrouped pallasites
Several more pallasites are recorded as ungrouped or anomalous because they match no group in their olivine, metal or oxygen isotopes. Some have unusually iron-rich or iron-poor olivine; others have metal chemistry unlike any known iron group. Many are small Antarctic or desert finds. Zaisho, the Japanese fall of 1898, has been classed as an anomalous main-group pallasite. The catalogue figure on this page counts those recorded as ungrouped.
Formation
The presence of pyroxene suggests that these pallasites formed in a region where the mantle was less purely olivine, perhaps nearer the crust, or on a body with a different bulk composition. Their slow-cooled metal shows they were buried deeply enough to cool over millions of years. Without more samples, the parent body's size and history remain uncertain.
How many pallasite parent bodies?
Counting the main group, the Eagle Station grouplet, the pyroxene pallasites and the distinct ungrouped specimens, pallasites sample perhaps five or more different parent bodies. This is surprising, because pallasite-like rock should be a small fraction of any melted asteroid, and it supports the view that collisions mixing metal into olivine were common early in Solar System history.
How to recognise one
Like other pallasites, these are heavy, magnetic mixtures of crystals and metal. The pyroxene grains are hard to tell from olivine by eye; identification depends entirely on laboratory analysis.
Specimens of the pyroxene pallasites are very rare in collections, and most material is held by research institutions. Ungrouped pallasites appear occasionally among desert finds, where careful classification is essential before any name is assigned.
Famous examples
- Vermillion Pallasite, ungrouped, United States, 1991 · 34.36 kg
- Yamato 8451 Pallasite, ungrouped, Antarctica, 1984 · 55 g
Related groups
- Main-group pallasites Nine in ten pallasites: magnesium-rich olivine in metal related to the IIIAB irons.
- Eagle Station pallasites A tiny grouplet of pallasites with iron-rich olivine and nickel-rich metal, from a separate parent body.
- Pallasites Gem-like olivine crystals set in iron-nickel metal.
Photos shared under Creative Commons licences (Wikimedia Commons).