Winonaites
Primitive achondrites related to the silicate inclusions in IAB irons.
- Classification
- Stony meteorites › Achondrites › Primitive achondrites › Winonaites
- Share of witnessed falls
- about 0.1%
- In our catalogue
- 28 meteorites, 1 seen to fall
- Heaviest
- Winona, 24 kg
- Earliest recorded fall
- Pontlyfni, 1931
- Type specimen
- Winona, found in Arizona in 1928
Named after the Winona meteorite, found buried in a Sinagua ruin in Arizona in 1928, winonaites have near-chondritic chemistry and some relic chondrules. Their oxygen isotopes match the silicate inclusions found in IAB iron meteorites, so both are thought to come from the same partly melted parent body.
Mineralogy and chemistry
Winonaites are fine- to medium-grained rocks of olivine and pyroxene that are very poor in iron, with plagioclase, iron-nickel metal, troilite and small amounts of graphite, chromite and phosphates. They are more reduced than ordinary chondrites but less than enstatite chondrites. Some show relic chondrules; others contain veins of metal and sulphide and areas depleted in plagioclase, where melt began to collect and drain.
Their oxygen isotopes lie just below the terrestrial line, with a Δ17O of about −0.5 per mil, the same as silicate inclusions in the IAB iron meteorites.
Parent body and formation
The leading idea is that winonaites and IAB irons come from a single partly melted asteroid. Heating melted metal and sulphide and a little silicate, but before the body could separate fully into layers, a large collision broke it apart while it was still hot. Molten metal mixed with chunks of partly melted rock, and the debris reassembled. The silicate-free parts became IAB irons, the silicate-rich parts became winonaites, and the silicate inclusions in IAB irons are pieces of winonaite-like rock caught in the metal. Radiometric dating shows these events happened within a few million years of the first solids.
Because the winonaites and IAB irons fit together, they offer a rare chance to reconstruct a whole broken parent body from its pieces. The IAB complex itself contains several subgroups with slightly different chemistry, and whether all of them share the winonaite parent is still being tested.
Abundance, type specimen and famous examples
Winona, the type specimen, was found in 1928 during the excavation of a ruin of the Sinagua people near Flagstaff, Arizona, where it had been buried in a stone cist, apparently as a sacred object; about 24 kg is known. Pontlyfni, which fell in Gwynedd, Wales, in 1931, is the only winonaite seen to fall, and only about 157 g was recovered. Mount Morris (Wisconsin) is listed in the catalogue as an anomalous IAB iron, a reminder of how closely the two are linked. A few dozen winonaites are known.
How to recognise one
Winonaites are grey, fine-grained, recrystallised rocks with metal flecks, often heavily weathered to brown in finds. They attract a magnet. They look much like strongly heated chondrites, and Winona itself was long thought to be an unusual chondrite. Confirmation needs mineral and oxygen isotope analysis.
Famous examples
- Winona Winonaite, United States, 1928 · 24 kg
- Pontlyfni Winonaite, United Kingdom, 1931 · 157 g
- Tierra Blanca Winonaite, United States, 1965 · 860 g
- Hammadah al Hamra 193 Winonaite, Libya, 1996 · 259 g
- Mount Morris (Wisconsin) Iron, IAB-an, United States, 1937 · 676 g
Related groups
- IAB irons Irons with stony inclusions, from a partly melted asteroid. Canyon Diablo and Campo del Cielo.
- Acapulcoites Fine-grained rocks that only just began to melt.
- Primitive achondrites Rocks that began to melt but were never fully processed: halfway between chondrites and igneous rocks.
Photos shared under Creative Commons licences (Wikimedia Commons).