NWA 18593 Winonaite Meteorite End Cut 279.74g Mirror Polished Primitive Achondrite

A mirror polished end cut of a winonaite

Northwest Africa 18593 is a winonaite, one of the primitive achondrites. These meteorites carry the recrystallized texture of an achondrite while keeping a bulk composition close to that of a chondrite, and they sample parent bodies that were heated to the point of partial melting early in the history of the solar system. This 279.74 g end cut has been mirror polished across its full cut face, where bright metallic grains are scattered through a dark silicate groundmass and change with every angle of the light.

Structure and features

The cut face is mirror polished, not etched, and carries no coating. Under direct light it reads silver and bright; turned away from the light it goes dark, with the metallic grains picking out points of reflection across the whole surface. The Meteoritical Bulletin records Fe-Ni metal, troilite and chlorapatite as accessory phases in this meteorite.

A network of fine fractures crosses the polished face, several of them with brown fill. The Bulletin records a moderate weathering grade and a moderate shock stage for NWA 18593, and the fractures are part of what this stone looks like after time in the desert. The reverse and edges keep the natural exterior, a knobby, weathered surface with light tan material in some of the recesses. The Bulletin describes the three stones as having weathered fusion crust.

In thin section, the Bulletin describes a granoblastic texture of olivine, low-Ca pyroxene, high-Ca pyroxene and plagioclase, with a mean silicate grain size of about 0.3 mm and grain boundaries often meeting at roughly 120 degree triple junctions. In parts of the section, rounded olivine is enclosed within larger poikiloblasts of low-Ca pyroxene. These are microscope observations from the classification section, not features visible on the polished face by eye.

Discovery and provenance

According to the Meteoritical Bulletin, three stones were found in 2023 in Mauritania and were later purchased online in 2025 by Brian McDonald from a meteorite dealer in Morocco. The recorded total known weight is 1986 g across three pieces.

Material was submitted by Daniel Sheikh and classified by D. Sheikh at the Cascadia Meteorite Laboratory, Portland State University, where the 29 g type specimen is held. The Bulletin records the main mass with Brian McDonald. The name was approved on 16 May 2026, and the meteorite is listed in Meteoritical Bulletin 115, which is in preparation and not yet a published volume. The Bulletin notes that information from MB 115 or later remains subject to change by the Meteorite Nomenclature Committee.

This end cut is not the main mass. It was cut from another of the three stones. Treasure Coast Meteorite Co. holds all recorded material of NWA 18593 apart from the type specimen at Cascadia.

Scientific context

Primitive achondrites include the acapulcoite and lodranite group and the winonaites, named for the Winona meteorite found in Arizona. Telling them apart has historically leaned on oxygen isotopes, which can overlap between groups. Stephant et al. (2023) proposed separating them with three mineral measurements instead: the fayalite content of olivine, the FeO/MnO ratio of olivine, and the potassium content of plagioclase. The same study defined a new related group, the tissemouminites.

NWA 18593 was classified on that basis. Its olivine measures Fa4.3 with an FeO/MnO ratio of 17, and its plagioclase is An21.7 Or3.5. The Bulletin states that the sample plots within the winonaite field on those three components, and that classification as a winonaite is favored over acapulcoite. That wording is carried here as the Bulletin gives it.

For more on how these groups fit into the wider picture, see what is an achondrite and how meteorites are classified.

Frequently asked questions

Is NWA 18593 an officially classified meteorite?

Yes. Northwest Africa 18593 is an official name in the Meteoritical Bulletin Database, approved on 16 May 2026 and listed in Meteoritical Bulletin 115, which is in preparation. Understanding the Meteoritical Bulletin explains what that status means.

What is a winonaite?

A primitive achondrite: a meteorite with the recrystallized texture of an achondrite and a composition still close to a chondrite. Winonaites are one of the named groups of primitive achondrites, alongside the acapulcoites and lodranites.

How many winonaites have been approved?

The Meteoritical Bulletin lists 109 approved meteorites classified as winonaite as of September 2026. That is a count of database entries. Pieces of the same fall recovered separately can be listed under different names until pairing is established, so the number of distinct meteorites may be lower.

Is this piece the main mass?

No. The Bulletin records the main mass of NWA 18593 with Brian McDonald, and this end cut was cut from a different stone of the three. Treasure Coast Meteorite Co. holds all recorded material apart from the 29 g type specimen at Cascadia.

Is the cut face etched or coated?

Neither. The face is mirror polished only. It has not been etched and has no coating or epoxy.

How should I care for it?

Keep it dry and handle the polished face as little as possible. The stone contains iron-nickel metal and already shows weathering, so humidity is the main thing to avoid. Do meteorites rust covers storage in more detail.

Does it come with documentation?

Yes. This specimen carries serial TC-00046 and has a permanent public specimen record with its weight and photographs. The record is a reference document, not a certificate of authenticity or an appraisal.

Who is selling this specimen?

Treasure Coast Meteorite Co., Hobe Sound, Florida, IMCA Member #3323.

Collector significance

At 279.74 g, this end cut is about 14 percent of the recorded total known weight of NWA 18593. It comes from the holder of the main mass and of every other recorded piece, with the full chain from the Bulletin writeup to this listing documented.

The complete classification is available in the Meteoritical Bulletin Database entry for Northwest Africa 18593. More stony material is listed in the stony meteorite collection. Treasure Coast Meteorite Co., IMCA Member #3323.