NWA 18592 Mesosiderite Meteorite Main Mass, Stony-Iron, 687.00g, Largest Stone

The main mass of NWA 18592, a mesosiderite stony-iron meteorite, at 687.00 grams

NWA 18592 is a mesosiderite, one of the two classes of stony-iron meteorites, built from metal and silicate rock broken and mixed together in roughly equal parts. The Meteoritical Bulletin records eleven stones and a total recorded mass of 903 grams, and lists the main mass with Brian McDonald. This 687.00 gram stone is the largest of the eleven and is that main mass.

The stone is dark brown to black across its exterior, lumpy and blocky, and crossed by a network of fine cracks. Pale tan desert sediment and small rounded sand grains are packed into many of those cracks and hollows. It was classified by D. Sheikh at Cascadia Meteorite Laboratory and approved in May 2026.

Structure and features

Every surface shown in the photographs is natural exterior, with no saw cut and no polished face. The exterior breaks into rounded, knobby blocks separated by thin cracks, a pattern that shows on every side of the stone. Orange and rust brown staining runs along the cracks and across the lighter areas. The stone carries no coating.

The Bulletin describes the stones as having visibly weathered metal, and the classification section shows why. The metal portion, kamacite and taenite, is almost entirely replaced by iron hydroxide formed by weathering on Earth. That replacement accounts for the dark, rust toned exterior. The Bulletin records the shock stage as moderate and the weathering grade as moderate.

In thin section the sample is brecciated, made of broken fragments, with metal and silicate in roughly subequal proportions. The silicate portion is mostly low-calcium pyroxene, in clasts up to 1 mm that sometimes hold inclusions of high-calcium pyroxene, with lesser plagioclase. Silica, merrillite, troilite and chromite are present as accessory minerals.

Discovery and provenance

NWA 18592 carries a Northwest Africa name, used for meteorites from the region whose exact find site is not documented. According to the Bulletin writeup, eleven stones were found in northwest Africa in 2024 and were purchased in 2025 by Brian McDonald online from a meteorite dealer in Morocco. Coordinates are recorded as unknown. More on how these names work is in NWA meteorites explained.

The meteorite was submitted by Daniel Sheikh, and D. Sheikh at Cascadia Meteorite Laboratory, Portland State University, carried out the petrography and classification. A 24.6 gram type specimen is held at Cascadia. The entry was approved on 16 May 2026 and is listed for Meteoritical Bulletin 115, which is in preparation and not yet a published volume.

The Bulletin records the main mass of NWA 18592 with Brian McDonald. Identification of this 687.00 gram stone as that main mass comes from Treasure Coast Meteorite Co. records. The recorded mass of 903 grams covers all eleven stones together and is not the weight of this stone.

Scientific context

Mesosiderites pair a silicate component that looks like crustal basalt and gabbro with iron-nickel metal, mixed together at high temperature and then broken and recombined. The silicate chemistry of NWA 18592 shows that crustal character. Low-calcium pyroxene averages Fs33.8 Wo3.7, high-calcium pyroxene averages Fs18.4 Wo38.5, and the plagioclase is highly calcic at An94.6. Those minerals are the same ones that make up the howardite, eucrite and diogenite meteorites, covered in the HED meteorites collection.

The other stony-iron class, the pallasites, pairs metal with olivine crystals instead. The two classes share a broad grouping by composition, not an origin. How meteorites are sorted into these classes is set out in how meteorites are classified.

What the research reports

The Meteoritical Bulletin classifies NWA 18592 as Mesosiderite, with no subgroup assigned. No published study of NWA 18592 itself is known beyond the Bulletin writeup. The study below concerns mesosiderites as a class and is included for the origin question it addresses. Its findings come from the authors' own samples and methods and are given as the study states them.

Haba, Wotzlaw, Lai, Yamaguchi and Schonbachler (2019)

M. K. Haba, J.-F. Wotzlaw, Y.-J. Lai, A. Yamaguchi and M. Schonbachler, Mesosiderite formation on asteroid 4 Vesta by a hit-and-run collision, Nature Geoscience (2019).

The authors dated zircon grains from mesosiderites by uranium-lead methods and found two ages: about 4,558.5 million years for formation of the original crust, and about 4,525.4 million years for the mixing of metal and silicate. Both ages line up with events recorded in the eucrite parent body, which they identify as probably the asteroid Vesta. In their words, "This chronological coincidence corroborates that Vesta is the parent body of mesosiderite silicates." They propose that the mixing came from a hit-and-run collision with a smaller body. That model is the authors' hypothesis; the origin of mesosiderites is still debated, and the Bulletin does not assign a parent body.

Frequently asked questions

Is this the main mass of NWA 18592?

The Meteoritical Bulletin records the main mass of NWA 18592 with Brian McDonald. This 687.00 gram stone is the largest of the eleven stones and is that main mass, per Treasure Coast Meteorite Co. records.

Is the 903 gram figure the weight of this stone?

No. 903 grams is the recorded mass for the meteorite as a whole, across eleven stones. This stone weighs 687.00 grams.

Has this stone been cut or polished?

No saw cut or polished face shows anywhere on the stone. Every surface in the photographs is natural exterior.

Why is the stone so dark and rusty?

The metal in NWA 18592 has been almost entirely replaced by iron hydroxide through weathering on Earth, as the Bulletin records. That weathered metal gives the exterior its dark brown, black and rust tones. Background on how metal in meteorites weathers is in do meteorites rust.

What is in the cracks on the surface?

Pale tan desert sediment and small rounded sand grains, lodged in the cracks and hollows of the exterior. They have been left in place.

What is a mesosiderite?

A stony-iron meteorite made of iron-nickel metal and silicate rock, broken and mixed together. The silicate part resembles the crust of a differentiated asteroid. Mesosiderites and pallasites are the two stony-iron classes.

Has NWA 18592 been published?

The entry was approved on 16 May 2026 and is listed for Meteoritical Bulletin 115, which is in preparation. The classification is official; the volume is not yet published.

How do I verify this is the same specimen shown in these photographs?

This stone is recorded under serial TC-00056. Its specimen record is a permanent page holding the classification data as recorded in the Meteoritical Bulletin, the recorded weight, and photographs of this exact stone. Meteorite surfaces are individual to each stone, so a specimen in hand can be compared against those photographs and weighed against the recorded figure.

Who is the seller?

Treasure Coast Meteorite Co., IMCA Member #3323, Hobe Sound, Florida. Background on the Bulletin and what its entries record is in understanding the Meteoritical Bulletin.

Collector significance

This is the main mass of an officially classified mesosiderite, the largest of its eleven stones, and it holds about three quarters of the meteorite's recorded mass. It is offered with its natural exterior as it was received.

As of November 2026, the Meteoritical Bulletin lists NWA 18592 as 1 of 306 approved meteorites classified as Mesosiderite.

This specimen is offered by Treasure Coast Meteorite Co., IMCA Member #3323, and is issued with a permanent specimen record and serial number, listed in the Specimen Registry. Browse more stony-iron meteorites in the collection.

Classification and provenance for this specimen follow the official Meteoritical Bulletin entry for NWA 18592, viewable at the Meteoritical Bulletin Database.