Howardites
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Eucrites

Volcanic basalt from the crust of Vesta.

Eucrite
Millbillillie, Eucrite-mmict, Australia
Classification
Stony meteorites › Achondrites › Asteroidal achondrites › HED meteorites: pieces of Vesta › Eucrites
Share of witnessed falls
about 3.4%
In our catalogue
681 meteorites, 34 seen to fall
Heaviest
Millbillillie, 330 kg
Earliest recorded fall
Stannern, 1808
Most recent fall
Bunburra Rockhole, 2007

Eucrites are basalts and gabbros from Vesta's crust, made mainly of pigeonite (a calcium-poor pyroxene) and calcium-rich plagioclase feldspar. Many have a shiny black fusion crust that contrasts with their pale grey interior. The name comes from the Greek for 'easily distinguished'.

They are among the oldest volcanic rocks known, about 4.5 billion years old. Most are brecciated by impacts. They divide into basaltic (lava flows), cumulate (slowly cooled, coarse-grained) and polymict varieties.

Mineralogy and chemistry

Eucrites are made of roughly equal amounts of pyroxene, mainly pigeonite, and calcium-rich plagioclase (anorthite to bytownite), with minor silica, chromite, ilmenite, troilite, phosphates and specks of metal. In many, the pyroxene contains fine parallel lamellae of calcium-rich pyroxene that separated as the rock cooled, a record of slow cooling or later reheating. Compared with Earth's basalts they are very dry and poorer in sodium and potassium.

Basaltic eucrites are fine- to medium-grained lavas or shallow intrusions; most belong to a 'main group', with the 'Stannern trend' richer in incompatible elements and the 'Nuevo Laredo trend' more evolved. Cumulate eucrites, such as Moore County and Serra de Magé, are coarse gabbros formed as crystals settled in a magma chamber. Polymict eucrites are breccias of several kinds.

Parent body and formation

Eucrites formed as Vesta's crust, either as lavas erupted from its magma ocean or as melt that collected near the surface. Hafnium-tungsten and manganese-chromium dating shows the crust formed within a few million years of the first solids. Many eucrites were later reheated, probably by burial under fresh lavas or impact debris, which recrystallised their minerals. Argon ages reset between about 3.4 and 4.1 billion years ago record a period of heavy bombardment. A few eucrite-like basalts, such as Ibitira and Northwest Africa 011, have different oxygen isotopes and must come from other bodies.

Abundance and famous examples

Eucrites are the most common achondrites, about 3.4% of all witnessed falls. The name, from Greek for 'easily distinguished', was given by Gustav Rose in 1863. Stannern fell in Moravia in 1808 as about 200 to 300 stones. Juvinas fell in the Ardèche, France, in 1821 (about 91 kg). Millbillillie fell in Western Australia in 1960, and abundant pieces found since have made it the most widely held eucrite. Pasamonte fell in New Mexico in 1933, after a dawn fireball seen across several states.

Eucrites make good teaching specimens: a single slice can show a fusion crust, an igneous texture and an impact breccia, and their glossy crusts are among the most beautiful of any meteorite. Pieces found in deserts must be collected in line with local law and with the landowner's permission.

How to recognise one

Look for a glossy black fusion crust, often with flow lines, over a pale grey, fine-grained interior that may show white feldspar and darker pyroxene, often as a breccia. Eucrites contain little metal and are only weakly magnetic, so a magnet test is not helpful.

Famous examples

Related groups

See all 681 Eucrites

Photos shared under Creative Commons licences (Wikimedia Commons).