Different Directions - Moon & Meteorites

The Stones

Like many of you, I began learning about Meteorites haphazardly: some reading here, some there, and some educational conversations with vendors and collectors.

But only when I began seeing my own meteorites, and settling in to their various stories, did I start making sense of them. From the outside crust to the structures inside, Meteorites are beautiful and are great teachers about our solar system.

They reveal cosmic elements not seen in earth stones, and it is this, most of all, that has drawn me into the science of Meteoritics and into the science of our solar system and its processes.

Which brings me to the “Stones,” the largest class of Meteorites, the ones most seen falling to our Earth.

While we can call them Stones, there are really two major types of stony meteorites, and they are defined by their internal structures. Their names are

Chondrites and
Achondrites.

The most common meteorite is called an “ordinary chondrite” (OC). It's considered ordinary because 85 percent of the observed stony meteorite falls are in this class; otherwise, any meteorite is by definition out of this world un-ordinary....

However, there are two other scientific classes of these “ordinary” Stones, the

Carbonaceous (C) and the
Enstatite (E).

These individual types have unique characteristics, so let's see what they are.

But First Chondrules

Chondrites, some of the most primitive rocks in the solar system, have not changed much from the asteroid they came from, 4.5 billion years ago.

And because they had not been melted, they have a very distinctive appearance made from droplets of silicate minerals, mixed together with small grains of sulphides and iron-nickel metal.

This structure of round millimeter-sized granules also gives chondrites their name, from the Greek for sand grains 'chondros,' which today translates to Chondrules.

Chondrules from the chondrite Grassland

Chondrules are not found in terrestrial rocks.

And while this unique feature is purely cosmic, a better way to classify Chondrites is by the elements that make up the meteorite. So then we need to look at how they were formed.

Current evidence indicates that chondrules formed from preexisting grains. These grains were reheated to melting, and then combined together to form chondrules.

While in a momentary liquid state, they quickly solidified and combined with other solar nebula material to form the matrix that arrives on the door step of Earth.

Here's another great example of chondrules and matrix. In this case, a carbonaceous chondrite:

Here's a quick summation about key features of OCs.

Ordinary chondrites are...

Grouped as	Indicating	Amount of change or Metamorphism
H	High metal, high total iron	3-6 in the chondrules
L	Less metal, low total iron	3-6 in the chondrules
LL	Low metal, low total iron	3-6 in the chondrules

H chondrites have the highest iron content - 27 percent total iron by weight. Commonly referred to as olivine-bronzite chondrites.
L chondrites have a lower iron content - 23 percent by weight
Are referred to as olivine-hypersthene chondrites.
LL chondrites represent "low iron" and "low metal" content - 20 percent total iron.
Sometimes referred to as amphoterites.

The Amount of Change is called a petrologic grade - which indicates the degree of chondrule alteration by heating.

Well defined, unaltered chondrules have a petrologic grade of 3 or 4.
Less distinct chondrules have a higher number of 5 or 6 – which indicates an increased level of metamorphism.

Carbonaceous Chondrites

Carbonaceous chondrites are the most primitive and unaltered type of meteorite known, with an elemental composition probably similar to that of the nebula from which the Solar System formed.

In addition to silicates, oxides, and sulfides, they contain, most distinctively, water or minerals that have been altered in the presence of water, together with large amounts of carbon, including organic compounds.

Different groups of carbonaceous chondrites have been identified that came from parent bodies in different parts of the solar nebula.

Carbonaceous Chondrites contain large amounts of the magnesium-rich minerals olivine and serpentine and a variety of organic compounds, including amino acids.

These rare chondrites account for only about 5% of chondrite falls, but they provide a great deal of information about the origin of the Sun and planets, and even of life itself.

See Allende and Murchison.

E-Chondrites

Also called Enstatite chondrites, they are the least common stones and account for only about 2% of the chondrites that fall to Earth.

They are also unusual in that the iron they contain, which may make up to a quarter of their mass, appears in the form of metallic iron-nickel and as minerals containing iron sulfide. This is in marked contrast to other types of chondrites in which iron is combined with oxygen as oxides and silicates.

Enstatite chondrites also contain a variety of other minerals that can only form in extremely oxygen-poor conditions.

Enstatite chondrites are...

Grouped as	Indicating	Amount of change or Metamorphism
EH	High metal,	3-6 in the chondrules
EL	Low metal	3-6 in the chondrules

EH small chondrules and high ratios of siderophile elements to silicon.
- More than 10% of the rock is composed of metal grains.
- About 3% by weight of the iron-nickel metal consists of elemental silicon.
EL contain larger chondrites and low ratios of siderophile elements to silicon.
- Only about 1% by weight of the iron-nickel metal consists of elemental silicon.

The low oxygen content of E chondrites is a clue to their origin, possibility from an asteroidal parent body which once orbited the Sun in the inner part of the Solar System, or perhaps within the orbit of Venus or even that of Mercury.

Achondrites

These are some of the rarest of the stony meteorites, and while chondrites help us understand the origin of parent bodies, achondrites help explain the igneous processes of those bodies.

Let's look at this more closely. In general, achondrites do not have chondrules, and so were formed differently.

Chondrites formed by the process of accretion: Chondrules and the matrix material gathered together, forming and expanding into a parent body. And they did so without the necessary heat that destroys chondrules.
Achondrites formed by a melting and recrystallization process on a chondritic parent body – they are in many respects the igneous rocks of other worlds....

Here's an illustration that will help us understand where many types of achondrites come from:

Side Trip

Differentiation

This is a scientific term. It means certain planets and planet-like bodies formed mineral zones, zones that were different from each other. And the above illustration shows this.

But the process is pretty cool. Here's how it worked.

When our original solar system was forming, the Earth and the other inner planets (Mercury, Earth, Venus, Mars) were similar in their composition, as was the chondrite meteorites.

They grew by accumulation of other planet-like bodies, kinda like water droplets freezing into tiny ice crystals, and these crystals clumping together to form hailstones.

The clumping planet bodies started to heat up through the constant impact of other meteoritic bodies, and the melting surface and near-surface rocks. As these rocky bodies grew larger they began releasing gravitational energy which also converted into heat, especially deep in the interiors of the bodies.

Another heat source came from the decay of radioisotopes trapped in the rocks.

Together all of these heat sources melted these rocky planets and planet-like bodies (planetesimals).

Ultimately, these semi-liquid bodies became giant gravitational separators:

Heavy metals, iron, nickel, some gold, platinum, and iridium gradual sank to the centers – forming heavy cores.
Lighter elements and minerals accumulated around the cores, forming dense basalitic rock – forming mantles.
The lightest minerals, feldspars, quartz, and micas, floated to the top – forming a thin, outer crust.

You see these zones in the illustration above: the core, the mantle, and the crust. There is more to this story, and when we talk about the rock planets and especially our Earth, we'll see that these zones can be further differentiated. But that's for another time.

In conclusion

Returning to our beginning, 85 percent of all known meteorites are Chondrites, formed by the process of accreciation with other pieces clumping together in our early solar system.

The remaining 15 percent of known meteorites are made up of Achondrites, Stony-irons, and Irons.

As you can see from our illustration, these three are similar – they were formed by melting, on or within a rocky planetary body. While they were once probably chondritic, that early structure was destroyed by the process of differentiation, the process of planet building during the early growth of our solar system.

And, part of that growth was the destruction of those early rocky bodies. When these bodies broke apart, pieces of them formed the Achodrites, Stony-irons, and the Irons - each coming from an area within the bodies of those planetoids.

Types of Achondrites

What follows is a quick list of currently known Achondrites. I say currently because meteorite hunters and scientists are always being surprised.

Here are seven types of achondrites: Aubrites; Diogenites; Eucrites; Howardites; Shergottites, including Nakhlites and Chassignites; Ureilites; and Lunar Meteorites.

Aubrite is an alternative name for enstatite achondrite. They are differentiated stone meteorites consisting predominantly of enstatite with very low iron content.

Cumberland Falls, Kentucky, USA; Fell April 9, 1919; Achondrite, Ca-poor. Aubrite (AU)
Copyright © 2002-2009 Meteorites Australia.

Howardites are achondrite breccias containing rock and mineral fragments of eucrites and diogenites.

Kapeota, Equatoria, Sudan; Fell April 22, 1942; Achondrite, Ca-rich, Howardite (AHOW)

From the R.A. Langheinrich Museum of Meteorites

Eucrites are a class of achondrites that formed as basaltic flows on their parent body. They consist mostly of plagioclase and pyroxene.

Millbillillie, Western Australia, Australia; Fell October 1960; Achondrite, Ca-rich, Eucrite (AEUC) – a basaltic achondrite of the HED family probably derived from asteroid 4-Vesta. White areas are plagioclase, and grey areas are clinopyroxene.

The reddish rind was produced by terrestrial weathering of the original fusion crusted surface.

Here's another Eucrite:

Pasamonte - A Brecciated "Eucrite," but not from Vesta – New Mexico, 1933
From The International Meteorite Collectors Association (IMCA Inc.)

Ureilites are a unique type of achondrite composed mostly of olivine and pyroxene. Some display very heavy shock metamorphism.

Roosevelt County 027, New Mexico, USA, Found 1984, Achondrite, Ca-poor, Ureilite (AURE)

From the Michael Farmer Catalog

Diogenites are achondrites composed primarily of cumulative pyroxenes. They consist of larger interlocking crystals than eucrites.

Johnstown, Colorado, USA; Fell July 6, 1924; Achondrite, Ca-poor, Diogenite (ADIO)

Image source: http://www.meteorite-times.com

Shergottites, nakhlites and chassignites are very different from the other achondrite groups.

Unlike other meteorites, they contain iron-rich silicates and iron oxides which indicate they formed in a rather oxygen-rich environment. They also contain small amounts of water-bearing minerals.

This group of meteorites, often referred to as the SNC group, is one of the youngest groups of meteorites with an estimated crystallization age of approximately 1.3 billion years. However, what makes this group extremely exciting is the fact that their composition closely resembles that of the planet Mars. In fact, the resemblance is so remarkable, scientists are all but sure that these meteorites had their origin on the red planet!

Zagami, Katsina Province, Nigeria; Fell October 3, 1962; Achondrite, Ca-rich, Eucrite (shergottite) (AEUC)
Image from the New England Meteoritical Services

Nakhla, Alexandria, Egypt; Fell June 28, 1911; Achondrite, Ca-rich, Nakhlite (ACANOM)
Image from the New England Meteoritical Services

Chassigny, Haute Marne, France; Fell October 3, 1815; Achondrite, Ca-poor, Chassignite (ACANOM)
Image from the Meteorite-times.com

Lunar meteorites have also been classified as a subgroup of the achondrites because they show evidence of the igneous processes that took place on the Moon.

Chassigny, Haute Marne, France; Fell October 3, 1815; Achondrite, Ca-poor, Chassignite (ACANOM)
Image from the Meteorite-times.com

Lunar meteorites have also been classified as a subgroup of the achondrites because they show evidence of the igneous processes that took place on the Moon.