Meteorite Monday: The Chondrule Controversy

The rounded objects in the Grassland chondrite are chondrules (image from Wikipedia).

The rounded objects in the Grassland chondrite are chondrules (image from Wikipedia).

One of the more heavily  debated topics within the field of meteoritics is the origin of chondrules. These are the small, spherical silicate inclusions from where we derive the technical name for the most common type of meteorites, the chondrites. With few exceptions, chondrules are found in all chondrite groups in varying quantities. Sometimes we’ll see chondrites that are nearly 70% chondrules and in other cases, we’ll see chondrites, such the Ivuna, that contain no chondrules. In the simplest of terms, chondrules are composed of olivine and/or pyroxene, occasionally glass and a smattering of feldspar. In not so simple terms, chondrules are a hot mess of textures and compositions- messy enough that I’m not going to cover it in this post, but I did delve into it a bit in this older Meteorite Monday post about our enigmatic friends. Continue reading

Meteorite Monday: Aubrites

Bustee Aubrite- Image courtesy of IMCA

Aubrites belong to a group of meteorites called achondrites. This group of meteorites is different from chondrites in that they don’t contain chondrules, the silica-rich sphere like inclusions for which chondrites are known. Aubrites originated from asteroids and have a brecciated texture. This fragmented texture is the product of a violent collision between the aubrites parent asteroid and another asteroid.

Mineralogically, aubrites are very similar to enstatite chondrites. They are composed primarily of the magnesium rich pyroxene, enstatite. The big difference is that aubrites come from asteroids that heated to the point that they experienced melting and underwent some form of chemical change. In the geological sciences this is called differentiation. Another defining characteristic is the addition of the mineral oldhamite (1). This calcium sulfide, for reasons I’m not sure of, doesn’t form naturally in terrestrial rocks. This quality makes it a useful mineral in determining the celestial origin of aubrites.

1. Smith, Caroline; Russell, Sara; Benedix, Gretchen. Meteorites. Firefly Books. 2009. P. 68

Meteorite Monday: Ordinary Chondrites

Before I move too far into this post I should define two important terms so that we’re all on the same page:

  • Chondrite- basically the sedimentary rock of the solar system. It’s an aggregate of the left over material from the formation of the solar system.
  • Chondrule- Kind of the vagabonds of the proto solar system. They started off as molten spheres of either pyroxene, olivine or another silicate mineral that glommed onto the nearest asteroid, cooled and became the round features so prominent in ordinary chondrites.

If you have further questions (or even corrections/critiques) post it in the comments and I’ll do my best to address it.

The term “ordinary chondrite” is a bit misleading. Rocks that formed during or even before the formation of the solar system are anything but ordinary. Rather, they are the most commonly found meteorite on the earth. Their stony composition makes them more resilient than their carbonaceous brethren and as such, there is a ton of material about them that one can study. I don’t want to get into the details in this post because I need a little more time to study it and do the topic justice. However, I will list the three classes of ordinary chondrites (from Wikipedia):

  1. H Chondrites- Highest total iron, high metal, but lower iron oxide in the silicates
  2. L Chondrites- Lower iron total AND metal content, but higher iron oxide in the silicates
  3. LL Chondrites- Low iron total and low metal content, but higher yet in iron oxides

So basically, what separates the three classes is the iron and iron oxide content. As one goes up the other goes down.

 

Ordinary Chondrite Northwest Africa 1756

Ordinary Chondrite Northwest Africa 1756- image from Northern Arizona University

Those round blobs in this image are the chondrules. In a later post I’ll give more information about this particular chondrite.

Carbonaceous Chondrite

Carbonaceous Chondrite

Carbonaceous chondrites are the rarest of all the meteorite specimens. Like most meteorites, there are composed of a matrix and chondrules. The matrix is basically the body of the meteorite.  In the case of the carbonaceous chondrite, it’s composed of soft minerals very similar to serpentine or montmorillonite (John A. Wood, The Solar System, 1979). Due to it’s composition, very few of these meteorites survive the entry into the earth’s atmosphere. Those that do face further weathering damage at the surface of the earth.

The chondrules are the rounded minerals that are studded into the matrix. They are mostly composed of olivine and orthopyroxene and are generally rounded in shape (like the picture shows). However, not all CC’s have chondrules. Some have these irregular inclusions that are composed of uncommon minerals such as spinel and grossular. These minerals have been enriched in “calcium, magnesium, aluminium and titanium relative to silicon” (Wood, 1979).

Sources:

John A. Wood, The Solar System, 1979

Carbonaceous Chondrite Meteorites http://www.astro.washington.edu/courses/labs/clearinghouse/labs/Meteors/meteors.html