Meteorites Under a Microscope: A Web Tour and Exercise!

When looked at through a microscope, Meteorites show many of the same features as rocks from the Earth, except that the stuff they are made of is very different. What we're going to do now is tour through some pictures of meteorite textures and makeup as seen through a petrographic microscope, to see what kinds of unique features they have.

The above photomicrographs (Left side in Plain Light; right under Crossed Polars) come from a CM3 (Mighei-type) Carbonaceous Chondrite. Carbonaceous Chondrites are the most primordial of all meteorites. They all contain some amount of carbon (either as Graphite, or in Organic compounds), which means they have not experienced temperatures much greater than about 300K (about 28íC, not much warmer than a Spring day in Florida). A useful byproduct of this low temperature history is that the grains that originally coalesced to make this meteorite 4.5 billion years ago are preserved, as is its original accretionary texture.
[A note on the classification scheme of chondrites: the classification acronyms always have two letters and a number: meteorite type, texture class, and metamorphic grade. For the stone above, CM3 means its a Carbonaceous Chondrite, Mighei texture type, with a metamorphic grade of 3 (on a scale of 1 (coolest) to 6 (hottest).]

Question 1: Based on your computer observations of Igneous and Sedimentary Rock thin sections, would you call the texture of this chondrite Clastic or Crystalline?
Question 2: Based on your chosen texture type, would it be a Sedimentary, Igneous, or Metamorphic rock?

Note the numerous round grains in this Chondrite. These are common in all chondrites, and are called Chondrules. Several different varieties of chondrules may be found in a chondrite. The round ones with banded crystals in them, like a washboard, are called Barred Olivine Chondrules.

The five crossed polars pictures above show us a Radiating Pyroxene chondrule, with a beautiful fanning array of pyroxene crystals. The pictures represent three different orientations of the slide, showing how the passage of light through the pyroxene crystals undulates as a function of crystal orientation.

More common than the other kinds are Porphyritic Pyroxene chondrules. In these chondrules, the pyroxenes form bladed or blocky grains. Some of these types of chondrules are not perfectly round.

Question 4: Describe the textures you see in the Chondrules: are they Crystalline or Clastic?

Link to Gallery of Chondrite images

Question 5: Look at the pictures below: These are images of Lunar Brown Glass collected on the Apollo 17 Moon mission. Lunar Brown Glass is droplets of molten rock formed either through violent volcanic eruptions, or during melting events related to large meteor impacts. In the frictionless vacuum of space they form tiny spheres, and the Quench Cool (i.e., they freeze very rapidly) upon eruption. If they do crystallize at all, they form Quench Crystals: long, delicate crystals that grow suddenly when the glass freezes, sorta like the rock version of snowflakes.

Now For the Question: Look at the Brown glass pictures, and the various chondrule pictures, and describe for me the conditions under which chondrules actually form!

Question 6: OK, let's do a thought exercise:

Chondrites are formed from the solids which condensed out of the Solar Nebula, the dust and gas and debris in orbit around the newly ignited Sun. Temperatures in the Solar Nebula would have varied from place to place much as temperatures in the Solar System today: hotter near the Sun, and colder farther away.

Given the temperatures at which Chondrules form, where in a Solar Nebula would you figure they'd come to be?

If your Chondrules and Chondrites didn't form in the same place in the Solar Nebula, how could they have gotten together (sidebar question and hint: was the Solar Nebula a sedate or windy place?)

Achondrite Textures: The two sets of images below are from a Ureilite, and a Shergottite, both Achondrites (stony meteorites which are not chondrites).

Ureilites are Olivine rich meteorites, which generally contain some form of carbon (either Graphite or Diamond). Given the fact that they contain carbon, it is believed that they formed from the melting of a Carbonaceous Chondrite.

--Question 7: Looking at the images, where is the Graphite in this Ureilite [Hint: in thin section, graphite is black, just like in a pencil].

Link to Gallery of Achondrite Images

This rock, EET79001, is a Shergottite: a basaltic meteorite believed to be derived from Mars. A characteristic constituent of Shergottites is a material called Maskelynite, which is a Shock glass formed from the mineral Plagioclase during an impact event (such as was necessary to knock it off its parent body and fling it toward Earth.)

Link to EET79001 Image Gallery

--Question 8: Try and identify the Maskelynite in this shergottite - where is it? [Hint: in plain light, glass is clear, but in crossed polars, it goes black (see the Brown glass pictures above)].

Link back to Rocks and Meteorites Introductory Page.

Link back to Page on Terrestrial Rocks under a Microscope.

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