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5.2.1 Moon rocks – a scientist’s view

Larry Taylor reflects on types of rocks on the Moon and the contrast between the lunar highlands and the maria. Note that he talks about another rock type, breccia. You’ll learn more about breccias later this week.

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I did a post-doc in Washington, DC, at the Carnegie Institute of Washington. And we were amongst the first to get the samples back at Apollo 11 and Apollo 12, et cetera. It was strange, because they didn't look any much different than they did from the rocks on Earth at first glance. But it's when you look at it closely with a microscope that you see that there's distinct differences. Probably 95% of the minerals are quite similar to what we have on Earth, but almost every lunar sample has metallic iron in it. On Earth, we have iron oxide type minerals, like magnetite and things like that. But on the Moon, we have a very reducing situation. So we have iron in the native state as well as plus two. So we have, as I say, metallic iron in every single lunar sample I've ever seen. The mare, the dark portion of the Moon, make up only 16-18% of the total Moon. So that means the 85% that's not represented by the mare are what we call the highlands. And we only went to one highland locality. That was Apollo 16. And it was the highlands type of mineral there, a feldspar, basically calcium sodium aluminium silicate that gave it its light colour. And what we discovered is that the highlands of the Moon, the white part of the Moon, was all part of what we call a big lunar magma ocean.
The outer portion of the whole Moon was a melt. And what happened was certain minerals, as they crystallised, sank to the bottom. Those were the ones with iron and magnesium. And the light ones, calcium sodium aluminium silicate, they floated. And the floated ones made the crust of the Moon. So it's all this white, light coloured mineral. If you go to almost any volcano on Earth, you'll see basalts, and you'll see rocks that are quite similar to the water in the mare of the dark portion. The Man in the Moon is the dark portion.
And they have the same basic minerals in them except that they have a little bit of a few oddities in there. We found an iron, titanium, magnesium, mineral that we named armalcolite, A-R-M for Armstrong, A-L for Aldrin, C-O-L for Collins, big PR, ok, armalcolite. And that mineral is not very abundant. But it's somewhat common in Apollo 11 and Apollo 17. The minerals that we're finding on the Moon as new minerals on the Moon we have then since found on Earth. Once you know where to look for something, it's a lot easier to find it.
We're geologic detectives. We come upon the scene long after it happens, OK? And we've got to check this and this. Well, we don't check footprints and blood and DNA stuff, but we check the nature of the rocks. What do they look like? Some rocks are breccias. They look like the cross section of a sidewalk. They're put together with all the little pieces all stuck together. That's a breccia. And so we find those, but we look at each individual portion of the breccia, and it comes from a different place, from a different rock. So one rock, one breccia, can have twenty different rocks inside. And it tells you a whole lot more than just normal.
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