I am a geologist living and teaching in the Seattle area. I recently had
a set of lunar rocks on loan from JSC and have a question about the
lunar basalt. The sample I had was embedded in disk # 032, and it’s
number is #15555 from Apollo Mission 15.
I understand that the lunar basalts are different from many Earth
basalts, but some of the differences were puzzling to me, so I hope you
can help!
1. The sample was much coarser in grain-size than most Earth basalts; in
fact without knowing its origin, I would have classified it as a gabbro,
based on texture alone.
2. The plagioclase feldspar in the sample seemed to be glassy in luster
(actually transparent) unlike any other plagioclase feldspar that I have
ever seen. Can you tell me why?
3. Could the lower lunar gravity have an effect on these observations?
If so, could you explain?
Thanks very much!
Linda K.

In reply to your question #1: The sample is fairly coarse-grained, with some plagioclase crystals up to 3 mm in size. The sample, however, was collected from a lunar mare (a flood basalt plain), so it was an extrusive rock (a basalt) rather than an intrusive rock (a gabbro). When it was erupted, the cooling rate was estimated to be 12 to 24 deg C/day. When olivine was growing, however, the cooling rate slowed to about 5 deg C/day.

In reply to your question #2: In general, the relative “freshness” of lunar minerals, including transparency, is partly a result of crystallization from relatively dry lavas. To truly answer the question, I would need to look at 15555 again (which I have not seen for several years). I might add, however, that any transparency of the plagioclase in 15555 may also be enhanced because cores of the crystals are unzoned and have very few inclusions.

In reply to your question #3: Gravity should not affect the transparency of the olivine, pyroxene, and plagioclase in this basalt. Nor should it affect the luster. The luster is a consequence of a spectacularly dry environment. Even though the lava erupted 3.3 billion years ago, it is fresher looking than most basalts on “wet” Earth.

Because you are a geologist, I suggest you also look at the description of sample 15555 in the Lunar Compendium (http://curator.jsc.nasa.gov/lunar/lsc/15555.pdf) and a large set of pictures of sample 15555 in the LPI-JSC Lunar Sample Atlas. The main page for that atlas is, but she can find the images of 15555 by going directly to. Some of the thin-section images are dramatic.

David Kring
Center for Lunar Science and Exploration
Lunar and Planetary Institute / NASA Lunar Science Institute

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