Review: “The Lunar Rocks”

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Written by Brian Mason and William Melson of the Smithsonian Institution, The Lunar Rocks was published in 1970 by Wiley-Interscience and weighs in at 179 pages all in. A couple of errors noted.

The book describes itself as:

“an attempt to provide a concise and coherent account of the scientific effort on the lunar [sic] samples and the interpretation of the results.”

Which it is, in spades. The Apollo program marks a definitive leap in our understanding of our Moon, and so what came before usually gets lumped into a single chapter, as is the case here. We start with the first great leap in Moon understanding when Galileo trained his instrument on the Moon and saw some unusal things that no one had noticed before. This started an optical arms race to try to produce better instruments to better see the Moon. In the early 1960s, the U.S. Geological Survey did some Lunar cartography, and identified four main periods in the Moon’s past:

-Copernican
-Eratosthenian
-Imbrian
-Pre-Imbrian

The authors also visit the scouts that we sent to survey the territory in anticipation of our Apollo visit, the Rangers, Surveyors and Orbiters that answered a lot of the major questions that were casting doubts on the program.

The second chapter looks at the Apollo program itself, laying out the major achievements and general objectives for flights 7 – 12, which were all that had occured at the time of publication.

The next chapter dives right into the Lunar Mineralogy. One thing to remember is the context and nature of the sample sites – Apollo 11 was a mare, and Apollo 12 was mare with some hills in the area. The rocks that were returned boiled down into three main categories:

Major (>10%): Pyroxene [(Ca, Fe, Mg)2Si2O6], Plagioclase [(Ca, Na) (Al, Si)4O8], and Ilmenite [FeTiO3]
Minor (1-10%): Olivine [(Mg, Fe)2SiO4], Cristobalite [Isometric SiO2], Tridymite [Hexagonal SiO2], and a surprise, Pyroxferroite [CaFe6(SiO3)7]
Accessory (<1%): lots of stuff, including more surprises like Armalcolite [(Fe, Mg)Ti2O5]

Of the major elements, the Pyroxene breaks down mainly into titanium and aluminum-rich augites and ferroaugites. What are augites you ask? Well thanks to Wikipedia we know that “Augite is a single chain inosilicate mineral described chemically as (Ca,Mg,Fe)SiO3 or calcium magnesium iron silicate”

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So is it useful? The Wiki article doesn’t say. Moving on…Plagioclase Feldspar makes up about 20-40% of the samples returned. The plagioclase has two major components – albite [NaAlSi3O8] (Ab) and anorthite [CaAl2Si2O8] (An), with most rocks showing a range of 60-100% An/0-40% Ab, covering anorthite (90-100% An), bytownite (70-90% An), and into labradorite (50-70% An).

The last major mineral is ilmenite, whose abundance surprised scientists. Here on Earth, ilmenite is common in basalts, but rarely exceeds 5% of content. Some of the Lunar rocks were in the 10-20% range, of a form close to the ideal, and with no detectable ferric iron but a number of substitutes.

Moving down to the minors, Olivine was up to 5% of returned samples, enriched in calcium and chromium, but deficient in nickel compared with terrestrial varieties. The SiO2 family of Cristobalite, Tridymite and Quartz are found on the Moon, but quartz not so much. These gave important clues as to the formation of the Moon, as cristobalite and tridymite are low-pressure/high-temperature forms of SiO2.

Pyroxferroite was one of several surprises for scientists. Here on Earth, Pyroxmangite is a manganese silicate with some ferrous iron (and a lovely pink color). On the Moon, pyroxferroite is a ferrous silicate with some manganese, but with an identical structure to pyroxmangite. It was found mainly in vuggy areas of the rocks, which leads to the question ‘what are vessicles and vugs?’

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Well, just like your soda, which is bottled under pressure, lava under the surface is under pressure from all the rocks above. Gases will be present, but diffused in the lava. When it erupts into the extremely low pressure environment of the Lunar surface, the gases start to bubble out, just like the carbon dioxide in your soda when you crack the seal. At the same time, the lava is cooling and hardening, so you’re left with solidified holes and partial holes. When the surface of the hole is smooth, it’s a vessicle. When it’s rough and bumpy with projecting crystals, it’s a vug.

All kinds of accessory minerals, often found in trace quantities, are then noted, from Troilite to Armalcolite, a new mineral named in honor of ARMstrong, ALdrin, and COLlins.

Summing things up, the authors note that:

“Lunar mineralogy as now known, while not extensive, is extremely interesting. It is clearly analogous to that of terrestrial basalts but reflects an extension into chemical compositions and physiochemical conditions of crystalization unknown in terrestrial rocks.”

The next chapter looks at the petrology of the igneous rocks. Scientists early on recognized three basic groups of sample types: crystalline igneous rocks, microbreccias, and fines (a/k/a regolith). The igneous basalts were divided into two groups – fine-grained (type A) and medium to coarse-grained (type B). The Apollo 11 samples showed only small ranges in relative mineral abundances, while Apollo 12 showed a wider range. By contrast, the Apollo 11 samples showed larger variations in grain size and texture. Scientists immediately noted distinct differences from terrestrial basalts, like the unusually high titanium content. The chapter looks at some of the ways that scientists have tried to classify the basalts, such as by mineralogical composition and chemical composition. Research into the viscosity of the erupting magmas was done, indicating that it was probably at about the viscosity of room-temperature glycerine (nowadays we say it was about as thin as motor oil), allowing it to flow long distances quickly.

Also noted is research into the crystallization sequences of the rocks. Determining which minerals formed when as the Moon cooled allows scientists to better understand the composition of the original magma, which helps them to try to pin down where exactly the material in the Moon came from. Also touched on is the study of the weak magnetic results from the Apollo samples.

Chapter five looks at the petrology of the Fines and the Microbreccia. Various aspects of the Lunar regolith are explored, even noting how regolith can serve as a radiation monitor over time. Breccia refers to a preponderance of angular rather than rounded grains, and is a perfect descriptor for the result of the impact ‘weathering’ that the Moon experiences. A lot of time is spent with the glasses, which are abundant in the samples in the form of tiny spheres. Six mechanisms are suggested for their formation:

(a) the expansion and tearing apart of large masses of molten glass formed near the center of major impacts
(b) the break-up of impact-produced liquid jets into droplet trains
(c) the splash and rebound from objects hitting molten masses of glass
(d) the drag of splattered glass over hard surfaces
(e) the condensation of droplets from a vapor cloud
(f) the vesiculation of impact-produced volcanic glass

The next chapter is a petrological comparison of Lunar rocks with terrestrial rocks, meteorites and tektites (which used to be thought of as having originated on the Moon). It restricts itself to the crystalline rocks from Apollo 11, as the fines and breccias are too dissimilar from typical Earth materials. Not so for the rocks, which are not all that different from their terrestrial counterparts in terms of abundances of compositional materials. Comparisons are also made to meteorites believed to have originated from the Moon to see if they pass muster, and tektites are touched upon at the end.

‘What are they made of, elementally?’ is the question visited in the next chapter, Lunar Geochemistry. This one takes a long stroll down the periodic table as it visits the various elements to be found in the Lunar samples. Even in the first samples scientists recognized elevated levels of the Lanthanide series elements, or Rare-Earth Elements. Apparently these don’t play well with other elements during the crystallization process, and so tended to accumulate in rocks in the highlands that came to be known as KREEP, from Potassium, Rare-Earth Elements, and Phosphorous. (so how did they get down on the mare?) The chapter finishes with a discussion of what the evidence tells us with regards to how the Moon came to be as it is today.

The final chapter looks at what all the data means and the Implications for Lunar History. Not just the major features, like the preponderance of mare on the near side, but also the origins. At the time there were four theories competing for recognition:

1) Fission Hypothesis – promulgated by Sir George Darwin, with the Moon having sloshed off from the Earth’s mantle. Where do you think the Pacific Ocean came from? Look at a globe – it’s huge! That must be where the Moon spun off from.
2) Double-Planet Hypothesis – where the Moon and Earth formed in close proximity from the same nebular materials feedstock.
3) Earth Capture – where the Moon formed elsewhere (say near Jupiter or Saturn) and got knocked into the inner Solar system where it was captured by the gravity of Earth.
4) Planetesimal Coalescence – the Earth once had a ring like the Gas Giants, and it coalesced into our Moon.

None of these fit the details that became known after Apollo, but the samples did point the way towards an alternative that drew from several of the theories, and one can see the early germination of the idea that is now known as the Big Whack Hypothesis.

The bibliography draws heavily from the Jan 30th, 1970 special issue of Science Magazine and the conference publication from the Apollo 11 Lunar Science Conference earlier that month. A glossary would have been good, as I came across a number of new words, like pleochroism, ophitic, fugacity (might have known that one in the distant past), and others.

Your friendly Lunar Librarian is not a geologist or mineralogist by training, so the book was a bit of a struggle, but doable. I’d certainly never considered how the size of an atom’s electron shell in angstroms determines its ability to be taken up into forming mineral crystals. The information is very preliminary, and so has been superseded by later results, but what makes the book interesting is the story of how the different pieces come together – abundance of siderophiles in the regolith compared with rocks, the likely crystallization schema of the mantle, the mixed-up breccias, and so on, to give the scientists the story of what really happened up there. Or at least a much better idea.

One thing that kept running through my head as I was reading this was ‘Okay, this is interesting, well, fascinating science, but what are the applications?’ This is, I think, the major weakness of all of the books to date – they focus on the data and science, and barely touch on the application. I recently saw reference to ‘economic geology‘, and I think one thing that is missing from the Moon writings to date is a translation of the pure mineralogy into economic selenology. Dennis Wingo sort of touches upon this in his book MoonRush, but primarily in relation to platinum-group metals. What I’d like to see is a book that says ‘here is how we can turn the stuff of the Moon into useful economic things’. Oxygen wrested from the rocks to supply cislunar transport needs. Rare-Earth Elements for advanced electronics applications. Zirconium for whatever it is one does with zirconium.

I rather enjoyed the book in spite of its limitations, and it has inspired me to tackle Judith Frondel’s Lunar Mineralogy next, so I’ll give The Lunar Rocks a waxing half Moon rating.

2 thoughts on “Review: “The Lunar Rocks”

  1. Ken,

    Good review of an almost paleolithic text!

    In re: your comment on the “economic geology” of the Moon, no one has written such a book. Part of the reason is because no one fully understands all we need to know to write such a text. For example, we know nothing about the physical and chemical state of ice at the poles of the Moon — where it is, how it occurs, what its physical state is. All of these parameters are the “meat” of economic geology and mining.

    In the meantime, a very good place to go is the book “Resources of Near-Earth Space” (1993) by J. Lewis, M. Matthews. and M Guerrieri (editors). This entire book is available FREE on-line! The Moon chapters are posted here:

    http://www.nss.org/settlement/spaceresources/resources2.html

    Cheers!
    Paul

  2. This is one of the few serious lunar science books that correctly shows the hard geochemical data that there’s a link between certain lunar specimens and terrestrial tektites. Despite post-Apollo reports to the contrary, there is a definite chemical association between more acidic lunar rocks and tektites. After considerable research, I have prepared a table showing a clear chemical link between Apollo specimens and tektites. This table appears in my book titled, “Inconstant Moon: Discovery and Controvery on the Way to the Moon” published by Xlibris/Random House.

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