Classification of igneous rocks revisited

Petrology, I must admit, was never my favourite subject even though I had to do quite a bit of it during my time as a student. Even my Honours project had a petrological component. An important tool for the classification of igneous rocks is the QAPF diagram which forms the basis of the IUGS classification of igneous rocks (Le Bas and Streckeisen, 1991). It was interesting to go back to this original publication to read about the considerations that led to the adoption of the classification schema. The general principles are solid science. In some of the finer points, however, it becomes apparent that this classification schema comes from the pre-digital age.

"One of the merits of the TAS system is that the boundaries are definitive although they could be criticized as over simplistic. Definitive boundaries remove the ambiguity in naming a rock which plots near a boundary between two adjacent rock types. The simple boundaries of the TAS system also enable the classification to be constructed in a few minutes by pencil and ruler"

Furthermore, the TAS classification system only takes into account the sum of Na₂O + K₂O over SiO₂ and does not account for any other major elements even though such classification systems had already been proposed (e.g. De La Roche et al. 1980).

"[It] was not adopted, partly because the two cation per mil parameters were difficult to calculate without electronic assistance"

Certainly, in the age before widespread use of spreadsheet calculation software, this might have been an argument. A bit more difficult to follow is the following argument:

"The most common interpretations made by geologists are petrogenetical, but there could also be aesthetic considerations."

Robert Huber, Jess Robertson and myself (JK) have started to look more closely at the classification of igneous rocks in the dawning age of data intensive research in the geosciences. Watch this blog for more on this subject.

Ackonwledgements

Thank you to Jess Robertson for pointing out this gem.

References

De la Roche, H., J. Leterrier, P. Grandclaude, and M. Marchal (1980), A classification of volcanic and plutonic rocks using R1R2-diagram and major-element analyses — Its relationships with current nomenclature, Chemical Geology, 29(1–4), 183–210, doi:10.1016/0009-2541(80)90020-0.

Le Bas, M. J., and A. L. Streckeisen (1991), The IUGS systematics of igneous rocks, J Geol Soc, 148(5), 825–833, doi:10.1144/gsjgs.148.5.0825. Online also available at https://www.utdallas.edu/~aiken/SHAKEBAKE/rockclassification.pdf

Petrified wood from the moon

Did Apollo 11 bring petrified wood from the moon? A treasured piece at the Dutch national museum - a supposed moon rock from the first manned lunar landing - is nothing more than petrified wood, curators say.



It was given to former Prime Minister Willem Drees during a goodwill tour by the three Apollo-11 astronauts shortly after their moon mission in 1969. The US agency gave moon rocks to more than 100 countries following lunar missions in the 1970s.

The Rijksmuseum, which is perhaps better known for paintings by artists such as Rembrandt, says it will keep the piece as a curiosity. "It's a good story, with some questions that are still unanswered," Xandra van Gelder, who oversaw the investigation that proved the piece was a fake, was quoted as saying by the Associated Press news agency.

A jagged fist-size stone with reddish tints, it was mounted and placed above a plaque that said: "With the compliments of the Ambassador of the United States of America . . . to commemorate the visit to The Netherlands of the Apollo-11 astronauts." The plaque does not specify that the rock came from the moon's surface.

It was on show in 2006, and a space expert told the museum it was unlikely NASA would have given away any moon rocks three months after Apollo's return.

But frankly, does it look like lunar basalt? Researchers from Amsterdam's Free University said they could see at a glance that the rock was probably not from the moon. They followed the initial appraisal up with extensive testing.

"It's a nondescript, pretty-much-worthless stone," geologist Frank Beunk concluded in an article published by the museum.

US officials said they had no explanation for the Dutch discovery.