Simulant Development: Igneous Rocks
The figures and information in this section are extracted from or redrawn from the IUGS Subcommission on the Systematics of Igneous Rocks (Le Maitre, 2002). For a full discussion of these figures and their application please see the referenced text. This reference is the internationally accepted standard for the classification of plutonic rocks and volcanic rocks.
Regrettably much if not most of the literature about lunar lithologic composition does not use the international standard. Technical literature dealing with terrestrial material written since the middle 1970's does use either this standard or earlier editions of it.
The naming and classification of rocks is based on the relative proportions of specific mineral groups. The specific measurement is the volume modal abundance. Modal abundance is obtained by measuring the abundance of actual minerals. This is in contrast to normative abundance, which is obtained by computing presumed mineralogy based on chemical abundance.
The mineral groups used are referred to as Q, A, P, F and M. The following is quoted from section 2.1.1 of the standard (Le Maitre, 2002).
"Q = quartz, tridymite, cristobalite
A = alkali feldspar, including orthoclase, microcline, perthite, anorthoclase, sanidine, and albitic plagioclase (An0 to An5)
P = plagioclase (An5 to An100) and scapolite
F = feldspathoids or foids including nepheline, leucite, kalsilite, analcime, sodalite, nosean, haiiyne, cancrinite and pseudoleucite
M = mafic and related minerals, e.g. mica, amphibole, pyroxene, olivine, opaque minerals, accessory minerals (e.g. zircon, apatite, titanite), epidote, allanite, garnet, melilite, monticellite, primary carbonate.
Groups Q, A, P and F comprise the felsic minerals, while the minerals of group M are considered to be mafic minerals, from the point of view of the modal classifications."
The first figure is the basic classification for igneous rocks. The most common terrestrial, coarse grained, igneous rocks fall in the center of the upper half of the diagram. There are abundant terrestrial occurrences within all fields of the diagram. Lunar rocks are almost exclusive within one field of the diagram, the diorite/gabbro/anorthosite field.
Click image to enlarge
Le Maitre, R.W., A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, P. Bateman, editors; 2002; Igneous Rocks: A Classification and Glossary of Terms : Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks; Cambridge University Press, 252p.
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