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And I wondered why students were being put off metamorphic geology... -----Original Message----- From: Jurgen Reinhardt [SMTP:[log in to unmask]] Sent: Monday, November 05, 2001 7:30 AM To: [log in to unmask] Subject: Re: Amphibolite Hi, There are some hang-ups about rock nomenclature that I don't quite understand. An amphibolite, as any reasonable textbook would state, is a rock consisting mainly of hornblende and plagioclase (not any amphibole plus plagioclase!). Additional phases can/should be added as qualifyers (epidote amphibolite, garnet amphibolite, Cpx (diopside- or whatever) amphibolite, etc.). "Metabasite" is a term referring to a bulk compositional category, with no information about mineral assemblage, metamorphic grade, nor structure. Hence, it is used in the same sense as metapelite, but is not acceptable as a specific rock name. The term "pyribole" as defined by J.B. Thompson is firmly entrenched in the literature. For that reason alone, I would not accept "pyribolite" as a rock term for any type of pyroxene-amphibole rock. As S. Banno says: no more new rock names, if it can be avoided. I agree with Jim Eckert that the term amphibolite should be used based on mineral assemblage only and not necessarily imply a facies. Keeping in mind, however, that any middle-of-the-road amphibolite would commonly have formed under amphibolite facies conditions, the minerals that (may) indicate otherwise should be included in the rock term, even if it becomes lengthy. I cannot see any problem in using "two-pyroxene amphibolite" (just like "two-mica granite"). It's certainly more informative than " amphibolite". There is, of course, only so much a rock name can tell you, and if the hormblende is prograde or retrograde remains a different matter. There are always borderline cases where standard nomenclature may be unsatisfactory. However, metamorphic rock nomenclature has the advantage that it can cater for a lot of variety, and if one wants to be precise, one can use as many qualifyers as necessary. It really depends on what one intends to communicate to others. The only real mishap in rock nomenclature is "granulite", a non-descriptive term that refers to conditions of formation only and can be applied to all sorts of rock compositions. Hence is not particularly useful as a stand-alone rock name. However, before I open another can of worms, I stop here. Cheers, Jurgen J. Reinhardt School of Geological & Computer Sciences University of Natal Durban, 4041 South Africa