Mineralogists (and petrologists), This is going to sound pedantic (and I am bracing for the rants in response), but I think its a legitimate question.... When do you use the term "hornblende gabbro" (igneous rock) and when do you use the term "amphibolite" (metamorphic rock)? The rock in question may or may not be from Shelby, North Carolina. The mafics include hornblende (some are dusted with opaques, and some aren't) + biotite (zero pyroxene). There is also no apparent deformation in the rock and relict igneous textures (cumulate plag) are present. Should I put this specimen on the Igneous Rocks Lab Exam, the Metamorphic Rocks Lab Exam, or on both? Thanks! Kent --- Dr. Kent Ratajeski Lecturer and Dice Mineralogical Museum Director North Hall 081 Department of Geology, Geography, and Environment Calvin University 3201 Burton St. SE Grand Rapids, MI 49546 (616) 526-6769 https://calvin.edu/directory/people/kent-ratajeski

Hi Kent, A lot depends on context, and if you don’t know that, it’s a tough call. Yes, there are essentially randomly-textured metamorphic rocks that look for all the world like an igneous rock. The assemblage and textures alone aren’t exactly diagnostic. A couple thoughts: How do you know the rock didn’t start out life as a pyroxene gabbro and was hydrated in the amphibolite facies? Many “igneous” rocks are metamorphic, in the sense that they have undergone chemical and mineralogical changes post-intrusion. In the chapter on plutons from “Petrology and Plate Tectonics” (Glazner and Kohn, due out this summer), Allen has a whole section on this topic. I’m happy to send that section to anyone who’s interested. Anyway, from a pedagogical point of view, I’m not sure it matters that much what you call it. In an igneous lab, you can talk about hornblende gabbros. In a metamorphic lab, you can make the point that not every metamorphic rock is foliated, especially metaigneous varieties. Best, Matt Note: While I may send email outside of regular business hours, I do not expect the same from others. ******************************************************************************************************************************* Dr. Matthew J. Kohn, University Distinguished Professor Department of Geosciences, Boise State University 1910 University Dr.; MS1535 Boise, ID 83725-1535 mattkohn@boisestate.edu https://www.boisestate.edu/earth/people/matthew-j-kohn/ fax: (208)-426-4061 Pronouns: He/him/his China book: https://www.amazon.com/dp/B0BRXWNTS6 <https://www.amazon.com/dp/B0BRXWNTS6>, https://www.amazon.com/dp/B0BRZ2XRV9 <https://www.amazon.com/dp/B0BRZ2XRV9> Optical Mineralogy Book: https://optical.minpet.org/ <https://optical.minpet.org/> Petrology and Plate Tectonics Book: https://global.oup.com/academic/product/petrology-and-plate-tectonics-9780190490645 <https://global.oup.com/academic/product/petrology-and-plate-tectonics-9780190490645> ******************************************************************************************************************************* > On Jan 6, 2025, at 2:13 PM, Kent Ratajeski via MSA-talk <msa-talk@minlists.org> wrote: > > Mineralogists (and petrologists), > > This is going to sound pedantic (and I am bracing for the rants in response), but I think its a legitimate question.... > > When do you use the term "hornblende gabbro" (igneous rock) and when do you use the term "amphibolite" (metamorphic rock)? > > The rock in question may or may not be from Shelby, North Carolina. The mafics include hornblende (some are dusted with opaques, and some aren't) + biotite (zero pyroxene). There is also no apparent deformation in the rock and relict igneous textures (cumulate plag) are present. > > Should I put this specimen on the Igneous Rocks Lab Exam, the Metamorphic Rocks Lab Exam, or on both? > > Thanks! > > Kent > > > --- > Dr. Kent Ratajeski > Lecturer and Dice Mineralogical Museum Director > North Hall 081 > Department of Geology, Geography, and Environment > Calvin University > 3201 Burton St. SE > Grand Rapids, MI 49546 > (616) 526-6769 > https://calvin.edu/directory/people/kent-ratajeski <https://calvin.edu/directory/people/kent-ratajeski> > _______________________________________________ > MSA-talk mailing list -- msa-talk@minlists.org <mailto:msa-talk@minlists.org> > To unsubscribe send an email to msa-talk-leave@minlists.org <mailto:msa-talk-leave@minlists.org>

Hello Kent, To your first point - texture and mineralogy are going to be your keys for separating hornblende gabbro and hornblendite on the igneous side of things from hornblende gneiss and amphibolites on the metamorphic side. As one can imagine, the boundary between undeformed gabbro or hornblendite and a gneiss is transitional, but an amphibolite is a high T rocks and will typically reflect that. Thin sections of the rocks should be more diagnostic. As for which lab exam to include them in, if the rock is igneous then it should be on that exam. But there is perhaps a more fundamental question, if you are uncertain on the classification of the rock is it fair to expect your students to be able to identify it? What exactly are you trying to assess by having it on the exam? These types of pedagogical questions should guide what you choose to include in your assessment. I hope this was helpful! Best, Bryant Platt (He/Him/His) Geologist U.S. Geological Survey Staff Profile<https://www.usgs.gov/staff-profiles/bryant-platt> ________________________________ From: Kent Ratajeski via MSA-talk <msa-talk@minlists.org> Sent: Monday, January 6, 2025 2:13 PM To: msa-talk@minlists.org <msa-talk@minlists.org> Subject: [EXTERNAL] [MSA-talk] hornblende gabbro vs. amphibolite This email has been received from outside of DOI - Use caution before clicking on links, opening attachments, or responding. Mineralogists (and petrologists), This is going to sound pedantic (and I am bracing for the rants in response), but I think its a legitimate question.... When do you use the term "hornblende gabbro" (igneous rock) and when do you use the term "amphibolite" (metamorphic rock)? The rock in question may or may not be from Shelby, North Carolina. The mafics include hornblende (some are dusted with opaques, and some aren't) + biotite (zero pyroxene). There is also no apparent deformation in the rock and relict igneous textures (cumulate plag) are present. Should I put this specimen on the Igneous Rocks Lab Exam, the Metamorphic Rocks Lab Exam, or on both? Thanks! Kent --- Dr. Kent Ratajeski Lecturer and Dice Mineralogical Museum Director North Hall 081 Department of Geology, Geography, and Environment Calvin University 3201 Burton St. SE Grand Rapids, MI 49546 (616) 526-6769 https://calvin.edu/directory/people/kent-ratajeski