Hi Kent and Bryant! The mineral zoning profiles should be different. For the hornblende gabbro, the hornblendes might high Ti, high Al cores zoning outward to lesser values toward the rims. The plagioclase should have normal to oscillatory zoning. An amphibolite will have none of this! Best wishes! Lawford Anderson Professor of Earth Science Department of Earth and Environment Boston University On Jan 7, 2025, at 11:43 AM, Platt, Bryant W via MSA-talk wrote:  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<mailto:msa-talk@minlists.org>> Sent: Monday, January 6, 2025 2:13 PM To: msa-talk@minlists.org<mailto:msa-talk@minlists.org> <msa-talk@minlists.org<mailto:msa-talk@minlists.org>> Subject: [EXTERNAL] [MSA-talk] hornblende gabbro vs. amphibolite 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 _______________________________________________ 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>