Geochemical alteration of pyrochlore group minerals; pyrochlore subgroup

Other| August 01, 1995 Geochemical alteration of pyrochlore group minerals: Pyrochlore subgroup Gregory R. Lumpkin; Gregory R. Lumpkin Australian Nuclear Science and Technology Organization, Advanced Materials Program, Menai, NSW, Australia Search for other works by this author on: GSW Google Scholar Rodney C. Ewing Rodney C. Ewing University of New Mexico, United States Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (7-8): 732–743. https://doi.org/10.2138/am-1995-7-810 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Gregory R. Lumpkin, Rodney C. Ewing; Geochemical alteration of pyrochlore group minerals: Pyrochlore subgroup. American Mineralogist 1995;; 80 (7-8): 732–743. doi: https://doi.org/10.2138/am-1995-7-810 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Primary alteration of uranpyrochlore from granitic pegmatites is characterized by the substitutions A☐Y☐ → ACaYO, ANaYF → ACaYO, and ANaYOH → ACaYO. Alteration occurred at ~ 450–650 °C and 2–4 kbar with fluid-phase compositions characterized by relatively low aNa+, high and aCa2+ high pH. In contrast, primary alteration of pyrochlore from nepheline syenites and carbonatites follows a different trend represented by the substitutions ANaYF → A☐Y☐ and ACaYO → A☐Y☐. In carbonatites, primary alteration of pyrochlore probably took place during and after replacement of diopside + forsterite + calcite by tremolite + dolomite ± ankerite at ~300–550 °C and 0–2 kbar under conditions of relatively low aHF, low aNa+, low aCa2+, low pH, and elevated activities of Fe and Sr. Microscopic observations suggest that some altered pyrochlores are transitional between primary and secondary alteration. Alteration paths for these specimens scatter around the trend ANaYF → A☐Y☐. Alteration probably occurred at 200–350 °C in the presence of a fluid phase similar in composition to the fluid present during primary alteration but with elevated activities of Ba and REEs. Mineral reactions in the system Na-Ca-Fe-Nb-O-H indicate that replacement of pyrochlore by fersmite and columbite occurred at similar conditions with fluid conpositions having relatively low aNa+, moderate aCa2+, and moderate to high aFe2+. Secondary alteration (<150 °C) is characterized by the substitutions ANaYF → A☐Y☐, ACaYO → A☐Y☐, and ACaXO → A☐X☐ together with moderate to extreme hydration (10–15 wt% H2O or 2–3 molecules per formula unit). Minor variations in the amounts of Mg, Al, K, Mn, Fe, Sr, Ba, and REEs are commonly observed as a result of secondary alteration. Major cation exchange for K, Sr, and Ba is a feature of samples from laterite horizons overlying carbonatites. In most cases U, Th, and B-site cations remain relatively constant. Radiogenic Pb is typically lost via long-term diffusion, but in some grains of uranpyrochlore 25–90% of the Pb is lost as a result of alteration. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.