The occurrence of högbomite in high-grade metamorphic rocks

Högbomite has generally been considered to be a rare accessory phase in metamorphic rocks. While investigating high-grade peraluminous metamorphites in the Benson Mine District, Adirondack Mountains, New York and the Manitouwadge Massive Sulfide District, Ontario, Canada, we have found several högbomite occurrences and believe that högbomite is more widespread in high-grade aluminous rocks than previously recognized. At Benson Mine, an iron-rich högbomite (Hög) occurs with K-feldspar-magnetite (Mt)-ilmenite (Ilm)-biotite-almandine (Alm)-sillimanite (Sil)-quartz (Qz)-hercynite (Hc)-corundum (Cor)-rutile (Ru). At Manitouwadge, Fe -Zn högbomite is found with gedrite-cordierite-staurolite-hercynite-magnetite±quartz ±ilmenite±rutile±biotite±cassiterite. Because composition varies with structure type, it is essential to determine the structure of högbomite utilized in specific reactions. Högbomite from Benson Mine has an 8H structure type, while that at Manitouwadge has a complex mixed structure. Both are more iron-rich than previously reported högbomites, and their composition can be approximated by the ideal formula Fe₅Al₁₆TiO₃₀(OH)₂. Proposed reactions for 8H-högbomite are Hög=Ilm+Hc+Cor+V, Hög=Ru+Hc+Cor+V, Hög+Ru=Ilm+Cor+V, and Hög+Ilm=Ru+Hc+V. These reactions can be combined with the experimentally determined reactions Alm+Sil=Hc+Qz and Ru+Alm=Ilm+Sil+Qz to derive reactions in the system FeO-Al₂O₃-TiO₂-SiO₂-H₂O that limit the stability of the assemblages Hög+Alm and Hög+Sil. Oxidation-sulfidation reactions define a wedge-shaped stability field for högbomite that is closed on the high f S₂ side.Contribution No. 456 from the Mineralogical Laboratory, University of Michigan, Ann Arbor, USA