Hydrothermal precipitation of sanidine (adularia) having full Al,Si structural disorder and specular hematite at Maunakea Volcano (Hawai'i) and at Gale Crater (Mars)

Abstract

A layer of weathering‐resistant material is located within the walls of an erosional gully of the Pu'u Poliahu cinder cone in the summit region of Maunakea volcano (Hawai'i). The volcanic cone, initially composed of unaltered basaltic material (tephra), was extensively altered throughout by hot, sulfuric‐acid solutions. The layer is a location where the alteration by hot water was particularly aggressive, cementing the volcanic sediment and causing extensive chemical and mineralogical changes. Instead of basaltic chemical and mineralogical compositions, altered tephra was enriched in iron from aqueous precipitation of the mineral hematite (Fe2O3) and was characterized by high sanidine with full structural disorder as the feldspar (instead of plagioclase, which was removed by dissolution) and by Mg‐rich phyllosilicates as additional precipitation products. Hematite, often present as a red pigment in geologic materials, was precipitated from the hot water as specular (i.e., gray) hematite. By analogy, high sanidine and specular hematite at Gale crater (Mars) can be interpreted as alteration products of preexisting Martian basaltic sediment by hot‐water solutions.