Nanoscale structure and compositional analysis of manganese oxide coatings on the Smithsonian Castle, Washington, DC

Abstract

The Smithsonian Castle is a historic building in Washington, D.C. that was erected 165 years ago and served as the first home for the Smithsonian Institution. Currently, the building exhibits dark surface discoloration due to manganese-rich coatings that are growing in patches across its Seneca sandstone exterior. Bulk and microanalysis of this coating indicate that it is consistent with rock varnish, but the coating is difficult to analyze without contribution from the underlying minerals that comprise the sandstone. To address this limitation, SEM/FIB-prepared cross-sections of the coating were analyzed using HRTEM, STEM-EDX, and SAED analyses in order to study the nanoscale structure and composition of the Mn-rich coating in isolation, without convolution from the sandstone. Structurally, the coating is <1 μm thick, polycrystalline, and composed primarily of birnessite, a layered manganese oxide. High spatial resolution data reveal that the coating is Ca-bearing, with only minor quantities of Fe, Si, and Al, distinguishing it compositionally from typical manganese rock varnish and classifying it instead as a manganese skin. TEM revealed layers of aeolian detritus below and a thin silica glaze above the Mn-rich coating, providing a relative timeline of formation. Nanoscale particulates of barite found within the Mn-rich coating, along with the micro- and nanoscale detritus below the coating, indicate atmospheric deposition as the main source for both Mn and Ba. The compositional and structural information gathered from these analyses helps constrain mechanisms of formation towards a biological origin; however, no direct evidence for Mn oxide producing microorganisms was observed.