Rock varnish on architectural stone: microscopy and analysis of nanoscale manganese oxide deposits on the Smithsonian Castle, Washington, DC

Manganese and iron rock varnishes have been noted to occur in a wide range of environmental
settings from well-studied examples in deserts around the globe to less studied examples
in rainforests of multiple continents, near active glaciers, and on rocks exposed
in rivers 7]. Given the latter, it is perhaps not entirely surprising to learn of rock varnish
formation in Washington, DC, which is in the humid subtropical zone according to the
Köppen climate classification. Average total precipitation in Washington is ~1000 mm
per year, or roughly ten times greater than that for an urban desert location, e.g.,
Las Vegas, NV.

Rock varnish on the Seneca sandstone described here shares certain similarities with
desert varnish: (1) high luster, nearly metallic in places; (2) Mn-rich particle sizes
in the nanometer range from 20 to 200 nm; (3) high enrichment in Mn as well as Pb,
Cu, and Zn (Table 2); (4) deposition on a lithological substrate containing a low bulk concentration
of Mn (Table 1); and (5) mineralogical association with Al-rich silicate dust minerals intimately
mixed with the Mn-rich coating (Fig. 8c).

Despite these parallels, rock varnish on the Smithsonian Castle sandstone possesses
differences relative to varnish formed in arid settings, namely: (1) Mn is found in
two zones, [i] dispersed along grain boundaries and pores in the uppermost 200–250 ?m
of the rock surface, and [ii] thin and discontinuous surface deposits ?1 ?m in thickness
or roughly three orders of magnitude thinner than typical desert varnish. In cross
section it is difficult to measure, or even identify, a surface deposit; (2) deposits
have no discernable vertically definable substructure as opposed to microstratified
desert varnish described in the literature 8], 13]. Differences in both the thickness and lack of microstratigraphy may be attributed
to the age disparity between the architectural varnish and those formed over geological
periods of time. In this regard Smithsonian varnish may serve as an example of the
earliest stage of desert varnish formation.

Varnish minerals are either poorly crystalline, present at levels below the detection
limit for x-ray diffraction (~5 volume%), or possibly both. Previous researchers have
encountered a similar lack of diffracted x-ray signal, even for more developed desert
varnish where the coating is significantly more massive compared to samples in this
study 8]. Many Mn oxides formed in nature, e.g., birnessite, are poorly crystalline in general,
which may well have compounded our inability to collect structural information 21]. Transmission electron microscopy or synchrotron-based x-ray methods represent useful
follow-up methods to characterize the rock varnish in this study 12], 22], 23].

The composition of the varnish likely represents multiple phases, a Mn-rich oxide
and an Al-rich silicate mineral. The aluminous phase is either intimately mixed with
Mn oxide or results from sampling a clastic mineral grain substrate. Unmixing the
convolved chemistry by extrapolating negatively correlated Mn–Si and Fe–Si to a zero
Si concentration end-member yields a Mn/Fe ratio of 20. Similar negative correlations
between Si and Mn have been noted in desert varnish 8], 24]. The Mn-rich component within the nanophase mineral mixture is most likely either
birnessite or todorokite.

Potter and Rossman 9] found that different terrestrial weathering environments produced distinct mineralogy.
If birnessite is the Mn-rich phase in the Smithsonian Castle varnish, the result is
consistent with subaerial Mn varnish formed in stream deposits where birnessite is
found in association with minor silicate minerals.

Apart from an extreme enrichment in Mn relative to the Seneca sandstone, the varnish
is additionally concentrated in a number of heavy metals, including Pb, Zn, Cu, and
Ni. These results are consistent with experiments that have determined divalent metal
preferential adsorption for Pb, Zn, and Cu on interlayer crystallographic sites in
birnessite 25]. Analytical studies also show a significant enrichment of Mn, Co, Pb, Ni, and Cu
in desert varnishes 24], 26].

According to previous investigators, e.g., 7], 10], the mechanism of formation of Mn rock varnishes involves multiple steps: (1) accumulation
of externally derived clay-bearing dust on a rock surface 10], (2) transport of Mn to the rock surface in solution within rain/fog droplets, and
(3) oxidation and precipitation of Mn oxide cementing previously unconsolidated clay
into a heterogeneous nanophase mixture. Step 1 was recently bolstered by the observation
that anticorrelated Mn and Si electron microprobe analyses for desert varnish have
a common Mn-free end-member regardless of the composition of the varnished rock 24]. Regarding step 2, one may question whether Mn is derived locally from the host rock
or externally via atmospheric transportation 8]. In this study we report the presence of hematite, titanite, and rutile in the Seneca
sandstone, which may all serve as internal reservoirs of Mn. However, to within the
sensitivity of our compositional imaging we find no evidence for solution transportation
of Mn in the sandstone. Based upon rare earth element fractionation observed in desert
varnish it has been argued that the degree of element fractionation observed is inconsistent
with isochemical leaching and reprecipitation within the same rock 26]. Therefore, the source of Mn must be delivered to the rock surface externally via
atmospheric transport and surface leaching/dissolution of airborne dust grains. The
final step 3 requires oxidation and precipitation of Mn either by a physiochemical
process under acidic oxidized conditions in rainwater 26] or via microbially assisted oxidation as favored by other investigators 7], 11], 27]. The microstructural imagery collected on the Smithsonian Castle varnish offers no
evidence for microbial entombment in Mn oxide, and the organisms observed on the surface
(Fig. 14) grew upon the varnish and therefore may not be related to Mn oxidizing microbes.
Apart from local compositional heterogeneity discussed above, another factor to consider
regarding the likelihood of varnish development is stone roughness. Rougher areas
provide increased surface area for condensation and therefore deposition of atmospheric
dust. However, no systematic associations between surface roughness and rock varnish
have been noted on the building stone and gateposts.

The patchy distribution of varnish on the Castle is less consistent with a chemical
process, which would lead to the development of a more uniform coating, and more suggestive
of biological colonization. It seems reasonable that biological mediation must play
some significant role in Mn oxidation, if in tandem with abiological processes, in
the development of rock varnishes in this study. Furthermore, we cannot state definitively
why varnish formation is occurring on the Smithsonian Castle Seneca sandstone as opposed
to other building stones on the National Mall. However, it has been noted that

“the irregular onset of bacterial colonization accounts for the puzzling inconsistency
in varnish development from stone to stone…” 11].

The rate of subaerially exposed Mn varnish formation in the western USA has been estimated
to range between 1 and 40 nm/a 28]. Given that the age of the Castle gatepost exposure to the atmosphere is known to
be just under 30 years, a rock varnish thickness range of ~28 nm–1.08 ?m would be
expected using the rates determined for the western USA. The observed surface coating
is closer to the lowest end of that thickness range, although Mn-rich particles sequestered
in pores within the sandstone do not contribute to the surface thickness estimate.
A rare 20–40 year old rock varnish has been reported forming on steel slag piles in
southern California 29], where the substrate material is rich in Mn, possibly influencing the rapid rate
of growth. Impressively, in a few short decades stone disfigurement of the Smithsonian’s
Castle is readily noticeable and a testament to the strong pigmentation of Mn oxides
even when present in small concentrations.

Removal of varnish from the stone may prove problematic. While laser cleaning can
be used to remove the exposed coating it will be more difficult to remove Mn oxide
within pores without damaging the sandstone.