SEM Imaging of the Outer Stone Surface

Outer surface fragments were analysed using the FEG-SEM. The surfaces of EE1 and EE3 are very similar. They are densely packed with fine grained mineral shards, which distort the shape of the detrital grains beneath (Figure 3-13). The surface of EE1 is also

combined with carbon rich material, identified by spot analysis. The carbon rich material is seen coating mineral grains on the surface making identification of these particles difficult (Figure 3-13).

Figure 3-13: FEG-SEM SE images of the outermost surface of EE1 and EE3.

The yellow arrows on EE1 are indicating the carbon rich material.

The surface of CS2 (mortar covering) has a “salt accretion” black crust formation on its surface, where salt crystals (mainly gypsum, identified by the chemistry and thin platy crystal shape) have combined with presumably atmospheric particulates to produce the black coloration of the crust. In general, the gypsum has formed in a chaotic manner over the surfaces although, in some areas, the crystals have some alignment and are found surrounding quartz grains (Figure 3-14). These thin platy crystals are also apparent on the surface of CS4 mixed with mineral fragments in an unorganised fashion.

Figure 3-14: FEG-SEM SE images of the outermost surface of CS2 and CS4.

Yellow G = Gypsum crystals surrounding quartz grains.

When polished block cross-sections of the outermost surfaces were viewed no black crust could be identified. This may have been due to damage during the sample preparation procedure. The surface of EE1 does appear to be slightly more densely packed with fragmented minerals occupying space between quartz grains. However this is difficult to distinguish as an individual layer.

3.4.3 Summary

Only a red rusty layer is detailed as an internal structure. The black crusts found on the surface are quite difficult to fully understand due to the mortar coverings which have possibly enhanced the production of salt crystals.

3.4.4 Discussion

Although the vast majority of samples surveyed have an outermost black crust, only a few have any defined internal layering profiles. Three main types of crust are evident: The first are crusts which are composed of a layer of small mineral fragments infilling all indentations on the stone surfaces and enveloping the detrital grains. The fine mineral detritus on the surface is most likely composed of kaolinite and quartz, which are most likely to have been transported from the interior of the stone (Cnudde et al., 2009). The black colour seen on the surface is most likely due to the mineral detritus combining with a proportion of atmospheric particulates of metal, such as Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn as collected in a study of Glasgow’s atmosphere by McDonald and Duncan (1979), as well as some salt crystals. The salts are difficult to identify due to the quantity and size

of the matter being observed. Crystals of gypsum are evident in the SVS samples, although these are the only sample set which had mortar repairs associated with it.

Mortar acts as a large store of calcium which could provide the source of the calcium ions to form salts. The black crusts are the most commonly occurring formation (Table 3-1).

They are normally less than 1 mm thick and are difficult to distinguish in cross-section, but this may be due to damage by the sample preparation methods. However, as this was consistent throughout all sample sets, it is unlikely to be wholly a preparation artefact.

The role the crusts play in weathering is difficult to determine, as the proportion of atmospheric pollutants is unknown. However, by restricting the pore space at the surface, they seem to provide a layer that is limiting water flow deeper into the sandstone and a limited supply may therefore protecting the sub-layers.

The second crust is the “silica glaze”, which appears only on UE1 and UE2 (Table 3-1) and is an extremely thin (~10 μm) veneer which leaves the block with a black and slightly glossy lustre. Dorn (1998) states that the presence of a silica crust on the surface will protect and strengthen the sandstone beneath.

The third main surface crust found was paint (Table 3-1). Paint on the surface of

sandstone provides an impermeable layer, and therefore has a detrimental effect on the sandstone beneath as any water which becomes trapped behind this layer cannot evaporate from within. However, the paint seen on these samples is often not laterally extensive and probably has caused minimal harm to the sandstone below.

The internal layers identified beneath the crust formations were: layer 1) a pale bleached zone which is normally very thin (<1 mm) and, in most situations, is formed just below the crust; layer 2) a red, rusty speckled zone which ranges widely in thickness from 1 mm to 6 mm and is most likely formed by the oxidation of any iron compounds present and layer 3) a darkened region, generally 3-4 mm thick. The process of formation for this third layer is as yet undetermined. The weathering profiles have no reoccurring pattern and only two samples show evidence of all three layers (Table 3-1). The most common layer is layer 2 and occurs in eight out of the ten samples which have any weathering layers present therefore appearing to be the most dominant weathering process in these

Scottish sandstones. To determine if the colour changes seen are due to mineral proportions point counting data was acquired, for all the cross-sections and will be discussed in greater depth within chapter 4. However, results show that those samples that develop a red rusty layer do not show larger quantities of oxide, or a loss of Fe-oxides from the internal regions. Nevertheless, very small amounts of strongly coloured minerals, such as iron oxides, can impart a strong colour on the rock.

Within the time frame of 150 years, a well developed crust has adhered itself to the surface of most of the sandstones, whether it be a mineral fragment crust or a silica glaze.

Although internal layers were found in many samples, well-developed internal

stratigraphy is much more rarely seen, suggesting that it takes a longer time frame to develop. Layer 2 (Fe-oxide rich layer) may be the first to establish as it is the most widely found.

Whether the crusts identified have a positive or negative effect on the decay of

sandstone beneath is not easy to determine from these results. However, once chemical analysis has been undertaken on the minerals beneath the crust, this can maybe be resolved.

Table 3-1: Key facts concerning the crusts and weathering profiles.

Table represents all samples analysed. --- denotes not present.

4 Mineralogical and Chemical