Geoscience Reference
In-Depth Information
113
large surface areas are more susceptible than large cubic
crystals (Evangelou, 1995). The rusting of iron sulfide
inclusions causes delamination and other deterioration
owing to expansion caused by the formation of iron
oxides (
113
). Rust deposits from pyrite often discolour
the cleavage surface (
114
).
Some slates contain significant quantities of carbonate
minerals, which may occur as discrete inclusions (see
106
), concentrated in veins (see
109
) or bands (see
110
),
or along bedding planes (
115
). Some rocks sold as slate
have been found to be metamorphosed marls, composed
almost entirely of dolomite and calcite (
116
). Carbonate
minerals are particularly susceptible to attack from acids
present in polluted atmospheres (
117
) and sulfuric acid
113
Weathering of pyrite inclusions (black) within
slate. The inclusion at the upper surface exhibits
brown rust deposits; XPT, ×150.
114
115
114
Discolouration of the upper slate surface. Rust
deposits from pyrite oxidation form a thin brown
layer; PPT, ×150.
115
Calcite (pink) concentrated along a bedding
lamination. Brazilian mudstone/siltstone being sold as
roofing slate; XPT, ×35.
116
117
116
Metamorphosed marl chiefly comprising
carbonate minerals (mainly dolomite). This was
imported from China as roofing slate; XPT, ×300.
117
Deterioration of carbonate minerals at the upper
surface of metamorphosed marl 'roofing slate' from
China; PPT, ×300.
