Geoscience Reference
In-Depth Information
OF ROOFING SLATE
from the surface may be referred to as inclusions. The
inclusions may be dispersed or concentrated in bands
along the grain. Common mineral inclusions include
quartz, carbonates, and iron sulfides, and different
minerals may occur together (
110
).
Of particular importance are several iron sulfide
minerals that can be present in slate, the most common
one being pyrite. Pyrite occurs in a number of
morphologies including large well formed cubic crystals
(
111
), which are usually relatively stable. Other pyrite
morphologies comprise smaller and less crystalline
rounded grains (
112
) that are often metastable. Less
common iron sulfide minerals include marcasite (a
polymorph of pyrite) and pyrrhotite.
The natural weathering of rock masses usually takes
place very slowly and leads to the breakdown and
eventual decomposition of rock at the Earth's surface.
Slate quarried for roofing would normally be extracted
from fresh rock beneath the weaker weathered layer.
Following production, roofing slates are placed on the
most exposed part of buildings and can be subjected to
severe weather conditions. To perform satisfactorily,
slates must resist physical deterioration from driving
rain, high winds, and extremes of temperature. Minerals
within slates are also subjected to chemical weathering
by a variety of reactions including ionization, hydrolysis,
and oxidation. The most significant chemical weathering
reactions are oxidation of iron sulfides and the reactions
of carbonate minerals with acids from rainwater and
polluted atmospheres. The oxidation of pyrite comprises
a series of reactions, some of which involve bacteria (e.g.
Thiobacillis ferrooxidans
) to produce iron oxides (rust)
and sulfuric acid. The disintegration process of pyrite
may be simplified as follows (Blanchard & Sims, 2007):
110
FeS
2
+ 9H
2
O
→
Fe(OH)
2
+ 2H
2
SO
4
+ 7H
2
Some forms of iron sulfide are more prone to weathering
than others. Pyrrhotite and marcasite are the most
reactive minerals and less crystalline forms of pyrite with
110
Spanish slate with a band of inclusions running
through the full thickness of the slate. The band runs
along the grain and the inclusions comprise pyrite
(black), quartz (white/grey), and calcite (pink); XPT, ×35.
111
112
111
Polished specimen of slate with cubic crystals of
pyrite (yellow); PPR, ×150.
112
Polished specimen of slate with anhedral lenses
of pyrite (yellow); XPR, ×75.
