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Slate
is a fine-grained metamorphic rock that splits readily into
thin, smooth-surfaced layers or slabs. As a result, slate has
the great capablitity of extension which adds to its strength
and durability.
The
most common building applications for this material are floor,
wall and roof tiles as well as fireplace surrounds. Tiles can
be split and cut-to-size with only the natural cleft surface,
or manufacturers will add value by calibrating the material to
ensure a consistent thickness throughout the piece.
The
original material was a fine clay, sometimes of sand or
volcanic dust, and the bedding of the sediment as originally
laid down may be indicated by alternating bands, differing in
colour or in lithological character, sometimes to be seen on
the cleavage faces of the slates. Cleavage is a super induced
structure, the result of pressure acting on the the rock at
the time when it was buried deep beneath the earth's surface.
On
this account slates are found chiefly among rocks of the older
geological systems, although some occur in regions where
comparatively recent rocks have been folded and compressed as
a result of mountain building movements in the earths crust.
Slates
may be black, blue, purple, red green or grey; dark slates
usually owe their colour to carbonaceous material or to finely
divided sulfide of iron, reddish and purple varieties to the
presence of oxide of iron in the form of hematite and green
varieties to the presence of much chlorite.
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