What is Slate?
Slate is a
fine-grained, foliated metamorphic rock that is created by the alteration of
shale or mudstone by low-grade
regional metamorphism. It is popular for a wide variety of uses such as roofing,
flooring and flagging because of its durability and attractive appearance.
Composition
of Slate
Slate is composed mainly of
clay minerals or micas depending upon the degree of metamorphism to which it
has been subjected. The original clay minerals in shale alter to micas with
increasing levels of heat and pressure. Slate can also contain abundant quarz
and small amounts of feldspar, calcite, pyrite, hematite and other minerals.
Color of Slate
Most
slates are gray in color and range in a continuum of shades from light to
dark gray. Slate also occurs in shades of green, red, black, purple and brown. The color of slate is often determined by the amount and type of iron and organic material that are present in the rock. How Does Slate Form?
The
tectonic environment for producing slate is usually a former sedimentary
basin that becomes involved in a convergent plate boundary. Shales and
mudstones in that basin are compressed by horizontal forces with minor heating.
These forces and heat modify the clay minerals in the shale and mudstone.
Foliation
develops at right angles to the compressive forces of the
convergent plate boundary
to yield a vertical foliation that usually crosses
the bedding planes that existed in the shale.Uses of Slate
Most of
the slate mined throughout the world is used to produce roofing slates. Slate
performs well in this application because it can be cut into thin sheets, absorbs minimal moisture and stands up well in contact with freezing water. A disadvantage is the cost of the slate and its installation in comparison with other roofing materials. As a result, in new construction slate is mainly confined to high end projects and prestige architecture . Slate is also used for interior flooring, exterior paving, dimension stone and decorative aggregate. Small pieces of slate are also used to make turkey calls. The photos and video on this page document several uses of slate. Historically slate has been used for chalkboards, student writing slates, billiard tables, cemetery markers, whetstones and table tops. Because it is a good electrical insulator it was also used for early electric panels and switch boxes. |
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