| Lithology of Irish Blue
Limestone:
The classic form of the Irish Blue Limestone
is a compact sedimentary rock comprising numerous crinoid fragments in a crystalline
calcite base. The scintillation of the grains, particularly on polished surfaces,
gives the stone its attractive appearance. Finely disseminated carbon within the
calcite matrix gives the distinctive blue colour that is best seen on the sawn surfaces.
As with all natural stones, there are subtle differences in colour
and texture between the Carlow, Kilkenny and Roscommon workings although the variability
is probably no greater than that found within a single quarry.
In addition to the crinoidal limestone, all the quarries
have finer grained darker coloured beds where the crinoid content is much reduced.
Some of these include many white bivalve shells. The finer grained material
will take a dense even polish while the bivalves make a startling and attractive contrast
to the black matrix particularly on polished surfaces.
This material is usually referred to as 'Shell' limestone.
Occasionally, the finer beds do not contain bivalves giving the fine even textured
'Black' limestone.
Chemical analysis of the limestones typically indicates a
Calcium Carbonate content of between 94% and 98% CaCO3. Traces of secondary minerals
are present with finely dissminated microcrystalline quartz being the most common.
Pyrite and iron oxides have been found but are not particularly common. As with
most limestones, there are traces of dolomite (MgCO3). The content can vary but is
generally less than 2% MgCO3.
Stylolites are an inherent feature of the stone. They
are usually in the same plane as the principal bedding but can be identified by the much
more irregular form. The various physical features of the Irish Blue Limestone are
examined in more detail in the section on quality
control and classification. |
