Carboniferous Limestone: Discovering the True Nature of Ireland’s Most Famous Stone

Millions of people worldwide have journeyed to Ireland to kiss the mystical Blarney Stone built into the battlements of Blarney Castle in an attempt to gain the gift of eloquence. Several legends attempt to explain the origins of this so-called enchanted stone. Some allege that the Blarney Stone is a piece of Stonehenge, while others maintain that it was part of the Stone of Destiny gifted from Robert the Bruce of Scotland. 21st-century geologists, however, have debunked these geographical origin tales.

Limestone is a carbonate, sedimentary rock predominantly composed of different crystal forms of calcium carbonate minerals like Calcite and Aragonite. This rock forms when the calcium carbonate minerals precipitate out of water containing dissolved calcium and fossils. Limestone can also have a significant percentage of dolomite, a calcium-magnesium carbonate. In 2014, geologists from the University of Glasgow analyzed a microscope slide containing a thin slice of the Blarney Stone. Determining that the carboniferous limestone identifiers were unique to the region, the scientists concluded that the stone is 100% Irish.[1]

Distinguishing carbonate minerals can be messy, destructive, and time-consuming. An alternative, non-destructive solution is to use a UV-VIS-NIR spectrometer which can provide results in a matter of seconds, right in the field or lab. Geologists can use the spectral reflectance from 350-2500nm to discriminate the different calcium carbonate minerals by looking at the absorption features and band positions. The band shapes and location can help to identify dolomite and calcites.

The plot below shows mineral spectra from 350-2500nm of the three most common rock-forming calcium carbonate minerals: Calcite (red), Aragonite (blue), and Dolomite (green). Calcite and dolomite are easily distinguished in two ways: first, in the 2300-2500nm region, calcite and dolomite can be discerned by the position of the carbonate bands. Here, we see that the bands in the dolomite spectra occur at shorter wavelengths than calcite. Additionally, the presence of Fe2+ in the 1200nm band, seen as a broad double absorption feature, can be indicative of calcium carbonate minerals. Aragonites, however, rarely display Fe2+. Instead, this mineral can be identified by the strong absorption feature centered at 2324nm and by a lack of features in the shorter wavelengths.

Modern science and technology provide methods of demystifying the past. But with myth, legend, and history so tied to our human experience, we must use caution in our pursuit of knowledge. UV-Vis-NIR spectrometers, such as Spectral Evolution’s oreX-™ series, allow geologists to study bygone times by using the absorption and reflection of light. Furthermore, compact, field-portable spectrometers may be carried directly to the sample, eliminating the need for physical sample collection. Non-destructive methods of studying geology will keep history alive, and knowledge abound for generations to come.