Black Markina: an ornamental basque limestone (Spain)

Black Markina is a limestone that has been exploited for more than 100 years as an ornamental and construction stone in the Basque Country (Spain). This stone is of great international interest and is well known for the intense black colour it acquires when it is polished. Nevertheless, in natural outcrops and in construction elements with other finishes the colour that characterises it is more or less dark greyish. 

The Markina Limestone Formation is located in the Basque Arc, to the north of the Basque-Cantabrian Basin. It was formed on a shallow subtropical carbonate marine platform during the Aptian/Albian. The main facies are biomicrites with a dark micritic matrix containing abundant fossils that contrast with the white colour: tabular corals in life position, as well as branching and hemispherical corals and Chondrodonta sp. colonies, rudists and gastropods. These facies are well stratified and sometimes a certain degree of transport is observed. Among these facies, those with few carbonate bioclasts and abundant orbitolines (binders) stand out. These facies correspond to the most commercially valued stone variety due to its more homogeneous black tone. 

The origin of the intense black colour of this lithology has not been resolved: on the one hand, the existence of organic matter disseminated in the matrix (on a microscopic scale) and in the interior of the fossils in the form of masses of pseudovitreous appearance (opaque under the optical microscope) has been confirmed. This organic matter already gives the matrix a dark basic tone. On the other hand, the presence of certain minerals such as nanometric magnetite or cubic and framboidal pyrite disseminated in the matrix, smaller than a micron in size, could be what intensifies the black colour when polished. 

The variety of ornamental stone known as Black Markina florid corresponds to facies with diagenetic features, mainly white, contrasting with the intense black of the matrix. In this sense, a generalised recrystallisation/neomorphism of the fossil remains is recognised and different late fracture systems filled with white calcite crystals also stand out. 

Markina village is an example of the use of local stone as a distinctive feature that defines its landscape. Numerous palaces (Ansotegi, 16th century; Gaitan de Ayala or Patrokua, 17th and 20th centuries), religious buildings (Church of La Merced, 18th century) and other architectural elements such as the sculpture in homage to Juan Antonio Moguel (20th century) or the cobblestones of several streets in the municipality have been preserved. Black Markina can also be recognised in elements from other countries as emblematic as the hall of the Empire State Building or the lectern of the United Nations headquarters, both located in New York, or the Kaaba of Mecca (Saudi Arabia).