On the microstructure, growth pattern and original porosity of belemnite rostra: insights from calcitic Jurassic belemnites

Calcitic belemnite rostra are usually employed to perform paleoenvironmental studies based on geochemical data. However, several questions, such as their original porosity and microstructure, remain open, despite they are essential to make accurate interpretations based on geochemical analyses.This paper revisits and enlightens some of these questions. Petrographic data demonstrate that calcite crystals of the rostrum solidum of belemnites grow from spherulites that successively develop along the apical line, resulting in a “regular spherulithic prismatic” microstructure. Radially arranged calcite crystals emerge and diverge from the spherulites: towards the apex, crystals grow until a new spherulite is formed; towards the external walls of the rostrum, the crystals become progressively bigger and prismatic. Adjacent crystals slightly vary in their c-axis orientation, resulting in undulose extinction. Concentric growth layering develops at different scales and is superimposed and traversed by a radial pattern, which results in the micro-fibrous texture that is observed in the calcite crystals in the rostra.Petrographic data demonstrate that single calcite crystals in the rostra have a composite nature, which strongly suggests that the belemnite rostra were originally porous. Single crystals consistently comprise two distinct zones or sectors in optical continuity: 1) the inner zone is fluorescent, has relatively low optical relief under transmitted light (TL) microscopy, a dark-grey color under backscatter electron microscopy (BSEM), a commonly triangular shape, a “patchy” appearance and relatively high Mg and Na contents; 2) the outer sector is non-fluorescent, has relatively high optical relief under TL, a light-grey color under BSEM and low Mg and Na contents. The inner and fluorescent sectors are interpreted to have formed first as a product of biologically controlled mineralization during belemnite skeletal growth and the non-fluorescent outer sectors as overgrowths of the former, filling the intra- and inter-crystalline porosity. This question has important implications for making paleoenvironmental and/or paleoclimatic interpretations based on geochemical analyses of belemnite rostra.Finally, the petrographic features of composite calcite crystals in the rostra also suggest the non-classical crystallization of belemnite rostra, as previously suggested by other authors.

On the integration of Argentinian activist art at exhibition venues

This article examines the institutional integration of activist art in the aftermath of the 2001 economic crisis in Argentina. Following this aim, we begin with a description of the most significant new groups of activist art in the City of Buenos Aires. Then, we elaborate the position these new groups take up in the field with respect to their dispute with the “light art” from the 90s, and in relation to the mutual accusations over the “trend” of political art. Finally, we analyse the reception of these groups of activist art in art exhibitions in the City of Buenos Aires and abroad, and we reflect upon the institutionalization of these specific and situated aesthetic and political practices.

The Devil and Views on the “Other” in Part One of Pedro Cieza de León’s "Crónica del Perú" (1553)

Many 16th century Spanish chroniclers and missionaries, arriving at what they interpreted as a New World, saw the Devil as a “hermeneutic wildcard” that allowed them to comprehend indigenous religions. Pedro Cieza de León, a soldier in the conquest of Peru, is a case in point. Cieza considers the Devil responsible for the most aberrant religious practices and customs of the Indians, although he views the natives in a positive light, as men susceptible to divine salvation. From a providentialist perspective of the history of the conquest, Cieza interprets that the evangelization and conversion of the Indians and the implantation of Christian civilization by the Spanish Crown, were able to defeat the Devil.