La anisotropía y el clivaje del cuarzo automorfo y sus posibles efectos sobre la talla: Una revisión bibliográfica [Anisotropy and cleavage of automorphic quartz and their potential effects on the knapping: A literature review]

Abstract

Anisotropy and cleavage of automorphic quartz and their potential effects on the knapping: A literature review

Over the last thirty years, archaeologists have begun to overcome their traditional lack of interest in lithic industries made from quartz, with a growing number of studies dealing with this raw material coming to light. However, most of these approaches have mainly or exclusively focused on the xenomorphic or “vein” quartz. Meanwhile, the automorphic quartz – traditionally called “rock crystal” or “quartz crystal” – has received comparatively less attention. In this paper, two basic characteristics of automorphic quartz are described succinctly: anisotropy and cleavage; with the aim of offering – to those researchers dealing with this raw material – a basis from which to increase their knowledge of the mechanical characteristics of rock crystal.

Automorphic quartz shows some specificity during knapping. This is due not only to the peculiar morphology and size of the blanks – single prismatic crystals, usually of small size – but also due to its anisotropic nature (a characteristic by which certain physical properties tend to vary according to the direction in which they are measured) and possibly also due to the presence of cleavage planes (structural weaknesses in the structure of materials along which they tend to break or split more easily). In this sense, the studies carried out by archaeologists and especially by geologists, mineralogists and crystallographers agree that quartz crystals show an uneven mechanical behavior depending on the direction of the forces to which these crystals are subjected. Thus, the mechanical forces (including those generated during knapping by percussion or pressure) tend to propagate more easily in those directions oblique to the longitudinal axis of the prismatic crystal, while their propagation is comparatively more difficult when it takes place in parallel to the aforementioned longitudinal axis. Regarding the presence of cleavage planes, their existence in quartz has been discussed for over a century without reaching any major consensus. Most researchers agree, nonetheless, on the existence of some type of crystallographic control of fractures in automorphic quartz, usually referring to it as an “imperfect” or “low energy” cleavage.

The impact of such physical characteristics over the knapping of automorphic quartz is far from being known in detail. The idea of anisotropy as a limiting factor for the exploitation of rock crystal was commonplace among the studies published in the last decade of the past century, this property even being considered as a possible explanation for an alleged residual use of this rock during prehistory. However, the archaeological record clearly contradicts such view of paucity, with automorphic quartz present in assemblages all over the world and even reaching an important percentage in Upper Paleolithic and Epipaleothic sets. Furthermore, the exploitation of automorphic quartz implies the use of relatively complex techniques, such as indirect percussion and pressure flaking. Thus, it seems reasonable to assume that either anisotropy or cleavage would have been an obstacle hindering the knapping of automorphic quartz. However, it is also true that the preponderance, among the cores of automorphic quartz, of knapping strategies exploiting those planes along which the fracture would be easier, suggests that prehistoric knappers were aware of the mechanical characteristics of automorphic quartz and that they used them to their advantage.