Mons Claudianus and the Problem of the 'Granito del Foro': A Geological and Geochemical Approach
Peacock, D. P. S., Williams-Thorpe, Olwen, Thorpe, R. S., Tindle, A. G., Antiquity
Granito del foro is a distinctive igneous rock, in fact a granodiorite rather than a granite, long known and named for its use in buildings of the Roman Forum. Exactly what is it? Where does it come from? Where else was it used and not used? What does the granito del foro say about ownership and empire?
Mons Claudianus in the Eastern Desert of Egypt was an important source of Roman monumental stone. Since 1987 an international team working under the aegis of the Institut Francais d'Archeologie Orientale has investigated the site with extensive excavations and also a detailed survey, which has included mapping the quarry field (Peacock 1993). To place this new work in its wider context, it is important to study the distribution and use of the Mons Claudianus rock, previously attempted by Kraus & Roder (1962) and more recently by Dodge (1988), Lazzarini (1987; 1992) and Galetti et al. (1992).
While the rock of the Mons Claudianus quarries has often been erroneously referred to as a granite, it has been classified variously as granodiorite (Lazzarini 1987), quartz diorite (Troger in Kraus & Roder 1962: 119), granodiorite/quartz diorite (Klemm & Klemm 1981; 1992), tonalite (Harrell 1989) and granodiorite/tonalite (Sabet et al. 1972; Greenberg 1981). New chemical analyses carried out for the present paper show the Mons Claudianus rock to be granodiorite following the classification of MacKenzie et al. (1982: 77), and this term will be used here.
To architectural historians, it has long been known as the granito del foro, because of its frequent occurrence in the Roman Forum. The granito del foro has been regarded as one of the most distinctive and easily recognized of the Roman ornamental stones; in theory it should be a simple matter to plot its distribution. However, thin-section examination of some typical specimens revealed an interesting point. While all were closely similar in mineralogy, some had markedly zoned plagioclase feldspars. Samples are available from all of the 130 quarries comprising the Mons Claudianus complex, and in these zoning is absent or weakly developed. Since zoning results from compositional changes in the parent magma when the rock was forming (and not, for example, from recent chemical alteration or weathering), it seems inescapable that two or more rock sources have been conflated under one name. The object of this paper is to explore ways in which grey plutonic rocks of similar appearance can be distinguished, and to examine the implications for Mons Claudianus.
Until recently, the scientific characterization of plutonic rocks used in antiquity has lagged far behind that of coloured or white marbles, but recent papers by Lazzarini (1987; 1992) and Galetti et al. (1992) have made significant advances. The latter, in particular, have attempted to define the chemical and petrographic traits of samples from all the major quarry sites. Our approach to the problem is different and complementary. We have concentrated on grey plutonic rocks of similar appearance, from a wide range of localities, in an attempt to find distinguishing criteria and to define sources. In contrast to Galetti et al. our approach included a wide range of promising outcrops, regardless of evidence for ancient exploitation. Our study includes visual comparison of rock samples with archaeological artefacts, complemented, where possible, with petrographic and chemical work. Fortunately the latter confirm the former, enabling us to use visual data in assessing the importance of the Mons Claudianus rock, although distinction between other sources is rarely possible without further analysis.
The grey plutonic rocks of the Mediterranean
FIGURE 1 shows the distribution of grey acid and intermediate plutonic rocks in the Mediterranean region; in other words potential sources of material that might be confused with that of Mons Claudianus and hence be classed as granito del foro. …