A novel approach in the mineralogy of Carpathian mahogany obsidian using complementary methods

Abstract

Carpathian obsidians can have various macroscopic features. They are typically black or grey and their transparency ranges from clear to opaque. The Tolcsva source, very rarely, can yield brown or red (‘mahogany’ type) obsidian. Archaeological, as well as geological pieces of mahogany obsidian were previously identified and characterised using Prompt Gamma Activation Analysis (PGAA). In 2007, the exact location of the red variant's outcrop was identified on the Szokolya hill (Tolcsva). The aim of this study was to better understand the possible reasons for the colouring of red obsidian. A novel approach was applied, using multiple methods for the analysis of the samples. For comparison, other Carpathian type, namely black obsidian from Tolcsva, and red obsidian from Bogazköy (Anatolia) were also studied.Besides the PGAA measurements of the bulk elemental composition, Mössbauer spectroscopy and transmission electron microscopy (TEM) were used to study the samples in order to identify the presence of ferrous or ferric iron. With the help of Small Angle Neutron Scattering (SANS), the bulk nanostructures of the samples have been investigated and their surface or volume fractal dimensions have been determined. Black obsidians showed isotropy, while mahogany samples displayed a considerable anisotropy in the bulk pore orientation. According to our results, a large amount of the iron is dominantly located in different phases in the case of mahogany and black obsidians. Based on the summarised results, the differences between the red and black variants can be also explained by the different oxidation states of the Fe-ions, which may explain the colour difference.