Rocks and Minerals

Stylianos Aspiotis

Monumental architecture and shared communication strategies are two of the most prominent aspects that characterise a human civilization. The latter includes, among other communicational expressions, the written language, which is arguably the major and most secure practice of transmitting knowledge through time. In particular, rock-based writing supports are thought to be the most persistent media in time and were introduced in the form of rectangular mould-based bricks (see Walls and Buildings) in Mesopotamia at least from the 4th or 3rd millennium BCE (Artioli 2010 and references therein). The most common rock-based writing supports are marble, limestone and granodiorite, which were firstly introduced in ancient Greece and Rome in the 1st millennium BCE and used predominantly as construction constituents in historical buildings, as well as epigraphic stele.

Rocks are generally any naturally occurring solid masses or aggregates consisting of one or more mineral phases. A mineral is a crystalline compound with a fairly well-defined chemical composition and a specific crystal structure. Rocks are commonly classified into three main categories – namely, sedimentary, magmatic and metamorphic in the way they are formed. In particular, sedimentary rocks are formed by accumulation or deposition of mineral or organic particles at the Earth’s surface, followed by the lithification or diagenesis of the deposited sediments. In contrast, magmatic rocks (also called igneous rocks) are formed by cooling and solidification of magma (a molten rock material with temperatures generally between 700°C and 1200°C) near the surface of the Earth. Finally, metamorphic rocks are formed by reconstitution of pre-existing rocks at elevated temperatures and pressures well beneath the surface of the Earth (for more details, see e.g. Best 2003). 

In the context of written cultures, the most frequently encountered rocks are marble, limestone, and granite, as well as diabase or dolerite (e.g. as modern headstones in various cemeteries and as construction constituents of many historical buildings). The straightforward relationship between limestone and marble is that the latter is the metamorphic counterpart of the former and that both rocks are composed mainly of calcite (calcium carbonate mineral; CaCO₃) or dolomite (calcium magnesium carbonate mineral; CaMg(CO₃)₂) in the case of dolostone. On the other hand, granite and diabase have a rather more complicated mineral-phase composition, as granite consists of almost equal amounts of quartz (SiO₂), alkali feldspars (KAlSi₃O₈) and plagioclase (NaAlSi₃O₈ – CaAl₂Si₂O₈) with minor quantities of accessory minerals, while diabase is a volcanic rock composed mainly of plagioclase being crystallised in a finer matrix of various silicate (such as pyroxenes, olivine, amphiboles and biotite) and iron oxides minerals (such as magnetite and ilmenite).

Another broad field that should be considered is various minerals and rocks used as pigments (see Mixed inks and Colour ink and paint), cylinder seals, and inscribed gems (see Ornaments) whose detailed analysis can assist in solving complicated archaeological problems (e.g. Bersani and Lottici 2016). In particular, inscribed gems are thought to be an easy and at the same time robust message-carrier over time. Given that they should be an easily-carved writing support, only specific mineral groups (such as quartz varieties and jade) and rock types (such as rocks containing mostly layered silicates) are suitable for this purpose.

Based on the possibility of sampling, the relative methods applicable to the analysis of rocks and minerals can be divided into two categories. The following analytical techniques can be used if the desired object can be sampled:

• Optical microscopy on prepared petrographic thin sections to characterise the mineral constituents of a rock based on their optical properties and thus reveal the origin and evolution of the parent rock.
• Powder XRD to identify different phases in a polycrystalline material (e.g. Aspiotis et al. 2021b; Bonnerot et al. 2016).
• The crystal chemistry and chemical composition of different mineral phases within a rock sample can be identified by FTIR through the preparation of pellets using the KBr technique (e.g. Aspiotis et al. 2021b).
• Wavelength-dispersive EMPA as the ultimate analytical tool for the determination of the chemical composition of rocks and minerals (e.g. Aspiotis et al. 2021b, 2022, 2023).
• Thick sections, as prepared for WD-EMPA, can be analysed by scanning electron microscopy for detailed determination of the morphological characteristics of rocks and minerals.
• Major and trace elements on whole rock powdered samples (e.g. Aspiotis et al. 2021a) can be further determined by XRF.
• For trace elements in the range of few ppm, mass spectrometry can be applied as well (e.g. Aspiotis et al. 2021a).

When sampling is prohibitive from the viewpoint of cultural heritage, analytical techniques operating in reflection mode can be applied:

• FTIR reflection (see FTIR Spectrometer Vertex 70) and Raman spectroscopy for the determination of the crystallochemical composition of rock-based written artefacts and of the mineral constituents of cultural-heritage objects (Aspiotis et al. 2021, 2023; Bersani and Lottici 2016; Mihailova et al. 2021)
• XRF reflection, in particular when portable spectrometers are used, for the semi-quantitative determination of the elemental composition of solid materials, such as mineral-based pigments (e.g. Howe et al. 2018) and metal alloys (e.g. Colomban et al. 2012).