Infrared Absorption Spectrum of α-Moganite from First-Principles

Abstract

α-Moganite is a polymorph of silicon dioxide that, together with α-quartz, comprises the inorganic component of flint/chert�a material of overwhelming importance in archeology, anthropology, and paleontology. Whereas α-quartz has been studied extensively, both theoretically and experimentally, αmoganite has received considerably less attention, possibly owing to an absence of pure-phase material. In this study, we address this gap by providing a detailed first-principles analysis of the infrared (IR) absorption spectrum of α-moganite and its relation to the underlying structure. We find that the 4-membered rings of silicon and oxygen atoms, that do not appear in α-quartz, constitute a fundamental structural motif of this mineral. This distinctive arrangement results in moganite-specific fingerprints in the IR spectrum of flint/chert. This opens the door to evaluation and quantification of α-moganite content in archeological samples, providing insights into flint/chert raw material preferences and stone tool production by hominins.Introduction. Flint, also known as chert, is a siliceous marine sedimentary rock. It plays an important role in diverse fields, including archeology, anthropology, paleontology, and paleoclimate reconstruction. 1 In the context of archeology and anthropology, early hominins pioneered the use of flint-based stone tools as a strategy to allow the extension of human capacities and ensure survival (e.g., hunting and butchering). Exceptional properties of flint, notably high hardness, resistance to wear, and the ability to break akin to glass to generate sharp edges from conchoidal fracturing, resulted in flint tools being the preferred material by hominins for millions of years, as evident from an extensive archeological record. 2 Flint is an inorganic−organic composite, 3 and its composition is origin specific. The inorganic component is comprised of two closely related polymorphs of silicon dioxide: α-quartz and α-moganite (henceforth moganite, see Figure 1). 4,5 The αquartz crystal is purely chiral (either left-or right-handed, space groups P3 2 21 or P3 1 21, respectively 6 ), with variable-size euhedral crystallites ranging from μm to nm. In contrast, the moganite crystal, 7 previously known as lutecite, 8 is racemic 9,10 (space group I2/a). 11,12 It is comprised of spherulitic nanocrystallites of size ranging from 100 to 200 nm 3 and its quantity in the flint matrix varies. 13 The need to deconvolute the contributions of the different components of flint hinders our broader understanding of its unique properties. In particular, the role of moganite, existing in variable quantities in flint, has not been fully explored. It is therefore imperative to investigate moganite in itself. However, this is very challenging as, to date, pure moganite samples have not been found in nature. Therefore, there are no absolute standards that can be used for, e.g., quantifying moganite content in flint/chert. At best, moganite-enriched material (65−89%) has been found in volcanic rocks of the Mogań For...