FTIR-Based Crystallinity Assessment of Aragonite–Calcite Mixtures in Archaeological Lime Binders Altered by Diagenesis

Abstract

Lime plaster and mortar are pyrotechnological materials that have been employed in constructions since prehistoric times. They may nucleate as calcite and/or aragonite under different environmental settings. In nature, aragonite and calcite form through biogenic and geogenic processes that lead to different degrees of atomic order. The latter is a result of defects in the crystal lattice, which affect the properties of crystals, including their interaction with infrared light. Using Fourier transform infrared spectrometry (FTIR) with the KBr pellet method, it is possible to exploit these differences and assess the degree of atomic order of aragonite and calcite crystals and thus their mechanisms of formation. Here we use FTIR to characterize the degree of short-range atomic order of a pyrogenic form of aragonite recently observed in experimental and archaeological lime binders. We show that pyrogenic aragonite has a unique signature that allows its identification in archaeological sediments and lime binders of unknown origin. Based on these results, we developed a new FTIR-based method to assess the integrity and degree of preservation of aragonite and calcite when they occur together in the same material. This method allowed a better assessment of the diagenetic history of an archaeological plaster and finds application in the characterization of present-day conservation materials, such as lime plaster and mortar, where different polymorphs may nucleate and undergo recrystallization processes that can alter the mechanical properties of binders.

Structural evaluation of chitosan-modified precipitated calcium carbonate composite fillers for papermaking applications

“…These characteristic peaks of aragonite polymorph have been reported by several researchers [24]. CO 3 2− ions remain inactive in the infrared region and that the band at 1081 cm −1 cannot be seen in the pure calcite phase of calcium carbonate has already been verified in the previous literature [27] [28]. Apart from stretching vibrations, out-of-plane and inplane bending of C-O bond resembling absorption band at 873 and 712 cm −1 are attributed to calcite phases in the sample [28].…”

“…CO 3 2− ions remain inactive in the infrared region and that the band at 1081 cm −1 cannot be seen in the pure calcite phase of calcium carbonate has already been verified in the previous literature [27] [28]. Apart from stretching vibrations, out-of-plane and inplane bending of C-O bond resembling absorption band at 873 and 712 cm −1 are attributed to calcite phases in the sample [28]. The IR spectra of industrially manufactured PCC used in this study have similar absorption bands as per the published literature data revealed from Fig.…”

“…In comparison to the unmodified one, those modified with chitosan displaying a slight displacement of peak around 1494 cm −1 toward lower absorption band at 1458 cm −1 is due to the differences in the crystal structure of calcite and aragonite. Rather it is assigned to the specific band for calcite [28]. The spectra display a broader peak at 3431 cm −1 mainly assigned to the OH or NH stretching vibrations related to chitosan.…”

The significance of aragonite in the interpretation of the microscopic archaeological record

“…Similarly, geogenic aragonite exhibits a different grinding curve compared with biogenic aragonite and aragonite precipitated from boiling water (Suzuki, Dauphin, Addadi, & Weiner, 2011). Toffolo, Regev, Dubernet, Lefrais, and Boaretto (2019) extended the method to pyrogenic aragonite and developed a procedure based on FTIR and XRD to determine degrees of atomic order in calcite–aragonite mixtures (Figure 3). Powdered samples may be analyzed also in attenuated total reflectance (ATR) mode (e.g., Loftus, Rogers, & Lee‐Thorp, 2015; Villagran, Strauss, Miller, Ligouis, & Oliveira, 2017).…”

“…Crystals may show different degrees of atomic order in mollusks, as shown by FTIR grinding curves of nacre, myostracum, ligament, and crossed‐lamellar aragonite (Suzuki et al, 2011). Figure 3 shows the grinding curve of a powdered Glycymeris insubrica (bittersweet clam) shell compared with other aragonite reference standards (Toffolo, Regev, et al, 2019). Stony corals (phylum Cnidaria, class Anthozoa, order Scleractinia) mineralize aragonite exoskeletons (Lowenstam & Weiner, 1989).…”