4.1.1. Calcium Oxalate

The differences between the monohydrate (whewellite, Wh) and dihydrate (weddellite, W) spectra of calcium oxalate have been documented by Conti et al. [32] and Petit et al. [33]. These authors point out that in the infrared spectrum, the calcium oxalate monohydrate (Wh) is characterized in the elongation zone of water molecules (3000–3600 cm−1) with several peaks at 3058, 3258, 3336, 3429 and 3483 cm<sup>−</sup>1. Furthermore, the pattern of calcium oxalate dihydrate has a more intense peak close to 3469 cm−1. In the studied species, there was an intense peak that varies from 3466 to 3327 cm−<sup>1</sup> (Figure 1A,C,E,G,I). Other characteristic vibrations of calcium oxalate dihydrate are a weak signal at 1475 and 915 cm−<sup>1</sup> that belongs to the symmetric vibrations of CO and CO + H2O, respectively [33], and were present from 1476 to 1466 and 913 to 910 cm−<sup>1</sup> (Figure 1B,D,F,H,J) in most tissues of the species studied. In the species studied, calcium dihydrate vibrations ranged from 774 to 762 cm<sup>−</sup>1, very close to the reported vibrations (770 cm<sup>−</sup>1), while the calcium oxalate monohydrate is assigned to 782 cm−<sup>1</sup> [33]. Based on these results, we consider that calcium oxalate dihydrate is the dominant biomineral in the studied species and is mainly distributed in the three tissues. Hartl et al. [12] documented the presence of this state of hydration for other species of *Ariocarpus* (W), *Astrophytum* (W), *Echinocactus* (W) and *Mammillaria* (Wh or W). *Mammillaria* is the largest genus of the Cacteae tribe with more than 100 species; to date, only six have been studied, Wh (3) and W (3) (12, 25), plus the two here reported. The study of other *Mammillaria* species is needed to understand the variability of calcium oxalate in terms of the state of hydration as well as the identification of other elements.
