4.1.4. Unknown Biominerals

In *Astrophytum asterias*, the spectra in the EH region showed a very broad peak in the 1027 cm−<sup>1</sup> region, which unlike the biominerals of *Echinocactus texensis*, they showed birefringence and were named here as prismatic crystals (see 4.1.1). However, in *Astrophytum asterias*, there was a high intensity peak in the vascular cylinder at 1028 cm−<sup>1</sup> and an intense peak at 1026 cm−<sup>1</sup> in the pith. These three peaks in *Astrophytum asterias* suggest a different biomineral composition, which according to Palacio et al. [39], the spectrum of the regions that presents the strongest link patterns could be assigned to some types of silicates (1100–950 cm−1) or phosphates (1100–1000 cm−1). We consider that these vibrations belong to silicates, probably aluminosilicates due to the presence of silicon and aluminum in biominerals detected by EDS in the hypodermis or aluminum oxides in the cortex (Figure 5). To support this finding, new FTIR spectra at 400–100 cm−<sup>1</sup> are needed [44]. It is important to mention that these aluminosilicate-weddellite prismatic crystals appear to be conservative for the genus since they have been observed in the other four *Astrophytum* species studied by EDS [45]. However, they were not detected by EDS in the other stem tissues as FTIR did here for *Astrophytum asterias*.

Here, we report for the first time the presence of silicates in *Mammillaria* biominerals. In *Mammillaria melanocentra* subsp. *rubrograndis*, the peaks assigned to silicates occurred in the vascular cylinder and pith. It should be noted that these biominerals were also birefringent. Peaks assigned to silicates were also present in three of the four *Mammillaria sphaerica* tissues (Figure 2). The surprising results were the peaks in the epidermis-hypodermis of *M. sphaerica* because no vacuolar biominerals were detected in this region by microscopy. Furthermore, no biominerals other than calcium oxalate have been described for the genus [12,25]. Our results suggest that other *Mammillaria* species should be studied as mentioned above to confirm biomineral diversity.

Magnesium was detected by EDS spectra (Figure 5) but not with FTIR (Figure 2). The characteristic peaks assigned to magnesium oxalate reported by Monje and Baran [17] for other cacti such as *Opuntia* were not detected in our study by FTIR but Mg is present in traces.
