**1. Introduction**

A biomineral is a crystalline or amorphous mineral product of the biochemical activity of an organism and the local accumulation of elements available in the environment [1]. In plants, the most common biominerals are amorphous silica, calcium oxalate and calcium carbonate salts [2–7]. Some species of the Cactaceae family accumulate up to 85% of their dry weight in calcium oxalate crystals [8,9]. These calcium oxalate crystals may have one of two states of hydration: monohydrate (CaC2O4·H2O; whewellite) or dihydrate (CaC2O4·2H2O; weddellite) [10–17]. In addition, other biominerals, such as magnesium oxalate (MgC2O4·2H2O; Glushinkite) [17], amorphous silica bodies (SiO2·nH2O; opal) [18] and silica in crystalline form (SiO2; α-quartz) [19], have been identified in cacti.

Like other plants, biominerals in Cactaceae develop mainly in the cellular vacuole of different epidermal, fundamental or vascular stem tissues. The accumulation of biominerals in a given tissue is usually highly specific in some species or genera [10,18,20–23]. Thirty-four species from the Cacteae tribe have been studied with techniques such as X-ray diffraction to detect calcium oxalates [11], by Raman spectroscopy in *Ferocactus latispinus* and *Coryphantha clavata* [24] and by Fourier transform infrared (FTIR) only in one species, *Mammillaria uncinata* [25]. In these techniques, the tissues were blended or macerated together, so they have not been analyzed individually in the Cacteae species.

For this reason, the FTIR technique can be a good option to study biominerals due to the minimum amount of sample and the speed of data acquisition. Therefore, the aims of this study were to characterize the biominerals and determine their hydration state in the different tissues of five Cacteae species. With this, it will be possible to identify other biominerals that are not calcium oxalate, magnesium oxalate or amorphous silica, in addition to the state of hydration of biominerals in the different tissues.
