## 3 Results and Discussion  
### XRD Analysis: Mineralogical Transformations  
Figure 1 shows the diffractograms obtained from the XRD test applied to Dt and Dt-Z, while a summary of the quantification by Rietveld method of the different phases is shown in Table 2. The diatomite used in this research has a fundamentally amorphous nature (81.2%), which makes it a material with, a priori, good zeolitization potential. Considering \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Diatomite** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **K\({}_{2}\)O** & **MgO** & **CaO** & **TiO\({}_{2}\)** & **Na\({}_{2}\)O** & **P\({}_{2}\)O\({}_{5}\)** & **SO\({}_{3}\)** & **LOI** \\ \hline As received & 76.8 & 3.8 & 1.5 & 1.0 & 0.2 & 0.8 & 0.4 & 0.7 & 0.3 & 0.03 & 14.5 \\ Calcined (Dt) & 89.8 & 4.4 & 1.8 & 1.1 & 0.2 & 0.9 & 0.4 & 0.8 & 0.4 & 0.03 & 0.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the diatomic residue used in this research as received and after calcination at 550 \({}^{\circ}\)C (data in percentage). the chemical composition shown in Table 1, such an amorphous phase would be mainly composed of SiO\({}_{2}\). In smaller proportions, Dt also presents opal-CT (11.1%) and quartz (7.7%) as the only crystalline phases present in the original sample. However, as can be seen in Figure 1 and Table 2, the alkaline hydrothermal treatment has produced very significant changes both in the proportions of these original phases and in the neoformation of new ones. Thus, Dt-Z contains only 39.2% and is amorphous phase; it does not contain opal-CT and its quartz proportion has also been reduced to 4.3 %. In addition, new minerals have crystallized. As a minor species, calcite (1.6%) has been neoformed, whose crystallization could have been due to the presence of Ca\({}^{2+}\) ions that have reacted with dissolved CO\({}_{2}\) during the hydrothermal agitation. However, calcite's contribution to the mineralogy of Dt-Z is not very relevant if compared to zeolite-P, whose crystallization fulfills the objective of this research, also being the predominant phase in Dt-Z, representing 55% of the total sample. Based on the database of zeolite structures published by the International Zeolite Association (IZA), zeolite P presents a GIS framework type [15; 16; 17], i.e., this is a zeolite topologically related to the mineral called gismodine, presenting a two-dimensional pore system with two intersecting eight-membered oxygen ring channels [15; 16]. Although increasingly in disuse compared to other zeolites, due to this structure, zeolite P has potential applications as a separator and adsorbent both at the industrial level and in environmental technologies [7; 9; 18]. Considering that calcite does not contain silicon in its structure, the zeolite P developed in this investigation would not have been formed solely from the chemical dissolution of the amorphous phase, but also from opal-CT and quartz, minerals that in theory could present \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Amorphous** & **Opal-CT** & **Quartz** & **Calcite** & **Zeolite P** \\ \hline Dt & 81.2 & 11.1 & 7.7 & – & – \\ Dt-Z & 39.2 & – & 4.3 & 1.6 & 54.9 \\ \(\Delta\) \% & \(-\)52.8 & \(-\)100.0 & \(-\)44.2 & 100.0 & 100.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Quantitative mineralogical composition of the diatomic (Dt) and the material obtained after hydrothermal treatment (Dt-Z). Data in percentage. \(\Delta\) % represent the percentage. Figure 1: XRD diagrams of Dt and Dt-Z. Q = Quartz; O = Opal-CT; Std = Standard (Al\({}_{2}\)O\({}_{3}\)); C = Calcite; Z = Zeolite P.  
greater difficulties for it. In fact, according to the data in Table 2, the alkaline treatment has been totally effective in dissolving the opal-CT in its entirety, while the original amorphous phase has been wiped out by 52.8% and the quartz by 44.2%. These results refute what has been published by other authors, such as [PERSON] et al.