Na-Alternative to Tinsleyite Obtained under Hydrothermal Conditions: Crystal Structure and Comparative Crystal Chemistry

: The synthesis and characterization of a new aluminophosphate, Na 2 Al 2 O(PO 4 ) 2 · 0.12H 2 O obtained as single crystals, is reported. Centrosymmetric tetramers built from AlO 5 polyhedra sharing edges and vertices, represent the distinguished feature of the compound. These tetrameric units of AlO 5 bipyramids are cross-linked by PO 4 tetrahedra to form two-periodic slabs alternating with Na + ions and a small amount of H 2 O molecules. The Na 2 Al 2 O(PO 4 ) 2 · 0.12H 2 O with an original crystal architecture is chemically and structurally related to the mineral tinsleyite, KAl 2 (PO 4 ) 2 (OH) · 2H 2 O. Similar clusters of Al-centered polyhedra are essential building blocks of both monoclinic structures. The main difference between them consists of the type of the Al coordination by O atoms: in tinsleyite, the clusters are designed from AlO 4 (OH) 2 and AlO 4 (OH)(H 2 O) octahedra. In both cases, alkali Na or K atoms signiﬁcantly distinct in size, act as structure regulating agents, determining the character of the developing crystal architecture. The ﬂexibility of aluminophosphate constructions allows them to self-organize around structure-forming Na + or K + ions into anionic layers in Na 2 Al 2 O(PO 4 ) 2 · 0.12H 2 O or a framework (tinsleyite). The synthesis of sodium aluminophosphate under mild hydrothermal conditions and the topological resemblance of its structure with that of the mineral tinsleyite suggest a high probability of a mineral equivalent of the Na 2 Al 2 O(PO 4 ) 2 · 0.12H 2 O in nature.


Introduction
Most rocks are formed by oxosalts, minerals containing complex anions, including phosphate anions, crystallized under the conditions of a lithophilic geochemical system. The latter is characterized by an excess of oxygen in it, both in the bound forms, and in the free forms of O 3 , O 2− , O 2 , etc. Rocks, as a rule, contain different types of water-from hydroxyl and crystallization in minerals to capillary, etc. It is with the active participation of water in rocks that various processes of their transformations occur. Statistical data on the chemical composition of natural oxosalts show that most of them are typical metals with amphoteric properties. These are, first of all, iron and aluminum, as well as manganese, copper, beryllium, zirconium, and some others. In high-alkaline medium-and low-temperature natural systems (late pegmatites and hydrothermalites), complexes of these elements often play the role of anion formers, creating, together with acid groups, anionic structures of a mixed crystal chemical nature, for example, aluminophosphate. Under these conditions, zeolite-like macro-and microporous minerals are quite common, which have the properties of sorbents, molecular sieves, ion exchangers, etc. Simulation of such environments in the laboratory and subsequent detailed study of synthetic analogs of minerals allows, on the one hand, to trace the regular relationships between the physicochemical parameters of the crystallization system and the specific features of the crystal structures of the forming phases (structural typomorphism), and, on the other hand, to obtain new compounds with promising properties.
The mineral tinsleyite, KAl 2 (PO 4 ) 2 (OH)·2H 2 O is a member of the leucophosphite group, with the general formula AB 2 (PO 4 ) 2 (OH)·2H 2 O (A = NH 4 , K; B = Al, Fe). This mineral group apart from leucophosphite, KFe 2 [PO 4 ] 2 (OH)·2H 2 O [1] and tinsleyite, also includes sphenscidite, NH 4 Fe 2 (PO 4 ) 2 (OH)·2H 2 O [2] and ammoniotinsleyite, (NH 4 ) 2 Al 2 (PO 4 ) 2 (OH) 2 ·2H 2 O [3]. The above-mentioned species are known to occur in two different paragenetic associations: late products of the hydrothermal alteration of primary phosphates in granitic pegmatites, or biominerals. Tinsleyite was first described from the Tip Top granitic pegmatite, in South Dakota. It is found in pods of highly altered triphylite in the intermediate zone of the pegmatite in association with leucophosphite, on which it commonly occurs as a morphologically continuous overgrowth [4]. Its crystal structure was established and refined by Dick [5], who used a synthetic crystal, obtained by the reaction of gibbsite with a potassium phosphate solution of pH = 7 at 423 K. Two steps in the thermal loss of water, at 341 and 471 K, were observed. A potassiumrich variant of tinsleyite was synthesized under hydrothermal conditions at 553 K from the water solution of KH 2 PO 4 and Al(OH) 3 [6]. Compared to the mineral tinsleyite, the new variant with the crystal chemical formula |K 1 differs not only in the quantity of K + cations in the framework channels, but also in the amount of H 2 O and in the way it is distributed in the structure. It was suggested that the capacity of the minerals of the leucophosphite group to accommodate the K + (or NH 4 + ) ions is coupled with the H 2 O content in the framework interstices, which is, therefore, variable. As it happens, the synthetic NH 4 ,Al end member of ammoniotinsleyite called AlPO 4 -15 has been obtained and investigated [7,8] 25 years before the mineral was discovered. This material was studied by means of a charge-density analysis [9] and first-principal calculations [10]. The same compound was obtained later and described in [11]. The crystal structure of leucophosphite, KFe 2 [PO 4 ] 2 (OH)·2H 2 O was refined by Dick and Zeiske [12] using a synthetic crystal. They found hydrogen atom positions by Rietveld refinement based on powder neutron-scattering data. The structure of the NH 4 ,Fe end-member sphenscidite, (NH 4 )Fe 2 [PO 4 ] 2 (OH)·2H 2 O, was refined by Yakubovich and Dadashov [13] with crystals grown at 423 K from a hydrogel containing an organic compound, urea or carbamide CO(NH 2 ) 2 , which is found in the urine of mammalia, birds, and some reptiles. In the presence of water, the urea gives off ammonia that enters the forming crystal structure. This is a usual way in which sphenscidite crystallizes in nature [2]. As part of our experimental study of synthetic analogues of minerals [14], we report here the new aluminophosphate, Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O, with original crystal architecture chemically and structurally related to tinsleyite and might be considered as its hypothetical alteration product of Na metasomatism.

Hydrothermal Synthesis and Crystallization
Single crystals of the new compound were synthesized under hydrothermal conditions. Chemical compounds of analytical grade were taken in the mass ratio NaCl:Al(OH) 3 = 2:3, which corresponds to 1g (17 mmol) NaCl and 1.5 g (19 mmol) Al(OH) 3 . This starting mixture was melted in a 10% solution of H 3 PO 4 (5.6 mL), sealed in a poly(tetrafluoroethylene) (PTFE)-lined stainless steel pressure vessel of 7 mL in volume (fill factor 80%). It was kept in a furnace at a temperature of 553 K and a pressure of 7 MPa for 10 days, followed by slow cooling to room temperature. The reaction products were colorless crystals with an irregular shape up to 0.3 mm long ( Figure 1). They were washed with water, dried and subjected to a SEM-EDX analysis and to single-crystal X-ray diffraction. The SEM-EDX analysis was carried out on a Jeol SEM (JSM-6480LV) Oxford X-Max N equipped with an energy-dispersive diffraction spectrometer (Laboratory of Local Methods for Studying Materials, Department of Petrology, Faculty of Geology, M. V. Lomonosov Moscow State University). The measurements were performed at 20 kV and 0.7 nA using the sample covered by a carbon film with a thickness of about 25 nm. The crystals were stable under these conditions. X-ray spectral semiquantitative analysis of unpolished samples revealed Na, P, Al, and O atoms in their composition with the P:Al:Na ratio close to 1:1:1, which is consistent with the data of our X-ray diffraction structural study. crystals with an irregular shape up to 0.3 mm long ( Figure 1). They were washed with water, dried and subjected to a SEM-EDX analysis and to single-crystal X-ray diffraction. The SEM-EDX analysis was carried out on a Jeol SEM (JSM-6480LV) Oxford X-Max N equipped with an energy-dispersive diffraction spectrometer (Laboratory of Local Methods for Studying Materials, Department of Petrology, Faculty of Geology, M. V. Lomonosov Moscow State University). The measurements were performed at 20 kV and 0.7 nA using the sample covered by a carbon film with a thickness of about 25 nm. The crystals were stable under these conditions. X-ray spectral semiquantitative analysis of unpolished samples revealed Na, P, Al, and O atoms in their composition with the P:Al:Na ratio close to 1:1:1, which is consistent with the data of our X-ray diffraction structural study.

X-ray Diffraction and Crystal Structure Determination
X-ray diffraction data were collected from the crystal of 0.05 × 0.15 × 0.26 mm in size at T = 150 K on an Oxford Diffraction Gemini single crystal diffractometer equipped with a CCD detector; Mo Kα radiation (λ = 0.71073 Å). The dataset was corrected for background, Lorentz and polarization effects, and absorption [15]. All calculations were performed within the WinGX program system [16]. The monoclinic crystal structure was solved by direct methods in the space group P21/c and refined in anisotropic approximation with the SHELX programs [17,18] using the F 2 data to residual R = 0.0221 [for 2063 reflections with I > 2σ(I)], S = 1.177. In Table 1, we report the crystallographic characteristics of the new aluminophosphate and the experimental conditions of data collection and refinement. Table S1 presents the final results of the atom positions and equivalent isotropic displacement parameters. Characteristic distances are given in Table 2. A bondvalence calculation (Table 3) was performed using the algorithm and parameters given by Brown and Altermatt [19]. Data from Table 3

X-ray Diffraction and Crystal Structure Determination
X-ray diffraction data were collected from the crystal of 0.05 × 0.15 × 0.26 mm in size at T = 150 K on an Oxford Diffraction Gemini single crystal diffractometer equipped with a CCD detector; Mo Kα radiation (λ = 0.71073 Å). The dataset was corrected for background, Lorentz and polarization effects, and absorption [15]. All calculations were performed within the WinGX program system [16]. The monoclinic crystal structure was solved by direct methods in the space group P2 1 /c and refined in anisotropic approximation with the SHELX programs [17,18] using the F 2 data to residual R = 0.0221 [for 2063 reflections with I > 2σ(I)], S = 1.177. In Table 1, we report the crystallographic characteristics of the new aluminophosphate and the experimental conditions of data collection and refinement. Table S1 presents the final results of the atom positions and equivalent isotropic displacement parameters. Characteristic distances are given in Table 2. A bond-valence calculation (Table 3) was performed using the algorithm and parameters given by Brown and Altermatt [19]. Data from Table 3

Interatomic Distances and Crystal Structure Description
The basic structural elements of the title compound are shown in Figure 2a. The Al 3+ ions in two symmetrically independent positions are surrounded by O atoms, forming trigonal bipyramids. The Al1-centered polyhedron has three close Al1-O distances that vary from 1.774(1) to 1.831(1) Å, and two longer distances to apical O atoms of 1.880 (1) and 1.898(1) Å. The distortion of the Al2-centered five-vertex polyhedron is different: all Al2-O distances lie in the interval 1.816(1)-1.844(1) Å. The pattern of distortion of the AlO 5 polyhedra is consistent with the bond-valence calculation ( Table 3). The asymmetric unit of the structure includes two P sites in tetrahedral coordination. In the P1 tetrahedron, there are two pairs of close P1-O bond lengths, one of 1.529 (1)

Crystal Chemical Regularities in the Family of Aluminum Phosphates/Aluminophosphates
Natural phases, formed within the Al-P-O-(H) compositions, are generally supergene minerals of sedimentary rocks; they also occur in weathering zones and late hydrothermal formations. Although these minerals contain water in a different form, they often have rather dense crystal structures formed by cationic layers (augelite, Al 2 (OH) 3 [20]. In the crystal structure of mineral berlinite, AlPO 4 (an indicator of high-temperature conditions of phase formation [21]) with the ratio Al:P = 1, the framework of which represents a superstructure based on quartz, Al atoms are in the tetrahedral coordination.      Incorporation of Na into the composition of aluminum phosphates is usually connected with metasomatic reactions. In nature, two minerals are known in the Na-Al-P-O-H system-wwardite and brazilianite. Both structures are based on mixed anionic frameworks of Al octahedra and P tetrahedra, but the topology of the frameworks differs significantly. Brazilianite, NaAl 3 (PO 4 ) 2 (OH) 4 is considered to form in granitic pegmatites as a product of Na-metasomatic alteration of primary pegmatite minerals of the montebrasiteamblygonite series. The crystal structure of brazilianite is designed from columns of AlO 4 (OH) 2 and AlO 3 (OH) 3 octahedra sharing edges; these columns are linked by PO 4 tetrahedra in a framework with cavities populated by Na atoms [22,23]. The mineral wardite, NaAl 3 (PO 4 ) 2 (OH) 4 ·2H 2 O of hydrothermal origin usually occurs in P-rich zones of granite pegmatites. In its tetragonal crystal structure, Al-centered octahedra sharing OH vertices are aligned in intercrossing chains along the {100} and {010} directions. These chains form cellular layers parallel to the ab plane with Na atoms occupying positions near their centers. The sheets of AlO 2 (OH) 4 , AlO 3 (OH) 2 (H 2 O) and NaO 6 (H 2 O) 2 polyhedra sharing edges and vertices are linked along the c axis via phosphate tetrahedra and hydrogen bonds [24,25].

Crystal Chemical Regularities in the Family of Aluminum Phosphates/Aluminophosphates
Natural potassium and aluminum phosphates are mainly weathering products of clay minerals under the action of phosphate-containing solutions of guano or anthropogenic fertilizers. Potassium aluminophosphate taranakite, K 3 Al 5 (HPO 4 ) 6 (PO 4 ) 2 ·18H 2 O mostly occurs in humid caves where phosphate-rich excrements of birds, penguins, or bats react with clay minerals. It is also known as a reaction product of soil clays with phosphate-containing fertilizers [26]. In the rhombohedral structure of taranakite, six [K 3 Al 5 (HPO 4 ) 6 (PO 4 ) 2 (H 2 O) 12 ] layers alternate along {001} with layers of H 2 O molecules. Non-exchangeable K atoms are trapped within the layers. Hydrogen bonds in taranakite act within the rigid layers, within the water interlayers, and between the layers and interlayers [27]. The product of taranakite dehydration, francoanellite, occurs at the contact of "terra rossa" with bat guano in the karst cave. The mineral crystal structure is built of the same "taranakite" [K 3 Al 5 (HPO 4 ) 6 (PO 4 ) 2 (H 2 O) 12 ] layers connected by hydrogen bonds. Hydrogen bonds in francoanellite are formed within the rigid layers and between them. It has been shown that single crystals of synthetic francoanellite could be obtained by topochemical dehydration of taranakite crystals with loosing of every second water interlayer, when a first-order staging product of the deintercalation of water from taranakite was formed [28].
Known to date, several structural topologies of aluminophosphates are derived from the assemblage of PO 4 tetrahedra and a number of types of Al-centered polyhedra (tetrahedra, octahedra, trigonal bipyramids, and tetragonal pyramids) in the framework. Five-vertex polyhedra formed by O atoms in the Al surrounding are not as common as usual AlO 4 tetrahedra or AlO 6 octahedra. There are compounds, for instance Na 6 Al 3 P 5 O 20 , in which the microporous framework is built from mutually AlO 6 and AlO 4 polyhedra, and PO 4 tetrahedra having vertex-bridging contacts [29]. Obtained by a high-temperature molten salt method, the Ba 3 Al 2 P 4 O 16 phase with an Al/P ratio of 1:2, is characterized by unique [Al 2 P 4 O 16 ] ∞ chains constructed from PO 4 , AlO 4 , and AlO 5 groups [30]. Two complex aluminophosphates with an Al:P ratio equal to 5 5 , and AlO 6 , and the interlayer space contains the amines and water molecules [31].
The title compound Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O represents an example of the layered sodium aluminophosphate built from PO 4 and AlO 5 polyhedra. Its monoclinic crystal structure is very similar to that of mineral tinsleyite, KAl 2 (PO 4 ) 2 (OH)·2H 2 O with K atoms in the framework channels. The main difference between the two structures consists of the type of Al coordination by O atoms. As shown above, clusters of AlO 5 bipyramids sharing edges and vertices are essential building blocks of the Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O. Similar clusters, but designed from AlO 4 (OH) 2 and AlO 4 (OH)(H 2 O) octahedra, form the tinsleyite crystal structure ( Figure 5). The topologies of both Al/P substructures within the bc layers are identical; the amount and distribution of alkali Na + or K + ions and the H 2 O molecules are different. Significantly distinct sizes of Na + and K + ionic radii define here not only a type of Al coordination by O atoms in order to arrange the necessary environment around one or another alkali metal. They also specify a periodicity of the aluminophosphate anionic construction: layers alternating along the {100} direction with Na atoms in the Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O or a framework with channels populated by a two times lower quantity of K atoms in the case of tinsleyite ( Table 4). The synthesis of sodium aluminophosphate under mild hydrothermal conditions and the topological resemblance of its structure with that of the mineral tinsleyite and a number of its structural analogues, suggest a high probability of the existence of a mineral equivalent of the Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O in nature. A certain structural resemblance between hydrothermally obtained sodium aluminophosphate hydroxide Na2Al3(OH)2(PO4)3 and the mineral minyulite, KAl2F(H2O)4(PO4)2 has been noticed earlier [32]. Minyulite was first synthesized by Haseman et al. [33], as well as a number of other hydrous phosphates, including tinsleyite, by the treatment of clays with phosphate anions at pH ranges appropriate for soil environments and at a temperature less than 95 °C. A synthetic hydroxide analogue of minyulite, namely, KAl2(OH)(H2O)4(PO4)2 could also be obtained by the reaction of gibbsite with a potassium phosphate solution of pH = 5.5 at 333 K [34].   A certain structural resemblance between hydrothermally obtained sodium aluminophosphate hydroxide Na 2 Al 3 (OH) 2 (PO 4 ) 3 and the mineral minyulite, KAl 2 F(H 2 O) 4 (PO 4 ) 2 has been noticed earlier [32]. Minyulite was first synthesized by Haseman et al. [33], as well as a number of other hydrous phosphates, including tinsleyite, by the treatment of clays with phosphate anions at pH ranges appropriate for soil environments and at a temperature less than 95 • C. A synthetic hydroxide analogue of minyulite, namely, KAl 2 (OH)(H 2 O) 4 (PO 4 ) 2 could also be obtained by the reaction of gibbsite with a potassium phosphate solution of pH = 5.5 at 333 K [34]. Crystal structures of these Na-and K-bearing pseudo tetragonal compounds look similar in the ab projection. The elementary blocks of the anionic layers of the mixed type in the minyulite structure are dimers consisting of two Al octahedra sharing F vertices (or OH vertices in the structure of its synthetic variant). These pairs of octahedra are connected in layers via PO 4 tetrahedra. Voids of the octagonal cross-section in layers parallel to the {001} plane contain K + ions. In the direction of the c axis, the aluminophosphate layers are linked by hydrogen bonds [35]. Blocks of sharing vertices Al-centered polyhedra crosslinked by PO 4 tetrahedra can be distinguished in the structure of Na 2 Al 3 (OH) 2 (PO 4 ) 3 ; however, in this case, two AlO 4 (OH) polyhedra and one AlO 4 (OH) 2 octahedron form an elementary cluster of a horseshoe appearance ( Figure 6). The Al-centered five-vertex polyhedra are moved up and down along {001} from the Al-centered octahedron. As a consequence, the Al 3 (OH) 2 O 12 trimers with the AlO 4 (OH) 2 octahedron at their centers occur disposed at two levels along the z axis ( Figure 6b); thus increasing the unit cell c parameter to 14.319 Å, as compared with c = 5.522 Å of minyulite having the layered structure (Figure 6c). In the framework structure of sodium aluminophosphate, the clusters are linked via phosphate tetrahedra not only in the ab plane (as in the minyulite structure), but also in the {001} direction. Each Al-centered polyhedron overlaps along the z axis with its counterpart at a distance of about 4.9 Å, and the trimeric clusters look like dimers in the xy projection (Figure 6a). Crystal structures of these Na-and K-bearing pseudo tetragonal compounds look similar in the ab projection. The elementary blocks of the anionic layers of the mixed type in the minyulite structure are dimers consisting of two Al octahedra sharing F vertices (or OH vertices in the structure of its synthetic variant). These pairs of octahedra are connected in layers via РO4 tetrahedra. Voids of the octagonal cross-section in layers parallel to the {001} plane contain K + ions. In the direction of the c axis, the aluminophosphate layers are linked by hydrogen bonds [35]. Blocks of sharing vertices Al-centered polyhedra crosslinked by PO4 tetrahedra can be distinguished in the structure of Na2Al3(OH)2(PO4)3; however, in this case, two AlO4(OH) polyhedra and one AlO4(OH)2 octahedron form an elementary cluster of a horseshoe appearance ( Figure 6). The Al-centered five-vertex polyhedra are moved up and down along [001] from the Al-centered octahedron. As a consequence, the Al3(OH)2O12 trimers with the AlO4(OH)2 octahedron at their centers occur disposed at two levels along the z axis ( Figure 6b); thus increasing the unit cell c parameter to 14.319 Å, as compared with c = 5.522 Å of minyulite having the layered structure (Figure 6c). In the framework structure of sodium aluminophosphate, the clusters are linked via phosphate tetrahedra not only in the ab plane (as in the minyulite structure), but also in the [001] direction. Each Al-centered polyhedron overlaps along the z axis with its counterpart at a distance of about 4.9 Å, and the trimeric clusters look like dimers in the xy projection (Figure 6a). The absence, at ambient conditions, of isostructural chemically similar but Na-or Kbased compounds is well-known and is mainly due to the difference in ionic radii of Na + and K + . The abovementioned examples demonstrate the crystal chemical variations between Na-and K-representatives of aluminophosphates, the structures of which include clusters of Al-centered polyhedra interconnected in the framework through oxygenbridging contacts of PO4 tetrahedra. In both cases, alkali Na or K atoms work as structure regulating agents, determining the character of the developing crystal structure, namely, the types of Al-centered polyhedra and the structure periodicity (layer or framework). The flexibility of aluminophosphate constructions, which include clusters of Al-centered octahedra and/or five-vertex polyhedra connected by phosphate groups, allows them to self-organize around structure-forming Na + or K + ions into anionic layers or frameworks. The implementation of such structures, also in nature in the form of minerals, is largely determined by the participation of water, in one form or another, which stabilizes the crystallizing phases due to the forming hydrogen bonds. The absence, at ambient conditions, of isostructural chemically similar but Na-or K-based compounds is well-known and is mainly due to the difference in ionic radii of Na + and K + . The abovementioned examples demonstrate the crystal chemical variations between Na-and K-representatives of aluminophosphates, the structures of which include clusters of Al-centered polyhedra interconnected in the framework through oxygenbridging contacts of PO 4 tetrahedra. In both cases, alkali Na or K atoms work as structure regulating agents, determining the character of the developing crystal structure, namely, the types of Al-centered polyhedra and the structure periodicity (layer or framework). The flexibility of aluminophosphate constructions, which include clusters of Al-centered octahedra and/or five-vertex polyhedra connected by phosphate groups, allows them to self-organize around structure-forming Na + or K + ions into anionic layers or frameworks. The implementation of such structures, also in nature in the form of minerals, is largely determined by the participation of water, in one form or another, which stabilizes the crystallizing phases due to the forming hydrogen bonds.

Conclusions
Various structural topologies of aluminophosphates are derived from the grouping of PO 4 tetrahedra and a number of types of Al-centered polyhedra (tetrahedra, octahedra, trigonal bipyramids, and tetragonal pyramids) in the framework. A novel representative Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O within this family was obtained as single crystals in middletemperature hydrothermal conditions. Its original layered crystal structure was established by low-temperature X-ray diffraction. A mineralogically probable phase is discussed as a sodium alternative of tinsleyite, KAl 2 (OH)(H 2 O)(PO 4 ) 2 ·H 2 O, a member of the leucophosphite mineral group, which also includes sphenscidite and ammoniotinsleyite. These minerals are known to occur in two different paragenetic associations: as late products of the hydrothermal alteration of primary phosphates in granitic pegmatites, or as biominerals.
The crystal structures Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O and tinsleyite contain similar fragments: clusters of four Al-centered polyhedra sharing edges and vertices, surrounded by PO 4 tetrahedra. In contrast to tinsleyite, where tetramers are built from AlO 4 (OH) 2 and AlO 4 (OH)(H 2 O) octahedra, analogous clusters of AlO 5 bipyramids are the main structural units of sodium aluminophosphate. We showed that alkali Na or K atoms, which have strongly distinct ionic radii, act as structure regulators, determining the character of the developing crystal structure, namely the types of Al-centered polyhedra and the structure periodicity. The flexibility of aluminophosphate constructions, which include clusters of Al-centered octahedra or five-vertex polyhedra connected by phosphate groups, allows them to self-organize around structure-forming Na + or K + ions into anionic layers or frameworks. We assume that the synthesis of sodium aluminophosphate under mild hydrothermal conditions and the topological resemblance of its structure with that of the mineral tinsleyite and a number of its structural analogues, suggest a high probability of the existence of a mineral equivalent of the Na 2 Al 2 O(PO 4 ) 2 ·0.12H 2 O in nature.