Structure evolution of fluorapophyllite-(K) under high pressure

ABSTRACT The high pressure structural evolution of a natural fluorapophyllite-(K) K0.70(NH4)0.20 Ca3.97Na0.07[Al0.04Si7.96O20]F0.99·(H2O)8.05, Z = 2, a = 8.9757(2), c = 15.7920(2) Å, space group P4/mnc, from Nidym river, East Siberia, Russia, compressed in penetrating (ethanol:water 4:1 mixture) and non-penetrating (paraffin oil) media up to 4.7 GPa, was studied by single-crystal X-ray diffraction with a diamond anvil cell. The compressibility is identical in both media. At the initial stage the compression proceeds mainly within the plane (xy) and less along the z-axis; above 3 GPa the compression becomes almost isometric. Within the whole pressure range there are no signs of the symmetry lowering. The main pressure-induced effect on the tetrahedral layer consists in a cooperative rotation of the 4-fold rings, which provides the structure compression within the (xy) plane. The compression along the z-axis proceeds through the shortening of the interlayer distance, whereas the thickness of silicate layer remains almost unchanged.


Introduction
Fluorapophyllite-(K), a member of apophyllite group, with an idealized composition KCa 4- [Si 8 O 20 ]F•(H 2 O) 8 (space group P4/mnc, Z = 2) [1], is a layered silicate, in which the silicate tetrahedra form the 4-fold and contracted 8-fold rings.The inter-layer space is filled by the Ca cations, connected to the terminal O atoms of adjacent layers and the anions F -, which are also located between the layers.The K + cations coordinated by eight H 2 O molecules occupy the large cages at the intersection of the neighbor silicate layers.Hydroxyapophyllite-(K) is isostructural with fluorine-bearing species [2], the structural position of the OH group being identical to that of fluorine in fluorapophyllite-(K).
A general formula of the apophyllite group members can be written as AB 4 (Si 8 O 20- )X•(H 2 O) 8 (A = K, Na, NH 4 , Cs; B = Ca (in apophyllite rootname members), Sr (in1 mcglassonite rootname member); X = F, OH).Apophyllite group minerals refer to layered silicates, but they have common features with the framework aluminosilicates zeolites.Moreover, it is often classified as zeolite [3].Zeolites contain the exchangeable cations and the H 2 O molecules, which can be reversibly or irreversibly removed on heating.Apophyllite group minerals and zeolites often occur together in low-temperature and low-pressure metamorphic rocks [4].
Upon the compression in water-bearing (penetrating) medium many zeolites undergo a so-called pressure-induced hydration (PIH) [5], when the H 2 O content in the structure increases.At a minimum, this leads to the decrease of the compound compressibility, compared to that in non-penetrating medium.
The high pressure (HP) behavior of zeolites has been characterized in detail in many works [6], whereas the studies of layered silicates in penetrating pressure-transmitting medium are scarce.The PIH effect was observed in kaolinite Al 2 Si 2 O 5 (OH) 4 [7,8] and its polymorph nacrite [9] under the compression in water medium.The penetration of the H 2 O molecules into the structure of the kaolin group minerals proceeds at 2.7 GPa and 200°С and leads to an approximately 31% volume increase [7,9].The minerals of smectite group beidellite, montmorillonite and nontronite behave differently under the compression in anhydrous and aqueous media [10].This is due to the incorporation of additional H 2 O molecules in the interlayer space when wetting these minerals.
The HP behavior of apophyllite-group minerals was studied by powder X-ray diffraction (XRD) [3,11] and Raman spectroscopy [12].The compressibility of hydroxyapophyllite-(K) was studied up to 10 GPa [3].The pressure transmitting medium was a 16:3:1 mixture of methanol-ethanol-water (so-called nominally penetrating medium).Hydroxyapophyllite demonstrated regular contraction; no anomalies in the pressure dependences of the unitcell parameters were detected.The structure of hydroxyapophyllite at high pressure was not determined.The study of fluorapophyllite-(K) under the compression in pure water up to 7.7 GPa [11] also provides only the compressibility data.The authors of [11] believe that a second-order phase transition cannot be ruled out based on the correlation between normalized pressure and strain.The results of the HP Raman spectroscopy study of fluorapophyllite-(K) [12] do not exclude the possibility of phase transition under pressure, as well.Our recent HP-HT Raman experiments with fluorapophyllite-(K) [13] show no difference between the compression behavior in penetrating (water-containing) and nonpenetrating (paraffin oil) media.
Our aim is to study the evolution of the apophyllite structure under pressures up to 5 GPa in penetrating and non-penetrating media.
The HP XRD experiments were performed in a Boehler-Almax diamond-anvil cell (DAC) [15] with anvil culets of 600 μm diameter.A 300 μm micro-hole in a 200 μm thick stainless steel gasket, pre-indented to a thickness of 120 μm, was used as the sample chamber.In all experiments, pressure was estimated from the shift of the ruby R1 band (±0.05 GPa) [16].An ethanol-water (4:1) mixture was used as the pressure-transmitting fluid in the first experimental series.Diffraction data were collected on an Oxford Diffraction Xcalibur Gemini diffractometer (MoKα radiation, 0.5 mm collimator, graphite monochromator).Data were collected using ω scan technique, with the scan step of 0.5°and a time of 60 s per frame with the strategy of CrysAlis Pro software [17] for the HP experiments in the DAC.Single-crystal X-ray diffraction measurements of first series were carried out at 8 pressure points.
After the first HP experimental series, the DAC was opened and dried to remove ethanol-water mixture from the sample chamber.Then the sample chamber was filled with paraffin oil (ROTH GmbH) as a non-penetrating medium.The second series (3 pressure points) of single-crystal X-ray diffraction measurements was carried out using the same crystal and the same experimental parameters as the first one.
After the second experimental series, an attempt was made to withdraw the crystal from the DAC to estimate the reversibility of pressure-induced changes in the apophyllite structure.However, the attempt failed and the crystal was lost.
The HP diffraction data were reduced using the CrysAlis Pro software.The reflections from diamond anvils were not included in the data processing.In the first experimental series, ice reflections were present starting from 2.7 GPa.The overlap of the reflections from the sample and diamond or ice was checked manually, and the overlapped reflections were removed.The absorption by diamonds, the gasket and the crystal was corrected with the Absorb-7 software [18].The structures were refined using the SHELXL-2018/3 package [19].For the first pressure point, the structural parameters were taken from [14] as the starting model; hereinafter, the starting model was the structural parameters obtained for the previous pressure point.
The full experimental details of the data collection and structure determination are given in Tables 1 and 2. The results of structural refinement are given as Supplementary materials (Tables S1-S4 and CIF files).

High pressure behavior of fluorapophyllite in water-containing medium
The crystal structure of Nidym fluorapophyllite at ambient conditions is shown in Figure 1.The silicate layers are formed by four-and eight-membered rings of SiO 4 -tetrahedra.In the tetrahedral layers, only the unshared oxygen atoms are bonded to the interlayer Ca 2+ ions.It can be seen that the oxygen (Ow) of the water molecule occupies a strategic position in the structure [1].The large ion (K + , (NH 4 ) + ) is coordinated by eight H 2 O molecules in the form of a squat tetragonal prism.Each Ow is shared with Ca 2+ which, in turn, is bonded to two Ow atoms of its seven nearest neighbors forming a one-capped trigonal prism.
The Nidym fluorapophyllite is deficient in large cations, which is confirmed by the chemical analysis and the structure determination.To maintain electroneutrality, the additional Na + cations reside in one of the two mirror-symmetrical positions (Figure 1 (b)) located 1.2 Å away from the K site, in four-fold planar coordination.
Unit-cell parameter variations with P are shown in Figure 2. Experimental data for fluorapophyllite weighted by their uncertainties have been fitted to the third order Birch-Murnaghan equation of state [20], using the EoSFit 7.0 GUI software [21].The results from a least-square fitting are K 0 = 33.9± 0.7 GPa, K´= 17.2 ± 0.8 for P-V data, where K 0 and KH are the isothermal bulk modulus and its pressure derivative, respectively.No signs of the symmetry lowering are observed up to the highest pressure studied.The c(P) dependence (Figure 2) is close to linear; the compressibility along the a axis decreases with pressure.At the initial stage, the compression of fluorapophyllite is essentially anisotropic: up to about 2 GPa, the compressibility along the a axis is 1.5-2 times higher than along the c axis.Upon the pressure rise the compression approaches isotropic.
The compression along the a axis is mainly determined by the deformation possibilities of silicate layers.The 4-membered rings, forming the silicate layer, are connected to each other through one shared O-atom, and therefore they can rotate relative to each other.This affects the aperture of the 8-membered rings formed at the junction of four 4-  HIGH PRESSURE RESEARCH membered rings (Figure 1(a)).Accordingly, the deformation of silicate layer consists, firstly, in the distortion of the 4-membered rings and, secondly, in their relative rotation.The 4-membered rings, forming the silicate layers, surround the four-fold axes, which restricts significantly their possible geometrical changes.As a result, the deformation of the ring is very small: the distance O1-O1, which determines the ring dimension (see insert in Figure 3), decreases from 6.97 Å at ambient conditions to 6.93 Å at 4.7 GPa.
At that the angle of ring rotation ψ decreases by 2.8°(Figure 3), which decreases the 8membered ring aperture from 3.47 to 3.00 Å.Such a significant compression of the ring results in the rapprochement of the atoms in a silicate layer; this, in its turn, limits  the further deformation of the rings within the plane (xy).We can suppose that this is the main reason for the decrease of the structure compressibility along the a and b axes.
A minor deformation and significant relative rotation of the 4-membered rings in silicate layer manifest in the changes of the Si-O-Si angles.For example, the Si-O2-Si angle at the bridge O-atom of the ring decreases by only 1°within 0.0001-4.75GPa (Table S3), whereas the Si-O1-Si angle, determining the relative rotation of the rings in silicate layer, diminishes by about 5°within the same pressure range.
The interlayer cations do not affect directly the rings rotation.The Ca 2+ coordination includes two O-atoms of one 4-membered ring of the layer, whereas the bond to the neighboring ring is organized via the H 2 O molecule.The large cations K + and (NH 4 ) + in A site are connected with the silicate layers also only through the hydrogen bonds of their hydrate shell.Hydrogen bond enables significant variation of the H•••O distances with a slight change of its bond valence.
On the contrary, the structure contraction along the c axis is solely determined by the change in the coordination of the interlayer cations.The silicate layers and interlayer space, filled with the cations coordinated by the H 2 O molecules and F -anions, alternate along the c axis.There is one silicate layer and one interlayer space for half of the cperiod of the unit cell.The distance between two mirror-symmetrical apical atoms O3, entering the Ca coordination, can be taken as the height of interlayer space.Accordingly, the thickness of silicate layer is equal to one half of the c-period minus the interlayer spacing.The thickness of silicate layer remains almost unchanged under compression (Figure 4), which is evidently related with the absence of deformation of the 4-fold rings forming the layer.At 4.7 GPa the height of the interlayer space decreased by 8% relative to the initial value; at that the parameter a decreased by only 3%.It follows that the major structural changes due to the compression occur in the environment of interlayer cations (Figures 5-6).assemblages: the distances A-Ow shorten by about 5% (by 0.15 Å) at 4.7 GPa compared to their initial value 2.98 Å.
All water molecules in apophyllite group minerals are crystallographically equivalent and show one strong (H1•••O3) and one weak (H2•••O2) hydrogen bonds [1,14,[22][23][24].Note, however, that the X-ray diffraction methods provide large errors in the determination of the hydrogen atom position in the structure, especially at high pressure.Therefore the changes in the system of the H 2 O-O bonds can be estimated more reliably from the distances Ow•••O2 and Ow•••O3.The decrease of the distance Ow•••O2 (by 0.13 Å) is comparable with that in the Ca coordination, whereas the distance Ow•••O3 shortened by 0.08 Å (Figure 7).
The results of structure analysis show that the composition of fluorapophyllite remains constant under the compression in penetrating aqueous medium, i.e. the effect of the pressure-induced hydration [5] is absent.

High pressure behavior of fluorapophyllite in paraffin oil
The study of fluorapophyllite compressed in paraffin oil shows that its behavior does not depend on the composition of the pressure-transmitting medium.First of all, the changes of the unit cell parameters and volume, observed under the compression in aqueous and waterless medium, are similar (Figure 2); the largest deviations are observed at maximal pressure, when the measurements errors are also maximal.The absence of crystal-fluid interaction is confirmed by the results of structure refinement, which show constant composition of fluorapophyllite within the whole studied range of pressures.The pressure dependences of the interatomic distances and angles, obtained in the experiments with paraffin oil and aqueous medium, coincide (Figure 3-6).The differences between the values obtained in two different pressure-transmitting media are obviously determined by the structure refinement inaccuracies.

The behavior of the apophyllite group minerals under high pressure
As it was noted, the behavior of fluorapophyllite-(K) compressed in water [11] and hydroxyapophyllite-(K) compressed in 16:3:1 mixture of methanol-ethanol-water was studied by X-ray powder diffraction method.The Figure 8 shows the pressure dependences of the unit cell volume of fluor- [11] and hydroxyapophyllite [3] compared to our results.The curve obtained in this work is close to the compressibility data for fluorapophyllite [11].Nonmonotonicity of the curve plotted according to the literature data, in our opinion, is explained by the incorrectness of the method for determining the position of the maxima of diffraction peaks used in [11], and the respective large errors in the calculation of the unit cell parameters.
The compressibility of hydroxyapophyllite is significantly less than that of fluorapophyllite, as is seen from Figure 8.Compared to fluorapophyllite, hydroxyapophyllite demonstrates a lesser compressibility in both the (xy) plane and the c axis.
Crystal structures of fluorand hydroxyapophyllite are very similar, the only difference is the presence of the anion F -or hydroxyl ion (OH) -, located in the same structural position and entering the Ca 2+ coordination.According to the structure data [2], the distance Ca-OH in hydroxyapophyllite structure is 2.435 Å, which exceeds by only 0.02 Å the distance Ca-F in the structure of studied fluorapophyllite.The mean distances < Ca-ligand > differ even less (by 0.01 Å).The authors of [3] report the unit cell parameters at different pressures; the structural changes in hydroxyapophyllite were not studied.The available  [11] and with hydroxyapophyllite-(K) (◊) compressed in a 16:3:1 mixture of methanol-ethanol-water [3].
data do not permit to explain the difference in the HP behavior of these members of the apophyllite group.

Structural evolution of layered silicates under high pressure
The structure of layered silicates determines a priori the anisotropy of their properties, including the compression anisotropy along the coordinate axes.At that the behavior of layered silicates under high pressure certainly depends on their structural peculiarities.The most detailed studies are devoted to the structure evolution of di-and trioctahedral micas.The results of studies of other layered silicates are reduced mainly to describe the bulk compression behavior (providing isothermal EoS).The similarity of the structure of micas determines their similar structure evolution at high pressure [25][26][27][28].The compressibility of three-layered tetrahedral-octahedral-tetrahedral sheets is determined by the changes in the octahedral layer, which are mainly represented by isometric compression of the polyhedra.The tetrahedra of silicate layers are joined to the octahedral layer through the O-atoms, and tetrahedral tilting is restricted by the compression of the octahedral layer.According to [26], tetrahedral tilting represents a preferential way, from an energy point of view, of reducing the misfit induced by the differing compressibilities of the tetrahedral and octahedral sheets.As a result, the compressibility of di-and trioctahedral micas within the sheet (plane xy) is significantly less than their compressibility perpendicular to the sheets (along the c axis).The ratios of linear compressibility coefficients β(GPa -1 ) along the a:b:c axes are 1.04:1:5.12for phlogopite [26], 1:1.17:4.60 for phengite [28], 1:1.15:3.95 for muscovite [25] and 1:1.03:2.37 for paragonite [29].
The structure compression along the c axis is determined by the narrowing of the interlayer space and, to a lesser extent, by reducing the height of the octahedral layer.The height of tetrahedral layer in micas, as well as in fluorapophyllite studied, practically does not change under pressure [26,29].For example, the height of the octahedral layer is reduced by 2% at 5 GPa in phlogopite structure [26] and by 4.5% at 5.38 GPa in phengite [28].As regards the interlayer space filled with large cations, its height is reduced by 11% in phlogopite and by 12% in phengite.
According to [26], the interlayer cation strongly controls the bulk compressibility of mica.The tetrahedral tilting, caused by different compressibility of the tetrahedral and octahedral layers, permits to fit their metric, but the size of the interlayer cation in the mica structure determines the actual evolution of this mechanism [26].It should be noted, however, that the cation size determines mainly the compression of the interlayer space and to a lesser extentthe compression of three-layered sheet within its plane.The increase in the cation size leads to a larger compressibility of the structure perpendicular to the layer and, at the same time, reduces the compressibility within the layer plane.For example, the compression of paragonite NaAl 2 (Si 3 AlO 10 )(OH) 2 along the a, b and с axes at 4 GPa makes 1.4, 1.5 and 3.8% [29].The unit cell parameters of Cs-mica CsFe 3 (Si 3 FeO 10- )(OH) 2 at 4 GPa are reduced by 1.0, 1.0 and 5.9%, respectively [30].In phlogopite KMg 3 (Si 3- AlO 10 )(OH) 2 the respective axial compression at 5 GPa is 1.2, 1.2 and 4.5% [26].
In contrast to micas, the apophyllite structure includes only highly corrugated tetrahedral layers.The structure response to high pressure is similar to that of the framework aluminosilicateszeolites.The layer in apophyllite, similarly to zeolite framework, compresses through mutual rotations of the tetrahedra forming the structure.As regards the interlayer cations, in apophyllite the interlayer sub-system is responsible for the structure compression along the c axis and weakly affects the compressibility of the layer within its plane.

Conclusion
The X-ray single crystal diffraction method was used to study the behavior of natural fluorapophyllite-(K) under compression up to 5 GPa in penetrating (water-containing) and non-penetrating (paraffin oil) media using DAC.The composition of the pressure-transmitting medium has no influence onto the structure evolution of mineral; its compressibility is identical in both media.Within the whole pressure range the compound compresses regularly, the structure symmetry is preserved.Within the plane (xy) the compression proceeds via mutual rotation of the 4-fold rings in silicate layer; along the с axis the structure compresses through the shortening of the interlayer distance, whereas the thickness of silicate layer remains unchanged.Comparison of the obtained results with the literature data on the HP behavior of hydroxyapophyllite-(K) indicates its significantly lower compressibility.To find out the reasons for this, it is necessary to study the structural evolution of hydroxyapophyllite.

Figure 2 .
Figure 2. Normalized lattice a/a 0 and c/c 0 parameters and the unit-cell volume V/V 0 of fluorapophyllite compressed in water-containing (full symbols) and anhydrous (open symbols) media.

Figure 3 .
Figure 3.The ψ angle rotation of 4-membered rings in the structure of fluorapophyllite compressed in water-containing (full symbols) and anhydrous (open symbols) media.

Figure 6 .
Figure 6.HP evolution of the K-Ow bond distances.Full and open symbols correspond to compression in water-containing and anhydrous media, respectively.

Figure 7 .
Figure 7. HP evolution of the Ow•••O distances.Full and open symbols correspond to compression in water-containing and anhydrous media, respectively.

Table 1 .
Parameters of data collection and structure refinement for fluorapophyllite compressed in ethanol-water mixture.

Table 2 .
Parameters of data collection and structure refinement for fluorapophyllite compressed in paraffin oil.