Phys Chem Minerals (1997) 24: 345–355
C Springer-Verlag 1997
OR IG I NA L P A P E R
M. E. Fleet ? G. S. Henderson
Structure-composition relations and Raman spectroscopy of high-pressure sodium silicates
Received Februar 6, 1996 y Revised, accepted November 6, 1996
Abstract e-Na2Si2O5, z-Na2Si2O5, Na2Si[Si2O7], and Na6Si3[Si9O27] have been synthesized using an MA6y8 superpress. Densification in high-pressure sodium silicates is effected largely by changes in packing. In the relaxed (1 bar) structures, cation polyhedra and thermaly displacement parameters are similar to those of lowpressure silicates, but the extra-framework cation positions are oversized. The two mixed [4]Si and [6]Si framework silicates of known structure (Na2Si[Si2O7] and Na6Si3[Si9O27]) belong to the limited homologous series Na2mSim[Sin–mO2n1m], with m,n. The structure-composition relationships of wadeite-type, A2Ge4O9-type, and Na6Si3[Si9O27] silicates and germanates depend on T-O distance and size of the large extra-framework cation. Characteristic features of the SiO4 tetrahedral units are present in micro-Raman spectra of mixed [4]Si and [6]Si framework silicates, but bands uniquely attributable to SiO6 octahedra are weak or obscured. However, [6]Si has a profound indirect influence on the Raman spectra, resulting in intense and complex low-frequency bands, assigned to symmetric bending modes with coupled displacements at both bridging oxygens and nonbridging oxygens bonded to [6]Si, and a shift to higher frequency and reduction in intensity of the high-frequency bands assigned to symmetric [4]Si-Onbr stretching vibrations. Raman spectroscopy does not appear to be a useful structural probe for small amounts of [6]Si in silicate glasses and melts.
Michael E. Fleet (✉) Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada Fax: 519–661–3198; e-mail:
[email protected] Grant S. Henderson Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada
Introduction Although there is only one mineral of sodium silicate composition (ertixiite, Na2Si4O9, Zhang et al. 1985) and magmas approximating to sodium silicate composition are unknown, the SiO2-rich portion of the Na2O-SiO2 system continues to be of interest to earth scientists, providing insights into silicate crystal chemistry, model structures for interpreting absorption spectra of silicate glasses and melts, and relatively low-temperature products for studying the physical and chemical properties of silicate glasses and melts (e.g., Furukawa et al. 1981; Henderson and Fleet 1991; Fleet and Henderson 1995a, b; Fleet 1996; Xue et al. 1991, 1994; Xue and Stebbins 1993; Kanzaki et al. 1996). The phase relations at 1 bar are well established (e.g., Morey 1964). In the subsolidus region, there are stable phases of composition Na2O, Na4SiO4, Na2SiO3, Na2Si2O5, and SiO2. There appear to be several roompressure modifications of sodium disilicate (e.g., Williamson and Glasser 1966) but only three of them have been completely characterized (a-, b- and dNa2Si2O5; Pant and Cruickshank 1968; Liebau 1961; Pant 1968; Jacobsen 1991). All of these sodium disilicates have structures based on sheets of six-membered rings of SiO4 tetrahedra. Indeed, all known monovalent cation disilicate structures are based on [Si2O5] disilicate sheets (e.g., Liebau 1985; de Jong et al. 1994; Veldman et al. 1995). Following Liebau (1985), structural complexity of layer-structure M12[Si2O5] phases arises from accommodation of the interlayer monovalent cation, and this is largely determined by cation size. The small cation structures (phyllosilicic acid, H2Si2O5 and Li2Si2O5) have very strongly folded (crenulated) disilicate sheets. In a-Na2Si2O5 (present above 7008C) the disilicate sheets are slightly less folded, and in b-Na2Si2O5 (present at 610–7008C) markedly less folded. De Jong et al. (1994) noted that the six-membered rings are in chair configuration in a- and b-Na2Si2O5 and boat configuration in d-Na2Si2O5.
346
Sodium tetrasilicate does not occur as a stable phase at 1 bar but is present as a metastable phase, with a54.9, ˚ , in annealed soda-silica glasses b523.4, and c515.4 A and gels (Mogensen and Christensen 1981; Neilson and Weinberg 1984; PDS file 38–20). Kanzaki et al. (1989, 1996) investigated phase relations in the Na2O-SiO2 system up to 14 GPa, and characterized products by powder X-ray diffraction, 29Si NMR and Raman spectroscopy. They reported epsilon sodium disilicate (e-Na2Si2O5) and coesite at moderate pressure and a second sodium disilicate polymorph (z-Na2Si2O5), sodium trisilicate (Na2Si3O7), sodium tetrasilicate (Na2Si4O9) and stishovite (SiO2) at high-pressure. Santarsiero et al. (1991) reported a partial structure for zNa2Si2O5 containing [6]Si:[4]Si in the ratio 1: 6. Our interest in the high-pressure structure-composition relations in the Na2O-SiO2 system arose from the need for more information on the structures of these crystalline sodium silicates to complement Si K-edge XANES spectroscopy of high-pressure vitreous products in the Na2O-SiO2 and K2O-SiO2 systems (Li et al. 1994, 1996). The crystal structures of three of the new high-pressure phases (epsilon sodium disilicate-eNa2Si2O5, sodium trisilicate-Na2Si[Si2O7], and sodium tetrasilicate-Na6Si3[Si9O27]) are reported elsewhere (Fleet and Henderson 1995a, b; Fleet 1996).
Experimental procedures High-pressure experiments were made on two bulk compositions on the Na2O-SiO2 join (33.3 mol% Na2O- Na2Si2O5; and, 20 mol% Na2O- Na2Si4O9) using the MA6y8 superpress at the University of Alberta. Starting materials were glasses prepared by melting stoichiometric mixtures of Na2CO3 and powdered SiO2 at 13008C for 1 h in covered platinum cups, and then air quenching. All highpressure experiments were run in 18 M low-T stepped graphite furnace assemblies. Temperature was monitored with a W-Re thermocouple. The sample capsule (2.5 mm OD Pt) loaded with glass powder was dried at 300–4508C for 2–3 hour before welding. The pressure assembly (without the sample capsule) was fired at 10008C in a N2-H2 gas mixture for 1 h. The observed phase relations (Table 1) are generally consistent with Kanzaki et al. (1996). The experiments were conducted in batches over a period of 14 months. The integrity of the products towards hydration was ensured by storing them in Nujol oil in sealed glass vials. In particular, only half of each capsule was sampled for preliminary phase characterization and single-crystal specimens. The remaining half was stored essentially intact, thus minimizing possible reaction with H2O. The susceptibility of the crystalline products towards hydration increased with increasing Na2O content. For grains in Nujol oil in contact with the laboratory atmosphere, the sequence of decreasing rate of hydration was Na2Si2O5-glass. e-Na2Si2O5..SiO2-rich-glass..z-Na2Si2O5.Na2Si[Si2O7].. Na6Si3[Si9O27], with the latter showing no visible effects after several years exposure. The products were characterized by transmitted-light microscopy, Gandolfi powder and single-crystal X-ray diffraction, electron microprobe analysis (EMPA), and micro-Raman spectroscopy (Table 2). e-Na2Si2O5 occurred as colourless crystals up to 0.15 mm in diameter that were markedly anisotropic with perfect {100} cleavage and weak birefringence normal to the a-axis and strong birefringence in orientations parallel to the a-axis. Sodium trisilicate occurred as colourless crystals up to 0.2 mm in diameter, with moderate birefringence. The 2V and optical sign in Table 2 are revised from Fleet and Henderson (1995b). Sodium
Table 1 High-Pressure Experiments Expt Pres- Tempe- Dura- Products1 sure rature tion (GPa) (8C) (h) Na2Si2O5 bulk composition 1889 1909 1985 1983 1916 1919 1831
9 9 8 8 8 7 4
1000 1200 1000 1100 1200 1100 900
24 12 12 12 12 12.3 24
z-NS21NS3 NS31glass e-NS21minor (NS31NS41z-NS2) e-NS21minor (NS31NS41z-NS2) glass1quench (NS31NS41z-NS2) e-NS2 e-NS2
Na2Si4O9 Bulk composition 1918 1888 1986 18272 2112
9 9 6 4 4
1300 1500 1000 900 1000
7.5 6 12 10 12
NS4 NS41glass NS4 coesite1glass coesite1glass
1 e-NS2 is epsilon sodium disilicate-e-Na2Si2O5; z-NS2 is incompletely characterized zeta sodium disilicate-z-Na2Si2O5; NS3 is sodium trisilicate-Na2Si[Si2O7]; NS4 is sodium tetrasilicateNa6Si3[Si9O27] 2 '1827 was held initially at pressure at 1500 8C and then decreased to 900 8C over 57 min
tetrasilicate occurred as colourless crystals up to 0.6 mm in longest dimension, with weak birefringence. Crystals grown in the presence of melt were doubly-terminated pseudo-hexagonal prisms, and were superficially similar to quartz, whereas those grown in the subsolidus region tended to fracture on pseudo-hexagonal {0001} forming tablet-shaped grains. z-Na2Si2O5 occurred as massive aggregates of small (,50 mm) sugary grains of low birefringence, and always in association with Na2Si[Si2O7] in experiments of bulk composition Na2Si2O5. Our limited EMPA results for it indicated a slightly more Na2O-rich composition (35.0 at% Na2O, 65.0 at% SiO2) than that assumed by Kanzaki et al. (1996). Subsolidus products tended to be sheared, but the effects of decompression were minimized for crystals grown in the presence of melt. Sample material for Raman spectroscopy was washed in acetone, dried with paper tissue then immediately placed on the micro-Raman sample stage. Each sample was examined optically and the exciting laser focussed onto crystal grains and grain aggregates. Parallel- and perpendicular-polarized spectra were collected using the experimental setup described in Henderson and Fleet (1995). Spectra were excited using an Ar1 laser operating at 500 mW and tuned to the 488 nm line. A charge coupled detector [Princeton Instruments LNyCCD (CSMA)] system was employed and individual spectra were counted for 30 minutes. Glass spectra were averaged over 1 h and all spectra were calibrated against a neon spectrum obtained from a neon discharge lamp
High-Pressure Structures e-Na2Si2O5. The structure of epsilon sodium disilicate is based on a disilicate [Si2O5] sheet formed of alternating six-membered rings of UUUUDD and DDDDUU (where U is upward- and D is downward-pointing) SiO4 tetrahedra and lying in the (100) plane (Fleet and Henderson 1995a; Fig. 1). The disilicate sheets are linked together by sodium cations (Na2 is five-fold coordinated to ˚ , and Na2 is four-fold coordinated to 2.55 A ˚ ) and 2.57 A
347 Table 2 Crystal data for highpressure sodium silicates
*Santarsiero et al. (1991)
Space group ˚) a (A b c b (8) ˚ 3) V (A Z Dm (g ? cm23) 2V (8) Optical sign
e-Na2Si2O5
Na2Si[Si2O7]
Na6Si3[Si9O27]
z-Na2Si2O5
Pbc21 5.580 9.441 8.356
C2yc 8.922 4.8490 11.567 102.64 488.29 4 3.295 ©5 negative
P21yn 10.875 9.326 19.224 90.18 1949.5 4 3.090 ©15 positive
R3m* 9.8988*
440.2 4 2.749 40–45 negative
have a gentle undulation in c-axis projection. The structure is similar to those of a- and b-Na2Si2O5 (Pant and Cruickshank 1968; Liebau 1961; Pant 1968), but differs significantly from these room-pressure structures in its relatively-small Si-Obr-Si bond angles. NaSi[Si2O7]. The sodium trisilicate structure accommodates silicon in both tetrahedral and octahedral coordination with oxygen in the ratio [6]Si:[4]Si51: 2 (Fleet and Henderson 1995b; Fig. 2). The SiO4 tetrahedra form a diorthosilicate [Si2O7] group and are linked by isolated SiO6 octahedra via shared corners into a framework of six- and four-membered rings of silicate polyhedra. The single [4]Si-Obr-[4]Si bond angle is 132.88. The sodium cations are accommodated in channel positions, within the framework, in irregular 6-fold coordination (,Na˚ ). O.52.511 A Na6Si3[Si9O27]. High-pressure sodium tetrasilicate is a second novel high-pressure structure of this project with silicon in both tetrahedral and octahedral coordination with oxygen, but in the ratio [6]Si:[4]Si51: 3. The tetrahedral silicate unit is now a nine-membered ring of SiO4 tetrahedra that is collapsed around and interconnected by isolated SiO6 octahedra (Fleet 1996; Fig. 3), forming a large tricluster. Layers of nine-membered rings are arranged in ABABAB…. stacking sequence along [010]. This is possibly the largest high-pressure silicate structure yet reported with three independent SiO6 octahedra and nine independent SiO4 tetrahedra. Another interesting feature of this structure is that although all nine SiO4 tetrahedra have similar nearestneighbour stereochemistries, the [4]Si-Obr-[4]Si bond angles vary markedly, from 130.6 to 172.18. Sodium cations are displaced to one side of irregular cavity positions. In transverse section, layers of nine-membered rings of tetrahedra alternate with layers of SiO6 octahedra and NaOn polyhedra (Fig. 3), as in the wadeite-type (e.g., Swanson and Prewitt 1984) and A2Ge4O9-type (Choisnet et al. 1973) structures. 29 Si NMR. The 29Si NMR results of Kanzaki et al. (1996) are remarkably consistent with the X-ray structures of e-Na2Si2O5, Na2Si[Si2O7], and Na6Si3[Si9O27] in respect to [6]Si:[4]Si ratio, and number and possible steric environment of [4]Si positions. They also give partial support to the [6]Si:[4]Si ratio (1: 6) deduced by Santarsiero et al. (1991) for z-Na2Si2O5.
13.0089 1103.9 uniaxial negative
Discussion Structure-composition relationships In respect to Si-O bond distances, O-Si-O bond angles, volume, and distortion parameters, the SiO4 tetrahedra in the three known high-pressure sodium silicate structures are indistinguishable from SiO4 tetrahedra in silicates and aluminosilicates crystallized at relatively low pressures (1 bar to 1 GPa; Table 3). The Si-O bond distances, bond angles, volume, and distortion parameters of the SiO6 octahedra in Na2Si[Si2O7] and Na6Si3[Si9O27] are typical of SiO6 octahedra in [6]Si-bearing silicates and silicate-phosphates that are linked by shared corners to other SiO6 octahedra andyor SiO4 and PO4 tetrahedra (Table 3; Finger and Hazen 1991). Thus, the SiO4 tetrahedra and SiO6 octahedra in the present high-pressure sodium silicate structures are neither compressed nor distorted relative to other silicate and aluminosilicate structures determined at room pressure. The NaOn polyhedra do not appear to be compressed relative to low-pressure structures either. Quantification of alkali metal polyhedra in silicates is not straightforward because of ambiguity in defining the effective ˚ bonding sphere. However, the number of bonds to 2.9 A and the range of bond distances in the high-pressure sodium silicate structures are typical of sodium in lowpressure silicates and aluminosilicates. Indeed, the framework cavities in Na2Si[Si2O7] and Na6Si3[Si9O27] are larger than required to accommodate the sodium cations. The nature of these framework cavities at the conditions of synthesis and their response to compression has a considerable bearing on the mobility of sodium in high-pressure framework structures and would seem to be desirable future studies. Densification in the present high-pressure sodium silicates is effected by changes in packing. In e-Na2Si2O5 (Dx52.749 g.cm–3), in which all silicon is [4]Si, the change in intermediate-range structure is manifest largely by decrease in Si-Obr-Si bond angles to 127.0–129.38, from 135.1–137.18 in b-Na2Si2O5 (Dx52.57 g.cm–1) and 138.9–160.08 for a-Na2Si2O5 (Dx52.50 g.cm–3) (Fleet and Henderson 1995b). This results in a gentle undulation in c-axis projection (Fig. 1) that allows the disilicate sheets to fit closer together. In fact, compared with b-
348 Table 3 Octahedral and Tetrahedral Sizes and Distortions
SiO6 Octahedra
Si1 Si2 Si3
Mean Si-O ˚) (A
Polyhedral volume ˚ 3) (A
1.789
7.63
1.774 1.790 1.778
7.44 7.63 7.49
Quadratic elongation1
Na2Si[Si2O7] 1.0007 Na6Si3[Si9O27] 1.0008 1.0010 1.0008
Bond length distortion2 (3106)
Bond angle variance
156
1
47 121 114
2 3 2
Finger and Hazen (1991) – Corner-Shared Polyhedra Minimum Maximum
1.758 1.863
7.22 7.86
1.000 1.005
0 8
SiO4 Tetrahedra
Si1 Si2
1 2
Robinson et al. (1971) Fleet (1976)
Si4 Si5 Si6 Si7 Si8 Si9 Si10 Si11 Si12
Mean Si-O ˚) (A
Polyhedral volume ˚ 3) (A
1.633 1.623
2.22 2.17
1.625
2.18
1.616 1.617 1.621 1.611 1.609 1.620 1.616 1.623 1.622
2.15 2.14 2.17 2.15 2.10 2.15 2.12 2.16 2.17
Fig. 1 Polyhedral representation of the structure of e-Na2Si2O5, a disilicate sheet structure with puckered six-membered UUUUDD and DDDDUU rings of SiO4 tetrahedra (shaded): closed circles are Na cations (after Fleet and Henderson 1995a)
Quadratic elongation1
e-Na2Si2O5 1.0039 1.0085 Na2Si[Si2O7] 1.0088 Na6Si3[Si9O27] 1.0066 1.0108 1.0062 1.0061 1.0140 1.0133 1.0103 1.0074 1.0062
Bond length distortion2 (3106)
Bond angle variance
Nonbridging oxygens
360 356
17 39
1 1
260
35
3
401 306 340 372 360 199 307 204 396
25 44 26 23 55 51 38 30 25
2 2 2 2 2 2 2 2 2
Na2Si2O5, the reduction in unit-cell volume in eNa2Si2O5 (93.4%) is accounted for exclusively by shortening of the normal distance between the disilicate ˚ in e-Na2Si2O5, and (axsinb)y sheets [a55.580 A ˚ 255.975 A in b-Na2Si2O5]. The individual six-membered rings of SiO4 tetrahedra are in boat configuration, forming half of each undulation, as in d-Na2Si2O5 (cf. De Jong et al. 1994). In the structures of Na2Si[Si2O7] and Na6Si3[Si9O27], densification is effected largely by placing a proportion of the silicon atoms in octahedral coordination with oxygen; the ratio [6]Si:[4]Si is 1: 2 in the densest phase Na2Si[Si2O7] (Dx53.295 g.cm–3) and 1: 3 in Na6Si3[Si9O27] (Dx53.090 g.cm–3). Undistorted and uncompressed SiO4 tetrahedra and SiO6 octahedra are connected via shared corners to form silicate frameworks (Fleet and Henderson 1995b; Fleet 1996; Figs. 2, 3). Further decrease in unit-cell volume results from crimping of Si-O-Si angles, but this is largely to accommodate sodium within the large framework cavities. As seems to be always the case where sodium is an extra-framework cation (e.g., Merlino 1984), the sodium cations are displaced to one side of the channel cavities in Na2Si[Si2O7]
349 Table 4 Comparative Data for Isotropic Temperature Factors (B) B(Obr)yB(Si) B(Onbr)yB(Si) B(Na) Reference ˚ 2) (A e-Na2Si2O5 NaSi[Si2O7] Na6Si3[Si9O27] Reedmergnerite Coesite 1 2 3 4 5
1.74 1.87 2.28 1.69 1.91
1.47 1.54 1.98
1.49 1.34 1.61 1.54
1 2 3 4 5
Fleet and Henderson (1995a) Fleet and Henderson (1995b) Fleet (1996) Fleet (1992) Smyth et al. (1987)
and Na6Si3[Si9O27]; in the latter structure, the sodium cations are displaced closer to one proximal (010) layer of SiO4 tetrahedral rings than the other. As in albite structures (Fleet 1992) and framework structures in general, the shape of the thermal ellipsoid of the sodium cations in these two high-pressure structures is determined by the spatial distribution of Na-O bonds, and the sodium cations, the most weakly bonded atoms in the two structures, have by far the greatest thermal vibration. Fleet and Henderson (1995b) suggested that densification of NaSi[Si2O7] is accomplished also by decrease in size of thermal ellipsoids. However, this observation is not supported by a survey of isotropic temperature factors (B, Beq) for silicate structures refined with roompressure reflection intensities. In general, thermal parameters vary markedly with refinement procedures (sinuy l, scattering factors, refinement of atomic charges, etc.), crystal structure and composition, crystal quality and, particularly, with orderydisorder (cf. Fleet 1992). The average isotropic temperature factors for sodium, [4] Si, and oxygen are similar, respectively, in all three high-pressure sodium silicate structures (e-Na2Si2O5, Na2Si[Si2O7] and Na6Si3[Si9O27]; Table 4) and comparable to their values in reedmergnerite (NaBSi3O8), which has the ideal low albite structure (Fleet 1992). B(O) values have been normalized by B(Si) in Table 4 to minimize differences from one refinement to another, although this is hardly necessary in the present case. Also, where appropriate, we have distinguished thermal parameters for bridging and nonbridging oxygens. Bridging oxygens in silicate and aluminosilicate structures invariably have exaggerated thermal motion in a direction transverse to the T-Obr-T bonding plane (e.g., Burnham et al. 1971; Downs et al. 1990) and this results in a significant increase in B (or Beq) for these atoms. In situ measurements have shown that applied pressure does have a small effect on the isotropic temperature factor (Finger and King 1978) but the results to date are not systematic (e.g., Ross et al. 1990). It is noteworthy that the isotropic temperature factors for the sodium cations are not significantly reduced in the three highpressure sodium silicate structures relaxed to 1 bar pressure, even though the NaOn polyhedra would have been
significantly compressed at the conditions of synthesis of the crystals (cf. Hazen and Finger 1982). The Na2Si[Si2O7] and Na6Si3[Si9O27] structures belong to the group of structures characterized by Finger and Hazen (1991) as having both tetrahedral and octahedral Si and stable at about 10 to 20 GPa. These mixed [4] Si and [6]Si structures are predominantly framework silicates, with the framework formed from the linking of SiO4 tetrahedral groups by isolated SiO6 octahedra via shared corners. The alkali and alkaline earth silicates with frameworks of corner-linked [4]Si and [6]Si are a distinct class of high-pressure silicate structures (Hazen et al. 1995). The tetrahedral groups now recognized range through isolated tetrahedron (CaSi[SiO4]O-titanite structure), diorthosilicate dimer (Na2Si[Si2O7]-sodium trisilicate), rings (three-membered: K2Si[Si3O9]wadeite structure, BaSi[Si3O9]-benitoite structure, BaSi[Si3O9]- barium germanate structure; and nine-membered: Na6Si3[Si9O27]-sodium tetrasilicate), and a sheet (Na1.8Ca1.1Si[Si5O14], Gasparik et al. 1995). In the Na2O-SiO2 system, possible corner-shared framework structures are limited by the bond valence requirements of the nonbridging oxygens. The single optimum coordination environment for a nonbridging oxygen is [4]Si1[6]Si12xNa. Thus, the optimum proportion of Na:[6]Si is 2: 1, and formulae for compositions mNa2O.nSiO2 belong to the homologous series Na2mSim[Sin–mO2n1m], with m,n. The two known structures (Na2Si[Si2O7] and Na6Si3[Si9O27]) may be the only representatives of this structure-composition series. Because Na2Si[Si2O7] and stishovite appear in the SiO2-rich portion of the system at 9–10 GPa, the possibilities for undiscovered structures appear quite limited. The formula Na2Si2O5 does not belong in the series because it corresponds to extra-framework oxygen (i.e., Na2Si[SiO4]O). The factors that are important in low-pressure tetrahedral (TO4) framework structures (composition and stoichiometry, T-O bond distance and bond strength, size of large extra-framework cation) are also important in mixed [4]Si and [6]Si framework structures. In particular, accommodation of the large extra-framework cation is of prime importance in determining details of structural topology. The very large monovalent cations (K, Rb, Cs, and Tl, with effective ionic radii of [8]K- 1.51, [8]Rb˚ , respectively; Shan1.61, [8]Cs- 1.74, and [8]Tl- 1.59 A non 1976) prefer cavity positions offering 8 to 12 bonds ˚ ) is too with oxygen. The sodium cation ([7]Na- 1.12 A small for large framework cavities. Usually, the introduction of sodium into an aluminosilicate framework results in distortion of the framework to yield an oval shaped cavity with sodium displaced to one end and bonded to fewer oxygens. There are three known structure types with the formula A2M[T3O9], where A is a large monovalent cation and M and T are SiyGe: namely, wadeite-type K2Si[Si3O9] (Swanson and Prewitt 1984; Fig. 4), A2Ge4O9-type for germanates with A5Na, K, Rb, Tl, Ag (Choisnet et al 1973; Fig. 5), and, Na6Si3[Si9O27] (Fleet 1996; Fig. 3).
350
The wadeite-type K2Si[Si3O9] and A2Ge4O9-type structures are both built of [T3O9] three-membered tetrahedral ring units cross linked by octahedra. In the transverse (c-axis) direction, the octahedra share faces with interspersed unoccupied polyhedra, forming columns. The principal topological difference between these two structures is the nature of the unoccupied polyhedron. In the wadeite-type structure, alternate octahedra along any column are rotated 1808 and the unoccupied polyhedron is a trigonal prism, as in the Na6Si3[Si9O27] structure. However, in the A2Ge4O9-type structure, the unoccupied polyhedron is an octahedron. The shared (triangular) octahedral faces, cross-linked by the [Ge3O9] units, are now contra-rotated, causing the [Ge3O9] rings to be twisted. In the K2Si[Si3O9] wadeite-type structure, potassium is accommodated in a large cavity position with 31313 coordination with oxygen, similar to its environment in tetrahedral aluminosilicate framework structures. Wadeite-structure germanates would have a larger cavity position, because of the longer Ge-O bond distance, which is evidently too large for even the biggest alkali cations. In the A2Ge4O9-type structure, the twist deformation of the [Ge3O9] three-membered ring of GeO4 tetrahedra reduces the size of the extra-framework cavity and adds a degree of freedom to the distortion of the mixed [4]Ge and [6]Ge framework, permitting a wide range in size of large monovalent cations to be accommodated. For alkali silicate compositions, the [Si3O9] tetrahedral ring is too rigid to permit the amount of twist distortion required by the A2Ge4O9-type structure, because of the short bond distance and high bond strength of the [4]Si-O bond. Hence, sodium tetrasilicate adopts a framework configuration with pliable five-membered [4] Si-[4]Si-[4]Si-[4]Si-[6]Si rings. In both Na2Si[Si2O7] and Na6Si3[Si9O27], the sodium cation is displaced to one side of the cavity position, and there appears to be a large fraction of unoccupied cavity space (Figs. 2, 3). Moreover, the cavity positions extend into channels, parallel to b-axis and c-axis in Na2Si[Si2O7] and Na6Si3[Si9O27], respectively. These stereochemical features point to a high mobility for the sodium cations in the relaxed room-pressure structures. Thus, mixed [4]Si and [6]Si framework structures of the smaller alkali metals may be fast ion conductors and have application to dry storage cells. However, the mobility of the alkali cations under the conditions of synthesis, and in mantle silicates in general, is an open question, because of the high compressibilty of large cation positions (e.g., Hazen and Finger 1982). The new sodium tetrasilicate structure (Fleet 1996) is timely confirmation of Finger and Hazen’s (1991) prediction of a complex crystal chemistry for high-pressure framework silicates. The structures of Na2Si[Si2O7] and Na6Si3[Si9O27] are striking confirmation that even transition zone pressures do not dominate over the stereochemical requirements of the large cations in determining the structures of the alkali and alkaline-earth aluminosilicates. Although the essential mineralogy of the
Fig. 2 Structure of Na2Si[Si2O7], a mixed [4]Si and [6]Si framework with [6] Si:[4]Si51: 2: isolated SiO6 octahedra are shaded; diorthosilicate units are unshaded; open circles are Na cations (after Fleet and Henderson 1995b)
Fig. 3 Structure of Na6Si3[Si9O27], a mixed [4]Si and [6]Si framework with [6]Si:[4]Si51: 3: a single nine-membered ring of nonequivalent SiO4 tetrahedra is high-lighted; three non-equivalent isolated SiO6 octahedra are shaded; open circles are Na cations (after Fleet 1996)
Fig. 4 Structure of wadeite-type K2Si[Si3O9]: three-membered rings of SiO4 tetrahedra are unshaded; isolated SiO6 octahedra are shaded; open circles are K cations (after Swanson and Prewitt 1983)
Earth’s mantle (e.g, Ito and Takahashi 1987) is no doubt well established, the recent studies on mixed [4]Si and [6] Si framework and closest-packed layer structures do open up the possibility of a complex mineralogy for minor elements in the transition zone. Early work on highpressure phases in the MgO-SiO2 system had conditioned earth scientists to anticipate only minerals of sim-
351
does occur metastably in the Na2O-GeO2 system (e.g., Sakka et al 1977; work in progress), the stable GeO2-rich phase at room pressure is sodium enneagermanate (Na4Ge9O20; with [6]Ge:[4]Ge54: 5 and a structural formula of Na4Ge4[Ge4O12][GeO4]O4). In this structure, four-fold clusters of GeO6 octahedra are linked by isolated GeO4 tetrahedra into columns that are interconnected by spiral chains of GeO4 tetrahedra (Fig. 6; Ingri and Lundgren 1963; Fleet 1990).
Fig. 5 Structure of metastable A2Ge4O9-type Na2Ge[Ge3O9]: twisted three-membered rings of GeO4 tetrahedra are unshaded; isolated SiO6 octahedra are shaded; open circles are Na cations. Note that alternate columns of SiO6 octahedra are contra-rotated (cf. wadeite-type K2Si[Si3O9], Fig. 4, and Na6Si3[Si9O27], Fig. 3) (work in progress)
Fig. 6 Structure of sodium enneagermanate (Na4Ge4[Ge4O12] [GeO4]O4): spiral chains of GeO4 tetrahedra are light shaded; four-membered clusters of GeO6 octahedra are dark shaded, and connecting isolated GeO4 tetrahedra are unshaded; open circles are Na cations (after Ingri and Lundgren 1963; Fleet 1990)
ple crystal chemistry in the Earth’s deep interior. The limiting factor, of course, is not whether exotic silicate structures will be stable at mantle pressures and temperatures but the extent to which the minor elements are accommodated in solid solution in essential minerals. Numerous other mixed [4]Si and [6]Si alkali silicate structures with a higher proportion of [6]Si are possible at higher pressures than presently investigated. However, these must contain oxygen not associated with tetrahedral units and, therefore, would not belong to the class of corner-linked framework structures discussed above. As was the case for the framework structures, the simple and restricted chemistry of alkali silicate systems limits the nearest-neighbour coordination of the oxygens and, therefore, possible structural topologies. Equalization of bond valence for the extra-tetrahedral oxygens bonded to [6] Si requires either three [6]Si neighbours or less favourable combinations like 2x[6]Si14xNa, etc. Thus, SiO6 octahedra would tend to occur in clusters. Once again, analogue germanate structures are helpful in suggesting the type of structures that may occur. Although sodium tetragermanate with the A2Ge4O9-type structure
Raman spectroscopy The Raman spectrum of epsilon sodium disilicate (Fig. 7a) is dominated by a low-frequency band at 642 cm–1 and a high-frequency band at 1076 cm–1. This spectrum is essentially the same as that collected by Santarsiero et al. (1991) and is similar to those of b-Na2Si2O5 (Brawer and White 1975) and other alkali disilicates. Following the band assignments of Brawer and White (1975), Furukawa et al. (1981), Matson et al. (1983), and Xue et al. (1991), and the disilicate layer structure of Fleet and Henderson (1995a), the high-frequency band is assigned to the symmetric Si-Onbr stretching vibration of Q3 SiO4 tetrahedra, and the low-frequency band to symmetric bending of Si-Obr-Si linkages. The two nonequivalent SiO4 tetrahedra and three non-equivalent SiObr-Si bond angles are apparently not resolved. This was not unexpected because the bridging-oxygen bond angles range over only 2.28 (cf. 2.08 in b-Na2Si2O5), and the two SiO4 tetrahedra are very similar to each other (Si2˚ , Si2-Onbr51.571 A ˚ , etc.; Fleet and HenOnbr51.580 A derson 1995a). The low-frequency band for the symmetric bending vibration is at much higher frequency than in the spectrum of b-Na2Si2O5 (525 cm–1 in b-Na2Si2O5 versus 642 cm–1 in e-Na2Si2O5), and this correlates with the smaller Si-Obr-Si bond angle in the high-pressure structure (135.1–137.18 versus 127.0–129.38, respectively). The dependence of the position of the prominent low-frequency band(s) in the Raman spectra of crystalline tectosilicates and alkali silicates and germanates on bridging-oxygen bond angle has been known for some time (e.g., Sharma et al. 1981; Furukawa et al. 1981; Henderson et al. 1985; McMillan 1984; Xue et al. 1991). Sharma et al. 1981, Henderson et al. (1985), and Santarsiero et al. (1991), among others, associated the decrease in bond angle inferred from the shift in band position to higher frequency with a reduction in size of SiO4 tetrahedral ring structures. However, Henderson and Fleet (1991) recognized that this shift in the Raman spectra of binary and ternary composition glasses represented a progressive change in intermediate-range structure associated with progressive closing of the bridging-oxygen bond angle, and did not necessarily imply a decrease in average ring size. The micro-Raman spectra of Na2Si[Si2O7] and Na6Si3[Si9O27] (Fig. 7b,c) are believed to be the first Raman spectra of mixed [4]Si and [6]Si silicates of known
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Fig. 7a–e Parallel- and perpendicular-polarized micro-Raman spectra of: a) e-Na2Si2O5; b) Na2Si[Si2O7]; c) Na6Si3[Si9O27]; d) z-Na2Si2O5; e) high-pressure (4 GPa) sodium silicate glass ('1827; Table 1)
structure in which the contributions of the non-silicate components can be discounted. Thus, they provide a unique opportunity to analyze the contributions of [4]Si and [6]Si to Raman spectra, and thereby assess the utility of Raman spectroscopy to the study of pressure-induced coordination change of silicon in silicate melts and glasses. Raman spectra may be reconstructed with varying degrees of quantification by theoretical calculation (e.g., Furukawa et al. 1981; Dowty 1987a; Chakraborty et al. 1987). Clearly, detailed study of the vibrational motion of complex solids requires oriented high-resolution single-crystal Raman spectra, as well as complementary infra-red spectra. Provisional band assignments have been made by computer simulation of the present spectra using VIBRAT (Dowty 1987a, b; personal com-
353
munication 1996) and comparison with previous work on [4] Si and [6]Si silicates. Semi-quantitative simulations of the Raman spectra of e-Na2Si2O5, Na2Si[Si2O7] and Na6Si3[Si9O27] were obtained using force constants and bond-polarizability factors for [4]Si-Obr, [4]Si-Onbr, and Na-O within the range of literature values (e.g., Lazarev 1972; Dowty 1987a, b). Force constants of about ˚ –1; (cf. Chakraborty et al. 1987) and bond3.0 mdyn.A polarizability factors of about 0.6 for [6]Si-Onbr were fitted by visual agreement with the observed spectra. The calculations embodied Si…Si and O…O interactions, but further complexity was not warranted by the quality of the present spectra; e.g., there were too few resolved bands for meaningful least-squares refinement of the fitting parameters. Atom motions were not calculated. The contributions of individual [4]Si, [6]Si, Obr, and Onbr atoms to spectral features were estimated qualitatively by suppressing their contribution to the crystal model through sequential reduction in bond-polarizability factors and force constants. Our Raman spectra of Na2Si[Si2O7] were all from single-crystal grains and exhibited some grain-to-grain variation in relative intensity of the characteristic bands (particularly of the high-frequency bands) due to variation in crystal orientation. All of the spectra had a strong band at 359 cm–1, and other important bands at 456, 527, 613, 701, 900, 951, and 1051 cm-1 (Fig. 7b). The SiO4 tetrahedron in the structure of Na2Si[Si2O7] has one bridging oxygen and three nonbridging oxygens. The single [4]Si-Obr-[4]Si bond angle is 132.88, and there are three non-equivalent [4]Si-Onbr-[6]Si bond angles that range from 123.4 to 158.48 (Fleet and Henderson 1995b). The triplet high-frequency bands are assigned to the symmetric Si-Onbr stretching vibrations of Q1 SiO4 tetrahedra. The band at 701 cm–1 is attributed to the symmetric Si-O stretching vibration at the bridging oxygen, that is often designated nsSiOSi (e.g., Lazarev 1972), but this mode also embodies a significant bending component (Dowty 1987b). The weaker band at 613 cm–1 corresponds to a stretching mode of the SiO6 octahedron (cf., Chakraborty et al. 1987). The low-frequency bands are attributable to modes involving symmetric bending of predominantly [4]Si-Obr-[4]Si and [4]Si-Onbr-[6]Si linkages. All of these bending modes involve coupled displacements and, therefore, structural assignments are complex. However, the displacements of the bridging oxygen (O4) appear to dominate the 359 cm–1 mode. The contributions from the more strongly-bonded diorthosilicate unit are evident in the Raman spectrum of Na2Si[Si2O7]. However, the spectrum is distinct from those of diorthosilicate glasses (e.g., McMillan 1984), crystalline diorthosilicates (e.g., Lazarev 1972) and the diorthosilicate unit [Si2O7] (Dowty 1987b), which have prominent bands at ©600–700 and ©900 cm–1. Clearly, when the nonbridging oxygens are bonded to a cation like [6]Si that forms strong bonds with oxygen, the characteristic symmetric Si-Onbr stretching bands are shifted to higher frequency and their relative intensity reduced. Also, displacements of both bridging and nonbridging
oxygens couple to yield relatively intense low-frequency bands. The micro-Raman spectrum of Na6Si3[Si9O27] is complex (Fig. 7c). There are two strong bands in the low-frequency region at 552 and 627 cm–1, other prominent bands of moderate strength at 360, 507, 535, 574, 611, 738, 1065 and 1097 cm–1, and numerous weak bands. The framework structure of Na6Si3[Si9O27] has 180 atoms per unit cell; there are nine non-equivalent [4] Si-Obr-[4]Si bond angles that range from 130.6 to 172.18, and 18 non-equivalent [4]Si-Onbr-[6]Si bond angles that range from 126.0 to 160.48 (Fleet 1996). Therefore, a complex Raman spectrum was expected. The high-frequency bands (at 1065 and 1097 cm–1) are assigned to the symmetric Si-Onbr stretching vibration of the Q2 SiO4 tetrahedra: the shift to slightly higher frequency and reduction in relative intensity being attributed again to nonbridging oxygens bonded to [6]Si. The weaker band at 738 cm–1 is attributed to the symmetric Si-O stretching vibrations at the bridging oxygens (nsSiOSi). The multiplet low-frequency bands are attributed to symmetric bending at the bridging and nonbridging oxygens. Because the crystal structure contains a very wide variation in [4]Si-Obr-[4]Si and [4]Si-Onbr-[6]Si bond angles, the prominent single band that characterizes the Raman spectra of silicates of simple structure (e.g., e-Na2Si2O5; Fig. 7a) is now replaced by fine structure. We had anticipated a correlation between band position and [4]Si-Obr[4] Si bond angle, but this was not evident in the computer simulations. However, displacements of bridging oxygens are the dominant contributions to bands at and above 574 cm–1, whereas displacements of non-bridging oxygens dominate for bands of lower frequency. In summary, the vibrational spectra of mixed [4]Si and [6] Si framework silicates are complex. The characteristic features of the tetrahedral silicate units are still recognizable, but possibly only with a priori knowledge of the crystal structure. The displacements of the nonbridging oxygens, that are now bonded to [6]Si, couple strongly with those of the [4]Si-Obr-[4]Si linkages to yield strong and complex low-frequency bands. Correspondingly, the high-frequency bands are shifted to higher frequency and reduced in intensity. The overall appearance of individual Raman spectra suggests a silicate of higher degree of polymerization than the corresponding tetrahedral silicate unit, and clearly reflects a strongly-bonded framework of mixed SiO4 tetrahedra and SiO6 octahedra. Direct, characteristic features of SiO6 octahedral units (e.g., nsSiOnbr) are weak and likely to be obscured by overlapping bands of bending modes. It is premature to interpret the Raman spectrum of z-Na2Si2O5 (Fig. 7d) in the absence of more complete information on its crystal structure. However, this spectrum, which is similar to that collected by Santarsiero et al. (1991), is consistent with a relatively simple structure dominated by Q1,Q2 SiO4 units and low [6]Si:[4]Si ratio, in general agreement with Santarsiero et al. (1991) and Kanzaki et al. (1996).
354
High-pressure glasses Raman spectroscopy does not appear to be a useful direct structural probe for small amounts of [6]Si in silicate glasses and melts. However, small amounts of [6]Si may be detected indirectly through concomitant change in the tetrahedral silicate unit(s). The micro-Raman spectra for glass from an experiment with Na2Si4O9 bulk composition at 4 GPa (Fig. 7e; Table 1) corresponds closely to the 5 GPa spectrum of Na2Si2O5 composition glass of Xue et al. (1991), with strong bands at 573 and 1086 cm–1, and weak bands at 775 and 940 cm–1. The strong bands correspond, respectively, to bending vibrations of [4]Si-Obr[4] Si linkages and stretching vibrations of Si-Onbr in Q3 tetrahedra (e.g., McMillan and Wolf 1995), as in the spectrum of e-Na2Si2O5 (Fig. 7a). However, the mean [4] Si-Obr-[4]Si bond angle is evidently relaxed in the glass, and estimated to be about 1338. The weak band at 940 cm–1 was tentatively assigned by Xue et al. (1991) to Q2 species produced by disproportionation of Q3: i.e., 2Q35Q21Q4. For Na2Si2O5composition glass, the high-pressure Raman spectra of Xue et al. (1991) were very similar to their 1 bar spectrum, although the ©940 cm–1 band does strengthen somewhat and the half width of the high-frequency band correspondingly increases with increase in pressure. Accordingly, Xue et al. (1991) interpreted these changes as pressure-induced disproportionation of Q3 species. They noted that the evidence for contributions of [6]Si to their Raman spectra for sodium silicate glasses was ambiguous. Change in the ©940 cm–1 band of quenched highpressure Na2Si2O5 composition glass was also reported by Farber and Williams (1996). The weak Raman band at 750–800 cm–1, that might be characteristic of symmetric Si-O stretching motion of octahedral SiO6 groups (cf. Hemley et al. 1986), also corresponds to bands in the 1 bar spectra of Xue et al. (1991). We suggest that none of the features in the Raman spectra of the quenched high-pressure sodium silicate glasses of Xue et al. (1991) and the present study is directly attributable to [6] Si. In contrast to the 29Si MAS NMR results of Xue et al. (1991), recent Si K-edge XANES spectra of highpressure sodium silicate glasses reveal very little [6]Si (Li et al. 1996). Six-fold coordinated silicon was marginally above background (#1.0 at %) in Na2Si2O5 and Na2Si4O9 composition glasses to 8 GPa, and present at only 3 at % in Na2Si4O9 composition glass 12 GPa. Farber and Williams (1996) have recently reported profound change in in situ Raman spectra of high-pressure Na2Si2O5 composition glasses and melts, which show progressive collapse of the high-frequency band and development of a band at ©750 cm–1. These features are certainly more consistent with the inferred presence of SiO5 or SiO6 polyhedra. However, a strong low-frequency band consistent with symmetric bending of coupled [4]Si-Obr-[4]Si and [4]Si-Onbr-[6]Si linkages was not present. Independent confirmation of the inferred change in glass and melt structure would seem to be desirable.
Acknowledgements We thank two unnamed reviewers for constructive comments, Y. Thibault and R.W. Luth for running the high-pressure experiments, Y. Pan for EMPA, the Ontario Laser and Lightwave Research Centre of the University of Toronto for access to and assistance with the Raman facilities and the Natural Sciences and Engineering Research Council of Canada for financial support.
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