II. EXPERIMENTAL

Twenty-seven scapolite samples between Me _6–93 from various localities were used in this study (Table I). Their descriptions and compositions were given by Shaw (Reference Shaw1960a, Reference Shaw1960b), Evans et al. (Reference Evans, Shaw and Haughton1969), and Lin and Burley (Reference Lin and Burley1973a, Reference Lin and Burley1973b, Reference Lin and Burley1973c) for some samples (Table I). The samples were analyzed using a JEOL JXA-8200 electron microprobe. The JEOL operating program on a Solaris platform was used for ZAF correction and reducing the data. The wavelength-dispersive operating conditions were 15 kV accelerating voltage, 20 nA beam current, and a beam diameter of 5 μm, and used various standards. The crystals are homogeneous based on optical observations and microprobe analyses of eight spots for each sample (≈0.2 mm in diameter). The 27 chemical compositions are given (Table II).

Crystals of scapolite (≈0.2 mm in diameter) were handpicked under a binocular microscope, and finely crushed in an agate mortar and pestle for synchrotron high-resolution powder X-ray diffraction (HRPXRD) experiments that were performed at beamline 11-BM, Advanced Photon Source, Argonne National Laboratory. Each sample was loaded into a Kapton capillary (0.8 mm internal diameter) and rotated during the experiment at a rate of 90 rotations per second. These data were collected to a maximum 2θ of about 40° with a step size of 0.0005° and a step time of 0.1 s per step. A silicon and alumina NIST standard (ratio of ⅓ Si: ⅔ Al₂O₃ by weight) was used to calibrate the 12-analyzer crystal detector system response, zero offset, and determine the wavelength used in the experiment (see Table I). Additional details of the experimental set-up and data analyses were given (Antao et al., Reference Antao, Hassan, Wang, Lee and Toby2008; Lee et al., Reference Lee, Shu, Ramanathan, Preissner, Wang, Beno, Von Dreele, Ribaud, Kurtz, Antao, Jiao and Toby2008; Wang et al., Reference Wang, Toby, Lee, Ribaud, Antao, Kurtz, Ramanathan, Von Dreele and Beno2008).

The unit-cell parameters for the 27 samples were obtained using the Rietveld method (Rietveld, Reference Rietveld1969), as implemented in the GSAS program (Larson and Von Dreele, Reference Larson and Von Dreele2000), and using the EXPGUI interface (Toby, Reference Toby2001). Structure refinements were conducted by varying parameters in the following sequence: scale factor, background (shifted Chebyschev), cell, zero shift, profile (type-3), atom positions, and isotropic displacement parameters. The chemical composition was used to fix the site occupancies and they were not refined. Finally, all of the other variables were refined simultaneously until convergence was achieved. Data for unit-cell and compositional parameters are reported, including the Rietveld R (F ²) values (Tables I and II). With these data, equations and correlations factors (R ²) for structural variations can easily be obtained, hence they are not reported. Structural parameters obtained by HRPXRD for a few scapolite samples were reported (Antao and Hassan, Reference Antao and Hassan2008a, Reference Antao and Hassan2008b, Reference Antao and Hassan2011a, Reference Antao and Hassan2011b; Antao et al., Reference Antao, Hassan, Wang, Lee and Toby2008). As an example, a typical Rietveld fit of a trace is shown in Figure 1 and the following Rietveld statistics were obtained [R (F ²) = 0.0688, wR _p (fitted) = 0.0710, χ ² = 2.338, N _obs = 2959, 2θ range = 2.5–40°, and λ = 0.402 43(2)].

Figure 1. HRPXRD trace for Lake Clear scapolite, Me _37.5, at 25 °C, together with the calculated (continuous line) and observed (crosses) profiles. The difference curve (I _obs–I _calc) is shown at the bottom. The short vertical lines indicate allowed reflection positions. The intensities for the trace and difference curve that are above 10° and 20° 2θ are scaled by factors of ×4 and ×30, respectively. The (021) reflection is indicated.

III. RESULTS AND DISCUSSION

The chemical analyses for 27 samples are plotted (Figure 2). The dashed lines indicate the ideal stoichiometry for scapolite, which is not achieved, and is often referred to as scapolite chemical anomaly. The Me% increases linearly with Al apfu [Figure 2(a)]. Figure 2(b) shows the variation of scapolite analyses as two series that meet at Me ₇₅. The thick solid lines are based on the ideal chemical compositions for Me ₀, Me ₇₅, and Me ₁₀₀, and the measured data points are not fitted [Figure 2(b)]. This variation is the same as that reported by Hassan and Buseck (Reference Hassan and Buseck1988) and shown as Fig. 188 in Deer et al. (Reference Deer, Howie and Zussman1992), based on chemical analyses available at that time.

Figure 2. Chemical composition across the scapolite series. (a) The Me% increases linearly with Al apfu. The least-squares fitted solid line does not pass through the points (Me ₀ and Al₃) and (Me ₁₀₀ and Al₆) because of scapolite chemical anomaly. (b) Composition shown as two series that meet at (Me ₇₅ and Al₅) and represented by two thick solid lines that correspond to the ideal theoretical compositions of Me ₀, Me ₇₅, and Me ₁₀₀. The data points are not fitted in (b). The dashed lines in (a) and (b) represent ideal stoichiometric compositions for scapolite solid solutions that do not occur. Data from the literature are included for comparison (insert).

Included for comparison in Figure 2 are data from Sokolova and Hawthorne (Reference Sokolova and Hawthorne2008), Sokolova et al. (Reference Sokolova, Kabalov, Sherriff, Teertstra, Jenkins, Kunath-Fandrei, Goetz and Jäger1996), and Sherriff et al. (Reference Sherriff, Sokolova, Kabalov, Teertstra, Kunath-Fandrei, Goetz and Jäger1998, Reference Sherriff, Sokolova, Kabalov, Jenkins, Kunath-Fandrei, Goetz, Jäger and Schneider2000). The chemical analyses from these studies and the data from this study are similar to each other. The solid straight line passes through the point (Me ₀, 3 Al), but does not pass through the (Me ₁₀₀, 6 Al) point [Figure 2(a)]. Compositions above Me ₉₃ or below Me ₆ do not occur naturally; so the synthetic end-members, Me ₀ and Me ₁₀₀, are shown.

Based on the chemical analyses, there is only one discontinuity at Me ₇₅ [or five Al apfu; Figure 2(b)]; no other discontinuities occur at any other points such as: Me _20–25 and Me _60–67, or Me ₁₈ and Me ₉₀, as were reported in the literature (see Introduction). Can the Me ₇₅ discontinuity be observed in the unit-cell parameters?

The unit-cell parameters for the 27 scapolite samples may be plotted against Me%, Al apfu, or V. These plots show similar features, irrespective of the variable chosen (Figure 3), three plots are discussed in detail. The a and c unit-cell parameters show discontinuities at 1120 Å³ [Figures 3(a) and 3(d)], which corresponds to Me ₇₅ [=five Al apfu; Figures 3(c) and 3(f)]. A maximum in the c value occurs at about 1106.5 Å³, near Me _37.5 [Figure 3(d)] [=four Al apfu; Figure 3(f)]. The data points were fitted by least-squares and are shown as solid quadratic curves or straight lines. It is possible to fit the c data points between Me _0–75 with two straight lines that meet at about Me _37.5, but a quadratic curve was chosen for the fit over the Me _0–75 series [Figure 3(d)].

Figure 3. Variation of the unit-cell parameters across the scapolite series. (a) a vs. V, (b) a vs. Me%, (c) a vs. Al apfu, (d) c vs. V, (e) c vs. Me%, (f) c vs. Al apfu, (g) c/a vs. V, (h) c/a vs. Me%, (i) c/a vs. Al apfu, (j) V vs. Me%, (k) V vs. Al apfu, and (l) V vs. Al apfu. A dashed vertical line is drawn at 1120 Å³, Me ₇₅, and five Al apfu where a discontinuity occurs. A maximum c value occurs at Me _37.5, and four Al apfu (d, e, and f). A continuous non-linear variation is shown for V vs. Me% or Al apfu (j and k), which rules out any phase transition in the scapolite series.

The unit-cell volume, V, varies smoothly with Me% [Figure 3(j)], and with Al apfu [Figure 3(k)]; in both of these cases no discontinuity occurs at Me ₇₅. However, it is possible to fit the same V data to show a discontinuity at five Al apfu [Figure 3(l)]. For a first-order transition, it is important for V to show a significant change, instead of changes in the a or c unit-cell parameters. Such a transition would also require additional confirmation, such as a change in space group at that particular point. Since the V does not show a discontinuity between Me _0–100 [Figures 3(j) and 3(k)], no first-order transition occurs in the scapolite series, but a second-order transition at Me ₇₅ cannot be ruled out. Therefore, point Me ₇₅ is considered as a discontinuity instead of a transition.

There is no 1:1 correspondence between V, Me%, and Al apfu because the dashed vertical lines should occur at the same position on the horizontal x-axis (Figure 3). For example, the dashed vertical line at Me ₇₅ occurs to the right of similar lines for V or Al apfu [Figures 3(a)–3(c)]. The V and Al apfu have a closer 1:1 correspondence than Me% [Figures 3(a) and 3(c)]. One expects the Me ₇₅ vertical line to coincide with the dashed vertical line at five Al apfu based on the Me ₇₅ formula, NaCa₃[Al₅Si₇O₂₄](CO₃); which is not the case [Figures 3(b) and 3(c)].

The net change in the c value between Me _0–100 is small; about 7.556–7.588 Å, a difference of 0.032 Å, which corresponds to 0.42%. The net change in the a parameter is large; about 12.033–12.232 Å, a difference of 0.199 Å, which corresponds to 1.65%, and is about six times larger than the change in c value. Large changes are measured more accurately than small changes. The present data and those of Sokolova and Hawthorne (Reference Sokolova and Hawthorne2008) agree quite well for the a parameter [Figure 3(a)], but they disagree for the c parameter near the end members [Figure 3(d)]. The largest differences between these datasets are seen between Me _75–100, where two samples (25 and 27 in Table I) were used in many studies. The data shown as open circles were obtained by Rietveld refinements and are closer to the values from this study than to those obtained from the single-crystal method [Figure 3(d)]. Unit-cell parameters obtained by the Rietveld method, especially with HRPXRD data, are typically more accurate than those obtained by the single-crystal method.

Unit-cell parameters are generally measured more accurately than chemical compositions, so when a, c, and c/a are plotted against V from different studies, they are closer to each other [Figures 3(a), 3(d) and 3(g)], but they are more spread out when they are plotted against compositions such as Me% or Al apfu (Figure 3).

Based on the c unit-cell parameter, a number of studies divided the scapolite series into three subseries with transitions at Me _20–25 and Me _60–67, but not at Me ₇₅. These data are included for comparison in the figures shown for this study, and all of the data appear to support the discontinuity at Me ₇₅. Between Me _75–100, the cell parameters are slightly different among various studies, but they fall on similar parallel trend lines [Figures 3(a) and 3(d)]. The present data do not support a transition at Me _60–67.

The data from Sokolova and Hawthorne (Reference Sokolova and Hawthorne2008) occur as a cluster of six points between Me _0–21, which may indicate a separate trend and a transition at Me _20–25, as they have reported (Figure 3). These six data points occur on a nearly vertical straight line, indicating that they are independent of V [Figures 3(a) and 3(d)], but they do vary with Me% (and Al apfu), which is unusual [Figure 3(j)]; the V for these six points is constant between Me _0–21. Hawthorne and Sokolova (Reference Hawthorne and Sokolova2008) suggested introduction of K atoms into the structure as a cause for the transition at Me _20–25. The data from this study indicate that a transition at Me _20–25 is unlikely (Figure 3). The large K atom should cause a significant change in V, which is not observed (Figure 3).

The normalized unit-cell parameters are shown in Figure 4. Both V/V ₀ and a/a ₀ vary linearly with Me% and show no discontinuity. The a ₀, c ₀, and V ₀ values used are 12.047 56(1), 7.563 21(1), and 1097.751(1) Å, respectively, for sample-1 (Table I). The non-linear variation of c/c ₀ shows a maximum value at about Me _37.5 and a minimum value at about Me ₇₅ (Figure 4).

Figure 4. Normalized unit-cell parameters. Both V/V ₀ and a/a ₀ vary linearly with Me%. The non-linear variation of c/c ₀ shows a maximum value at about Me _37.5 and a minimum value at about Me ₇₅.

Some studies did not report a discontinuity at Me ₇₅ and appear to incorrectly identify phase transitions in the ranges Me _20–25 and Me _60–67 (Sokolova et al., Reference Sokolova, Kabalov, Sherriff, Teertstra, Jenkins, Kunath-Fandrei, Goetz and Jäger1996; Teertstra and Sherriff, Reference Teertstra and Sherriff1996, Reference Teertstra and Sherriff1997; Zolotarev, Reference Zolotarev1996; Sherriff et al., Reference Sherriff, Sokolova, Kabalov, Teertstra, Kunath-Fandrei, Goetz and Jäger1998, Reference Sherriff, Sokolova, Kabalov, Jenkins, Kunath-Fandrei, Goetz, Jäger and Schneider2000; Sokolova and Hawthorne, Reference Sokolova and Hawthorne2008). In this study, a discontinuity occurs at Me ₇₅ (=five Al apfu), and a maximum c value occurs at Me _37.5 (=four Al apfu). Maximum order of Al–Si, Cl–CO₃, and Na–Ca, together with APBs occur at Me _37.5 (Hassan and Buseck, Reference Hassan and Buseck1988; Antao and Hassan, Reference Antao and Hassan2011a).

Why do the c values show some scatter between 1103 and 1108 Å³, that is, near Me _37.5 [Figure 3(d)]? This scatter is related to APBs and the averaging of different domains containing different clusters that form with Cl and CO₃ ions, including minor SO₄ ²⁻ and K⁺ ions, and the different percentages of such clusters.

Unit-cell data across the scapolite series, particularly the c parameter, contain a discontinuity at Me ₇₅, Na₂Ca₆[Al₁₀Si₁₄O₄₈](CO₃)₂, which does not correspond to a first-order transition; it marks the point where the series is divided into two with distinct exchange reactions (Evans et al., Reference Evans, Shaw and Haughton1969; Hassan and Buseck, Reference Hassan and Buseck1988). Note that type-b reflections were observed between Me _18–90 (Seto et al., Reference Seto, Shimobayashi, Miyake and Kitamura2004). At Me ₇₅, the A site ideally contains only CO₃ groups and no Cl atoms.

A detailed explanation of the unit-cell variations across the scapolite series with composition must come from bond distances and angles and will be addressed in a separate paper, but a qualitative explanation is given here. The height of a TO₄ tetrahedron is about 2.5 Å, and the height of three tetrahedra is 7.5 Å, which is similar to the c parameter value in scapolite (Table I; Figure 5). The variation in the c parameter across the series is related to the Al–Si order. A maximum c value occurs where there is complete Al–Si order at Me _37.5 (Antao and Hassan, Reference Antao and Hassan2011a). From Me _0–100, Al replaces Si [Al–O > Si-O], larger CO₃ group replaces Cl, longer M (Ca, Na site) −A (Cl,CO₃ site) distances occur, hence a and V increase. This increase in a and V is restrained by stronger <Ca–O> bonds that decrease and cause the TO₄ tetrahedra to rotate, resulting in decreasing T–O–T angles, as Ca replaces Na from Me _0–100.

Figure 5. (Color online) Part of the structure of scapolite showing a central cage containing the interstitial framework M cations and A anions and a few five-membered rings in space group P4₂/n. The c unit-cell parameter is related to the Al–Si order in the T sites and a maximum c value occurs at Me _37.5 (=four Al apfu) where complete Al–Si order is shown (T1 and T3 = Si, and T2 = Al; Antao and Hassan, Reference Antao and Hassan2011a).

Scapolite forms two solid solutions that meet at Me ₇₅, Na₂Ca₆[Al₁₀Si₁₄O₄₈](CO₃)₂, where a discontinuity occurs in the unit-cell parameters and chemical compositions. Division of the scapolite series into two at Me ₇₅ was as previously observed (Evans et al., Reference Evans, Shaw and Haughton1969; Hassan and Buseck, Reference Hassan and Buseck1988). Me ₇₅ is a valid end-member.