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Article

Al-Si Order and Chemical Composition Model across Scapolite Solid Solutions with Evidence from Rietveld Structure Refinements

by
Sytle M. Antao
Department of Earth, Energy and Environment, University of Calgary, Calgary, AB T2N 1N4, Canada
Minerals 2024, 14(8), 812; https://doi.org/10.3390/min14080812 (registering DOI)
Submission received: 16 July 2024 / Revised: 2 August 2024 / Accepted: 9 August 2024 / Published: 11 August 2024
(This article belongs to the Special Issue Crystal Structure, Mineralogy, and Geochemistry of Scapolite)
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Abstract

:
Scapolite forms solid solutions between the end members marialite, Na4[Al3Si9O24]Cl = Me0, and meionite, Ca4[Al6Si6O24]CO3 = Me100. Al-Si order and chemical composition models are proposed for the scapolite solid solutions. These models predict the chemical composition, Al-Si order, and average <T–O> distances between Me0–Me100. These models are based on the observed order of clusters and on two solid solutions that meet at Me75 coupled with predicted chemical compositions and <T–O> distances. The [Na4·Cl]3+ and [NaCa3·CO3]5+ clusters are ordered between Me0–Me75, whereas the clusters [NaCa3·CO3]5+ and [Ca4·CO3]6+ are disordered from Me75–Me100. To confirm the structural model, the crystal structure of 27 scapolite samples between Me6–Me93 has been obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. The structure was refined in space group P42/n for all the samples. The <T–O> distances indicate that the T1 (=Si), T2 (=Al), and T3 (=Si) sites are completely ordered at Me37.5, where the 1:1 ratio of [Na4·Cl]3+:[NaCa3·CO3]5+ clusters are ordered and gives rise to antiphase domain boundaries (APBs) based on Cl-CO3 order instead of Al-Si order. The presence of APBs based on Cl-CO3 order and cluster order indicate that neither space group P42/n nor I4/m are correct for the structure of scapolite, but the lower symmetry space group P42/n is a good approximation for modeling the average structure of scapolite. The complete Al-Si order at Me37.5 changes in a regular and predictable manner toward the end members: Me0, Me75, and Me100. The observed unit cell and several structural parameters show a discontinuity at Me75, where the series is divided into two. There is no structural evidence to support any phase transition in the scapolite series. The T1 site contains only Si from Me0–Me37.5; from Me37.5–Me100, Al atoms enter the T1 site and the <T1–O> distance increases linearly to Me100.

1. Introduction

Scapolite is a group of volatile-bearing framework aluminosilicate minerals, (Na,Ca,K)4[(Al,Si)3Al3Si6O24](Cl,CO3,SO4), that occur in a variety of metamorphic and igneous rocks. They exhibit many compositional and structural complexities. Scapolite stores volatiles in the lower crust and upper mantle [1], and may be an indicator of the activities of the volatile components [2,3,4,5]. The volatile components in scapolite indicate that they may play a role in capture and storage of greenhouse gases and may be of interest in Carbon capture and storage (CCS). A number of recent studies on scapolite-group minerals are available [6,7,8,9].
Scapolite forms solid solutions between the end members marialite, Na4[Al3Si9O24]Cl = Me0, and meionite, Ca4[Al6Si6O24]CO3 = Me100 [10]. A silvialite end member, Ca4[Al6Si6O24]SO4, is known [11]. The meionite percentage [Me% = 100 × Ca/(Na + Ca +K)] is used as an index to indicate the composition of scapolite [12]. Scapolite forms two series that meet at Me75, and the composition varies by replacement of [Na4·Cl]Si2 for [NaCa3·CO3]Al2 between Me0–Me75, and by the replacement of [NaCa3·CO3]Si for [Ca4·CO3]Al between Me75–Me100 [10,13,14]. Using high-resolution transmission electron microscopy (HRTEM) and image simulations, it was shown that the cage clusters [Na4·Cl]3+ and [NaCa3·CO3]5+ are ordered, whereas the clusters [NaCa3·CO3]5+ and [Ca4·CO3]6+ are disordered [14]. In addition, chemical analyses were represented by two straight lines that meet at Me75, indicating two series; see Figure 12 in reference [14] or Figure 188 in reference [10]. The Ca-Na cations disorder on heating, but the Cl-CO3 anion order remains at 900 °C [15,16,17]. Other studies using similar experimental methods are available [18,19,20,21,22,23,24]. The Me37.5 (midway between Me0–Me75) composition, Na5Ca3[Al8Si16O48]Cl(CO3), has a 1:1 ratio of clusters [Na4·Cl]3+:[NaCa3·CO3]5+, where complete cluster and Al-Si order occurs. A maximum intensity of the type-b (h + k + l = odd) reflections occur at Me37.5 [25,26] (or see Figure 2 in reference [14]). However, Sokolova and Hawthorne [27] indicate that complete Al-Si order is not attained in the scapolite series. This study confirms the prediction of Hassan and Buseck [14] and Lin [25] that complete Al-Si order exists for Me37.5. A number of recent studies divided the scapolite series into three subseries at Me20–Me25 and Me60–Me67 [11,27,28,29,30,31,32,33,34,35].
Using transmission electron microscopy (TEM) and the type-b reflections, three subseries with different boundaries were identified by Seto et al. [36]. In series Me0–Me18 and Me90–Me100, type-b reflections are absent and indicate space group I4/m. Type-b reflections are present in series Me18–Me90 and indicate space group P42/n. The antiphase domain boundaries (APBs) imaged with type-b reflections in Me18–Me90 samples are of various shapes and sizes and are attributed to Al-Si order [36] instead of Cl-CO3 order [14,37].
The division of the series into three is debatable and has been addressed [38,39]. These studies observed a discontinuity in unit-cell parameters at Me75 where a change in the compositional trend occurs. Where the scapolite series is divided into two series at Me75, chemical exchange reactions show how the composition changes, which is not the case where the series is divided into three.
Ideal solid solutions between end members in a binary system generally give rise to simple relations between structural parameters and compositions. No simple relation was observed for scapolite solid solutions because of complications from APBs, Ca-Na, Cl-CO3, and Al-Si order across the series. However, scapolite-group minerals are excellent materials to investigate the relation between composition and structural parameters. This study shows that simple relations do exist across the scapolite series.
In this study, Al-Si order and compositional models are proposed for scapolite solid solutions; these models predict the composition and Al-Si order based on <T–O> distances between Me0–Me100. The series is divided into two: Me0–Me75 and Me75–Me100. At Me37.5, complete Al-Si and complete [Na4·Cl]3+ and [NaCa3·CO3]5+ cluster order are present. To confirm the model experimentally, the crystal structure of 27 scapolite samples between Me6–Me93 were obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements.

2. Scapolite Crystal Structure

The framework structure of scapolite, M4[T12O24]A, consists of AlO4 and SiO4 tetrahedra, which form two types of four-membered rings (Figure 1a). Type-1 rings consist only of T1 tetrahedra that point in the same direction, whereas type-2 rings consist of tetrahedra that point alternately up and down. In space group P42/n, each of the T1, T2, and T3 sites has a multiplicity of eight (Figure 1b). In space group I4/m, the T2 and T3 (=T2′) combine to give a multiplicity of 16 and the topology is the same as that for space group P42/n. Five-membered rings occur along columns parallel to the c axis (Figure 1b). The continuous oval-shaped channels contain Na+ and Ca2+ cations with minor K+ (=M site), and the large cages contain Cl−, CO32−, and SO42− anions (=A site). Each A anion is coordinated by four M cations in a square planar configuration (Figure 1b). The resulting anion–cation cage clusters, if disordered, are centered at the corners [for example, P1 = (0,0,0)] and center [for example, P2 = (½,½,½)] of a unit cell. However, the clusters are ordered in series-1 (Me0–Me75) scapolite, so [Na4·Cl]3+ and [NaCa3·CO3]5+ clusters order on the positions P1 and P2, and neither space group I4/m nor P42/n are appropriate for scapolite. The order of these clusters also gives rise to antiphase boundaries (APBs), where the occupancies of P1 and P2 positions by the clusters are switched [14,37]. The Na-Ca cations are ordered in series-1 scapolite and they can be disordered on heating [15,16,17]. The CO32− and SO42− groups are positionally disordered in the scapolite structure.

3. Scapolite Solid Solutions

Scapolite forms two solid solutions: (1) series-1 is from Me0 to Me75 (=NaCa3[Al5Si7O24]CO3) because at Me75, ideally, the A site is completely filled with CO3 and no Cl is present; and (2) series-2 is from Me75 to Me100 [13,14]. The scapolite composition varies by replacement of [Na4·Cl]Si2 for [NaCa3·CO3]Al2 in series-1, whereas series-2 varies by the replacement of [NaCa3·CO3]Si for [Ca4·CO3]Al. When scapolite chemical analyses are plotted in terms of two solid solutions that meet at Me75, the scapolite “chemical anomaly” no longer exists [38]. The cage clusters [Na4·Cl]3+ and [NaCa3·CO3]5+ are ordered because of their size and charge differences, whereas the clusters [NaCa3·CO3]5+ and [Ca4·CO3]6+ are disordered [14]; in series-1, NaSi replaces CaAl, as in plagioclase feldspars. Cluster order and APBs are energetically favorable [40]. Additional clusters are also present depending on the scapolite composition, for example, SO42−-bearing clusters. The highly charged [NaCa3·CO3]5+ and [Ca4·CO3]6+ clusters stabilize Al-O-Al linkages that occur toward Me100. The Al-O-Al linkages are less stable compared to the Al-O-Si and Si-O-Si linkages, but they are not unstable [41]. The Al-O-Al linkages occur in aluminate sodalites that contain highly charged clusters and are stable to quite high temperatures [42], similar to those for scapolite [15,16,17].
Across the scapolite series, the variations are controversial. Based on changes in unit-cell parameters, space groups, and Al-Si order, the series was divided into three subseries [11,27,28,29,33,36,43]. Antao [38] observed discontinuities in unit-cell parameters at Me75, where changes in the chemical composition trend occur for two separate series.

4. Space Groups and Antiphase Domain Boundaries (APBs)

There are two space groups for scapolite based on single-crystal X-ray diffraction studies. The structure is described in space group I4/m near the end members Me0 and Me100, whereas the intermediate members are described by space group P42/n [11,26,27,33,43,44,45,46,47,48,49,50,51,52].
In contrast, based on TEM studies, scapolite between Me0–Me75 has space group P4 or P4/m [14,37,53]. These space groups allow for cluster order, which were directly observed in HRTEM images [14]. The space groups P42/n and P4/m are incorrect for scapolite that contains cluster order, but they are good approximations because of APBs arising from Cl-CO3 order.
The type-b reflections [h + k + l = odd; for example (021)] are absent in space group I4/m but are present in space group P42/n. Type-b reflections give rise to APBs. The interpretation of the APBs is either based on Al-Si order [36,48] or Cl-CO3 order [14,37]. There is direct evidence of cluster order in HRTEM images obtained by Hassan and Buseck [14]. On heating, the Na-Ca order is destroyed, Cl-CO3 order remains as inferred from the presence of the type-b reflections, and Al-Si order increases [15,16,17]. There is no evidence that shows the APBs arise from Al-Si order that is present to some extent from Me0 to Me100. Al-Si disorder does not occur in the scapolite series.
Based on TEM observations by Seto et al. [36], from Me0 to Me18 and from Me90 to Me100, type-b reflections are absent and indicate space group I4/m. Type-b reflections are present from Me18–Me90 and give rise to APBs of various shapes and sizes and are attributed to Al-Si order and indicate space group P42/n [36]. Seto et al. [36] also observed type-c (00l, l odd) reflections in some scapolite specimens that give rise to APBs and violate the space group P42/n. Seto et al. [36] interpreted the APBs as arising from Al-Si order because of a transition where they suggested that the high-temperature structure forms as high-symmetry I4/m and transforms to low-symmetry P42/n on slow cooling, irrespective of composition. Rapid cooling preserves the metastable I4/m structure [36]. If this is correct, for a particular composition, the high-temperature I4/m structure should have a more disordered Al-Si distribution and no type-b reflections compared to an ordered Al-Si distribution for the low-temperature P42/n structure. Therefore, quenched metastable I4/m scapolite should have more disordered Al-Si distribution, which is not observed. Based on published structure refinement data, all scapolite samples show partial or complete Al-Si order [26,27,45,46,47,54]. High-temperature studies on scapolite showed that the Na-Ca cations are disordered at about 300 °C, and near 900 °C the Al-Si order increases for the T1 site and disorder in the T2–T3 sites in both P- and I-type scapolite, but the type-b reflections in P-type scapolite were present at all temperatures [15,16,17].
If the type-b reflections give rise to APBs that arise from Al-Si order that occurs to various extents throughout the series, then the type-b reflections and APBs should also occur throughout the series, but that is not the case. The average X-ray structure of scapolite indicates continuous and regular change in Al-Si order across the series. There is never complete Al-Si disorder in scapolite. If Al-Si disorder is complete and coincides with the disappearance of the type-b reflections, then there may be a relation between type-b reflections and Al-Si order. The APBs arise from cluster order that was directly observed in HRTEM images. Type-b reflections occur from Me18–Me90 [36]. From Me75 to Me100, ordered clusters from series-1 may be present as inclusions in a host series-2 continuous framework structure, as was observed for Me79.6 scapolite, and give rise to type-b reflections [14]. If the type-b reflections are unobserved near Me0 and Me100, it does not mean that type-b reflections are really absent; the experimental technique to observe weak type-b reflections may be lacking. This fact was stated in previous single-crystal structure refinements where space group P42/n, instead of I4/m, was used to refine the structures of Me93 and Me19.4 although the authors clearly stated that no type-b reflections were observed [46,47].

5. Al-Si Order and Compositional Model for Scapolite Solid Solutions

Using the cluster order that was directly observed by Hassan and Buseck [14], and average <Si–O> and <Al–O> distances of 1.6100(2) and 1.7435(2) Å, respectively, for sodalite [55,56], Al-Si order and chemical composition models are proposed for scapolite solid solutions (Table 1, Figure 2). This model explains the chemical compositional trends and Al-Si order based on <T–O> distances for the T1, T2, and T3 sites. The average <T–O> distance in sodalite, Na8[Al6Si6O24]Cl2, was chosen because Al-Si order is well established, and the composition is similar to that of scapolite because it contains large anions, unlike plagioclase. Charge-balanced compositions for scapolite at different points in the series and using different ratios for the anion clusters are listed (Table 1). A minimum of four points at Me18.5, Me37.5, Me56.25, and Me75 are needed for the model, but additional points are given (Table 1). From these four points, the model is constructed graphically (Figure 2). Similar types of Al-Si order models were considered by Lin and Burley [47,54] and Otterdoom and Wenk [48]. The mean <T–O> distances from Sokolova and Hawthorne [27] (x symbols) are shown in Figure 2 as well as in other figures for comparison and are not fitted; their data fall close to the theoretical mean <T–O> distance (thick black line in Figure 2). The mean <T–O> distances from this study are fitted (R2 =0.9124). At Me37.5, because of its unique composition, Na5Ca3[Al8Si16O48]Cl(CO3), maximum Al-Si order occurs with the T1 and T3 sites containing only Si atoms, and with the T2 site containing only Al atoms. At this point, maximum intensity is observed for the type-b reflections [25,26,54], and the ratio of ordered clusters [Na4·Cl]3+:[NaCa3·CO3]5+ is 1:1 [14]. The Na-Ca atoms are also ordered [16]. Above Me37.5, additional Al atoms cannot enter the T2 site that is already filled with Al atoms and must enter the T1 and T3 sites (Si atoms enter the T2 site above Me37.5; Figure 2). As complete order of the clusters only occurs at Me37.5, away from Me37.5, the clusters are ordered as best as possible, and the “excess” clusters are distributed randomly.
The average <T2,3–O> distances are shown (green line in Figure 2). A thick black line is drawn through the mean <T–O> values at Me37.5 and Me56.25 (Table 1). The T1 site contains only Si atoms from Me0 to Me37.5 and thereafter, Al atoms enter the T1 site from Me37.5 to Me100, and the Al-Si exchange is represented by the magenta straight line that increases with Me% (Figure 2). Between Me37.5–Me56.25, the <T1–O> and <T3–O> distances follow the same path. From Me56.25 to Me75, the <T3–O> and <T2–O> distances are “mirror” images.
The model in this study is based on the fact that the clusters are ordered so theoretical chemical formulae can be written down, and that scapolite forms two binary solid solutions that meet at Me75, where a discontinuity occurs. This proposed model is tested experimentally by using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements.

6. Scapolite Samples and Experimental Methods

A total of 27 scapolite samples from various localities and sources were used in this study; their descriptions, localities, unit-cell parameters, and chemical analyses are given elsewhere [38]. Crystals of scapolite 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. Additional details of the experimental set-up are given elsewhere [55,57,58].
The crystal structure of scapolite was modeled using the Rietveld method [59], as implemented in the GSAS program [60], and using the EXPGUI interface [61]. Initial structural parameters for a P42/n scapolite were taken from Levien and Papike [45]. The structure refinement was carried out by varying parameters in the following sequence: scale factor, background, unit cell, zero shift, profile, atomic positions, and isotropic displacement parameters. The chemical composition was used to fix the site occupancies. Finally, all variables were refined simultaneously until convergence is achieved.
The CO3 and SO4 groups are disordered in the scapolite structure. In many refinements of the scapolite structure, the O atoms of these anion groups and C were not refined, or they were fixed by geometry and not refined [29,30,32,44,46,47,49,51,54]. The exception is where the CO3 group was refined as a rigid body [45]. Single-crystal structure refinements are available for many scapolite samples across the series [27]. Because of cluster order and APBs, the Na-Ca and Cl-CO3 order cannot be modeled in either space group P42/n or I4/m. In the structure refinements in this study, Cl, C, and S atoms are placed on the A site, and one 8-fold position is chosen, denoted O7C, to represent O atoms from both the SO4 and CO3 groups. The O7C position and displacement parameter are refined.
The structure of all the scapolite samples refined quite well. An example of an HRPXRD trace for scapolite sample-14 is shown (Figure 3).
The unit-cell parameters, compositions, and other information regarding data collection and structure refinement are given in Tables 1 and 2 from Antao [38]. The atom coordinates and isotropic displacement parameters are given in Table S1, and the bond distances are given in Table S2. Tables S1 and S2 are available online as Supplementary Materials.

7. Results and Discussion

7.1. Normalized Unit-Cell Paramters

The variations of unit-cell parameters for scapolite across the series were discussed [38]. In this study, the unit-cell parameters are normalized to scapolite sample-1, which has the smallest unit cell, and the results are shown as d/d0 ratio (Figure 4). The trend lines, based on least-squares fits, are for the data from this study. Dashed vertical lines are drawn where discontinuities in unit-cell parameters are expected. The a/a0 and V/V0 ratios increase linearly across the scapolite series, whereas the c/c0 ratio increases to a maximum value at Me37.5, thereafter decreases to Me75, and increases again to Me100 (Figure 4). Maximum order of Al-Si, Ca-Na, and Cl-CO3 occurs at Me37.5.
Sheriff et al. [29] indicated breaks in unit-cell parameters and Al-Si order at two different points [Al3.6Si8.4O24 and Al4.7Si7.3O24; complete compositions were not given] where they divided the series into three subseries. Ulbrich [62] suggested breaks in unit-cell parameters between Me65–Me66 (see Figure 1 in Ulbrich [62]), whereas Levien and Papike [45] and Eugster et al. [63] described linear variation of cell parameters with Me%. Sokolova and Hawthorne [27] showed that their unit-cell data indicate three separate trends with phase transitions at Me20–Me25 and Me60–Me67. In a similar way, three separate trends were reported by Zolotarev [34] and Teertstra and Sherriff [33,43]. In this study, the only significant break in unit-cell parameters occur at Me75.
The data from this study do not support any breaks in unit-cell parameters for compositions less than about Me18. Hawthorne and Sokolova [28] suggested the introduction of K atoms into the structure as a cause for a transition below Me18. The larger K atoms should cause an increase in volume, which is not observed experimentally.
Based on the unit-cell data from this study, there is a reliable discontinuity at Me75 that marks the division of the series into two. Therefore, Me75 is a valid end member and deserves a proper mineral name. In general, a and V parameters increase with Me% because of the increase in the number of larger Al atoms compared to Si atoms. The c parameter achieves a maximum value at Me37.5, where maximums occur for cluster order, Al-Si and Na-Ca order, and intensities of type-b reflections.
Other researchers have reported that the a and V cell parameters increase approximately linearly with increasing Me% [26], or with a trend inversely proportional to Me% [64]. However, Baker [65] showed that this variation is nonlinear, especially for the more calcic compositions. The unit-cell parameters vary with framework composition and Al-Si order, but the influence of the interframework ions is only minor [33]. Sokolova and Hawthorne [27] investigated the variation in the unit-cell parameters with several structural parameters.
The c unit-cell parameter shows large variations in values at about Me37.5 (Figure 5). The Ca-Na and Al-Si contents should be fixed and should have no effect on such variations. The explanation is related to the APBs and the averaging of the different domains containing different clusters with Cl and CO3, including SO4, and the percentages of such clusters. The different types of clusters in the samples give rise to the scatter in the c parameter at about Me37.5. A discontinuity in the c parameter is clearly shown at Me75 (Figure 5b; dashed vertical line), whereas the a and V parameters vary smoothly with the mean <T–O> distance (Figure 5a,c). Beyond Me75, there are three samples (solid triangles) that belong to the Me75 to Me100 series based on the chemical analyses [38]. The Al-Si substitution is partially responsible for the smooth increase in a and V parameters as Al replaces Si atoms from Me0 to Me100.

7.2. Anion Groups

The CO3 and SO4 groups are positionally disordered in scapolite and their order with Cl atoms gives rise to APBs. This is complicated further by cluster order that causes the space group to be lower than those used in all refinements of the scapolite structure. However, placing the C atom together with Cl and S on the same A site gives rise to C–O and S–O distances close to 1.29 and 1.5 Å as expected for these bonds, respectively (Table S2).

7.3. Average <M–O> and <M–A> Distances

The M site is bonded to two O(S/C) atoms of the anion groups or a Cl atom. In addition, the M site is bonded to seven framework oxygen atoms and the variations in these average <M–O>[7] distances are shown (Figure 6). The average <M–O> and M–A distances vary with volume, V (Figure 6a,d), Me% (Figure 6b,e), and Al apfu (Figure 6c,f). A discontinuity occurs at Me75 as indicated by the dashed vertical lines (Figure 6). In general, the <M–O> distances decrease with increasing Me% as more Ca replaces Na atoms, which causes the framework tetrahedra to rotate.
The M–A distances of 3.0788(9) Å in marialite are long for Na-Cl bonds compared to 2.730(1) Å observed in sodalite [66]. In marialite, the Cl atom is surrounded by four Na atoms in a square-planar configuration, whereas in sodalite, Cl is in a tetrahedral configuration. The M–A distances vary continuously with Me% and these show discontinuities at Me75 (Figure 6d–f). The M–A distances increase with increasing Me% as larger CO3 anion groups replace Cl atoms. The M–A distance is a good structural parameter to test the internal consistencies between unit-cell parameters, Me%, and M atom positions, and the reliability of the values obtained from the structural refinements.

7.4. Average <T–O–T> Bridging Angle and <T–O> Distance

The variations of the average <T–O–T> bridging angle with three different parameters are shown (Figure 7). The average <T–O–T> bridging angles decrease with increasing Me% (Figure 7b), which indicates TO4 rotations and closing of the oval-shaped channels. Data from reference [27] indicate some discrepancies, especially near the Me0 and Me100 end members (Figure 7).
Figure 8 displays the variations in average <T–O> distances (Table S2) with three different parameters on the x-axis, and the agreement with the proposed structural model shown in Figure 2 is excellent. In comparison, although some published data indicate some discrepancies, especially near end members Me0 and Me100, they generally support the structural model from this study. If the T2 and T3 sites are considered equivalent throughout the series, in general the [(<T2–O> + <T3–O>)/2] = <T2,3–O> distance varies linearly from Me0 to Me100 (Figure 2 and Figure 8). However, the T2 and T3 sites are distinct from Me18.75 to Me75 (Figure 9). From Me0 to Me18.75, the average <T2,3–O> distance becomes enriched slightly in Al atoms whereas the T1 site contains only Si atoms. From Me18.75 to Me37.5, the T3 site becomes enriched in Si atoms whereas the T2 site becomes enriched in Al atoms in a reciprocal relation (Figure 9). The slopes of the lines representing Al-Si exchange are quite steep and complete Al-Si order is achieved in a narrow interval from Me18.75 to Me37.5. The T2 site contains only Al atoms whereas the T1 and T3 sites contain only Si atoms at Me37.5 (Figure 9). From Me37.5 to Me56.25, the T2 site begins to incorporate Si atoms and the T3 site incorporates Al atoms in a reciprocal relation, but the slopes of the lines representing the Al-Si exchange are less steep. Order in the T2 and T3 sites is achieved from Me18.75 to Me37.50, and disorder is achieved in going from Me56.25 to Me75. From Me75 to Me100, the <T2,3–O> distance indicates slight enrichment in Al atoms. The T2 and T3 sites have a reciprocal relation and may be visualized as an average of the T2 and T3 sites (=<T2,3–O>, Figure 9) throughout the series, especially at about 900 °C [16,17].
The T1 site contains only Si atoms from Me0 to Me37.5. From Me37.5 to Me100, Al atoms are incorporated into the T1 site, and the Al-Si exchange is represented by a straight line (magenta). At Me100, the T1 site contains [Al0.42Si0.58] corresponding to a <T1–O> distance of 1.6667 Å (Table 1, Figure 9). At Me75, the average <T1–O> distance is 1.6434 Å and the average <T2,3–O> distance is 1.6768 Å, whereas the T1 site contains [Al0.25Si0.75] and each of the T2 and T3 sites contains [Al0.5Si0.5]. Based on the model in this study, the T2 and T3 sites at Me100 contain [Al0.54Si0.46], where Al atoms are greater than the amount of Si atoms, violating the Al-avoidance rule. However, for Me76.9 (sample-26) and Me92.9 (sample-27), the <T2,3–O> distances are nearly the same at about 1.672 Å and correspond to a site occupancy of [Al0.44Si0.57], whereas for Me92.9, the <T1–O> distance is 1.661 Å and corresponds to a site occupancy of [Al0.38Si0.62] (Table S2). It appears that the transition at Me75 is necessary to reduce the excess of Al over Si in the T2 and T3 sites at the Me-rich end of the series. The amount of Si is greater than Al atoms towards the Me-poor end of the series, so no transition occurs towards Me0.
Data from Sokolova and Hawthorne [27] (Figure 9) indicate that full Al-Si order is never complete at any point in the series (see their Figure 7c). Other single-crystal data from the literature and this study clearly show that Al-Si order is complete at Me37.5. The average <T1–O> distances from reference [27] fall on or close to the predicted model for the T1 site, and their <T2,3–O> distances fall close to the green line throughout the series (Figure 9). However, data from reference [27] for the average <T2–O> and <T3–O> distances match the model closely at some compositions away from Me75 or Me18.75. Above Me75 and below Me18.75, structures were refined in space group I4/m [27], and their data show more scatter (Figure 9). The main reason for such mismatch close to Me75 or Me18.75 is that the ordering type-b reflections are very weak and structure refinements are biased and give misleading average <T2–O> and <T3–O> distances. For example, compare the <T–O> distances for the composition of Me76.9 (P42/n) and Me78.4 (I4/m) from reference [27]; the large differences for their average <T2–O> and <T3–O> distances clearly arise from the choice of space group because their average <T2,3–O> distances are nearly identical. Similar mismatch with the model appears to be associated with Me66.7 and Me69.6 data [27]; both refined in P42/n, but their average <T2,3–O> is as expected. By observing the average <T2,3–O> distances throughout the series, data points are close to the green line (Figure 9). However, the average <T2,3–O> distances show a break at Me75, where a transition is present. Based on high-temperature structural studies [16,17], throughout the scapolite series, the T2 and T3 sites appear completely disordered at about 900 °C, where the T1 site contains only Si atoms.

7.5. Oval-Shaped Channels and Tetrahedral Rotations

In Figure 10, an oval-shaped channel is displayed at room temperature. The ratio of the short axis (Oa–Ob) to long axis (Oc–Od) (Figure 10) decreases linearly with Me% (Figure 11). With increasing Me%, the oval-shaped channels “close up” as the short diameter decreases and cause the TO4 tetrahedra to rotate “in” (Figure 10), as indicated by Levien and Papike [45]. Increasing temperature has the opposite effect on the short axis and causes it to increase with temperature and the TO4 tetrahedra rotate “out” [16,17]. The TO4 tetrahedra rotate in opposite directions with increasing Me% and increasing temperature. However, the main reason for the expansion/contraction of the scapolite structure arises from the expansion/contraction of the <M–O> and M–A distances that cause the TO4 tetrahedra to rotate “in” or “out”, and give rise to opening/closing of the oval shaped channels, resulting in a more open/closed framework structure (Figure 10 and Figure 11).

8. Concluding Remarks

A new structural model is proposed for the scapolite solid solutions and predicts Al-Si order and <T–O> distances across the series from Me0 to Me100. This model is based on observed order of clusters and two binary solid solutions that meet at Me75, where a discontinuity occurs [14,15]. This Al-Si order and compositional model is confirmed by crystal structural parameters obtained for 27 samples of scapolite using HRPXRD data and Rietveld structure refinement. The powder diffraction data used in this study are of such high quality that they produce structural data similar to that of single-crystal quality. For Me0–Me18.25 and Me75–Me100, several researchers refined the scapolite structure in space group I4/m because the type-b reflections were unobserved. Seto et al. [36] observed type-b reflections and APBs from Me18 to Me90, and it is likely that they disappear only in the extreme end members. The <T–O> distances indicate that the T1 (=Si), T2 (=Al), and T3 (=Si) sites are fully ordered at Me37.5, where complete cluster order also occurs and gives rise to APBs. Thereafter, the Al-Si order changes in a regular and predictable manner towards the end members Me0, Me75, and Me100. The T1 site contains only Si atoms between Me0–Me37.5. Between Me37.5–Me100, Al atoms enter the T1 site and the <T1–O> distances change linearly to Me100. The unit-cell parameters and several structural parameters show a discontinuity at Me75 indicating that scapolite forms two solid-solution series that meet at Me75. In series-1 scapolite, the clusters are ordered, whereas in series-2 they are disordered. Inclusions of series-1 scapolite occur in series-2 [14]. Type-b reflections give rise to APBs that arise from cluster order instead of Al-Si order. The anions remain ordered to at least 900 °C where the Al-Si order is destroyed in the T2 and T3 sites (T1 contains only Si atoms) and the type-b reflections still exist. The Na-Ca order is destroyed at a lower temperature [16,17].

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min14080812/s1: Table S1: Atom positions and isotropic displacement parameters (Å2) for 27 scapolite-group samples; Table S2: Selected bond distances (Å) and angles (°) for scapolite-group minerals.

Funding

This research was funded by an NSERC Discovery Grant to S.M.A., grant number 10013896.

Data Availability Statement

Data supporting reported results in this study can be found in reference [38].

Acknowledgments

The three anonymous reviewers are thanked for comments that helped to improve this manuscript. D.M. Shaw and J. Post are thanked for providing some of the scapolite samples. The HRPXRD data were collected at the X-ray Operations and Research beamline 11-BM, Advanced Photon Source (APS), Argonne National Laboratory (ANL). Use of the APS was supported by the U.S. Dept. of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Conflicts of Interest

The author declares no conflicts of interest.

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Minerals 14 00812 g001
Figure 1. (a) Scapolite structure projected down the c axis showing the framework T and interstitial M and A sites, as well as the four-membered rings and oval-shaped channels for space group P42/n. The scapolite structure in the higher symmetry space group I4/m has the same topology with T2′ = T2 + T3. (b) Part of the structure of scapolite in space group P42/n showing a central cage containing A and M sites; the A anion is coordinated by four M cations in a square-planar configuration. Uncommon five-membered rings are shown.
Figure 1. (a) Scapolite structure projected down the c axis showing the framework T and interstitial M and A sites, as well as the four-membered rings and oval-shaped channels for space group P42/n. The scapolite structure in the higher symmetry space group I4/m has the same topology with T2′ = T2 + T3. (b) Part of the structure of scapolite in space group P42/n showing a central cage containing A and M sites; the A anion is coordinated by four M cations in a square-planar configuration. Uncommon five-membered rings are shown.
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Minerals 14 00812 g002
Figure 2. A model for Al-Si order across scapolite solid solutions based on average tetrahedral distances: T1 = <T1–O>, T2 = <T2–O>, T3 = <T3–O>, <T2,3–O> = (<T2–O> + <T3–O>)/2, and mean <T–O> = (<T1–O> + <T2–O> + <T3–O>)/3. The <T–O> distances with Me%, Si apfu, and T site occupancies [=Al/(Al + Si)]] are given (Table 1). The thick black line is based on data points at Me37.5 and Me56.25 (open circles). The experimental mean <T–O> distances (solid squares from this study) are fitted by the thin black straight line (R2 = 0.9124). At Me37.5, a maximum occurs for Al-Si order, cluster order, and intensity of type-b reflections.
Figure 2. A model for Al-Si order across scapolite solid solutions based on average tetrahedral distances: T1 = <T1–O>, T2 = <T2–O>, T3 = <T3–O>, <T2,3–O> = (<T2–O> + <T3–O>)/2, and mean <T–O> = (<T1–O> + <T2–O> + <T3–O>)/3. The <T–O> distances with Me%, Si apfu, and T site occupancies [=Al/(Al + Si)]] are given (Table 1). The thick black line is based on data points at Me37.5 and Me56.25 (open circles). The experimental mean <T–O> distances (solid squares from this study) are fitted by the thin black straight line (R2 = 0.9124). At Me37.5, a maximum occurs for Al-Si order, cluster order, and intensity of type-b reflections.
Minerals 14 00812 g002
Minerals 14 00812 g003
Figure 3. HRPXRD trace for a typical scapolite sample-14 from Fishtail Lake, Ontario (Me40.1) at room T, together with the calculated (continuous line) and observed (crosses) profiles. The difference curve (Iobs–Icalc) is shown at the bottom. The short vertical lines indicate allowed reflection positions. The (021) reflection is indicated and shown in the insert. All the data beyond 10 and 20° are multiplied by 5 and 30, respectively.
Figure 3. HRPXRD trace for a typical scapolite sample-14 from Fishtail Lake, Ontario (Me40.1) at room T, together with the calculated (continuous line) and observed (crosses) profiles. The difference curve (Iobs–Icalc) is shown at the bottom. The short vertical lines indicate allowed reflection positions. The (021) reflection is indicated and shown in the insert. All the data beyond 10 and 20° are multiplied by 5 and 30, respectively.
Minerals 14 00812 g003
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Figure 4. Variation of normalized unit-cell parameters (V/V0, solid squares; a/a0, solid triangles; c/c0, solid circles) with (a) volume, V; (b) Me%; (c) Al apfu; and (d) mean <T–O> distance. Lines shown are least-squares fits to the data from this study (solid symbols). Dashed vertical lines are drawn where a discontinuity occurs for the c/c0 ratio, but the a/a0 and V/V0 ratios increase linearly. The Al-Si atoms are partially responsible for the increase in a/a0 and V/V0 as Al replaces Si atoms in going from Me0 to Me100. Al-Si order is responsible for the variation in the c/c0 ratio that has a maximum value at Me37.5 and a minimum value at Me75 in (b). The change in c parameter is small compared to that for a and V. Discontinuities occur at 5 Al apfu in (c) and mean <T–O> distance in (d). Data from reference [27] are displayed.
Figure 4. Variation of normalized unit-cell parameters (V/V0, solid squares; a/a0, solid triangles; c/c0, solid circles) with (a) volume, V; (b) Me%; (c) Al apfu; and (d) mean <T–O> distance. Lines shown are least-squares fits to the data from this study (solid symbols). Dashed vertical lines are drawn where a discontinuity occurs for the c/c0 ratio, but the a/a0 and V/V0 ratios increase linearly. The Al-Si atoms are partially responsible for the increase in a/a0 and V/V0 as Al replaces Si atoms in going from Me0 to Me100. Al-Si order is responsible for the variation in the c/c0 ratio that has a maximum value at Me37.5 and a minimum value at Me75 in (b). The change in c parameter is small compared to that for a and V. Discontinuities occur at 5 Al apfu in (c) and mean <T–O> distance in (d). Data from reference [27] are displayed.
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Figure 5. Variation in unit-cell parameters with mean <T–O> distances across the scapolite series: (a) a parameter; (b) c parameter; (c) volume, V. Lines shown are fitted to the data from this study (solid symbols). A dashed line is drawn where a discontinuity occurs for the c parameter in (b), but a and V increase non-linearly (R2 = 0.9224 in (a) and R2 = 0.9301 in (c)). Al-Si order is responsible for the variation in c parameter that has a maximum value at Me37.5 in (b) where complete Al-Si order occurs in the scapolite series. Data from reference [27] are displayed.
Figure 5. Variation in unit-cell parameters with mean <T–O> distances across the scapolite series: (a) a parameter; (b) c parameter; (c) volume, V. Lines shown are fitted to the data from this study (solid symbols). A dashed line is drawn where a discontinuity occurs for the c parameter in (b), but a and V increase non-linearly (R2 = 0.9224 in (a) and R2 = 0.9301 in (c)). Al-Si order is responsible for the variation in c parameter that has a maximum value at Me37.5 in (b) where complete Al-Si order occurs in the scapolite series. Data from reference [27] are displayed.
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Figure 6. Average <M–O>[7] and M–A distances as a function of V (a,d); Me% (b,e); and Al apfu (c,f). Solid lines are least-squares fits to the data from this study. A dashed line is drawn at V = 1120 Å3 (a,d), Me75 (b,e), and 5 Al apfu (c,f) where discontinuities are observed. The <M–O>[7] distances decrease from Me0 to Me100 because of the replacement of Na by Ca atoms, whereas the M–A distances increase because of the replacement of Cl by CO3 groups. Data from reference [27] are displayed.
Figure 6. Average <M–O>[7] and M–A distances as a function of V (a,d); Me% (b,e); and Al apfu (c,f). Solid lines are least-squares fits to the data from this study. A dashed line is drawn at V = 1120 Å3 (a,d), Me75 (b,e), and 5 Al apfu (c,f) where discontinuities are observed. The <M–O>[7] distances decrease from Me0 to Me100 because of the replacement of Na by Ca atoms, whereas the M–A distances increase because of the replacement of Cl by CO3 groups. Data from reference [27] are displayed.
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Figure 7. Average of six tetrahedral bridging angles, <T–O–T>, as a function of (a) V; (b) Me%; and (c) Al apfu. Solid lines are least-squares fits to the data from this study. Average <T–O–T> angle decreases from Me0 to Me100 because of the replacement of Na by Ca atoms, which causes the TO4 tetrahedra to rotate. Data from reference [27] are displayed.
Figure 7. Average of six tetrahedral bridging angles, <T–O–T>, as a function of (a) V; (b) Me%; and (c) Al apfu. Solid lines are least-squares fits to the data from this study. Average <T–O–T> angle decreases from Me0 to Me100 because of the replacement of Na by Ca atoms, which causes the TO4 tetrahedra to rotate. Data from reference [27] are displayed.
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Figure 8. Average tetrahedral distances, <T–O>, as a function of (a) V; (b) Me%; and (c) Al apfu. Solid lines are least-squares fits to the data from this study (solid symbols). The mean <T–O> distance [mean T = (<T1–O> + <T2–O> + <T3–O>)/3] increases linearly from Me0 to Me100 in (b). The average <T1–O> distance increases slightly from Me0 to Me37.5 in (b), or 3 to 4 Al apfu in (c) even though the T1 site contains only Si atoms in this range. From Me37.5 to Me100 in (b), or 4 to 6 Al apfu in (c), the average <T1–O> distance increases linearly as Al atoms begin to enter the T1 site. The average <T2,3–O> distance also increases slightly from Me0 to Me37.5, or 3 to 4 Al apfu, and thereafter it levels off [<T2,3–O> = (<T2–O> + <T3–O>)/2]. Predicted mean <T–O> distances are shown as dashed lines in (b,c). Data from reference [27] are displayed.
Figure 8. Average tetrahedral distances, <T–O>, as a function of (a) V; (b) Me%; and (c) Al apfu. Solid lines are least-squares fits to the data from this study (solid symbols). The mean <T–O> distance [mean T = (<T1–O> + <T2–O> + <T3–O>)/3] increases linearly from Me0 to Me100 in (b). The average <T1–O> distance increases slightly from Me0 to Me37.5 in (b), or 3 to 4 Al apfu in (c) even though the T1 site contains only Si atoms in this range. From Me37.5 to Me100 in (b), or 4 to 6 Al apfu in (c), the average <T1–O> distance increases linearly as Al atoms begin to enter the T1 site. The average <T2,3–O> distance also increases slightly from Me0 to Me37.5, or 3 to 4 Al apfu, and thereafter it levels off [<T2,3–O> = (<T2–O> + <T3–O>)/2]. Predicted mean <T–O> distances are shown as dashed lines in (b,c). Data from reference [27] are displayed.
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Figure 9. Average tetrahedral distances for the T sites with Me%, Si apfu, and Al/(Al + Si). The data from this study are shown in magenta (T1 = <T1–O>), red (T2 = <T2–O>), blue (T3 = <T3–O>), and green (<T2,3–O> = (<T2–O> + <T3–O>)/2) symbols. Data from Sokolova and Hawthorne [27] are shown in black symbols. All data are placed on the graph and are not fitted. The average <T–O> distances from this study match the model quite well. Samples near Me37.5 have complete Al-Si order. At Me0, <T1–O> = 1.6100 and <T2,3–O> = 1.6601 Å. At Me100, <T1–O> = 1.6667 and <T2,3–O> = 1.6834 Å.
Figure 9. Average tetrahedral distances for the T sites with Me%, Si apfu, and Al/(Al + Si). The data from this study are shown in magenta (T1 = <T1–O>), red (T2 = <T2–O>), blue (T3 = <T3–O>), and green (<T2,3–O> = (<T2–O> + <T3–O>)/2) symbols. Data from Sokolova and Hawthorne [27] are shown in black symbols. All data are placed on the graph and are not fitted. The average <T–O> distances from this study match the model quite well. Samples near Me37.5 have complete Al-Si order. At Me0, <T1–O> = 1.6100 and <T2,3–O> = 1.6601 Å. At Me100, <T1–O> = 1.6667 and <T2,3–O> = 1.6834 Å.
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Figure 10. Expansion of the scapolite structure with Me% and temperature (T). Double-headed black arrow indicates opening of the oval-shaped channels that arise from rotation of the TO4 tetrahedra “out” as indicated by the black arrow heads in the four-membered rings because of thermal expansion. The red arrows correspond to closing of the channels with increasing Me% by rotation of the TO4 tetrahedra “in” as indicated by the red arrow heads in the four-membered rings. These openings/closings are caused by expansion or contraction of the <M–O> and M–A distances and give rise to a more open/closed framework structure with T or Me%.
Figure 10. Expansion of the scapolite structure with Me% and temperature (T). Double-headed black arrow indicates opening of the oval-shaped channels that arise from rotation of the TO4 tetrahedra “out” as indicated by the black arrow heads in the four-membered rings because of thermal expansion. The red arrows correspond to closing of the channels with increasing Me% by rotation of the TO4 tetrahedra “in” as indicated by the red arrow heads in the four-membered rings. These openings/closings are caused by expansion or contraction of the <M–O> and M–A distances and give rise to a more open/closed framework structure with T or Me%.
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Figure 11. Linear variation of channel width/length ratio (short axis Oc–Od/long axis Oa–Ob in Figure 10) with Me%. The solid line is a least-squares fit to the data from this study (R2 = 0.8161).
Figure 11. Linear variation of channel width/length ratio (short axis Oc–Od/long axis Oa–Ob in Figure 10) with Me%. The solid line is a least-squares fit to the data from this study (R2 = 0.8161).
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Table 1. Scapolite solid-solution model: meionite % (=Me%), chemical formulae, ratios of clusters, average <T–O> distances, and occupancies of T sites.
Table 1. Scapolite solid-solution model: meionite % (=Me%), chemical formulae, ratios of clusters, average <T–O> distances, and occupancies of T sites.
Me%Formulae and ClustersT1 SiteT2 SiteT3 SiteMean <T–O>
0Na8[Al6Si18O48]Cl28Si3Al + 5Si3Al + 5Si
[Na4·Cl]3+ = 11Si0.375Al + 0.625Si=T2
Tetrahedral distances1.6100 Å1.6601 Å1.6601 Å1.6434
9.38Na7.25Ca0.75[Al6.5Si17.5O48]Cl1.75(CO3)0.258Si3.25Al + 4.75Si3.25Al + 4.75Si
[Na4·Cl]3+:[NaCa3·CO3]5+ = 1.75:0.251Si0.406Al + 0.594Si=T2
Tetrahedral distances1.6100 Å1.6642 Å1.6642 Å1.6461
18.75Na6.5Ca1.5[Al7Si17O48]Cl1.5(CO3)0.508Si3.5Al + 4.5Si3.5Al + 4.5Si
[Na4·Cl]3+:[NaCa3·CO3]5+ = 1.5:0.51Si0.438Al + 0.563Si=T2
Tetrahedral distances1.6100 Å1.6701 Å1.6701 Å1.6501
37.5Na5Ca3[Al8Si16O48]Cl(CO3)8Si8Al8Si
[Na4·Cl]3+:[NaCa3·CO3]5+ = 1:11Si1Al1Si
Tetrahedral distances1.6100 Å1.7435 Å1.6100 Å1.6545
56.25Na3.5Ca4.5[Al9Si15O48]Cl0.5(CO3)1.51Al + 7Si7Al + 1Si1Al + 7Si
[Na4·Cl]3+:[NaCa3·CO3]5+ = 0.5:1.50.125Al + 0.875Si0.875Al + 0.125Si0.125Al + 0.875Si
Tetrahedral distances1.6267 Å1.7268 Å1.6267 Å1.6601
75Na2Ca6[Al10Si14O48](CO3)22Al + 6Si4Al + 4Si4Al + 4Si
[NaCa3·CO3]5+ = 10.25Al + 0.75Si0.5Al + 0.5Si=T2
Tetrahedral distances1.6434 Å1.6768 Å1.6768 Å1.6656
87.5NaCa7[Al11Si13O48](CO3)22.7Al + 5.3Si4.15Al + 3.85Si4.15Al + 3.85Si
[NaCa3·CO3]5+:[Ca4·CO3]6+ = 1:10.338Al + 0.663Si0.519Al + 0.481Si=T2
Tetrahedral distances1.6567 Å1.6793 Å1.6793 Å1.6718
100Ca8[Al12Si12O48](CO3)23.4Al + 4.6Si4.3Al + 3.7Si4.3Al + 3.7Si
[Ca4·CO3]6+ = 10.425Al + 0.575Si0.538Al + 0.463Si=T2
Tetrahedral distances1.6667 Å1.6834 Å1.6834 Å1.6779
These values in Table 1 were used to construct Figure 2. Mean <T–O> = (<T1–O> + <T2–O> + <T3–O>)/3.
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Antao, S.M. Al-Si Order and Chemical Composition Model across Scapolite Solid Solutions with Evidence from Rietveld Structure Refinements. Minerals 2024, 14, 812. https://doi.org/10.3390/min14080812

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Antao SM. Al-Si Order and Chemical Composition Model across Scapolite Solid Solutions with Evidence from Rietveld Structure Refinements. Minerals. 2024; 14(8):812. https://doi.org/10.3390/min14080812

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Antao, Sytle M. 2024. "Al-Si Order and Chemical Composition Model across Scapolite Solid Solutions with Evidence from Rietveld Structure Refinements" Minerals 14, no. 8: 812. https://doi.org/10.3390/min14080812

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